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The CHROMacademy Essential guide to
Translating GC Methods from Helium to Hydrogen Carrier Gas


Translating GC methodsThe Essential Guide from LCGC’s CHROMacademy presents the definitive guide in translating your GC methods for use with Hydrogen Carrier. In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) consider the impact of the global issues with Helium supply on Gas Chromatography methods. We consider the use of hydrogen as an alternative carrier gas to Helium and present the facts around Hydrogen safety and use within the laboratory. We study method translation, including all of the important method and instrument parameters to ensure retention, selectivity and resolution are maintained. We consider the use and limitations of hydrogen carrier with mass spectrometric detectors and finally highlight some of the advantages of working with a carrier capable of producing high efficiency at higher flow rate.

Topics include

  • Why change at all?
  • Using hydrogen safely – generators and cylinders
  • Maintaining retention time
  • Avoiding selectivity changes in temperature programmed GC
  • Effects of changing carrier on detector performance & sensitivity
  • Using hydrogen with MS detectors
  • Re-optimizing methods for speed & efficiency

Find out more about this Month's Essential Guide Webcast »

 

Parker Balston

   
 


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If you have any more questions on this or any other topic - please post them on our forum >>

Questions and answers from the CHROMacademy Essential Guide Webcast on
Translating GC Methods from Helium to Hydrogen carrier gas with John Hinshaw and Tony Taylor.


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In our lab we use Helium which is expensive - I want to look at changing to Hydrogen, however this would render he EI libraries useless - wouldn't it?

Hydrogen is a little under half as viscous as Helium. This makes it slightly more difficult to pump away for high vacuum equipment. Secondly, hydrogen shows minimum plate heights at higher linear velocities than helium. Thirdly, when the outlet pressure drops to almost zero (i.e. a vacuum) the flow of hydrogen into the instrument may be large.

A large volumetric gas flow into the instrument when using a direct interface will mean a lower vacuum level is attainable. This may increase the number of background molecules which can collide with the ions formed, leading to a potential reduction in sensitivity and a change in the relative abundance of ions within the mass spectrum.

This all being said, if we use smaller internal diameter columns (0.15, 0.18 or 0.20mm), higher linear velocities are achievable at lower volumetric flow rates – thus we can maintain the vacuum levels within the system and there should be no wholescale changes in the appearance of the spectra.

As John Hinshaw remarked during the webcast, Hydrogen is a reactive gas whereas helium is not, and there is always an exception to the rule which states that we do not typically see ion / carrier gas reactions in GCMS, one that regularly appears in the forums is the dehalogenation of chlorinated compounds.


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If you enable gas saver on the GC you can drop the split demand significantly right?

Using a ‘gas saver’ GC mode reduces the split flow after the injcector is cleared of sample components and does indeed allow one to save gas by reducing the flow through the split line from, say. 200 mL/min. to a much lower value, 10 mL/min. for example. Whilst maintaining some split flow is desirable to flush the inlet and reduce baseline noise, a considerably lower flow than that used during the injection phase of the analysis is usually acceptable. This is a great way to conserve Helium.


---------------------------------------------------------------------------------
The hydrogen will collect in pockets near the ceiling, won't it?

Hydrogen is less dense than air and will collect at the ceiling – which is where most folks fit their hydrogen detectors – i.e. at ceiling level.


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With flow control possible, doesn't it make more sense to optimize flow instead of pressure?

Yes. Most instruments allow the user to select a desired flow, linear velocity or pressure. In temperature programmed analysis constant flow and in some cases constant linear velocity can be selected. When changing from helium to hydrogen, the calculator mentioned during the webcast will allow us to translate the initial pressure, linear velocity or flow and we can program our EPC units to adopt these initial settings from where they will calculate the required pressure to maintain contant flow or linear velocity during the temperature program

You can find the method translator at:
http://www.agilent.com/chem/heliumupdate


---------------------------------------------------------------------------------
Is there an optimum column bore for high split flow hydrogen methods?

This really depends upon your objective.

If you want fast analysis – the highest linear velocities are attainable on the smallest internal diameter columns. These columns also give high plate numbers even when relatively short column length (10-20m) are used. Smaller internal dimeter columns also require relatively high head pressures to attain the required linear velocity – which makes for more reproducible methods where the EPC units are not trying to reproduce flows at very low applied head pressures.

By keeping the phase ratio constant, the resolution of methods should be preserved.

We use 0.18, 0.20 and 0.25 mm i.d. columns pretty successfully with Hydrogen carrier gas.


---------------------------------------------------------------------------------
Can you please explain again what the concerns were converting to hydrogen as carrier gas with a megabore column?

With megabore columns, the head pressure required to achieve a given carrier linear velocity may be relatively low – a 20m x 0.53mm column running at 40 cm/sec. may require a head pressure of as little as 2 psig. EPC units don’t run so reproducibly at these very low pressures and this can cause reproducibility issues.


---------------------------------------------------------------------------------
Any suggestions on using hydrogen with a packed column? I've been dropping the makeup to keep column + makeup to be about 45ml/min.

Yes – use nitrogen! Nitrogen tends to give the best efficiency at the linear velocities typically required for packed column work.

If you’re running micro-packed columns or are stuck with hydrogen, you’re doing the right thing in managing the make-up flow to acheiev a total hydrogen flow of around 30-50 mL/min. which is optimal for most manufacturers instruments. Some instruments even have a facility to automatically adjust the make-up flow to achieve a constant column + make up flow during temperature programmed operation in constant pressure mode.


---------------------------------------------------------------------------------
How about just not using a makeup gas?

It’s certainly an option – you may not get an optimal response and you should check carefully for deviation from linearity at higher analyte concentrations, however it the method is fit for purpose you won’t be harming the instrument at all.


---------------------------------------------------------------------------------
We have an FID detector, we use Helium as carrier gas, are you saying we would still need Helium or nitrogen as a make up gas?

Yes. Using hydrogen as make-up gas usually swamps the detector and alters the stoichiometric ratio of fuel (hydrogen) to oxidizer (air) gases causing a reduction in detector sensitivity – consider, helium, nitrogen or no make-up gas


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Tony, this is Craig Marvin from Agilent. Thank you for your comments regarding our translation tools. Customers can visit http://www.agilent.com/chem/heliumupdate to access current method translator (compliant up to Windows XP) and other information related to hydrogen conversion. We expect a Win 7 compliant translator to be available shortly. If you have questions, please contact me at Craig_Marvin@agilent.com
---------------------------------------------------------------------------------


---------------------------------------------------------------------------------
Does "hydrogen wear" effect (metals become brittle when saturated with hydrogen) endanger turbo pumps? Are oil diffusion pumps preferable with hydrogen?

You know I’ve heard this suggested several times and yet I don’t really have a definitive answer. Perhaps a manufacturer representative reading this post will help us to understand a little more (if they have studied this phenomenon.....)

I know that the use of copper tubing is not recommended when using hydrogen carrier gas due to this phenomenon.


---------------------------------------------------------------------------------
Please comment on the use of a hot cathode ionization gauge to measure MSD pressure when using H2 as carrier gas.

My understanding is that most Pirani gauges are in sealed bulbs these days with a permeable membrane used to measure the background current (and hence vacuum) – I’m presuingt therefore that they will be safe. However, as always with hydrogen and GCMS I would very strongly recommend that you contact your supplier to get their instructions prior to making the switch.


---------------------------------------------------------------------------------
Can you use wider bore capillary columns with nitrogen with GC-MS?

Yes – nitrogens viscosity makes it relatively easy to pump away and achieve a good vacuum. However, I’m not sure that I’d want a whole bunch of nitrogen around - remember there will be many nitrogen molecules per cm3 even at the best vacuum levels achievable by bench top instruments. It’s a reasonably reactive gas and one might not easily predict the background / analyte ion reactions which occur in the MS ion source and analyser, thus giving rise to possible anomalies in the mass spectra obtained compared with the less reactive helium.


---------------------------------------------------------------------------------
We normally swap septa without cooling down the inlet. Can we still do that when we switch to hydrogen?

I would not see this as an issue – if you turn off the split flow or reduce your instrument total flow to a low value whilst carrying out the operating the amount of hydrogen liberated will be minimal and will not reach the explosive limit of 4% in air. Please ensure that you are in compliance with any local safety rules however and you should perhaps consider fitting a snorkel above the instrument to vent any hydrogen released during this operation or which comes from the split vent or any FID detectors that you might be using.


---------------------------------------------------------------------------------
You mentioned that carrier gas should show a high efficiency over a wide range of linear velocity for temperature programmed GC. But is this really required if you work in constant flow?

Yes – if you want to carry out fast separations, high initial flow / linear velocity can push the analytes through the column at the required rate. It;s not so much a case of high linear velocity being developed at the top of the GC temperature program – more that high flow / linear velocity can be used from the get go when running ‘fast’ or ‘high throughput’ methods and still get very highly efficient peaks.


---------------------------------------------------------------------------------
I am using only H2 and air for my TCD and FID GC. For the FID the makeup gas choices are Helium and Nitrogen only. What should be my makeup gas Hydrogen flow rate if I enter Helium?

The hydrogen makeup gas flow that should go through the FID or TCD for is the same as for helium or nitrogen. It would appear that you are using an EPC type of pneumatic controller and can only choose helium or nitrogen as the makeup gas type. You have two choices: either 1) use nitrogen or helium makeup gas or 2) enter a lower makeup flow rate, measure the actual flow rate, and then adjust the entered flow so that the desired flow is achieved. Pre-calculation of the entered flow is not advisable because the type of flow controller in use will dictate what is the effect of entering a makeup gas type different than the gas in use. Also bear in mind that for a FID the sum of Carrier + Makeup + Fuel hydrogen must equal the required total hydrogen flow through the detector at all times.


---------------------------------------------------------------------------------
What tubing do you recommend for piping your Hydrogen to the GC?

Stainless steel is preferred, especially for long-term installations, however new copper tubing is acceptable as well. Any tubing used for GC must be properly cleaned to chromatographic standards.


---------------------------------------------------------------------------------
I have the Agilent MS Quickswap sysetem. I've heard conflicting reports as to how safe it is to use it with hydrogen as when switching columns purge gas leaks out of the device. Is it a major risk having <30 ml/min leaking for a few minutes?

Disable autoignition, cool down the detectors and inlets, and turn off any detector hydrogen flows beforehand. Allow sufficient time for accumulated hydrogen to dissipate from the oven after completing the changeover before closing the oven door.


---------------------------------------------------------------------------------
Is there a way to incorporate H2 safely into a mobile laboratory situation without the use of an H2 generator? Safety shutoffs or other options for the carrier gas line?

This is the same situation as would arise for use of a FID in a mobile lab with hydrogen pressurized tank supply. Ventilate any hydrogen exit flows to the outside, restrain tanks firmly to prevent shifting during transport, use a loop in each gas line to relieve stress, install a hydrogen detector in the lab space, and install another leak detector in the GC oven. If possible, connect the alarm relays to a cut-off valve at the low-pressure output side of the gas tank regulator.


---------------------------------------------------------------------------------
How do you get rid of the hydrogen gas coming from the split vent, purge vent and TCD detectors?

Optionally vent these exit flows to the outside with a vent snorkel or route them into an exhaust hood.


---------------------------------------------------------------------------------
So there really is no safety need to route the split and purge flows to an exhaust hood? My company will likely insist on that. Does somebody just remember the Hindenburg fire in 1937?

A layered safety scheme is best. Even if routed to an exhaust hood, what happens to hydrogen flows if the hood fan fails? If depending on room ventilation, same question. For better security install a hydrogen detector set to 1 – 2 % (less than half the explosive limit) that will turn off a valve at the hydrogen tank regulator output and keep it off until manually reset. Then if the power, HVAC, or exhaust fan fails or if a potentially dangerous amount of hydrogen accumulates, you are protected.


---------------------------------------------------------------------------------
Can H2 be used in the analysis of VOC by purge and trap methods?

Hydrogen is not recommended for purge and trap methods due to the accumulation of hydrogen in the enclosed purge vessel gas space.


---------------------------------------------------------------------------------
Are there recommendations for using hydrogen as a carrier gas for thermal desorption techniques?

Hydrogen is not recommended for thermal desorption due to the possibility of sample hydrogenation at normal desorption temperatures, especially in the presence of metal desorption tubes.


---------------------------------------------------------------------------------
According to another vendor, use of hydrogen as a carrier can be a problem when using methylene chloride as a solvent and injector temperatures greater than 275C, due to potential for forming HCl. Any thoughts on this?

See answer to previous question above.


---------------------------------------------------------------------------------
What about the generation of HCl from Dichloromethane solutions?

Dichloromethane solvent is not recommended for use with hydrogen carrier as HCl can be generated in certain situations where liquid solvent is present at elevated temperatures with hydrogen carrier.


---------------------------------------------------------------------------------
We're using our GCMS with a head space sampler. What do you think about translating to H2 for vials pressurization in the Head space?

Pressurization of headspace vials with hydrogen is not recommended as it will create an enclosed pressurized volume of flammable gas. Hydrogen carrier gas can be used with headspace samplers if a separate helium or nitrogen pressurization gas can be utilized.


---------------------------------------------------------------------------------
What issues would you expect when using an static headspace sampler?

No issues are expected if hydrogen carrier and a non-flammable pressurization gas are used.

 

---------------------------------------------------------------------------------


If you have any more questions on this or any other topic - please post them on our forum >>

 

 

FREE CHROMacademy Tutorial
available to Lite & Premier Members

The CHROMacademy Essential guide to
Translating GC Methods from Helium to Hydrogen Carrier Gas

The definitive guide in translating your GC methods for use with Hydrogen Carrier.

In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) consider the impact of the global issues with Helium supply on Gas Chromatography method

We consider the use of hydrogen as an alternative carrier gas to Helium and present the facts around Hydrogen safety and use within the laboratory. We study method translation, including all of the important method and instrument parameters to ensure retention, selectivity and resolution are maintained. We consider the use and limitations of hydrogen carrier with mass spectrometric detectors and finally highlight some of the advantages of working with a carrier capable of producing high efficiency at higher flow rate.

Topics include

  • Why change at all?
  • Using hydrogen safely – generators and cylinders
  • Maintaining retention time
  • Avoiding selectivity changes in temperature programmed GC
  • Effects of changing carrier on detector performance & sensitivity
  • Using hydrogen with MS detectors
  • Re-optimizing methods for speed & efficiency



Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
 

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In gas chromatography many gases including nitrogen, helium and hydrogen have been used as the carrier gas. [1, 2] Nowadays, helium is the most widely used of all of them. The reasons for this are many and include:

  • Inert gas (no reaction with sample or column)
  • High efficiency separations
  • Safe to use

In spite of being the second most abundant element in the known universe, helium is rather rare on Earth.  In fact, helium is a non-renewable source and most terrestrial helium has been created by the natural process of radioactive decay of heavy elements such as thorium and uranium.

Equation 1: Helium production by radioactive decay or uranium

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s

 

Helium is refined from natural gas deposits using cryogenic fractional distillation in a relatively complex process and is not something we can easily obtain or produce. 

Undoubtedly the United States is world’s leading supplier of helium.  The United States National Helium Reserve accounts for almost 30% of the world’s helium; however, this reserve is expected to be depleted by 2018.  Other sites with large concentrations of helium include Algeria (second leading producer) and Qatar, however some experts believe we could face total depletion within a generation.

Plant maintenance, the sale of the US Helium reserve and low natural gas demand has led to sparse supplies in some regions and an increase in price.

In spite of being explosive, hydrogen seems to be the ideal replacement for helium as a GC carrier gas. The advantages of using hydrogen instead of helium are well known and include:

  • Reduced production costs: hydrogen is produced from renewable sources
  • Availability:  hydrogen can be produced in situ in the analytical lab (via a hydrogen gas generator)
  • Method translation: the translation from helium to hydrogen is relatively straight forward
  • High separation efficiency: lower plate height at higher linear velocities than any other common GC carrier gas
  • Faster separations: hydrogen provides faster separations at lower pressures than any other GC carrier gas
  • Achieve lower temperature separations: At the faster elution times, it might not be necessary to increase the column temperature run rate. Lower maximum temperatures are needed for the analysis or remain at those temperatures for shorter periods.
  • Longer column life: Lower temperatures lead to less column bleed and can increase column life. In addition, hydrogen is a reducing gas and can remove potential acidic sites inside the column. The removal of these sites leads to less sample absorption and less generation of phase breakdown (column bleed). The result is a longer usable life for the column.

Broadly speaking, hydrogen carrier gas is suitable for almost all GC methods, except of course for the analysis of hydrogen as a component in a mixture. The GC method has to be converted to conditions suitable for hydrogen, and detectors may require special consideration. For example, a flame ionization detector (FID) also uses hydrogen as a support gas; total FID hydrogen flow should be kept constant and equal to the manufacturer's specified amount. Some electron capture detectors (ECD) support hydrogen carrier gas while others may not; it is best to consult the manufacturer in this case. Thermal conductivity detectors (TCD) will function well, but the size of the peaks will not be the same as with helium carrier.

As can be clearly seen from the van Deemter curves (Figure 1) for efficiency against carrier gas linear velocity, there is less dependency of column performance on the average carrier gas linear velocity with hydrogen compared to that with helium. The range of linear velocities over which column efficiency lies close to the optimum (within 25% of the optimum is the usual range considered) for any particular solute is broader than with helium.

 

Figure 1.  Influence of carrier gas and linear velocity on theoretical plate height. The solid lines indicate the regions in which the plate height (H) are within 25% of the minimum plate height value (Hmin) which is often known as the Optimum Practical Gas Velocity (OGPV).  Theoretical data for a 50 m x 0.25 mm column at 100oC for an analyte of k’ = 10.0. 

 
 

Hydrogen gas seems to be a convenient replacement for helium; however, many chromatographers still have questions about method translation, performance, cost and safety. This Essential Guide will address these questions and will also explain practical aspects of using hydrogen instead of helium as a GC carrier gas.

Readers should note that this Essential Guide does not consider column stationary phase selection.  Our starting point is that you already have a helium based GC method which produces satisfactory resolution (selectivity) and that an equivalent hydrogen based method is required.  For more information on “GC column stationary phase selection”, please visit the links below: [3, 4]

 

The Theory and Instrumentation of GC - GC Columns >>

Column Choice for Capillary GC >>

 
 

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Hydrogen is not only the lightest of all elements but is also the most common substance in the universe.  Due to its reactivity, atomic hydrogen is rather rare on Earth; however, it can be found combined with other elements (forming water, organic molecules, inorganic acids, etc.).

Hydrogen is a flammable, colourless and odourless gas; it poses fire and explosive hazard when its concentration in air exceeds 4%. Proper safety precautions should therefore be used in order to prevent an explosion. [5, 6]

 

Warning: hydrogen is a flammable gas that burns with an invisible flame.  It is much lighter than air and at high concentrations could cause suffocation.

 

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
   
Figure 2.  Comparison of hydrogen and hydrocarbon flames
 

The concentration of hydrogen is very unlikely to exceed 4% even in the smallest laboratory due to the large volume of air against the rate at which hydrogen is produced by or consumed during a typical GC analysis.

See subsequent sections for more on Hydrogen safety.

 
 

Hydrogen Generation

Hydrogen is industrially produced from the steam reforming of natural gas; however, it can also be produced by electrochemical means. [6, 7]  The electrolysis of water, a very energetic process, provides the means to generate hydrogen gas for a multitude of applications. In fact, hydrogen generators for gas chromatography use electricity to split water into hydrogen and oxygen.  See equation 2.

Equation 2: Electrolysis of water

Equation 2  shows that water can be decomposed by the passage of a direct electric current through an ionic medium (due to the low conductivity of water, a strong water soluble electrolyte such as sodium hydroxide is usually added).  Two electrodes immersed in the ionic medium are powered by an external source and provide the physical interface for the electrochemical process to take place.  Those electrodes constitute the anode and cathode of the electrolytic cell.

 

Anode:

Cathode:

 

Or equivalently:

 

Anode:

Cathode:

Electrodes of different materials (metallic, graphite, semiconductor material) and geometries are currently available.

 
 

Figure 3.  Electrolysis of water (oversimplified).

 
 

Figure 4.  Hydrogen Generator with a desiccation unit.

 
 

Figure 5.  Hydrogen desiccant cartridges require periodic replacement. 

 
 

As hydrogen desiccant cartridges require periodic replacement, instrument manufacturers have developed more efficient approaches.

A very interesting alternative to ultrapure hydrogen production uses porous palladium electrodes.  These electrodes: [7]

  • provide an adequate interface for electrochemical production of ultrapure hydrogen
  • eliminate the need for desiccation units
  • conform a reliable source of pure hydrogen (99.99999%)
Figure 6 depicts a palladium based system for hydrogen production.
 

Figure 6.  Hydrogen generation with a palladium membrane based system.  Reproduced with permission. [8]

 
 

Fuel cells are electrochemical devices that produce electrical energy from the conversion of hydrogen.  Basically, an oxidant is fed to the cathode while hydrogen is infused into the anode.  The electrolyte supports the transfer of ions between the electrodes.  A general sketch of a fuel cell can be found in figure 7.

 

Figure 7.  General fuel cell construction (oversimplified).

 
 

Fuel cells technology can be used for hydrogen production (by reversing the process depicted in figure 8; this is, hydrogen and oxygen can be produced from water).

A Proton Exchange Membrane (PEM) is a polymeric semi-permeable membrane designed in such a way that only protons can diffuse through it.  PEM technology is currently used in the production of fuel cells and hydrogen generators.  A general PEM design for hydrogen generation is shown in figure 9.

 

Figure 8.  Proton Exchange Membrane fuel cell design (oversimplified).

 
 

Figure 9.  Hydrogen production by using a Proton Exchange Membrane.

 
 

Figure 10.  Hydrogen production by using proton exchange membranes.  Courtesy of Parker. [9]

 

Remember: If your GC system uses copper tubing for the delivery of hydrogen, then consider replacing it with stainless steel as reduction of copper may occur.  Likewise, never use cast iron or black steel pipe to supply gases to GC systems.

Hydrogen generators for GC are a reliable source of pure hydrogen.  In order to keep your generator working you should:

  • replace desiccation cartridges (whenever required)
  • use deionised water (follow manufacturers recommendation for the required water resistivity)
 
 

Hydrogen Cylinders vs. Generators

Due to the pressures required and the volume of gas typically consumed, gas cylinders are typically employed to supply GC instruments. [10]

Most GC labs use 25 or 50 L cylinders for hydrogen storage at 200 bar.  A small laboratory would contain approximately 50,000 L of air (at STP), so you would need to discharge almost half of the content of a 25 L cylinder (or almost a quarter of a 50L cylinder) to reach the explosion limit (4%), assuming the room is hermetically sealed, so explosions are unlikely due to hydrogen leaks. However, having relatively large quantities of gas are always less favourable than producing the gas on demand, from a generator for example.

Most hydrogen generators will implement several safety features including:

  • Low volumetric output detection
  • Low reservoir volumes (usually below 100 mL) and some produce gas on demand with no reservoir.
  • Back pressure monitoring (to constantly monitor for leaks)
  • Internal pressure monitoring to assess production run-away or over production of gas
 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s

Most gas chromatographs are manufactured with spring-loaded doors and perforated or corrugated metal column ovens. These help to either dissipate released gas or minimise the explosive force if an explosion were to occur.

When using hydrogen you should:

  • Provide adequate ventilation
  • Assess the gas output from the GC (detector, split and septum purge) and where appropriate plumb the outlets to an exhaust system
  • Check for leaks on a regular basis (use an electronic leak detector)
  • Use hydrogen generators whenever possible
  • Install a hydrogen sensor near any source of hydrogen (if your generator does not have on board), in a location near the laboratory ceiling and/or within the GC oven
  • Make sure the GC oven is equipped with a hydrogen-leak shutdown safety system (i.e. EPC below pressure safety shutdown system)
  • Avoid the use of copper tubing as, over extended time periods, it will react with hydrogen

Hydrogen generators produce small amounts of hydrogen at low pressure over a certain period of time.  Most modern hydrogen generators would hold something like 50 mL of hydrogen at a pressure not exceeding 4 bar (this is almost 200,000 times less than the actual content of a gas cylinder).  In this way, the risk of an explosion is drastically reduced.  In general terms, hydrogen generators will:

  • provide a reliable source of clean hydrogen
  • eliminate the inconvenience of dealing with hydrogen cylinders (or helium if changing carrier gas)
  • promote a safe utilization of hydrogen in the lab
 

Figure 11.  Hydrogen generator for GC analysis.  Maximum capacity 300 mL/min (99.99999% purity).  Courtesy of Parker Hannifin (UK) Limited. [14]

 
 

Most modern hydrogen generators will use plain deionised water to produce hydrogen of high purity (at least 99.9995%) which is sufficient even for GC/MS applications.  For more information consult your system manufacturer.

Example 1: 
Calculate the time required for a gas generator running at 300mL/min and 4 atm to reach 4% in air in a hermetically sealed laboratory say 500 m3 (500,000L).

Solution:
For mixtures of gases the molar and volume fractions are almost equal. 
So, the amount of hydrogen produced will correspond to 4% of 500,000L (this is 20,000L)

 

Bear in mind that hydrogen generators can improve safety by eliminating the use of high-pressure gas cylinders within the laboratory or stored in an outside location; however, when dealing with flammable gases such as hydrogen, safety is a must.

 
 

Figure 12.  Hydrogen cylinders tend to be red but other colours are used in different countries.  Check the label before using it.

 

Remember:  In order to prevent the interchange of fittings between cylinders containing flammable gases and non-flammable gases, the cylinder valve outlets are threaded to opposite hands. Therefore, cylinders containing:

  • flammable gases (such as: H2, acetylene, propane, etc.) have left-hand threads
  • non-flammable gases (such as:O2, N2, Ar, etc.) have right-hand threads
 
 

Gas Regulators

Gas regulators are of two types: single and double stage regulators.  In essence, double stage regulators are nothing but two regulators in the same device.

Gas cylinders are equipped with a two-stage regulator.  In GC, these devices work in such a way that:

  • the first stage reduces the pressure of the gas from the cylinder, usually from 200 to 30 bar
  • the second stage reduces the pressure of the gas from the cylinder, usually from 30 bar to the desired line pressure (usually in the order of 5 bar)

Warning:  When using gas regulators, please bear in mind the following pointers:

  • The use of single-stage gas regulators on gas cylinders is not recommended
  • Never remove a two-stage regulator from a gas line with a high pressure in the first stage
  • Gas generators operate at pressures lower than the pressures delivered from cylinders; therefore, single stage regulators are suitable for gas generators
  • Always use gas regulators for their intended application
  • Make sure you have read the manufacturer’s instruction before using your regulator
 

Figure 13.  Two stage cylinder regulator.

 

For more information in gas management, please visit the link below

The Essential Guide to Troubleshooting Gas Supply and Inlet Issues >>

 
 

GC Safety Considerations with Hydrogen Carrier

Most GC instruments are equipped with electronic pressure regulators which monitor carrier and detector gas pressures. If these devices sense a drop in pressure they will automatically shut down the instrument for safety reasons. The only instance in which this check might fail would be a column break at or near the detector, hence maintaining back pressure at the inlet. To guard against this, column installation should be carefully and properly carried out and the column should never rest against the internal oven walls as this may reduce its mechanical strength – leading to possible breakage. [1, 11]

Leak testing may also be carried out using an on-board gas detector or portable device.

 

Figure 14.  External hydrogen sensor.  Courtesy of SIM. [12]

 
 

Figure 15.  GC oven with gas sensor head.  Courtesy of SIM. [12]

 
 

Figure 16 shows a portable detector from SRI Instruments (Torrance, California, USA). The device can be connected to a multi-meter for quantitative readings and the flexible probe can be easily positioned within a GC oven or near the detector. [13]

 
Figure 16.  Hydrogen detector probe.  Courtesy of SRI Instruments.
 
 

Analytical Chemists

  • I feel empowered to fix things
  • I can troubleshoot effectively
  • I know where to go for help
  • I understand my analyses
  • I know where to get applications
  • I’m up to date
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  • My career is progressing
 

Laboratory Managers

  • Improved equipment utilization
  • Faster method development/problem solving
  • Flexible workforce with a common standard
  • Better quality data
  • Get up to speed quicker
  • Lower T&E
  • Less reliant on me
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A useful equation that describes the contributing factors to GC retention time and the ways in which the analysis speed might be altered is shown in Equation 3.

Equation 3: Factors influencing retention in gas chromatography.

Where:

tR = retention time (as an indicator of analysis speed)
L = column length (m)
ū = carrier linear velocity (cm/sec)
k = retention factor

When changing from helium to hydrogen as the GC carrier gas, changes in retention time can be expected.  Reasons for this include:

  • Hydrogen and helium separations optimise at different carrier gas velocities
  • The retention factor is influenced by the physicochemical properties of the gas  (mobile phase)

The best way to minimise those changes, is to keep the same carrier gas velocity and phase ratio as in the original method.

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
 
 

Switching from Helium to Hydrogen Carrier

In general terms, the appearance of the chromatogram (retention time, efficiency, resolution, and selectivity) is influenced by the choice of mobile phase.  In GC, however, this effect is much less important that in HPLC.  This is because the GC carrier gas rarely interacts chemically with the stationary phase or with the analyte.  In fact the role of the carrier gas is mainly to transport the analyte in the gas phase through the column and to act as the second phase in the partitioning mechanism.

Undoubtedly two properties of a gas play a major role in the GC process: diffusivity and viscosity.  The diffusivity of hydrogen and helium are roughly the same but hydrogen is a bit less than half as viscous as helium at the same temperature (Figure 17).  For this reason hydrogen requires a lower pressure drop to achieve the same average carrier gas velocity as for helium.

 

Figure 17.  Relative viscosity of common GC carrier gases at various temperatures. [22]

 
 

The relationship between linear velocity and pressure drop is somewhat nonlinear because of effects related to the compressibility of the carrier gas, but in general, the effect of switching from helium to hydrogen on retention time will be to cut retention times roughly in half if the inlet pressure is unchanged.

Retention times in gas chromatography (GC) are controlled by several factors: the distribution coefficient (K) of a solute in the column, which is not affected by the choice of carrier gas; the column dimensions (length, inner diameter and stationary film thickness) all of which we will keep constant in this discussion; and the average carrier gas linear velocity (ū). As the linear velocity increases, isothermal retention times decrease in exact proportion, so doubling the velocity cuts retention times in half. We will look at the effects of changing carrier gas for three situations: constant velocity, constant inlet pressure and constant flow-rate and then we will see what happens if the column is temperature programmed.

In addition to its dependencies upon the column dimensions and pressure drop, the linear velocity is influenced by the viscosity of the carrier gas. As we have seen above, hydrogen is a bit less than half as viscous as helium or nitrogen at the same temperature and so for this reason hydrogen requires a lower pressure drop to achieve the same average carrier gas velocity as for helium or nitrogen. For example, a 50 m × 250 μm column will deliver an average velocity of 60 cm/s at 100 °C with 58.6 psig (404 kPa) of helium or with 27 psig (186 kPa) of hydrogen; the situation is similar for nitrogen. If the velocity is unchanged with a hydrogen versus helium carrier, then retention times remain the same too, while the inlet pressure that is required with hydrogen will be about half as much as with helium.

Conversely, if the same inlet pressure were applied with hydrogen as with helium, then the hydrogen carrier gas would cause peaks to be eluted in less time, because the linear velocity would be faster than with helium. In the previous example, helium at 27 psig would have an average linear velocity of 29.4 cm/s and so with hydrogen carrier at the same pressure, all of the peaks' retention times would decrease in the proportion 29.4/60 ≈ 0.5.

At constant flow-rates, the situation is intermediate between the effects at constant velocity and those at constant pressure. At 27 psig of helium, the column flow-rate is going to be 1.43 cm3/min and the average velocity will be 29.4 cm/s. The hydrogen carrier pressure required for the same flow-rate will be about 16.3 psig (112.4 kPa), but now the average velocity will increase to 37.5 cm/s. Therefore, in this example, retention times will decrease by a factor of 29.4/37.5 = 0.78 for hydrogen compared with helium at the same column flow-rate.

In summary: for isothermal operation at constant average linear velocity, retention times are not affected by changing the carrier gas. At constant inlet pressure hydrogen carrier will cause peaks to be eluted in about half the time and at constant flow-rate retention times will be about 78% with hydrogen compared with helium.

 
 

What if column temperature programming is used?

If a constant linear velocity can be maintained during the temperature programme by an electronic pneumatic system, then retention times will not change, but this feature is not available in all pneumatic systems. At constant inlet pressure, hydrogen will cause peaks to be eluted earlier, but their elution temperatures will also be reduced and this can change the relative retentions of peaks with divergent chemical characteristics, such as hydrocarbons compared with polar compounds. The same effects will occur, but to a lesser degree, when comparing hydrogen with helium at constant column flow-rate. In general, if the column is temperature-programmed then each peak should be re-identified if the carrier gas is changed and it might be a good idea to optimize the temperature programme ramp rate by increasing it to restore, as closely as possible, the elution temperatures that were obtained with the helium carrier.

For ease of method translation one might use a method translation software tool such as Method Translation Software from Agilent Technologies (Santa Clara, California, USA) which can be found here, or a similar tool. This tool allows the translation of not only the carrier gas settings pressure / flow settings but can also recommended a temperature programme to preserve elution order/resolution.  Important: in order to use this software, the stationary phase chemistry must remain the same for the old and translated methods.

 
 

Example 2:  It was found that for a 30 m × 0.25 mm × 0.5 μm column, the optimum carrier gas flow rate (helium) was 0.6 mL/min, the injection volume was 0.1 μL (splitless injection).  The separation was performed under isothermal conditions (T = 120oC).  Translate to hydrogen carrier using the same column.

 

Table 1.  Tabulated information.

 
Element Information
Column: 30 m × 0.25 mm × 0.5 μm
Oven Temp: 120oC
Helium flow rate:* 0.6 mLn/min
Injection volume: 0.1 μL (splitless)

* The subscript n indicates that the flow rate (mL/min) was measured at normal conditions of temperature and pressure (0oC and 1.013 bar).

  • Find an equivalent GC method using the same column with Hydrogen carrier
  • We used Agilent’s “GC Method Translation” software tool to calculate an equivalent method with the same average liner velocity (19.40 cm/sec in this case).  Please refer to figure 18
  • This is possible by using the “None” translation criterion of the software as retention time is kept constant
  • This isn’t ideal because 19.40 cm/s flow rate is well below the optimum linear velocity for hydrogen – should really optimise at twice this linear velocity (retention times will be approx. half).  By using the “Best Efficiency” criterion, the optimum carrier gas speed by using hydrogen is approximately 32.8cm/s.  However, we are not looking to speed up the separation (at least for the moment)
  • In splitless injection hydrogen is preferred over helium as it carries the solute from the inlet into the column faster.  So, sharper peaks are obtained.  This allows for lower detection limits
 

Figure 18. 
Method translation from helium to hydrogen on a 30 m × 0.25 mm × 0.5 μm column, by using the “GC Method Translation” software from Agilent Technologies.

 
 

Figure 19 compares the separation under the two scenarios: hydrogen versus helium as the carrier gas and at the same linear velocity.

Figure 19.  Comparison of analysis of commercial paint remover formulation using hydrogen and helium carrier gas maintaining constant resolution. 
Courtesy of Agilent Technologies, Santa Clara, Ca.

Note that the separation can be further optimised; this will lead to decreased analysis time and increased efficiency.

 
 

Q&A – Columns & Flow Programming


Is there an optimum column bore for high split flow hydrogen methods?
This really depends upon your objective.
If you want fast analysis – the highest linear velocities are attainable on the smallest internal diameter columns.  These columns also give high plate numbers even when relatively short column length (10-20m) are used.  Smaller internal diameter columns also require relatively high head pressures to attain the required linear velocity – which makes for more reproducible methods where the EPC units are not trying to reproduce flows at very low applied head pressures.
By keeping the phase ratio constant, the resolution of methods should be preserved.
We use 0.18, 0.20 and 0.25 mm i.d. columns pretty successfully with Hydrogen carrier gas.

 

If you enable gas saver on the GC you can drop the split demand significantly right?
Using a ‘gas saver’ GC mode reduces the split flow after the injcector is cleared of sample components and does indeed allow one to save gas by reducing the flow through the split line from, say. 200 mL/min. to a much lower value, 10 mL/min. for example.  Whilst maintaining some split flow is desirable to flush the inlet and reduce baseline noise, a considerably lower flow than that used during the injection phase of the analysis is usually acceptable.  This is a great way to conserve Helium.


With flow control possible, doesn't it make more sense to optimize flow instead of pressure?
Yes.  Most instruments allow the user to select a desired flow, linear velocity or pressure.  In temperature programmed analysis constant flow and in some cases constant linear velocity can be selected.  When changing from helium to hydrogen, the calculator mentioned during the webcast will allow us to translate the initial pressure, linear velocity or flow and we can program our EPC units to adopt these initial settings from where they will calculate the required pressure to maintain contant flow or linear velocity during the temperature program.
You can find the method translator at:www.agilent.com/chem/heliumupdate.


Is there an optimum column bore for high split flow hydrogen methods?
This really depends upon your objective.  If you want fast analysis – the highest linear velocities are attainable on the smallest internal diameter columns.  These columns also give high plate numbers even when relatively short column length (10-20m) are used.  Smaller internal dimeter columns also require relatively high head pressures to attain the required linear velocity – which makes for more reproducible methods where the EPC units are not trying to reproduce flows at very low applied head pressures.  By keeping the phase ratio constant, the resolution of methods should be preserved.  We use 0.18, 0.20 and 0.25 mm i.d. columns pretty successfully with Hydrogen carrier gas.


Can you please explain again what the concerns were converting to hydrogen as carrier gas with a megabore column?
With megabore columns, the head pressure required to achieve a given carrier linear velocity may be relatively low – a 20m x 0.53mm column running at 40 cm/sec. may require a head pressure of as little as 2 psig.  EPC units don’t run so reproducibly at these very low pressures and this can cause reproducibility issues.
Any suggestions on using hydrogen with a packed column? I've been dropping the makeup to keep column + makeup to be about 45ml/min.
Yes – use nitrogen!  Nitrogen tends to give the best efficiency at the linear velocities typically required for packed column work.If you’re running micro-packed columns or are stuck with hydrogen, you’re doing the right thing in managing the make-up flow to acheiev a total hydrogen flow of around 30-50 mL/min. which is optimal for most manufacturers instruments.  Some instruments even have a facility to automatically adjust the make-up flow to achieve a constant column + make up flow during temperature programmed operation in constant pressure mode.

 
 

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When changing from helium to hydrogen as the GC carrier gas, changes in retention time can be expected.  The best way to minimise those changes when performing temperature programmed GC, is to keep the same carrier gas velocity, temperature program and phase ratio as in the original method.  Please bear in mind that after method translation further optimization might still be required.

We should add the caveat that if the velocity is too high, solutes don’t spend enough time in the column for an efficient separation; if the velocity is too low, solutes broaden excessively by diffusion through the mobile phase. Further, large changes in carrier-gas velocity during temperature programmed analysis can affect relative peak spacing (selectivity) and retention order may swap. If your instrument allows; chose to have the carrier programmed in ‘constant linear velocity mode’, which will avoid selectivity change issues.

If the instrument is operated at a constant head pressure, as the temperature increases, then the column flow decreases due to an increase in the viscosity of the carrier (Figure 17). By taking advantage of computerized inlet pneumatics (sometimes called Electronic Pneumatic Control or similar), then as the temperature increases, the instrument increases the carrier pressure to maintain constant column flow (increasing linear velocity), which results in earlier elution of the more highly retained sample components. [24]

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
 

Figure 20.  Theoretical effects of column temperature on: (a) corrected helium carrier gas flow and (b) average linear velocity with constant pressure drop.
Column: 30 m × 0.53 mm, column flow 6.0 mL/min, column outlet pressure 1 atm.

 
 

A computerized pneumatic system can increase the column pressure drop sufficiently to maintain a constant corrected carrier-gas flow-rate as the temperature increases.

Figure 21.  Theoretical effects of column temperature on: (a) corrected helium carrier gas pressure drop and (b) average linear velocity while maintaining constant corrected column flow. Column: 30 m × 0.53 mm, column flow 6.0 mL/min, column outlet pressure 1 atm.

 

One should note again that this may cause analytes to elute at a lower temperature than in constant pressure mode – which may again require some adaptation of the temperature programme to ensure the required selectivity between analytes, especially the more retained components.

 
 

Example 3:  It was found that for a 30 m × 0.25 mm × 0.5 μm column, the optimum carrier gas flow rate (helium) was 2.5 mLn/min, the injection volume was 0.1 μL, the split ratio was 1000:1.  The thermal program can be found in table 2.

 
Table 2.  Original temperature program.
  Ramp Rate (oC/min) Final Temperature (oC) Final Time (min)
Initial   120 1.17
Ramp 1 25 160 0
Ramp 2 10 160 0
Ramp 3 150 300 40
 

Translate to hydrogen carrier using the same column.

 

Table 3.  Tabulated information.

Element Information
Column: 30 m × 0.25 mm × 0.5 μm
Oven Temp: 120oC
Helium flow rate:* 0.6 mLn/min
Injection volume: 0.1 μL (splitless)
 
  1. Find an equivalent GC method using the same column with Hydrogen carrier
  2. We used Agilent’s “GC Method Translation” software tool in ‘Translate Only’ mode to calculate an equivalent method; note the translated average liner velocity is almost 1.5 times faster (79.12 cm/sec) than in the original method (51.45 cm/sec).  Please refer to figure 22.
 

Figure 22.  Method translation from helium to hydrogen on a 30 m × 0.25 mm × 0.5 μm column,
by using the “GC Method Translation” software from Agilent Technologies.

 
 

So, by running 3.1 mL/min of hydrogen (head pressure 20.25 psi) on 30 m × 0.25 mm × 0.5 μm column and by using the calculated temperature program, a faster separation that preserves retention time order is obtained.

You would get a similar separation.  Figure 23 illustrates the two situations.

 

Figure 23.  Method translation from helium to hydrogen on a 30 m × 0.316 mm × 0.25 μm column.

 
 

Q&A – Sample Introduction with Hydrogen

 

We're using our GCMS with a head space sampler. What do you think about translating to H2 for vials pressurization in the Head space?
Pressurization of headspace vials with hydrogen is not recommended as it will create an enclosed pressurized volume of flammable gas. Hydrogen carrier gas can be used with headspace samplers if a separate helium or nitrogen pressurization gas can be utilized.

 

What issues would you expect when using an static headspace sampler?
No issues are expected if hydrogen carrier and a non-flammable pressurization gas are used.

 

Are there recommendations for using hydrogen as a carrier gas for thermal desorption techniques?
Hydrogen is not recommended for thermal desorption due to the possibility of sample hydrogenation at normal desorption temperatures, especially in the presence of metal desorption tubes.

 

Can hydrogen be used in the analysis of VOC by purge and trap methods?
Hydrogen is not recommended for purge and trap methods due to the accumulation of hydrogen in the enclosed purge vessel gas space.

 

We normally swap septa without cooling down the inlet. Can we still do that when we switch to hydrogen?
I would not see this as an issue – if you turn off the split flow or reduce your instrument total flow to a low value whilst carrying out the operating the amount of hydrogen liberated will be minimal and will not reach the explosive limit of 4% in air.  Please ensure that you are in compliance with any local safety rules however and you should perhaps consider fitting a snorkel above the instrument to vent any hydrogen released during this operation or which comes from the split vent or any FID detectors that you might be using.

 
 

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The role of hydrogen in GC is not limited to use as a carrier gas but extends to participating in the detection process either as a fuel or make-up gas.  The use of hydrogen as a make-up gas and/or fuel gas is very detector and application dependent. [35 - 37]

As all GC detectors use make-up gas but only combustion detectors require fuel gas, we have organised this tutorial accordingly.  Table 4 lists typical maximum hydrogen flow rates typically encounter with most GC systems.  This table also gives an indication of the gas flows which need to be considered when specifying the volumetric flow rate requirements for hydrogen generators.  The maximium instantaneous flow rate requirements should be multiplied by the number of GC instruments being supplied.

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
 

Table 4.  Maximum hydrogen gas flow rates typically encounter with most GC systems.

Hydrogen Flow Typical Upper Limit (mL/min)
Make-up gas 30
Fuel gas* 35
Carrier gas 10
Split flow 250
Septum Purge 10

* require oxidising gas; usually air at flow rates not exceeding 400 mL/min.

 
 

Q&A

 

If you enable gas saver on the GC you can drop the split demand significantly right?
Using a ‘gas saver’ GC mode reduces the split flow after the injector is cleared of sample components and does indeed allow one to save gas by reducing the flow through the split line from, say. 200 mL/min. to a much lower value, 10 mL/min. for example.  Whilst maintaining some split flow is desirable to flush the inlet and reduce baseline noise, a considerably lower flow than that used during the injection phase of the analysis is usually acceptable.  This is a great way to conserve Helium.

 
 

Combustion Detectors

Before switching to hydrogen, you should bear in mind that GC combustion detectors (FID, NPD, FPD) work with hydrogen as the fuel gas, so any GC instrument equipped to work with such detectors is already fit to work with hydrogen (tubing and safety measurements in place). [38 - 40]  However, one should ensure the fittings and tubing are of the appropriate type for using hydrogen as the carrier or the make-up gas.

Combustion detectors use four different gases:

  • Carrier
  • Make up (detector and application dependent)
  • Fuel (usually hydrogen)
  • Oxidising gas (usually air)

The purity of hydrogen as a carrier gas is critical to achieve optimum results; however, this is not the case when hydrogen is used as a fuel or make up gas.

If you are using hydrogen as the make-up gas, then you should consider it in conjunction with the carrier gas and the fuel gas to optimise detector response.

 
 

The Make-up Gas

Used to meet the flow requirements of the detector, the make-up gas is delivered in the GC system between the column and the detector. 
The use of a make-up gas will:

  • improve the transfer of sample between the column and the detector
  • help to sweep the detector volume fast thus reducing peak broadening and distortion

The make-up gas must be selected in such a way that it does not affect the fuel to oxidant ratio.  Likewise, it should not affect the concentration of the analyte of interest.

In general terms, hydrogen is not the ideal make-up gas for combustion detectors as the stoichiometry of combustion (hydrogen to oxygen ratio) will determine signal response, in such cases, nitrogen (99.9999%) should be the gas of choice.

 
 

FID Considerations

The stoichiometry of combustion (hydrogen to oxygen ratio) is vital in determining the sensitivity of the instrument.  This ratio can be optimised for each analysis.  The use of hydrogen as a carrier gas and as a fuel will impose restrictions to the make-up gas; this is, for as long as the stoichiometry of the combustion (hydrogen to oxygen ratio) is not affected, hydrogen is a valid make-up gas; otherwise an inert gas (such as nitrogen or helium) would be a better choice.

From the previous paragraph, it necessarily follows that hydrogen should not be used as the make-up gas for an FID detector.  In fact, nitrogen and helium are the most common make-up gases for FID.  See table 5.

Remember: In order to reduce noise, the make-up gas purity must be the same as the carrier gas.

 
 

Figure 24. Gases for FID.

 
 

Figure 25. Response variation due to different fuel/makeup combinations for an FID.

 
 

A list of carrier and make-up gases typically used with combustion detectors can be found in table 4.

 

Table 5.  Gases for combustion detectors.

Detector Carrier Gas Make-Up Gas
First Choice Second Choice
FID Hydrogen Nitrogen Helium
Helium
Nitrogen
Argon
NPD Helium Nitrogen Helium
Nitrogen
FPD Hydrogen Nitrogen  
Helium
Nitrogen
Argon
 
 

Q&A - Detectors

 

How about just not using a makeup gas?
It’s certainly an option – you may not get an optimal response and you should check carefully for deviation from linearity at higher analyte concentrations, however it the method is fit for purpose you won’t be harming the instrument at all.

 

We have an FID detector, we use Helium as carrier gas, are you saying we would still need Helium or nitrogen as a make up gas?

Yes.  Using hydrogen as make-up gas usually swamps the detector and alters the stoichiometric ratio of fuel (hydrogen) to oxidizer (air) gases causing a reduction in detector sensitivity – consider, helium, nitrogen or no make-up gasI am using only H2 and air for my TCD and FID GC. For the FID the makeup gas choices are Helium and Nitrogen only. What should be my makeup gas Hydrogen flow rate if I enter Helium? The hydrogen makeup gas flow that should go through the FID or TCD for is the same as for helium or nitrogen. It would appear that you are using an EPC type of pneumatic controller and can only choose helium or nitrogen as the makeup gas type. You have two choices: either 1) use nitrogen or helium makeup gas or 2) enter a lower makeup flow rate, measure the actual flow rate, and then adjust the entered flow so that the desired flow is achieved. Pre-calculation of the entered flow is not advisable because the type of flow controller in use will dictate what is the effect of entering a makeup gas type different than the gas in use. Also bear in mind that for a FID the sum of Carrier + Makeup + Fuel hydrogen must equal the required total hydrogen flow through the detector at all times.

 
 

Non-Combustion Detectors

Non combustion detectors such as TCD and ECD will only use carrier and make-up gases.  The selection of make-up gas is very instrument and application dependent. [38 - 42]

TCD Considerations

Due to its inertness and high thermal conductivity, helium has been traditionally used as the carrier gas of choice for TCD; however, other gases such as nitrogen, argon and hydrogen can also be used.  The important thing is that thermal conductivity detectors work best when there is a large difference in thermal conductivity between the sample and the carrier gas.  As a consequence, the use of hydrogen for TCD is application dependent.

The sensitivity of a TCD, however, is affected by the carrier gas. TCD response depends on a change of thermal conductivity between pure carrier carrier gas and a mixture of carrier gas and a peak as it is eluted. Since hydrogen has about 1.2-times higher thermal conductivity than helium, peaks other than hydrogen or helium would be expected to be that much larger with hydrogen carrier, as long as pneumatic conditions were adjusted so that the peak shapes and positions were the same as with helium.

ECD Considerations

For optimum ECD operation, the carrier and make-up gas should be ionisable.  Neither hydrogen, nor helium ionise under the normal ECD operating conditions and should not be used as the make-up gas.

A list of carrier and make-up gases typically used with non-combustion detectors can be found in table 6.

 

Table 6.  Gases for non-combustion detectors.

Detector Carrier Gas Make-Up Gas
First Choice Second Choice
ECD Hydrogen Argon / Methane Nitrogen
Helium
Nitrogen Nitrogen Argon / Methane
TCD Hydrogen *** ***
Helium
Nitrogen

***  Must be same as carrier and reference gas.

 
 

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Perhaps the most widely used interface design for the GC-MS coupling is the 'capillary direct' interface.  In this design, the column is inserted directly into the ionisation chamber of the mass spectrometer.  Therefore, the column’s effluent (analyte plus carrier gas) is fully delivered inside the MS source.  For optimum sensitivity, the carrier gas must be pumped away by the MS vacuum system.  As expected, the vacuum level within the MS instrument as well as the carrier gas type will affect solute ionization.

The direct interface gives the highest sensitivity of all GC-MS interfaces; however, changing the GC column may be a time consuming process unless curtain gas devices are fitted.

 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s
 

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Figure 26. The GC-MS direct interface.
 

The typical flow for a 10 m × 0.53 mm column is approximately 10 mL/min, which exceeds the pumping capacity of most benchtop systems ( 2-4 mL/min).  In situations like this, gas concentrators and not direct interfaces should be used for the GC-MS coupling.  Table 7 lists typical flow rates and carrier gas linear velocities for selected capillary columns.

 
 

Table 7.  Linear velocity and flow rate for selected capillary columns.

Colum Dimensions Linear Velocity (cm/s) Flow rate (mL/min)
60m x 0.53mm 30 11
30m x 0.32mm 32 2.5
60m x 0.25mm 30 1.3
30 x 0.25mm 30 0.7
40m x 0.15mm 35 1.2
20m x 0.15mm 35 0.6
25m x 0.1mm 40 1.0
12.5m x 0.1mm 40 0.4
 
 

Q&A

 

In our lab we use Helium which is expensive - I want to look at changing to Hydrogen, however this would render he EI libraries useless - wouldn't it?

 

Hydrogen is a little under half as viscous as Helium.  This makes it slightly more difficult to pump away for high vacuum equipment.  Secondly, hydrogen shows minimum plate heights at higher linear velocities than helium.  Thirdly, when the outlet pressure drops to almost zero (i.e. a vacuum) the flow of hydrogen into the instrument may be large.
A large volumetric gas flow into the instrument when using a direct interface will mean a lower vacuum level is attainable.  This may increase the number of background molecules which can collide with the ions formed, leading to a potential reduction in sensitivity and a change in the relative abundance of ions within the mass spectrum.
This all being said, if we use smaller internal diameter columns (0.15, 0.18 or 0.20mm), higher linear velocities are achievable at lower volumetric flow rates – thus we can maintain the vacuum levels within the system and there should be no wholescale changes in the appearance of the spectra.
As John Hinshaw remarked during the webcast, Hydrogen is a reactive gas whereas helium is not, and there is always an exception to the rule which states that we do not typically see ion / carrier gas reactions in GCMS, one that regularly appears in the forums is the dehalogenation of chlorinated compounds.

 
 

Note that large amounts of hydrogen in the MS ion source can lead to ion molecule reactions (reduced sensitivity). 

The carrier gas flow rate through the column is influenced by the pressure difference between the inlet and outlet column ends.  This difference is usually larger in GC-MS than in any other forms of GC, so columns tend to deliver larger amounts of carrier gas to MS detectors than non-MS detection systems.

Some column dimension / flow rate combinations will lead to very low inlet pressure requirements.  This problem is worst when performing split injection (>10:1 split).  So, issues with pressure stability and retention time reproducibility may result.

Figure 27 plots of inlet pressure vs. column length.  Consider a 0.53 mm ID column delivering carrier gas at 40 cm/s to an FID detector (outlet pressure close to one atmosphere).  Even a 50m column would require an inlet pressure of only 3 psig; shorter columns will require even smaller inlet pressure values.  The use of a smaller ID column will help to minimise this problem.

 

Figure 27: Plots of inlet pressure vs. column length, with hydrogen carrier gas and vacuum compensation off. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32, (d) 0.53; column temperature: 50 °C; average linear velocity: 40 cm/s.

 
 

Unfortunately, the situation gets even worse if the same column was delivering to an MS detector.  In such case, the maximum inlet pressure would be negative (around -10 psig).  In such cases, proper flow control is not achievable.  This situation in which the required inlet pressure is below zero is depicted in the blue zone in figure 28 for various column /length internal diameter combinations with vacuum conditions at the column outlet.

So, it is not surprising that adjustments to the column-inlet pressure settings are in order when a separation is moved from a detector that operates near atmospheric pressure to an MS detector, or when changing an existing MS detection method from helium to hydrogen. Chromatographers can use built-in EPC calculations to establish the inlet-pressure setting that produces the same constant flow or average velocity at the starting oven temperature with a vacuum outlet, as was discussed earlier for a configuration without a vacuum outlet.

 

Figure 28: Plots of inlet pressure vs. column length, with hydrogen carrier gas and vacuum compensation on. Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32, (d) 0.53; column temperature: 50 °C; average linear velocity: 40 cm/s.

 
 

Alternatives to the direct interface that help alleviate the underwater effect include the open split interface and the use of restrictors.

By diverting only a portion of the column’s effluent into the detector, the open split interface uses a restrictor tube to transfer the column’s effluent and make-up gas to the MS source.  In this design, the column exit remains close to atmospheric pressure. As only a fraction of the solutes enter the source, sensitivity is reduced.

 

Figure 29. The open split interface (only a fraction of the columns effluent and make-up gas are delivered into the MS source).

 
 

It is also possible to connect a restrictor at the column outlet with a zero-dead-volume connector and then pass the restrictor through a direct interface into the MS source. The restrictor length and inner diameter need to be chosen according to the column flow rate, so that the column outlet pressure will be high enough to bring the inlet pressure above atmospheric levels. This arrangement is comparatively simple, but it will cause EPC display of the column flow and velocity to be in error.

 

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Figure 30. The GC-MS Restrictor.

 
 

The safety measurements that are required in GC are also necessary but not sufficient in GC-MS; the main reason being the fact that hydrogen can accumulate in the mass spectrometer leading to a subsequent combustion.  Reasons for hydrogen accumulation in the MS detector include:

  • Mass spectrometer off: a mass spectrometer shutdown does not necessarily shutoff the flow of carrier gas
  • GC off: a GC shutdown (either accidental, deliberate or by a system failure) does not necessarily shut down the carrier gas flow
  • Power failure: when power fails, both the GC and will shut down; however, the carrier gas flow is not necessarily shut down

In order to reduce the risk of an explosion, the pump-down process of the MS detector should be performed immediately after its starting, so any hydrogen accumulated within the instrument would be pumped away.  This practice is highly recommended especially after a power failure.

 

Warning:  When using hydrogen, please bear in mind the following rules:

  • Check for leaks on a regular basis (use an electronic leak detector)
  • Remove from the lab as many ignition sources as possible
  • Fast expansion of hydrogen could lead to self-ignition (do not allow hydrogen from a high pressure container to vent directly to atmosphere)
  • Turn off the hydrogen every time you shut down the GC or MS
  • Turn off the hydrogen every time there is a power or instrument failure
  • Turn off the hydrogen every time you vent the MS
  • Use hydrogen generators whenever possible

 

 
 

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The major benefit of using hydrogen as a carrier gas is that separations can be achieved much faster than when using any other carrier gas.  So when changing from helium to hydrogen, there is the potential to have the separation performed in a fraction of time; as expected, some additional changes, such as selecting a different flow rate or using a different temperature program, would also be required.

In many modern analytical laboratories, there is a growing demand for faster results without compromising the quality of the separation.  Several approaches have been used to achieve such end: [15]

  • To increase carrier gas flow rate
  • To decrease column length
  • To decrease internal diameter
  • To modify the temperature Programme –Increasing temperature heating rates
  • To use a faster carrier gas (such as hydrogen)
  • To reduce stationary phase thickness
 

Simplification of the ASTM D3606 Method
Headspace Analysis for the Quanitative Determination of VOC’s

There are other important aspects to consider when speeding up a GC separation, especially when using smaller internal diameter columns, such as:

  • Reducing system dead volume / restricting diffusion
  • Rapid introduction of sample from the inlet
  • Making sure that sample is injected as a narrow band (cryo-focussing)
  • Consider sample volume and concentration
  • Rapidity of oven heating and cooling
  • Detector sampling rate

The largest extra column volumes in the typical GC system will be the inlet liner and internal volume of the split/splitless inlet; and the void volume into which the column emerges in the detector.

We have presented a webcast and have written an extensive tutorial on this topic – for more details see the on-demand webcast within CHROMacademy;

Developing_Fast_Capillary_GC_Separations >>
 
 

Analytical Chemists

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  • I know where to go for help
  • I understand my analyses
  • I know where to get applications
  • I’m up to date
  • I’m more employable
  • My career is progressing
 

Laboratory Managers

  • Improved equipment utilization
  • Faster method development/problem solving
  • Flexible workforce with a common standard
  • Better quality data
  • Get up to speed quicker
  • Lower T&E
  • Less reliant on me
  • I spend less time on training
 

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  1. The CHROMacademy Essential Guide to Developing Fast Capillary GC Separations
  2. Eva Matisová, Milena Dömötörová. “Fast Gas Chromatography and its Use in Trace Analysis” Journal of Chromatography A, 1000 (2003) 199–221
  3. “GC Columns” from CHROMacademy’s “The Theory and Instrumentation of GC”
  4. The CHROMacademy Essential Guide Tutorial to Column Choice for Capillary GC
  5. Hydrogen Material Safety Data Sheet. Air Products. MSDS# 1009
  6. Gil Posada Jorge Omar and Hall P. “Porous Magnesium for Hydrogen Storage” University of Strathclyde. 2005.
  7. Reginald J. Bartram and Peter Froehlich. “Considerations on Switching from Helium to Hydrogen” LCGC North America. Vol 28. No 10. Pp 1-8. 2010.
  8. Reginald J. Bartram and Peter Froehlich. “Considerations on Switching from Helium to Hydrogen” LCGC North America Volume 28 Number 10. October 2010.
  9. Ultra High Purity Hydrogen Generators. Parker Balston.
  10. The CHROMacademy Essential Guide Tutorial to GC Troubleshooting Gas Supply and Inlet Issues.
  11. “Gas Supply and Pressure Control” from CHROMacademy’s “The Theory and Instrumentation of GC”.
  12. Scientific Instruments Manufacturer GmbH. http://sim-gmbh.de/
  13. SRI. http://www.srigc.com/
  14. Hydrogen Generators for Gas Chromatography. Parker Hannifin (UK) Limited. www.parker.com/pag
  15. M.S. Klee, L.M. Blumberg, J. Chromatogr. Sci. 40 (2002) 234
  16. “Chromatographic Parameters” from CHROMacademy’s “The Theory and Instrumentation of GC”.
  17. “Band Broadening” from CHROMacademy’s “The Theory and Instrumentation of GC”.
  18. R.J. Bartram and P. Froehlich, LCGC N. Am., 28(10), 890 (2010).
  19. J.V. Hinshaw, LCGC N. Am., 28(3), 218 (2010).
  20. J. Heseltine, LCGC N. Am., 28(1), 16 (2010).
  21. J.V. Hinshaw, LCGC N. Am., 26(11), 1100 (2008).
  22. J.V. Hinshaw, Column Connections, LCGC Asia Pacific, 12(2), 1100 (2009).
  23. J.V. Hinshaw, Column Connections, LCGC Europe, Jan 1 (2011).
  24. J.V. Hinshaw, Column Connections, LCGC Europe, Apr 1 (2008).
  25. Novel concepts for fast capillary gas chromatography, Marieke van Deursen. - Eindhoven : Technische Universiteit Eindhoven, 2002. ISBN 90-386-2873-0.
  26. Mark Sinnott. “Secrets of GC Column Dimensions” Agilent Technologies.
  27. Ken Lynam. “Practical Faster GC Applications with High-Efficiency GC Columns and Method Translation Software” Agilent Technologies. Pittcon. United States of America 2008.
  28. Leonid M. Blumberg & Matthew S. Klee. “Practical Approach to Porting GC Methods to Columns of Smaller Dimensions” Hewlett-Packard Co. United States of America. June 1998.
  29. Approaches to Increasing GC Speed, Resolution and Response. Sigma Aldrich Co. 2009.
  30. Practical Faster GC Applications with High-Efficiency GC Columns and Method Translation Software. Agilent Technologies.
  31. Agilent Technologies Application Note, ‘Predictable Translation of Capillary Gas Chromatography Methods for Fast GC’ (5965-7673).
  32. J.V. Hinshaw, LCGC Europe, 24(1) (2011).
  33. A. Hoffmann, B. Tienpont, F. David, P. Sandra, Ultra-Fast Determination of the Hydrocarbon Oil Index by Gas Chromatography using a Modular Accelerated Column Heater (MACH).
  34. David P. Rounbehler, Eugene K. Achter, David H. Fine, George B. Jarvis, Stephen J. MacDonald, David B. Wheeler, and Clayton D. “Wood. High Speed Chromatography” U. S. Patent 5,808,178. September 15, 1998.
  35. “GC Detectors” from CHROMacademy’s “The Theory and Instrumentation of GC”.
  36. The CHROMacademy Essential Guide to GC Troubleshooting - Column & Detector Issues.
  37. The CHROMacademy Essential Guide to Understanding GC Detectors.
  38. Agilent 6890 Series Gas Chromatograph -Operating Manual Volume 3. Detectors.
  39. Gas Recommendations For Agilent GC’s.
  40. A Guide to GC Setup. Copyright © 1994, 1998 Restek Corporation.
  41. Thermal conductivity detector. Gas chromatography with HiQ® specialty gases. Linde Application sheet. http://hiq.linde-gas.com
  42. Hints for the Capillary Chromatographer –Using Electron Capture Detectors. The Restek Advantage. June 1994, Restek Corporation.
  43. The CHROMacademy Essential Guide Understanding 2D Gas Chromatography.
 
 

Analytical Chemists

  • I feel empowered to fix things
  • I can troubleshoot effectively
  • I know where to go for help
  • I understand my analyses
  • I know where to get applications
  • I’m up to date
  • I’m more employable
  • My career is progressing
 

Laboratory Managers

  • Improved equipment utilization
  • Faster method development/problem solving
  • Flexible workforce with a common standard
  • Better quality data
  • Get up to speed quicker
  • Lower T&E
  • Less reliant on me
  • I spend less time on training
 

Subscribe for $399 per/year and access:

  • The entire e-Learning archive
  • All Essential Guide Webcasts and Tutorials
  • LCGCs archive of articles and webcasts
  • Expert troubleshooting advice when needed
subscribe now

Learn chromatography from the experts

Whether you work in a lab or manage a lab, you will benefit from being a member of CHROMacademy.

As a member of CHROMacademy, you will get access to our vast e-Learning archive full of great interactive content and animations.
All our Essential Guide Webcasts and tutorials and LCGCs archive of magazine articles and webcasts from your favourite authors - John Dolan, John Hinshaw, Mike Balough, and Ron Majors. Plus vendor application notes, electronic laboratory tools and calculators and with our 'Ask the Expert' function - help is always at hand.

 

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Laboratory Managers

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Learn chromatography from the experts

Whether you work in a lab or manage a lab, you will benefit from being a member of CHROMacademy.

As a member of CHROMacademy, you will get access to our vast e-Learning archive full of great interactive content and animations.
All our Essential Guide Webcasts and tutorials and LCGCs archive of magazine articles and webcasts from your favourite authors - John Dolan, John Hinshaw, Mike Balough, and Ron Majors. Plus vendor application notes, electronic laboratory tools and calculators and with our 'Ask the Expert' function - help is always at hand.

 

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The Essential Guide from LCGC’s CHROMacademy presents the definitive guide in translating your GC methods for use with Hydrogen Carrier. In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) consider the impact of the global issues with Helium supply on Gas Chromatography methods. We consider the use of hydrogen as an alternative carrier gas to Helium and present the facts around Hydrogen safety and use within the laboratory. We study method translation, including all of the important method and instrument parameters to ensure retention, selectivity and resolution are maintained. We consider the use and limitations of hydrogen carrier with mass spectrometric detectors and finally highlight some of the advantages of working with a carrier capable of producing high efficiency at higher flow rate.

John V. Hinshaw
Senior Scientist
BPL Global Ltd
Tony Taylor
Technical Director
Crawford Scientific

Translating GC methods from Helium to Hydrogen Carrier Gas

  • Why change at all?
  • Using hydrogen safely – generators and cylinders
  • Maintaining retention time
  • Avoiding selectivity changes in temperature programmed GC
  • Effects of changing carrier on detector performance & sensitivity
  • Using hydrogen with MS detectors
  • Re-optimizing methods for speed & efficiency

Key Learning Objectives:

  • Have a balanced discussion on the safety implications of using hydrogen for GC
  • Learn about the physical and chromatographic differences between helium and hydrogen as carriers
  • Understand how to successfully translate methods so that retention, selectivity and resolution are preserved or enhanced
  • Learn how to ensure that selectivity differences are minimized when transferring gradient temperature programmed methods
  • Understand the implications of using hydrogen on FID detector performance
  • Appreciate the considerations and limitations of using hydrogen as a carrier for GC-MS instrumentation
  • Learn how to develop faster separations using hydrogen carrier

Who Should Attend:

  • Anyone wishing to adopt hydrogen as their carrier gas of choice to overcome current or future issues with the supply of helium
  • Learn about the physical and chromatographic differences between helium and hydrogen as carriers
  • Understand how to successfully translate methods so that retention, selectivity and resolution are preserved or enhanced
  • Anyone who would like to optimize their separation quality or throughput by taking advantages of the high efficiency nature of hydrogen as a carrier gas