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Helium to Hydrogen - a change would do you good

John V. HinshawChange is always a scary concept but let us dispel some of the fear associated with changing your helium carrier gas to hydrogen by answering some of the most common questions posed to us.  John V. Hinshaw our GC department editor has written a comprehensive article answering the questions below in great detail.  Here we have highlighted the key points to help get you started, however, for a more in depth discussion the full article can be read here »

Performance

Hydrogen carrier gas has many advantages over helium or nitrogen.  Higher plate numbers can be achieved at high linear velocities as well as being able to achieve higher linear velocities while utilising lower pressures.  Extra hydrogen in the system may, however, affect detectors that use hydrogen fuel gas (i.e. FID) and MS detectors.

 

Does hydrogen carrier gas affect retention times?

Yes and no.  As linear velocity increases, isothermal retention times decrease in exact proportion, so doubling the velocity will reduce retention times by half. 
Three particular situations that need to be considered are:

  1. Constant velocity - for isothermal operation at constant average linear velocity, retention times are not affected by changing carrier gas.
     
  2. Constant inlet pressure - hydrogen carrier gas will reduce elution times by about half.

  3. Constant flow rate - isothermal retention times will be unchanged with hydrogen compared to helium.

For temperature programming

  • Maintaining constant linear velocity during the temperature program will maintain retention times.
  • With constant inlet pressure hydrogen will cause peaks to be eluted earlier, but their elution temperatures will also be reduced which can change the relative retentions of peaks with divergent chemical characteristics.
  • With constant flow rate a similar situation for constant inlet pressure will be observed, but to a lesser degree.
  • Always revalidate each peak in your chromatogram and optimise the temperature program ramp rate using method translation software (see you may also like below).

How can I manage hydrogen fuel flow in my flame ionisation detector with hydrogen carrier gas?

The simple answer is to reduce the hydrogen fuel gas flow so that the total hydrogen flow through the detector remains close to the manufacturer’s specified optimum flow – usually between 40 – 50 cm3/min.  If the column oven temperature drops during the run the flow rate may decrease.  With narrow bore columns this may be small enough that the ionisation detector will remain within the optimum range, however, with wide bore columns the FID sensitivity may be significantly affected.  Electronic pneumatics can be used to overcome this problem, by maintaining a constant total flow of hydrogen through the detector.

What should I use for detector makeup gas if I use hydrogen carrier gas?

If your flame ionisation detector requires additional makeup gas supply beyond the normal hydrogen fuel gas you should always use an inert gas and not more hydrogen.  Either helium or nitrogen will perform well as makeup gas with hydrogen carrier.  The makeup purity must be the same as the carrier gas to avoid an increase in detector noise. Using the same make-up gas as the carrier gas is often most convenient, but see the tables below for exceptions. If you are ever in doubt make sure to consult your manufacturer regarding equipment compatibility and required flow rates.

Makeup Gas   Makeup Gas

Table 1:  Gases for combustion detectors. 
*** No recommended second choice.

 

Table 2:  Gases for non-combustion detectors. 
** Must be same as carrier and reference gas.


Does hydrogen deliver higher plate numbers or separation efficiency at high separation speeds?

As can be seen from the van Deemter plot in Figure 1 the optimum linear velocity for hydrogen is almost 40 cm/s compared to 30 cm/s for helium.  The minimum theoretical plate heights for hydrogen and helium are close to each other, however, so at optimum linear velocity little difference in the theoretical plate number would be expected for significantly retained peaks.

Interestingly hydrogen behaves better at higher linear velocities than helium.  The solid portions of the lines in Figure 2 show the useful linear velocity range over which the plate height stays within 25% of the optimum value which equates to a maximum loss of resolution of about 12%.  Hydrogen works better at higher velocities and covers a wider range of velocities while remaining close to optimum.  If a faster speed of separation was required by increasing the linear velocity, then hydrogen is a logical choice.  At 60 cm/s, using the data in Figure 2, hydrogen produces about 33% more plates than would helium at the same velocity, so hydrogen performs better at high speeds.

 

 

linear velocity

 

 

 

 

 

 

 

 


Figure 1: The solid lines indicate regions in which the plate heights (H) are within 25% of the minimum plate height value (Hmin) . The optimum practical gas velocity (OGPV) lies roughly at the right-hand extent of the solid lines for each carrier gas. Theoretical data for a 50 m x 0.25 mm column at 100 °C for an analyte ofk = 10.0.


Why not use nitrogen instead of helium or hydrogen?

Nitrogen has a minimum plate height that is about 10% lower (better) than helium or hydrogen (Figure 2). This means that if we were to operate at the optimum velocity, then nitrogen would deliver around 10% more theoretical plates.  The effect on peak resolution is negligible, however, because resolution is related to the square root of the plate number. What is not so good about nitrogen is the much narrower velocity range over which the plate height remains close to optimum, from about 8 to 20 cm/s in this case. The optimum linear velocity depends upon the solute and temperature, so for different solutes, and as the temperature changes during programming, the optima for each solute will appear at velocities somewhat different than the theoretical minimum. With nitrogen it is more likely that, for a given solute, the plate height will be farther from optimum than with carrier gases like helium or hydrogen for which the plate height does not depend as strongly upon the linear velocity.


Will hydrogen react with my unsaturated or aromatic solutes?

Hydrogen is a reactive compound that will hydrogenate unsaturated and aromatic compounds under the influence of temperature, pressure, and a catalyst. Sufficient conditions for hydrogenation do not exist inside conventional wall-coated fused-silica capillary columns, but the use of nickel columns with hydrogen carrier gas at higher temperatures might better be avoided if reactive solutes are to be separated.


How pure does hydrogen carrier gas need to be?

Hydrogen carrier gas should be of the same purity level as helium carrier — "research" grade, or 99.9999% purity for trace work under 1 ppm, or at least 99.9995% for most normal analyses.  The manufacturer's literature will specify the purity and suitable applications.


Do I need to filter hydrogen carrier gas from a generator or a tank?

Hydrogen carrier is just like any other high-purity carrier gas and requires the same types of filters to be installed close to each supplied GC system. The filters are not needed to purify hydrogen as it leaves the generator or tank; they are needed to trap any contaminants introduced from regulators, connecting tubing, and fittings, as well as to isolate multiple instruments from each other.


If I use a hydrogen generator, will there be water or oxygen in the carrier gas?

Ultrahigh-purity hydrogen carrier gas generators remove water and the oxygen that is formed by passing the generated hydrogen through a palladium membrane that prevents passage of substances other than hydrogen.


Do hydrogen generators need specifically purified water and chemical consumables?

Yes. Hydrogen generators use deionized water that will need to be refilled regularly. Continuous flow water supplies are also available. Some carrier-gas grade generators will need periodic replacement of a deionizer bag, but normally this only needs to be done once or twice a year. Refer to the product manual for exact maintenance requirements.


How much hydrogen flow does a GC system consume?

A single-channel GC system with FID and split–splitless injection will consume 100–500 cm3 /min of hydrogen carrier and fuel gases, depending upon the split flow rate and column consumption. Gas-saver operation — turning off the split flow when not required — will reduce carrier gas consumption considerably. In general, a hydrogen generator should run at under 80% of its rated capacity, so for example, a generator capable of delivering 1 L/min could operate two single-channel GC systems with split flows up to 300 cm3 /min each, or possibly three GC systems at lower split flows. It is better to over-specify a generator and, thus, be sure not to run out of generating capacity for future planned applications.


I already have hydrogen cylinders that supply detector fuel gas in my laboratory: Can I use these for carrier gas as well?

Hydrogen for detector fuel gas might not be pure enough for carrier gas use. However, UHP or research-grade purity hydrogen cylinders or generators can be used for fuel gas as well.


Safety

Most GC laboratories now use flame ionization or other detectors, such as flame-photometric or nitrogen–phosphorus types, that consume hydrogen as a fuel gas. If your laboratory now uses or plans to use hydrogen as fuel or carrier gas, you should review safety equipment and procedures related to the use of hydrogen. It is also a very good idea to consider the hazards of non-flammable compressed gases such as nitrogen, air, and helium. Consult with instrument and hydrogen generator manufacturers, local safety authorities such as the fire marshal in your city, and in the U.S. with federal and state regulatory bodies. Many companies will have a safety policy that must be adhered to as well. In all cases, prepare the laboratory suitably before introducing any flammable, compressed, or otherwise hazardous gases. The use of electronic carrier gas controls is strongly recommended because such automated systems will detect many potentially unsafe fault situations that could result in leakage, and will shut off carrier gas flow.

A safety plus point for hydrogen generators is that they only store a small amount of hydrogen. The exact amount depends upon the generator capacity but typically is less than 0.20 L. Compared with the 6000–8000 L of gas that is stored in a full-size laboratory gas cylinder when new, it should be obvious that a hydrogen generator presents a considerably lower hazard in terms of the amount of stored flammable gas. Even so, this does not change the rate and total amount of hydrogen that is consumed nor does it remediate the hazards of using flammable gases, it just minimizes the total amount that is present in the laboratory at any one time.

Cost

The cost of a hydrogen generator, when used to replace helium carrier gas, will pay for itself in a relatively short time. It is not possible to review exact purchase-versus-lease figures or return on investment payback times in this article: these will vary considerably depending upon individual circumstances and usage, and on future helium costs.

In our lab we use Helium which is expensive - I want to look at changing to Hydrogen, however, would this would render the EI libraries useless?

Hydrogen is a little under half as viscous as Helium (Figure x).  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.  Care should be taken when analysing “fragile compounds, compounds at trace levels, and reactive compounds (alcohols, aldehydes, ketones).

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 wholesale changes in the appearance of the spectra.

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, however, one that regularly appears is the dehalogenation of chlorinated compounds.

 

Gas Viscosities

Figure 2:  Influence of carrier gas and temperature on viscosity.

The full article by John V. Hinshaw our GC department editor can be read here »

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