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The CHROMacademy Essential Guide:
Translating GC and GCMS Methods

Thursday 18th July 2013 - 11am EST / 4pm GMT

The Essential Guide from LCGC’s CHROMacademy presents the definitive guide in translating your GC and GCMS methods. In this session, Dr. John Hinshaw
(Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) explore the various reasons for translating methods including improved chromatography, faster throughput, adopting modern column geometries and switching to hydrogen carrier from helium.
We explore all aspects of instrument hardware and method variable translation and you will leave with a ‘toolkit’ approach for translating your own methods –
saving you time, money and improving your chromatography!

GC-MS/MS

Topics covered include:

  • Key reasons and benefits of translating GC and GCMS methods
  • Translating for speed, efficiency or to hydrogen from helium
  • Maintaining retention time
  • Avoiding selectivity changes in temperature programmed GC
  • Effects of changing carrier on detector performance & sensitivity
  • MS detector considerations

Who Should Attend:

  • Anyone who would like to optimize their separation quality or throughput by to taking advantages of the high efficiency nature of reduced internal diameter capillary columns or hydrogen as a carrier gas
  • Anyone who needs a template for method translation using improved method conditions, reduced dimension capillary columns or using an alternative carrier gas
  • Those who need information on how to avoid selectivity changes when transferring methods
  • Anyone who would like to speed up their GC separations but isn’t sure how to translate the method variables or which column to use
  • Those who wish to avoid issues with detectors when translating GC methods

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

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The CHROMacademy Essential Guide Tutorial
Translating GC and GCMS Methods

Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) explore the various reasons for translating methods including improved chromatography, faster throughput, adopting modern column geometries and switching to hydrogen carrier from helium.

They explore all aspects of instrument hardware and method variable translation and you will leave with a ‘toolkit’ approach for translating your own methods – saving you time, money and improving your chromatography.

Topics include:

  • Key reasons and benefits of translating GC and GCMS methods
  • Translating for speed, efficiency or to hydrogen from helium
  • Maintaining retention time
  • Avoiding selectivity changes in temperature programmed GC
  • Effects of changing carrier on detector performance & sensitivity
  • MS detector considerations

  ask the CHROMacademy experts

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Translating GC and GCMS Methods: Why?

There are many reasons for wanting to change your current GC and GCMS method:

  • Faster analysis times
  • More efficient chromatography
  • Greater analyte resolution
  • Lower temperature separations
  • Helium shortage
  • Longer column lifetimes
  • Cost savings/increased profit

Although you may already have a well-established method that provides you with good results there are always improvements that can be made.  Figure 1a shows a chromatogram of 16 polynuclear aromatic hydrocarbon (PAH) compounds separated using a conventional 30 m x 0.25 mm ID TR-5MS column with a 0.25 μm film.1  This separation was carried out in a reasonable time scale (<25 mins) with good resolution of all the peaks, however, by simply switching the column to one with a more modern geometry (10 m x 0.10 mm x 0.10 μm) the analysis time can be significantly reduced (approx. 12 mins, Figure 1b).  There is also the added advantage that the later eluting peaks (14 – 16) exhibit an increase in peak height due to the reduction in peak broadening on the short, thin film column.    

 

Figure 1: 1a: Chromatogram showing Polynuclear Aromatic Hydrocarbons (PAHs) using a conventional 30 m x 0.25 mm ID TR-5MS column with a 0.25 micron film. 1b: Chromatogram showing Polynuclear Aromatic Hydrocarbons (PAHs) using a FAST TR-5MS (10 m x 0.10 mm ID, 0.10 μm). 1

1. Naphthalene     7. Fluoranthene   12. Benzo(k)fluoranthene
2. Acenaphthyene   8. Pyrene   13. Benzo(a)pyrene 
3. Acenaphthylene   9. Benzo(a)anthracene   14. Indenol(1,2,3,-c,d)pyrene
4. Fluorene     10. Chrysene   15. Dibenzo(a,h)anthracene
5. Phenanthrene   11. Benzo(b)fluoranthene   16. Benzo(g,h,l)perylene
6. Anthracene        
 
 
Moderate Speed Increases with Existing Equipment

Going to faster speeds of analysis with GC doesn't always require a new or upgraded instrument, but not all existing instruments are suitable.  It all depends upon how fast a separation is required.  A modest increase of two to four times shorter retention using conventional capillary columns with inner diameters of 150 - 200 μm can be achieved quite reasonably on a wide range of conventional laboratory instruments (Figure 2).

 

Figure 2: Halving analysis time of a spearmint oil sample using simple means.  Top chromatogram: Polydimethylsiloxane phase 30 m x 0.25 mm x 0.25 μm,
He carrier (25 cm/s).  Bottom chromatogram: Polydimethylsiloxane phase 20 m x 0.18 mm x 0.18 μm, H2 carrier (47 cm/s). 2

 
 
Four Speeds of Fast GC

Making the decision to go to higher speeds is just the start of what can be an extended method development and validation exercise. A high-speed capable instrument is a platform upon which to deploy a suitable column and method. It might be able to inject and record very narrow peaks while ramping up the column temperature at impressive rates, but without the necessary separation method, it will not deliver the desired results.

We also need to consider other rate limiting factors such as sample preparation or data analysis time which may ultimately be the rate limiting step(s) in any analysis.

Table 1 shows John Hinshaws’ summary of the four proposed ‘speeds’ within capillary GC.

Table 1: Four proposed ‘speeds’ of Capillary GC (reproduced with permission).

 
 

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Translating GC and GCMS methods involves the consideration of several key aspects relating to the hardware used and the conditions under which the separation is achieved. 

When embarking upon the translation of your current GC and GCMS methods it is important to take a focussed and stepwise approach in order to maximise the benefits and to help with the revalidation of any existing methods.

A practical approach to the translation of a method is to start with existing, proven methods.  Already optimised methods and industry standard methods, i.e. ASTM, GPA, IFP, and UOP methods will be easier to translate and further optimise.  In your current methods look for separations that have insufficient or excess resolution as these are parameters that are convenient to optimise.

A simple step-by-step method for deciding where to make changes/improvements and how to do this are as follows.  Using your standard method:

  • Investigate use of alternative carrier gas
  • Alter column dimensions, flow, temperature
  • Use Method Translation (MTL) software to calculate faster conditions
  • Investigate alternatives to the standard method
  • Consider column selectivity
  • Translate temperature programming
 
 
Carrier Gas: Changing from Helium to Hydrogen

In gas chromatography many gases including nitrogen, helium, and hydrogen have been used as the carrier gas. 3,4  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

Although it is the second most abundant element in the known universe, helium is somewhat 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).

 

Equation 1: Helium production by radioactive decay of uranium.

 
 

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 the 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.

The global impact of hydrogen shortage has not only reached the scientific community, (Chemical and Engineering News - July 16, 2012 (Page 32-34)), but has also been discussed in mainstream media (http://www.bbc.co.uk/news/uk-19676639).

 
 

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 3) for efficiency against carrier gas linear velocity, there is less dependency on 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 3: Influence of carrier gas and linear velocity on theoretical plate height.  The solid lines indicate 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.

 
 
GC Safety Operation with Hydrogen

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, colorless and odorless 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.
 
 

Most GC instruments are equipped with electronic pressure regulators which monitor carrier and detector gas pressures (Figure 4). 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.

Newer GC’s include flow-limiting hardware in the carrier lines to reduce the amount of possible hydrogen accumulation.  Leak testing may also be carried out using an on-board gas detector or portable device (Figure 5).  Additionally you may consider venting split outlet/septum purge to a fume hood or snorkel.

When working with a mass spectrometer (MSD) some specific safety considerations include:

  • Hydrogen carrier gas flow must be cut off before powering down a mass spec detector to avoid filling the detector / pumping system with hydrogen
  • Always purge and pump down a MSD before starting carrier gas flow
 

Figure 4: GC oven with gas sensor head.  Courtesy of SIM. 7

 
 

Figure 5: Hydrogen detector probe.  Courtesy of SRI Instruments. 8  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.

 
 
Sample Inlet and Autosampler

One of the biggest factors that are going to affect the inlet/autosampler of a GC instrument is the change from helium to hydrogen gas.  There are several factors that must be taken into consideration when changing the carrier gas including:

  • Split Ratio
  • Sample Diffusion
  • Column Loading
  • Head pressure vs. liner flow

Relative Gas Velocities and Diffusivity: Effect on Inlet Pressure and Liner Selection

Undoubtedly two properties of a gas play a major role in the GC process: diffusivity and viscosity.  The diffusivity of nitrogen and helium are roughly the same but hydrogen is a bit less than half as viscous as helium at the same temperature (Figure 6).  For this reason hydrogen requires a lower pressure to achieve the same average carrier gas velocity as for helium.

 
 

Figure 6 Relative viscosity of common GC carrier gases at various temperatures. 9

 
 

With EPC systems, choosing the correct gas type and required linear velocity results in an automatic inlet pressure adjustment.  Some column dimension/flow rate combinations will lead to very low inlet pressure requirements (Figure 7). 

 

Figure 7: Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32, (d) 0.53; column temperature: 50oC; average linear velocity: 40cm/s.

 
 

This can be particularly problematic with split injection (>10:1 split) due to the increase in inlet head pressure that occurs when the split flow or split ratio is increased (Figure 8).  Issues with pressure stability and retention time reproducibility may result.

 

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Figure 8: Increasing the split flow or split ratio increases the inlet head pressure, reduces sample transfer time and reduces diffusion of the resulting gaseous sample.
 
 

Some practical points to remember that will help when changing carrier gas depending on the sample injection type you are using are:

  • Split Injection - When changing from helium to hydrogen it might be necessary not only to adjust the split ratio.  Method translation may mean a change in column flow rate.
  • 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.
  • Direct Injection - There is no concern on the conversion when using either hydrogen or helium.

Liner selection is another factor that is of great importance when changing the carrier gas.  Due to the higher diffusivity of hydrogen compared to helium, the analyte has a greater potential to diffuse in the inlet, which will result in peak broadening.  Reducing the inlet liner dimensions from 4 mm to 2 mm i.d. will give sharper peaks in the chromatogram (Figure 9).   Using a high split flow will also ensure that the sample is transferred quickly and efficiently to the GC column – reducing the time available for diffusion of the sample plug.

Whenever one reduces the liner internal diameter, the internal volume available for analyte expansion is reduced accordingly. To mitigate this, it is also typical to increase the split ratio in order to reduce analyte residence time within the inlet. The higher head pressure required to achieve these high split ratio’s will help to restrict the expansion of the analyte plug.

An additional benefit to increasing the split ratio is a reduction in the actual amount of sample transferred to the column.  As column internal diameter (and/or stationary phase film thickness) is reduced the sample capacity of the column reduces. Increasing the split ratio or decreasing the injection volume will reduce the absolute amount of analyte introduced into the column help to avoid overload characteristics such as peak tailing or fronting.

 

Figure 9: Peak width resulting from an injection 0.1 μL of acetone in liners of differing i.d. 10

 
 

A method translation calculator (Figure 10) can be used to calculate the optimum injection volume for an inlet liner of 2 mm internal diameter, by entering 100 for the liner length and 2 for the liner i.d.  It is important that the total volume of gas created by the sample does not exceed the total liner volume - this is represented visually on the liner graphic where we use 0.5 x the total liner volume as the ‘red zone’ indicator.

 

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Figure 10: Select the liner dimensions, solvent and other operating conditions to calculate gas expansion volume.

 
 
Column and Column Oven

Changing the column dimensions (length, diameter, phase ratio, and film thickness) can all have an effect on the analysis time for a GC or GCMS method.  Column heating will also play a part in increasing the speed of analysis. 

Oven Heating and Cooling

Conventional GC ovens suffer from the inordinate length of time taken to both heat and (especially) cool the GC column using forced air heating/cooling ovens which typically have relatively large thermal mass.  If this issue can be overcome, there would be increased scope for decreasing analysis time in combination with the other approaches mentioned above. It should be noted that there are also some significant disadvantages to increasing the temperature programme rate for certain separations. These typically include: separations involving high boiling analytes, convoluted samples with many critical peak pairs and samples which include thermally labile analytes. 12
In gradient temperature programmed GC, the analyte rate of migration through the column will double for every 30oC rise in the oven temperature – this is demonstrated by the smooth curve in Figure 11.

 

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Figure 11: Peak migration profile (as both a step function and overlaid with the actual analyte elution profile) for an analyte in temperature programmed GC with an elution temperature of 265oC.

 
 

In simple terms – the analyte moves through half of the column (volume) in the 30oC prior to elution (265oC in this instance), three quarters of the column volume in the 60 oC prior to elution etc.  This is true for all analytes – and all analytes spend exactly the same length of time moving through the GC column in temperature programmed analysis! Thus all analytes have approximately the same peak width – they are dispersing within the column for the same period of time.

Extrapolating back – all analytes are initially immobilized at the head of the column (focussed), where they remain until the column temperature is suitable for them to begin to vaporize and partition through the column. As each analyte has a different relative vapour pressure, the mechanism of separation in temperature programmed GC relies on this difference in vapour pressure vs. temperature for each analyte. Once the analyte begins to chromatograph, as explained above, it accelerates through the column at the same rate as all other analytes.  Therefore, in fast GC temperature programming, where the temperature profile is increased more rapidly, analytes with closely related chemistry/vapour pressure may begin to chromatograph at very similar times, and therefore separation may be reduced.

 
 

Achieving Fast GC Column/Oven Heating

There are essentially three methods of obtaining very fast temperature programming rates in capillary GC, (1) wrapping the capillary with electrically heated resistive tape (2) using multi-capillary bundles with heating wires inserted between the very small internal diameter capillaries, and (3) using GC oven inserts.
To adopt resistive heating of a single capillary the column can be coated with a conductive material, a coil can be wrapped around or run in parallel to the column, or the capillary column can be inserted into a metal tubing (Figure 12). The columns used are short (<10m) and are typically operated at higher carrier gas linear velocity. 13-15

 

Figure 12: Column heating via resistive wire (left, courtesy of Thermo Scientific, West Palm Beach, Florida, USA) and column jacket (right, courtesy of Thermedics, Chelmsford, Massachussetts, USA).

 
 

The introduction of the multicapillary bundle column in 1997 16 brought a new approach to rapid temperature programming which offered higher total flow rate, the higher sample capacity and, consequently, the lower minimum detectable concentration of the solutes (Figure 13). These bundles consist of many (circa. 900) short, narrow i.d. (40 μm) columns that are heated by electrically resistive wires placed between the capillaries within the bundle.

 

Figure 13: Typical cross sectional area of a multicapillary bundle GC column containing up to 900 separate coated capillaries. 17 
Agilent Technologies (Santa Clara, California, USA).

 
 

Using a GC oven insert (Figure 14) it is possible to increase oven temperature programme rates beyond what is normally possible with the standard configuration. The insert, which fits inside the GC oven, reduces the overall volume so that the column and sample heat more quickly, giving faster chromatography. Conversely, reducing oven volume can also decrease cool-down times, significantly lowering overall GC cycle times.

 

Figure 14: Oven insert.  Agilent Technologies (Santa Clara, California, USA).

 
Most conventional GC ovens can now heat at 120oC/min over at least part of the temperature range.
 
 

A good rule of thumb for optimum heating rates appears to be 10oC per void (hold-up) time of the instrument to avoid drastic loss of resolution and increase of upper elution temperatures.

 
 

Analytical Chemists

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  • 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

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  • 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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Practical Considerations for Increasing Analysis Speed

A useful equation that describes the contributing factors to GC retention time and the ways in which the analysis speed might be increased is shown in Equation 2.

 

tR = retention time (s)
L = column length (m)
ū = carrier gas linear velocity (cm/sec)
k = retention factor

Equation 2: Factors influencing retention in gas chromatography. 11
 
 

Practical ways to reduce retention (analysis) time can be derived from Equation 2 and can be summarized as:

Reduce L (column length) - which reduces the number of theoretical plates (efficiency, N) in direct proportion but affects resolution less (L is proportional to √Rs) and so most approaches to speeding up GC analysis use shorter GC columns (≤ 15m).

Increase ū (carrier linear velocity) – this can be achieved by running at higher carrier gas flow-rate (for a fixed column i.d.), increasing the diffusivity of the carrier by changing to hydrogen gas, using low pressure (vacuum) GC techniques or decreasing the internal diameter of the GC column.

Decrease k (retention factor) – this can be achieved in a number of ways including increasing the oven temperature or rate of temperature increase during gradient temperature programming; increase column internal diameter and decrease film thickness; and changing to an alternative stationary phase.

 
 

Reducing GC Column Dimensions: Resolution

 
 
Efficiency  
  • N = f (gas, L, rc)
  • Retention  
  • N = f (T, df, rc)
  • Selectivity  
  • N = f (T, phase)
  • where: L = length, rc = column radius, df = film thickness, T = temperature

    Equation 3: Standard resolution equation.

     

    The standard resolution equation (Equation 3) shows the three parameters that affect chromatographic resolution.  The main parameters that can be altered to optimize the speed of analysis are column length (metres), internal diameter (millimetres) and film thickness (the thickness of the stationary phase layer coated onto the inside of the capillary wall, microns).

     
     

    Decrease Column Length

    By decreasing the column length analysis times can be significantly decreased (Figure 15 – 16).  Prior to reducing column length it is advisable to make sure that you have good resolution of all your peaks.  Decreasing the column length will decrease analysis time and resolution.  From the resolution equation (Equation 2) it can be seen that L is proportional to √Rs.  Therefore, decreasing column length will decrease resolution by a factor of the square root of the length.

     
    Figure 15: DB-5 30 m x 0.53 mm I.D. x 0.5 μm.
     

    Figure 16: DB-5 15 m x 0.53 mm I.D. x 0.5 μm.

     
     

    Phase Ratio (β)

    One of the most useful concepts in method translation is that of Phase Ratio (β) which relates the column internal diameter to the stationary phase film thickness through the following equation:

    Where r is the column radius and df the film thickness (both in microns). This parameter can be used to predict how changes in column dimensions might affect analyte retention (or efficiency) and by keeping the phase ratio constant one can select a column which will produce a separation in the same timeframe but with increased efficiency.

     
     

    Consider the situation in Figure 17 below:

    30 m x 0.32 mm x 0.5 μm  

    β = 160 / 2 x 0.5 = 160

    10 m x 0.10 mm x 0.2 μm  

    β = 50 / 2 x 0.2 = 125

     

    Within the constraints of readily available column geometries, the phase ratio is well matched and these two columns are expected to produce a comparable separation in terms of analyte resolution, however the 10 m x 0.10 mm i.d. column will elute analytes around 3 x faster than the 30 m column providing carrier linear velocity and temperature programme are optimized.

     

    Figure 17: Comparison of traditional capillary GC and fast GC for the analysis of impurities in styrene monomer.  Agilent Technologies (Santa Clara, California, USA).

     
    1. Ethylbenzene   5. o-Xylene   9. a-Methylstyrene
    2. p-Xylene   6. n-Propylbenxene   10. Phenylacetylene
    3. m-Xylene   7. p/m-Ethyl toluene   11. b-Methylstyrene
    4. Isopropyl benzene    8. Styrene   12. Benzaldehyde
     
     

    Figure 17 demonstrates the power of using smaller internal diameter capillary GC columns which can then be shortened to reduce analysis time.  By keeping the phase ratio approximately constant, resolution is maintained but analysis time is reduced by a factor corresponding to the ratio of the original and final column lengths (in this case 3x).

     
     

    Decrease Film Thickness (df)

    Column stationary phase film thickness (df) affects five critical GC parameters; retention, inertness, capacity, resolution and bleed.  Under isothermal conditions film thickness is directly proportional to retention time.  Under temperature gradient conditions the proportionality is approximately 1.5:1; therefore, by decreasing the film thickness the retention time of your analytes will decrease (Figure 18).  Another advantage of reducing the film thickness may be that the retention of highly adsorbed analytes (late eluting high boiling point or high molecular weight analytes) may be reduced. 

     

    Figure 18: GC chromatrogram produced using a (Top) DB-5 30 m x 0.53 mm I.D. column, df = 1.5μm and (Bottom) DB-5 30 m x 0.53 mm I.D. column, df = 1.5 μm.  Carrier: Helium, 36cm/sec at 40oC.  Oven: 40oC for 3 min, 5o/min to 100oC.
    Analytes: 1. Benzene, 2. Toluene, 3. Ethylbenzene, 4. m,p-xylene, 5. o-xylene.

     
     

    Decrease Column Internal Diameter

    N (efficiency)is a function of both column length and internal diameter. Doubling the column length will double the efficiency (plate number) and result in a 1.4x increase in resolution, which may seem good value, however this will double analyte retention times, significantly increase cost of column purchase and add to the head pressure required to obtained the required carrier gas flow-rates. As a general rule of thumb, column length is dictated by the number of components of interest within your sample, with complex samples requiring longer columns in order to generate enough theoretical plates to achieve the separation. Reducing the column internal diameter is a much better approach to increasing column efficiency and resolution, and most importantly for reducing retention time. Halving the column internal diameter will double the column efficiency, this in turn will allow the column length (and retention time) to be halved without loss of efficiency (Table 2).

    Reducing column dimensions will reduce analysis time, however, the trade off can be in the sample capacity of the column.  Exceeding the column capacity will result in poor chromatography with skewed peaks and decreased resolution.  Therfore, it is important to be aware of the capacity limitations of your column and adjust accordingly.  You may need to reduce the sample volume or sample concentration used.  Increasing the split ratio will also lower the column loading reducing the likelihood of overloading.

     

    Table 2:  Various column geometries which can be used to generate a column effciency of 112,000 plates and the corresponding plates per metre measurement . The red line indicates a column internal diameter below which specialist GC equipment may be required.  (Reproduced with permission of Agilent Technologies, Santa Clara, California, USA.).

     
     

    Increase Linear Velocity

    The major benefit of using hydrogen as a carrier gas is that separartions can be achieved much faster than with any other carrier gas (Figure 19).  Hydrogen also demonsrates a high edfficiency over a wide range of linear velocities.

     

    Figure 19:  Van Deemter plot of Average Linear Velocity vs. Efficiency.

     
     

    Analytical Chemists

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    • I know where to go for help
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    • My career is progressing
     

    Laboratory Managers

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    • Get up to speed quicker
    • Lower T&E
    • Less reliant on me
    • I spend less time on training
     

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    When translating a GC method the selectivity for peaks of divergent polarities can shift when changing carrier gas if temperature programming is used.  It is best to use method translation software to determine new conditions that will yield closely comparable separations.  Software solutions assume exactly the same stationary phase before and after translation.  You will need to revalidate the identify of all peaks post translation.

     

    Translating GC Methods: Hints & Tips

    Set reasonable default values for scalable flows (flow rates measured at 25oC and 1 atm)

     
    Flow Rate = F100 μm x 0.01 x Diameter (μm)
     
        F100 μm(ml/min)
    Hydrogen   1
    Helium   0.8
    Nitrogen   0.25
     

    Example:

        FHe(ml/min)
    250 μm   2
    320 μm   2.56
    530 μm   4.24
     
     

    Calculate optimal flow rate

    • Use constant pressure mode if method will be scaled later
    • Use Method Translation software to find void time (hold-up time)

    Example calculation of optimal ramp rate.18

    if void time = 3.18 min

     
     

    Translating GC Methods: Software

    As previously mentioned a practical approach to the translation of a method is to start with existing, proven methods.  Figure 20 shows the chromatogram of CLP-pesticides, using traditional column geometry, helium carrier gas, and a programmed temperature method.  Chromatographically the peaks are already fairly well resolved with an analysis time of ~16 minutes.  However, there is scope for improvement in the resolution of peaks 9 – 11 and the analysis time can be shortened.

     

    Figure 20.  Chromatogram of CLP pesticide separation using a DB-XLB column, 30 m x 0.32 mm i.d., 0.25μm.  Agilent Technologies (Santa Clara, California, USA).

     
    1. Tetrachlorom-xylene (SS)   9. γ -Chlordane  

    16. Endosulfan II

    2. α-BHC           10. α-Chlordane   17. 4,4’-DDT
    3. γ -BHC   11. Endosulfan I  

    18. Endrin aldehyde

    4. β-BHC   12. 4,4’-DDE   19. Endosulfan sulfate
    5. Heptachlor              13. Dieldrin   20. Methoxychlor
    6. δ-BHC            14. Endrin        21. Endrin ketone
    7. Aldrin   15. 4,4’-DDD   22. Decachlorobiphenyl

    8. Heptachlor epoxide

           
     
     

    Method translation software allows the optimisation of your temperature programmed method by following the steps below (Figure 21):

    1. Reduce the column dimensions, while keeping the phase ratio constant.
    2. Translate the method for a different carrier gas – helium to hydrogen.
    3. Increase the linear velocity
    4. Optimize the temperature program
     

    Figure 21:  Example of method translation using the “GC Method Translation” software from Agilent Technologies.

     
     

    Using the stepwise translation method detailed above the CLP-pesticide method can be improved by reducing the column dimensions and film thickness (phase ratio remains approximately constant), changing the carrier gas from helium to hydrogen, and increasing the linear velocity to 105 cm/sec.  This results in a speed gain of 3x while still retaining good chromatographic resolution of all peaks (Figure 22 – 23).

     

    Figure 22:  Method translation using the “GC Method Translation” software from Agilent Technologies for the CLP-pesticide method shown in Figure 20.

     
     

    Figure 23: Chromatogram of CLP-pesticides post method translation.  Agilent Technologies (Santa Clara, California, USA).

     
     

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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. 19-21

     

    Detector considerations: 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 (Table 3), so any GC instrument equipped to work with such detectors is already fit to work with hydrogen (tubing and safety measurements in place). 22-24  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 usually have an optimum hydrogen flow rate (30-40 mL/min is typical for the FID detector for example).  The stoichiometry of combustion (hydrogen to oxygen ratio) is vital in determining the sensitivity of the instrument (Figure 24).  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.

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

     

    Table 3:  Gases for combustion detectors.  *** No recommended second choice.

     
     

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    Figure 24. Gases for FID (left) and response variation due to different fuel/makeup combinations for an FID (right).

     
     

    As speed of analysis increases peak width will decrease and the data sampling rate will need to match so that peaks are properly modelled and important data is not missed.  This especially important in quantitative analysis where peak area reproducibility is particularly important.

    Whilst most modern GC detectors will have appropriate sampling rates, it is always good practice to check the number of data points across your peaks.  Set data rate to give 10 points across half width of peak. Peak is 0.23 sec at 1/2 width, so use 50 Hz (Figure 25).

    When using MS detection scan range and threshold settings will dictate the scan speed and therefore the sampling frequency and particular care needs to be taken to ensure sufficient data points per peak are obtained.

     

    Figure 25:  Apparent peak width and height change as a function of detector sampling rate. 28

     
     

    Detector considerations: Non-Combustion detectors

    Non combustion detectors such as TCD and ECD will only use carrier and make-up gases (Table 4).  The selection of make-up gas is very instrument and application dependent. 22-26  A list of carrier and make-up gases typically used with non-combustion detectors can be found in table 4.

     

    Table 4:  Gases for non-combustion detectors.  ** Must be same as carrier and reference gas.

     
     

    Electron Capture Detector

    For optimum ECD operation, the carrier and make-up gas should be ionisable (Figure 26).  Neither hydrogen, nor helium ionise under the normal ECD operating conditions and should not be used as the make-up gas.  An ECD still requires nitrogen or argon/methane working gas and, if designed for hydrogen carrier applications, its sensitivity may be affected significantly

    If in doubt always consult the manufacturer for their recommendations.

     

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    Figure 26: Electron capture detector.

     
     

    Thermal Conductivity Detectors

    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 (Figure 27).  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 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.

     

    Figure 27: Thermal conductivity detector.

     
     

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    Hydrogen for MS Detectors: Practical Considerations

    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 ionization 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.  Most bench-top MS detectors can pump 2-4 cm3 of helium carrier gas but somewhat less with H2.  Changes in vacuum levels within the ion source may cause changes to ionization efficiency, reduce the resolving power of the mass analyzer / detector and change fragmentation patterns due to collisional effects.  It is important to consult manufacturers as changes to pumps, instrument hardware or firmware may be required.

    Problems may be experienced with the ionization of “fragile compounds, compounds at trace levels, and reactive compounds (alcohols, aldehydes, ketones) due to the reactive nature of hydrogen.  This will result in a change in ion ratio, extra peaks in the chromatogram, or reduced sensitivity (Figure 28).

    The use of methylene chloride can also be problematic due to the formation of HCl at higher temperatures (>280 °C), which may cause unwanted reactions with analyte molecules.

     

    Figure 28:  Mass spectra of Lindane in helium (top) and hydrogen (bottom). 29

     

    When converting a standard GCMS system to work with hydrogen it is always best to consult with the manufacturer to determine component compatibility with H2
    i.e magnet and draw out lens.

     
     

    Column dimensions

    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 29 shows a plot 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 50 m 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 minimize this problem.

     

    Figure 29: Column inner diameters (mm): (a) 0.20, (b) 0.25, (c) 0.32, (d) 0.53; column temperature: 50 oC; average linear velocity: 40 cm/s.  The blue shaded area designates negative inlet pressures.

     
     

    Column Pressure/Flow Calculators (Figure 30) can be used to help avoid the problem of low inlet pressures and provide correct inlet pressure to meet pump requirements.  Good practice is to determine max flow of H2 into MS that will give sufficient source pressures.

     

    Choose column dimensions at initial oven temp of method to give:

    • a flow < max column flow for vacuum pump
    • a flow > min column flow for efficiency
    • an inlet pressure of at least 5 psig
     

    Figure 30: Agilent Column Pressure/Flow Calculator. 27

     
     

    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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    We hope that this webcast and tutorial have provided you with an understanding and a practical toolkit to help you increase the speed of your analyses without the need for costly instrument modifications.  The tables below summarize the improvements that can be made and any advantages or disadvantages of each modification.

     
     
     
     
     

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    1. Thermo Scientific FAST GC Columns White Paper: DSGSCFASTGC 0509.

    2. Novel concepts for fast capillary gas chromatography, Marieke van Deursen. - Eindhoven : Technische Universiteit Eindhoven, 2002. ISBN 90-386-2873-0.

    3. The CHROMacademy Essential Guide to Developing Fast Capillary GC Separations

    4. Eva Matisová, Milena Dömötörová. “Fast Gas Chromatography and its Use in Trace Analysis” Journal of Chromatography A, 1000 (2003) 199–221.

    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. Scientific Instruments Manufacturer GmbH. http://sim-gmbh.de/

    8. SRI. http://www.srigc.com/

    9. J.V. Hinshaw, Column Connections, LCGC Asia Pacific, 12(2), 1100 (2009).

    10. Agilent Technologies Application Note, ‘Predictable Translation of Capillary Gas Chromatography Methods for Fast GC’ (5965-7673).

    11. K. Mastovska and S. J. Lehotay, J. Chrom. A, 1000 153–180 (2003).

    12. J.V. Hinshaw, LCGC N. Am., 28(3), 218 (2010).

    13. J. Heseltine, LCGC N. Am., 28(1), 16 (2010).

    14. J.V. Hinshaw, LCGC N. Am., 26(11), 1100 (2008).

    15. J.V. Hinshaw, LCGC Asia Pacific, 12(2), 1100 (2009). (http://chromatographyonline.findanalytichem.com/lcgc/article/articleDetail.jsp?id=581360&sk=&date=&pageID=3)

    16. J.V. Hinshaw, LCGC Europe, 24(1) (2011). (http://chromatographyonline.findanalytichem.com/lcgc/GC/Hydrogen-Carrier-Gas-amp-Vacuum-Compensation/ArticleStandard/Article/detail/709613)

    17. 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).

    18. L.M. Blumberg and M.S. Klee, Anal Chem, 1998, 70, 3828-3839.

    19. “GC Detectors” from CHROMacademy’s “The Theory and Instrumentation of GC”.

    20. The CHROMacademy Essential Guide to GC Troubleshooting - Column & Detector Issues.

    21. The CHROMacademy Essential Guide to Understanding GC Detectors.

    22. Agilent 6890 Series Gas Chromatograph -Operating Manual Volume 3. Detectors.

    23. Gas Recommendations For Agilent GC’s.

    24. A Guide to GC Setup. Copyright © 1994, 1998 Restek Corporation.

    25. Thermal conductivity detector. Gas chromatography with HiQ® specialty gases. Linde Application sheet. http://hiq.linde-gas.com

    26. Hints for the Capillary Chromatographer –Using Electron Capture Detectors. The Restek Advantage. June 1994, Restek Corporation.

    27. http://www.chem.agilent.com/en-US/promotions/Pages/GCapp.aspx.

    28. http://www.sepscience.com/Techniques/GC/Articles/1072-/GC-Solutions-30-Pay-Attention-to-Acquisition-Rate-and-Detector-Range.

    29. http://www.chem.agilent.com/Library/slidepresentation/Public/ASTS_MidAmerica_Converting_Agilent_GCMSD_To_Hydrogen_Carrier_Gas.pdf.

     
     

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    In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific) explore the various reasons for translating methods including improved chromatography, faster throughput, adopting modern column geometries and switching to hydrogen carrier from helium.

    Dr Peter Tranchida

    Tony Taylor
    Technical Director
    Crawford Scientific

    Josep Miquel Serret

    John V Hinshaw
    Senior Scientist
    BPL Global Ltd

    Key Learning Objectives:

    • Identify the reasons and benefits for method translation in GC
    • Discuss how to select columns for improving separations and reducing analysis time
    • What factors need to be considered when translating to smaller column i.d., higher carrier velocity or the use of hydrogen as carrier gas
    • A logical approach to translating method variables and instrument hardware
    • How fast can you go?
    • Avoiding GC method translation pitfalls
    • Avoiding selectivity changes and problems with detectors