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Nitrogen as a Carrier Gas for Capillary GC

Helium is the most routinely used carrier gas for capillary GC, however, with the pressure of limited worldwide supply resulting in its increasing cost there have been moves to use alternatives.  One of the biggest trends has been the use of hydrogen as a carrier gas as it provides many benefits, such as faster, more efficient analyses. 

Yet there is still some reticence to implement the use of hydrogen due to safety concerns (although if you already use hydrogen safely in your laboratory then these concerns are mitigated). 

Therefore, some laboratories are turning to often forgotten nitrogen.  Although there has been little favor shown to nitrogen in recent years due to the desire for faster analyses (nitrogen produces longer run times) it is a highly applicable carrier gas for many capillary GC separations. 

 

Nitrogen vs. Hydrogen or Helium

There are several disadvantages to using helium and hydrogen as carrier gases. 

Helium is expensive and in finite supply.  However there are several detection systems which prefer helium due to the detection principle, these include mass spectrometers and pulse discharge based detectors (pulsed discharge ionization detector, PDD; helium ionization detector, HID; barrier discharge ionization detector, BID).  However, if it is critical that helium is used over hydrogen or nitrogen then helium conservation through the use of gas saver options within GC systems is an excellent option. 

One of the limitations with hydrogen are the inherent safety concerns, however, this is mitigated by the fact that most instruments will use hydrogen as one of the detector gases, and therefore, most labs will already be safely set up with hydrogen sources (cylinders or generators - with generators being the safer alternative).  From a chemistry point of view hydrogen is reactive and could react with analytes during analysis under certain conditions.  For, example, care should be taken when using chlorinated solvents with hydrogen carrier gas because of the risk of hydrochloric acid (HCl) formation, which can affect column efficiency through formation of pores in the stationary phase.  If you are using MS detectors hydrogen can cause high background noise and may affect ionization efficiency due to the change in vacuum characteristics.

Nitrogen may be considered a slow gas but it does produce very efficient GC separations.  It can also be generated in situ in the lab with high purity using a generator (making it very cost effective).


 

Producing Nitrogen In the Lab

GC requires a carrier gas purity of 99.999%. Nitrogen can be generated in excess of this purity within the lab using nitrogen generators. Generators operate using two different modes.

1. Membrane Generator

In a membrane generator air is forced into a vessel which contains a hollow tube surrounded by a polymeric hollow fiber.  The hollow fibers selectively permeate O2, water vapor, and other impurities out of the side wall allowing N2 to flow through the center (Figure 1).

Figure 1: Membrane nitrogen generator.

 

2. Pressure Swing Adsorption (PSA) Generator

A pressure swing generator separates nitrogen molecules from other gas molecules using a carbon molecular sieve.  Two columns containing molecular sieve are alternately pressurized; O2, moisture, hydrocarbons, CO2, and other contaminants are adsorbed leaving N2 to flow into an accumulation tank where it is pressurised and delivered to the instrument (Figure 2).  Pressure swing generators are capable of generating nitrogen with a purity of 99.9995%.

Figure 2: Pressure swing adsorption (PSA) nitrogen generator.

 

What Does Van Deemter Say?

The van Deemter equation (Equation 1) tells us nitrogen is not the best carrier gas for capillary GC. However, is that really true or have we just been blindly following van Deemter without giving nitrogen a chance to produce the separation we require?

The van Deemter equation tells us that the highest efficiency separations (for any gas) will be produced when H (the height equivalent to a theoretical plate) is at a minimum (Figure 3). All this means is that each individual gas will have an optimum but at a different linear velocity - H2 = 30-50 cm/sec, He = 20-40 cm/sec, and N2 = 10-15 cm/sec. Capillary GC separations using 0.25 and 0.32 mm i.d. columns are generally run at 30-50 cm/sec, therefore, nitrogen would not be the most efficient gas at this flow rate, however, slowing the flow rate to 10-15 cm/sec will give very efficient separations. In fact a comparison of the van Deemter curves (Figure 3) shows that nitrogen utilised at its optimum linear velocity produces the most efficient separations of all the carrier gases.



Figure 3: Van Deemter plot.

 

It should be noted that both helium and hydrogen give relatively flat van Deemter curves in comparison to nitrogen - this flat curve profile allows us to operate out with the linear velocity without too much loss of separation. 

Therefore, it would seem that in this respect nitrogen would be a poor choice, however, due to the use of constant flow or linear velocity modes we have greater control over the linear velocity and there should be little change throughout the separation making this point moot.

If we slow our carrier gas flow rate to give the optimum separation using nitrogen our analysis time is going to be 2-2.5 longer, however, utilizing hydrogen at the same velocity and oven conditions as He will give the same chromatogram with only slightly broader and less intense (30%) peaks. 

This means that if you have a chromatogram that has extra resolution and you can afford to lose a few plates (most of us can afford to lose more than we think) then nitrogen used under the same conditions as helium is an excellent alternative.  Care of course should be taken if a loss of sensitivity for already low intensity peaks will be detrimental to analytical results. 

One drawback of the loss of efficiency when using nitrogen is that broader peaks will merge faster upon column ageing which reduces the number of analyses that may be able to be performed on a column resulting in more columns being required which ultimately increases the price per analysis.


Improving Efficiency and Resolution

The loss of efficiency and resolution when using nitrogen as a carrier gas can be compensated for by using a shorter, narrower internal diameter column. As i.d. decreases the optimum linear velocity increases (regardless of carrier gas, Figure 4). Narrower i.d. columns also give the advantage of producing flatter van Deemter curves allowing separations to be performed out with the optimum linear velocity without sacrificing resolution.

This means that that a slightly higher linear velocity can again be employed when using nitrogen which will help to reduce analysis time. In fact analysis times can be maintained by adjusting the linear velocity to match the holdup time of the original method (more on this later).

Figure 4: Van Deemter curve for different column internal diameters.

 

When using shorter, narrower i.d. columns the mass of sample which can be loaded onto the column must be taken into consideration.  Shorter, narrower columns are smaller and contain less stationary phase, therefore, less sample can be loaded onto them.  You may also have to adjust your injection volume.  Mass or volume overload on a column will result in deterioration of chromatographic peak shape, most commonly fronting peaks will be observed.

Efficiency and resolution can also be improved by using application specific stationary phases (i.e. pesticides, FAME) which are designed to produce optimum separations for their particular class of analytes. 

 

Putting the Theory Into Practice

When using the same column for a method and only changing from helium to nitrogen then just leave everything the same, including the head pressure; this will result in the same chromatogram being produced in the same amount of time.

If you are changing the column, carrier gas, and are working with more complex temperature programmed methods then using method translation software (the links below are free) is the best option to easily create a new method.

//www.restek.com/ezgc-mtfc

https://www.agilent.com/en-us/support/gas-chromatography/gcmethodtranslation

Translation software has been used to translate a method for analyzing a fragrance mixture from helium to nitrogen.  Table 1 details the method parameters and the resulting chromatograms are shown in Figure 5.

 
Original Translated
Carrier:
Helium
Carrier:
Nitrogen
Column:
30 m x 0.25 mm x 0.25 µm
Column:
20 m x 0.15 mm x 0.15 µm
Phase Ratio:
250
Phase Ratio:
250
Carrier Control:   Carrier Control:  
Column flow                   
Average linear velocity
Hold-up time                           
Inlet pressure         
Outlet pressure         
1.40 mL/min
32.07 cm/sec
1.56 min
15.28 psi
14.70 psi
Column flow                   
Average linear velocity
Hold-up time                           
Inlet pressure         
Outlet pressure         
0.35 mL/min
21.37 cm/sec
1.56 min
17.06 psi
14.70 psi
Temperature Program:
40 °C for 0.1 min
6.4 °C/min to 250
Temperature Program:
40 °C for 0.1 min
6.4 °C/min to 250
Control Method:
Constant flow
Control Method:
Constant flow
Run Time:
32.91 min
Run Time:
32.91 min

Table 1: Original and translated GC method using nitrogen as a carrier gas and altering column dimension.

Some points to note about the translated method and the different column dimensions are:

  • 20 m columns work well to convert methods which originally used 30 m columns

  • 0.18 or 0.15 mm i.d. columns are nice narrow bore columns which do not usually require any adaptations to the instrument, such as the use of smaller volume inlet liners

  • Using a reduced film thickness (0.15 µm in this case) in conjunction with the reduced column i.d. allows the phase ratios of the original and new columns to be matched which results in the selectivity of the separation being preserved

  • The flow and pressure can be adjusted to match the hold-up time for the two columns which maintains the original analysis time

Figure 5: Analysis of a fragrance mixture using He (top) and nitrogen (bottom).1

 

Conclusion

Nitrogen carrier gas has been utilized in a wide variety of applications such as pesticide, FAME, biodiesel, acrylate, di- and triamine, BTEX, non-aromatics, primary aryl alcohols, ethylacetate, ethanolamines, alcohols, acetic acid, and light hydrocarbons analysis. It can be easily and cost effectively generated in the lab at the correct purity for GC analyses.

Although using nitrogen at flow rates required for capillary columns results in longer less efficient analyses, simple changes to the column dimensions without the need for any change to instrument hardware (i.e. ferrules, liners) can remedy these losses.

Furthermore, the use of application specific stationary phases can also improve chromatographic efficiency and resolution. While some detection systems may not be suited to the use of nitrogen as a carrier gas (pulse discharge based detectors), mass spectrometers can be used, however, some adjustment to the GC-MS tune parameters may be required to optimize sensitivity.

References

  1. de Zeeuw, J; Cochran, J. The Column, Oct 2015, Volume 11, Issue 19.
 

For more information on carrier gases for GC:

Webcasts
Nitrogen as a Carrier Gas for Capillary GC
Translating to Hydrogen Carrier for GC - A Practical Perspective
Translating GC Methods from Helium to Hydrogen Carrier Gas
Gas Quality for GC

Quick Guides
Helium to Hydrogen - A Change Would Do You Good
Crimes Against GC Gas Filters

 

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