Enabling technology allowing for the unrestricted use of nitrogen as a Gas Chromatography (GC) carrier gas

Is hydrogen the only option when it comes to replacing helium as the carrier gas for GC-MS/MS applications?

For gas chromatography (GC) the role of the carrier gas is to move vapor phase solutes through a column to the detector. The most commonly used carrier gas is helium, followed by hydrogen and nitrogen.?The carrier gas for GC affects both retention times and the separation so the choice of a carrier gas comes down to the speed (linear velocity) of the gas and whether it might react with components of the flow path or the analytes of interest. To avoid this the carrier gas should be inert.?Availability and the cost of the gas is also a consideration as well.??

Why is there a discussion around GC carrier gas?

Helium is a finite gas and the extraction and availability of this gas is inter-linked to the recovery of natural gas. Helium of course is not solely used for the purposes of gas chromatography, we are all familiar with its use in party balloons, but more importantly it is routinely used in medical diagnostic equipment, manufacturing and it has various uses in scientific research. The wide use of helium, along with the fact it is non-renewable means uncertainty for laboratories around the future availability and costs of this gas. This uncertainty has led to many laboratories considering the alternative gases such as hydrogen and nitrogen to replace helium as the carrier gas for GC-MS applications. Whilst there has been a lot of discussion recently around the use of hydrogen as the replacement carrier gas for GC-MS, nitrogen should not be overlooked.?

The reason that nitrogen has received less attention is because it is often described as a slow gas, due to the fact its optimal linear velocity is lower than helium and hydrogen. Another reason is that the majority of current GC-MS technology has been optimized for use with helium, and as a result has limitations around the use of nitrogen as a carrier gas. Nitrogen does, however, have some advantages, it is an inert gas, renewable and widely available. Waters Atmospheric Pressure ionization source for GC-MS (APGC?) allows laboratories to obtain the benefits from use of nitrogen as a carrier gas without the restrictions around its use that current electron ionization (EI), GC-MS technologies have.

What is APGC?

APGC is an ionization technique which is similar to atmospheric pressure chemical ionization (APCI). It’s considered a soft technique compared to electron ionization, which is currently the most common ionization technique used for GC-MS applications.??

Waters commercialized APGC source technology in 2008, coupling the technology to both tandem/triple quadrupole (TQ) and quadrupole time of flight (QTof) mass spectrometry (MS) systems. The launch of the Waters’ Xevo TQ-S system (2010) saw the introduction of StepWaveTM ion optics, which when combined with APGC, allowed the system to sample a greater volume of gas and bring about an increase in sensitivity.

In the 12 years since, the sensitivity of tandem/triple quadrupole mass spectrometry technology has come a long way. The APGC source combined with the Xevo TQ-S micro, Xevo TQ-XS and Xevo TQ Absolute has resulted in an even greater sensitivity enhancement for GC applications when APGC technology is used. The benefits of this increased sensitivity not only allow for lower achievable quantitation limits, but also allows for lower on-column sample volumes which can reduce the amount of maintenance required for the GC inlet, column, and ion source.

Unrestricted use of nitrogen as a GC carrier gas

APGC is an atmospheric pressure ionization (API) technique which means in contrast to vacuum based EI GC-MS systems there are far less restrictions on carrier gas flow rate. This has benefits such as the use of fast-GC to reduce analysis times, but importantly allows for the use of nitrogen without the downsides resulting from the lower linear velocity than helium. With the appropriate GC column dimensions and correct method scaling, nitrogen can produce comparable data to that produced with helium.

Concluding thoughts

APGC technology offers laboratories a powerful alternative to current EI based GC-MS technology, enabling low limits of detection and confident compound identification and confirmation. Alongside these benefits, APGC can reduce laboratory costs, through reduced system maintenance and addresses any concerns a laboratory may have about the availability and cost of a carrier gas.

To learn more about APGC visit www.waters.com/NextGenGCMS

References:

American Chemical Society (2022). Green Chemistry, Helium https://www.acs.org/content/acs/en/greenchemistry/research-innovation/endangered-elements/helium.html

Dorman. F, Stevens. D, Jones. R (2022), APGC: A Better Future with Nitrogen, The Analytical Scientist https://theanalyticalscientist.com/techniques-tools/apgc-a-better-future-with-nitrogen