Impact of GC Parameters on The Separation: Part 6 - Choice of Carrier Gas and Linear Velocity
by Jaap de Zeeuw, Restek Corporation, Middelburg, The Netherlands.
In part 1-5 of this series we focused on the selection of the column dimensions and stationary phase. We also saw that temperature has a big impact on the chromatography. The last two factors that are somewhat related, are the choice of carrier gas and the setting of linear gas velocity, as shown in Figure 1.
Type of Carrier Gas
In GC there are only a few choices for the carrier gas. The most popular ones are helium, hydrogen and nitrogen. For some specific applications argon is used.
The carrier gas in itself does not impact the separation in GC. This is quite different from LC, where the mobile phase can be modified many ways to enhance selectivity. In GC the choice of carrier gas is defined by a number of factors:
Availability: This may seem an obvious factor, but in recent years, availability for helium has become a bigger challenge. Gases like N2 and H2 have no issues at all, as they can even be produced in the lab using generators. Helium cannot be manufactured and must be extracted from a natural resource like natural gas. Because of the insecure supply, labs are looking for alternatives.
Price: each carrier gas also has a cost. Also here helium is a challenge as it’s already expensive and expectation is, that the prices will only go up. As a result there is a strong interest in using other types of carrier gas.
Purity: This is an interesting parameter in GC. We always choose the most “pure” carrier gas. You can buy carrier gases with high grade being 99.9999 % pure. You also can buy 99.999 % pure and the price difference is interesting. Some groups buy a less expensive carrier gas and completely rely on the capability of the in-line filters. Bottom line is, that for trace analysis, one needs to optimize the whole system. You need to be assured the carrier gas used to work an expensive GC-MS setup, is clean.
Speed of Analysis: If analysis speed is important, the carrier gas choice will be directed to the gas that delivers shortest analysis time. Without any question, that would be hydrogen. Figure 2 shows the van Deemter curve, where hydrogen has an optimum velocity of about 40‑50 cm/s. Once hydrogen is chosen, questions will pop up on safety. How dangerous is hydrogen in reality when used as a carrier gas?