Hydrogen as a Carrier Gas for GC and GC–MS

Jan 01, 2010
Volume 28, Issue 1, pg 16–27

Ronald E. Majors
In gas chromatography (GC), the carrier gas serves as the mobile phase and carries (moves) the solutes down the column. The selection and linear velocity (flow rate) of the carrier gas influences efficiency and retention time. A carrier gas must be inert to solutes and stationary phase, must be free of detectable contaminants, must be readily available at a reasonable price, and must have a leak-free and prodigiously precise pressure–flow delivery system for precise qualitative and quantitative data from the gas chromatograph.

Figure 1
Commonly used and popular carrier gases include helium, hydrogen, and nitrogen, although argon, ammonia, and carbon dioxide also have been used to a minor extent. Also, hydrogen has long been used as a fuel gas for flame ionization detection (FID) as well as other detection methods in GC. In this application, the hydrogen purity with respect to oxygen and water is not particularly critical. However, the hydrocarbon content of this gas must be minimized to keep low background noise. Helium has been the most widely used carrier gas for GC due to its inertness, good purity, excellent performance, and well-established methodologies. In recent years, the demand for helium has outstripped the supply, resulting in limited supplies in certain geographies, increased costs, and uncertain delivery. Normally produced by radioactive elemental decay, it is more expensive to isolate compared to nitrogen or hydrogen. Although it displays the lowest minimum plate height compared with that of helium or hydrogen, nitrogen has a much narrower velocity range and a steeper van Deemter curve, so at higher flow rates, solute efficiency drops off dramatically (Figure 1). With the shortage and expense of helium and the less desirable properties of nitrogen, gas chromatographers have been looking toward hydrogen as a carrier-gas alternative.

Choice of Carrier Gas in GC

The choice of carrier gas can have an effect on the appearance of the chromatogram and plays an important role in GC optimization. Compared to high performance liquid chromatography (HPLC), in which the mobile phase has profound influence on the chromatography, the carrier gas in GC has a much smaller effect because it rarely interacts chemically with the stationary phase or solutes and serves mainly to volatilize the solutes under the influence of temperature. Two properties of a gas play a role in the chromatographic process: diffusivity and viscosity. The diffusion speed of an analyte in the gas determines the speed of GC. Analytes need to spend time in both the stationary phase and the carrier gas to separate. The diffusivity of hydrogen and helium are roughly the same, but nitrogen has a value that is roughly three to four times lower than helium, which means that the slower diffusion results in longer separation times. If one tries to speed up the flow rate with nitrogen carrier, then the van Deemter curve (Figure 1) shows that the efficiency will fall off dramatically.

The viscosity of the carrier gas will determine the inlet pressure required for a given gas velocity. High inlet pressure compresses the gas at the column inlet, and as the gas moves through the column, its linear velocity will change. Normally, for capillary columns, one tries to achieve an average linear velocity of around 20–25 cm/s. Hydrogen has about half the viscosity of helium and nitrogen, and at typical column lengths in the range of 15–30 m, it provides the best combination of efficiency and speed over the widest range of linear velocities. Its optimum carrier-gas velocity is around 60 cm/s so, relative to the other popular gases, separations are usually faster. So, if hydrogen has favorable properties and is less expensive, why hasn't it been endorsed universally by gas chromatographers? The answer is "Safety"!

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