Have you wondered how the gas chromatography (GC) carrier gases helium, hydrogen, argon, and nitrogen are transformed from their natural conditions or precursors into highly purified compressed states inside laboratory gas cylinders or generators? Here, we track the genesis of the top four carrier gases before they start their journey through a GC system.
Unlike liquid chromatographers, gas chromatographers have only a few mobile phases to apply to their separations. The mobile phase in liquid chromatography (LC) plays an active role in determining solutes' retention characteristics, and it does so in relation to the chemical nature of the stationary phase. To that end, liquid chromatographers employ various solvents, solvent mixtures and gradients, buffers, and other additives in combination with a wide array of stationary phase modes and types that give them degrees of control over their separations that the rest of us might dream of.
In gas chromatography (GC) the situation is much more limited. The role of the GC mobile phase is reduced to simply carrying solute molecules along the column while they are not dissolved in or adsorbed on the stationary phase. All of a GC separation's selectivity can be attributed to the stationary phase alone. The GC mobile-phase job description of "inert" and "carrier" gas sounds about as dull as it can be. And also unlike in LC, GC carrier gases come with some nonintuitive behaviours that can be daunting to understand at first, such as the effects of gas compressibility on flow and separation. These side effects of a gaseous mobile phase have been dealt with in innumerable publications and discussions, and I won't go into them any further in this article. Also, for space reasons the use of carbon dioxide for supercritical fluid chromatography (SFC) will not be addressed in any detail.This month's instalment asks the following question: Where exactly do the various GC carrier gases come from? The obvious and wrong answer is, "from a gas cylinder". Instead, consider the elemental or natural states of the primary carrier gases helium, nitrogen, hydrogen, and argon. Some of these are quite common in nature; others are rare. How are the carrier gases obtained from nature and purified to better than 99.9999%?
Argon is the next most abundant atmospheric carrier gas, followed by carbon dioxide as used for SFC. The carbon dioxide level in Table 1 is the reported monthly mean value at Mauna Loa in Hawaii as of October 2013 (1). The trace gases neon, helium, krypton, hydrogen, and xenon, which taken together account for about 25 ppm of the atmosphere's content, are listed below carbon dioxide. Krypton and xenon are and will remain too expensive to be taken seriously as GC carrier gases, but it is possible that desperate gas chromatographers with no helium on hand may have tried one of the noble gases if they could find a cylinder. Neon seems like a logical candidate. If helium prices continue to increase then perhaps neon, which costs more than five times the price of helium, will become another helium replacement like hydrogen.
Beyond simple cost considerations, there are performance reasons to prefer certain carrier gases. Hydrogen, for example, is sometimes preferred because of its low viscosity and high diffusivity. The low viscosity means that less pressure is required to attain any particular flow rate with hydrogen, which translates to better performance at higher flows and with narrower-bore columns. Higher diffusivity produces more efficient separations around the optimum carrier gas velocity region. Neon, on the other hand, has high viscosity and low diffusivity, which make it less attractive.
Helium, a nearly nonrenewable natural resource, is light enough that, along with hydrogen, it attains escape velocity after it is released into the atmosphere and simply leaks off into space where it is swept away by solar wind. Not that these elements are scarce — by some accounts hydrogen comprises 73.9% of the known baryonic matter in the universe, followed by helium at 24%. Both are replaced in the Earth's atmosphere at slow rates; the balance between replenishment and loss accounts for their average natural concentrations in air shown in Table 1. A chromatography laboratory with helium carrier gas in use would be above average — aren't they all — with a considerably higher background helium level. It's the higher levels of hydrogen in the laboratory that are of real concern and that drive the installation of hydrogen detectors and better laboratory ventilation.