There is a sense of worry about the future availability of helium, which puts it into the group of top endangered elements.1 Helium is not easy to come by. Most terrestrial helium derives from the decay of radioactive uranium (U238) and thorium (Th232) in the earth’s crust. Because the decay process for radioactive sources of helium is time-consuming (a billion years), the rate of production is incredibly slow. To make it more challenging, because it is a lighter gas, most of the helium produced over the years has diffused to the surface and escaped from the earth’s atmosphere. A small fraction that has been contained in the layers of rock is found alongside natural gas.2
In general, helium reminds us of parade and party balloons, but it is also a key element for research, medical technology, welding, semiconductor manufacturing, space exploration, national defense, and more. Being the largest producer of helium, the United States has controlled the exploration and availability of helium through government policies and amendments since 1925. As the ongoing search for helium continues worldwide, and contradictory news and views about helium resources surface in the media and literature, the community of analytical chemists often finds itself entangled in the debate of helium versus hydrogen in the field of gas chromatography. My favorite hydrogen versus helium trivia is that the Hindenburg was designed to be filled with helium, but because the US controlled most of the world’s supply of helium and the Hindenburg was used as a propaganda tool by the Nazis, the US wouldn’t sell the helium to Germany. As we all know, the story of the Hindenburg didn’t end so well.3,4
Most gas chromatographs (GCs) run on helium as a carrier gas. The advantages include non-inflammability, inertness, and moderate speed of analysis. Hydrogen, in contrast, is flammable and may be reactive under favorable conditions, but it produces high-speed analysis and generates sharper peak shapes. The last two characteristics are a matter of rejoicing for many gas chromatographers—those who value time and enhanced sensitivity. Furthermore, hydrogen is approximately 2.5 times less expensive than helium. Every gas chromatographer’s dream is to get analysis done quickly and cost-effectively with an improved detection limit. But there is a catch: the risk of setting the lab on fire!
Hydrogen is inherently dangerous and has a history of many unpleasant events in laboratories. Recently, a postdoctoral student at the University of Hawaii, Manoa, lost her arm in an explosion generated by a mixture of hydrogen, oxygen, and carbon dioxide.5 I also heard about an earlier incident of a hydrogen-related GC explosion in my current lab, and I myself survived a heavy hydrogen leak not many months ago.
So, do I still personally prefer hydrogen for GC? The answer is yes!
In spite of the fact that hydrogen is flammable (4-74%) and explosive (18.3-59%) in air, a hazardous mixture can be easily be avoided in the standard laboratory conditions. Because the rate of diffusion of hydrogen in air is very rapid, when it is released in open laboratory space, it quickly dilutes below the dangerous concentration. Being the lightest of the gases, hydrogen rises with a speed of 45 miles/hour, more quickly than any other gas.6 So, building up hydrogen to its flammable concentration is not something that usually happens unless there is a heavy leakage (pressure regulator failure, transfer line leakage, etc.) from a large, compressed source. Such situations can be avoided by laboratory-scale hydrogen generators that can produce a sufficient amount of the gas at any given time but do not store much of it.
But in a confined space like a GC oven, hydrogen may create problems. Unanticipated column breakage in a GC oven, leading to the buildup of a critical concentration and the subsequent attempt in detector ignition, was one of the major causes of explosions in the earlier days. Fortunately, most modern GC systems are smart enough to go into “shut down” mode when they sense a sudden discrepancy in flow or pressure pattern. And the introduction of metal columns has greatly reduced the amount of column breakage.
The reactivity of hydrogen gas is another concern that deters gas chromatographers from choosing it. Hydrogen is a reducing agent that can promote hydrogenation (the addition of hydrogen to a double or triple bond). Hydrogenation is favored at high temperatures and high pressure, and in the presence of a metallic catalyst such as nickel, platinum, or palladium. Fortunately, favorable conditions for hydrogenation do not exist inside regularly used open tubular fused silica capillary columns. But precautions must be taken in the use of nickel or alumina (Al2O3) columns. Moreover, carrying out pyrolysis GC with hydrogen as the carrier gas may cause some hydrogenation of the unsaturated polymers, provided there is metal contamination (from catalyst use during production). And solid phase micro extraction fibers, if the sample contains a metal core and divinyl benzene as a polymer, causes a considerable amount of hydrogenation.7-9
There are both proponents and critics of hydrogen in the GC community. And after much contemplation, irrespective of helium availability and price, many prefer hydrogen to other carrier gases. During my graduate school performance of GC and valve-based GCxGC analyses, I have used hydrogen extensively with no mishap. I still run GC both on helium and hydrogen for my microchip thermal gradient gas chromatography research. Like many other analysts, I really believe that if the safety aspects and possible consequences (unwanted hydrogenation) are identified well, with current available technology, hydrogen is not a threat to our lives and labs, but rather a gas chromatographer’s strength.
PS: Good news for helium lovers! A vast reservoir of helium was uncovered in Tanzania recently.
5. DOI: 10.1126/science.caredit.a1600049
8. Anal. Chem. 2016, 88, 5462−5468
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