John V. Hinshaw
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Recently, the use of hydrogen carrier in gas chromatography (GC) has been discussed, reviewed, analyzed, and praised multiple
times in this magazine (1–4) as well as many other publications. With the prospects of shortages and ever increasing costs
for helium, hydrogen offers some desirable economic and logistical advantages, as well as performance changes for columns.
The latter topic is the main focus of this installment, but first let's review some of the economic and logistical benefits
and concerns associated with hydrogen use in the laboratory.
Economics and Logistics:
First, hydrogen as a replacement carrier gas is less expensive than helium. Switching to hydrogen also offers significant
logistical advantages, because it makes it possible to reduce the number of tanks of compressed gas in the laboratory by installing
a gas generator instead. In laboratories where multiple GC systems equipped with split inlets and flame ionization detection
(FID) systems together consume large quantities of hydrogen gas, the deployment of hydrogen generators is an attractive proposition:
The high rate of consumption means an early break-even point on the initial cost of the generators. Also, gas generators allow
safety personnel to sleep better knowing that the amount of stored flammable gas is limited and that there are fewer high-pressure
gas cylinders chained to the walls. It can be argued that the presence of any high-pressure gas cylinders represents a greater
hazard than that imposed by a self-generated hydrogen carrier. This argument comes to a logical conclusion with the installation
of zero-air generators, in addition to hydrogen generators, that clean up compressed air for use with FID systems. Thus, switching
from helium to hydrogen means that high-pressure cylinders can be eliminated for routine GC–FID applications as well as for
gas chromatography–mass spectrometry (GC–MS) systems that use electron ionization (EI). Chemical ionization (CI) does require
a cylinder of collision gas such as methane or ammonia, neither of which are particularly amenable to in-situ generation.
Safety:
Although switching to hydrogen carrier can improve safety by making it possible to reduce the number of high-pressure gas
cylinders in the laboratory, hydrogen does pose safety risks. Reasonable precautions should be taken to prevent and to detect
the accumulation of hydrogen from multiple split vent flows or other significant sources, whether using tanks or gas generators.
The built-in leak detection features of modern electronic pressure control (EPC) pneumatic systems along with appropriate
awareness and procedural training for those who use hydrogen in the laboratory can reduce the likelihood of an accident to
low levels. The best practice is to install a hydrogen sensor high up in each laboratory. You may also want to consider installing
small dedicated hydrogen sensors in the GC ovens, especially if not using an EPC system with hydrogen-leak shutdown.
Performance:
Beyond the economic and logistical gains to be had from gas generators and hydrogen, it has some chromatographic performance
advantages over a helium carrier. There is less dependency of column performance on the average carrier gas linear velocity
with hydrogen compared to that with helium. The range of linear velocities over which column efficiency lies close to the
optimum for any particular solute is broader than with helium. This comparison has been made many times before. Two years
ago "GC Connections" showed an example (4) where a solute with k = 10.0 on a 50 m × 0.25 mm column at 100 °C had an optimum average linear velocity range of 18–45 cm/s for helium versus
25–65 cm/s for hydrogen, where "optimum" was defined as the region in which the plate height lies within 25% of its minimum.
This wider optimum range gives chromatographers more latitude in setting average velocities with hydrogen carrier. The optimum
range for hydrogen covers faster linear velocities too, which can yield higher analysis speeds because peaks can be eluted
in less time without sacrificing as much performance as when pushing helium carrier to the same higher speeds. Hydrogen also
has the advantage of requiring lower inlet pressures to deliver a given average velocity, due to its reduced viscosity.
All is not so simple, however, when moving an existing method from helium to hydrogen carrier either with conventional or
MS detectors; there are several key considerations that GC and GC–MS users should understand.
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