June 2013 » Chromatography Techniques CT13
Figure 2: The unchecked flow of a standard DOT 3AA-2400 6300 L hydrogen gas cylinder compared
with the maximum hydrogen flow rate of 600 cc/min Proton OnSite Hydrogen G600P into a 3,000
ft3 laboratory space with 12 air changes/hr.
Purity is key
Carrier gas purity is key for GC applications.
Hydrogen carrier gas should be of the same purity
level as a helium carrier, at 99.9999 percent purity
for trace work less than 1 part per million, or at least
99.9995 percent for most normal analysis.
Cylinder use will require practitioners to install
external hydrogen filters onto their systems. If the carrier
gas contains even trace amounts of water or oxygen it
may damage the GC column stationary phase, resulting in bad peak shape and excessive column bleed. Gas
produced on-site can be assured to be pure and without
contaminants because most generators, including Proton
OnSite’s family of hydrogen generators, have built-in,
efficient palladium membrane purifiers.
For practitioners used to ordering and managing
helium cylinders, it may make sense to just amend the
order with the gas supplier from helium to hydrogen
and continue to manage deliveries. But, for those labs
that have had to manage large numbers of helium cylinders in the past, or for those that question the purity
of delivered hydrogen gas, an on-site hydrogen generator
may be beneficial. An on-site generator can make sense
from a financial standpoint and it ensures optimal purity.
Regardless of the source, switching to hydrogen can improve long-term
results. Hydrogen acts as a scrubbing gas and can remove contamination
in unpurged areas of the GCMS system. Initially, this will result in an elevated hydrocarbon background in the chromatogram, but this will reduce
over time, resulting in a system that keeps away contaminant buildup and
needs less frequent cleaning of the ion source.
than the viscosity of helium, so it is easier to maintain higher linear velocities with hydrogen in the laboratory.
Using hydrogen as a carrier gas in GCMS systems presents new operational challenges for practitioners switching over from helium. Hydrogen
calls for different flow rate and column pressure considerations.
Most GC systems control the flow rates of the carrier gas electronically
using mass flow controllers. When these systems are used with different
carrier gases than the originally calibrated one, a conversion factor must be
used to calculate the correct flow. The new carrier gas can usually be selected
in the gas chromatography software and the computer automatically sets the
conversion factor of the gas. But if a practitioner needs to calculate the gas
conversion factor manually, the following approximation can be used:
Cf = n1 x Cp1/n2 x Cp2 where Cf = the conversion factor, n1, 2 = density of
gas, Cp1, 2 = heat capacity at constant temperature.
Because hydrogen has a different viscosity than helium and the maximum column efficiency occurs at a different gas velocity, hydrogen column settings will be different when run under optimum conditions.
With a flame ionization detector, the column outlet is at ambient pressure, whereas in a GCMS system, the column outlet pressure is at high
vacuum, enabling decreased column head pressure in order to maintain
a specific column flow rate. Because of the lower viscosity of hydrogen
resulting in a lower column head pressure, injection of the sample in
split-less mode will cause a much bigger, uncontrolled solvent plume. This
leads to imperfect chromatographic results, such as peak tailing and peak
To overcome these issues, the use of a narrower GC column (0.18 µm
diameter) is highly recommended. Using a narrow-diameter GC column
also helps to avoid flooding the MS. The turbo-molecular pumps achieving the high vacuum in the MS manifold were originally designed to
pump away more dense gases, i.e. helium, so a narrow column allows for
the pumps to better handle the lighter hydrogen.
It’s worth noting that MS manufacturers are recognizing the industry
trend toward hydrogen carrier gas and are installing turbo-pumps with
special-designed drag stages into their new systems, which create high
compression ratios for light gases.
A safer lab
GC practitioners need to take precautions to ensure the safety of their
laboratory if they decide to switch to hydrogen gas. An unintended leak
due to downstream equipment failure can quickly create an explosive
atmosphere. Several incidents of leaking hydrogen cylinders have been
reported in laboratories. Frequently replacing depleted high-pressure
hydrogen cylinders or unattended open taps can also lead to accidents.
A leaking cylinder can quickly create a dangerous atmosphere.
Hydrogen mixed in air can ignite when exposed to the energy in a static
discharge, as low as 0.017 mJ. The lower flammability limit (LFL) is 4 percent hydrogen in air, by volume. Leaking hydrogen cylinders can quickly
and easily create a dangerous situation.
More than one standard hydrogen cylinder exceeds code minimum
safety limits, imposing special building construction, separation distances
and increased facility costs, especially when co-located with other research
gases. One standard hydrogen cylinder, storing 6,300 liters of gas, has the
explosive potential of 35 lb of TNT.
In comparison, an on-site hydrogen generator cannot create an explosive atmosphere within an average-sized, ventilated laboratory. The Proton
OnSite Hydrogen G600P hydrogen gas generator contains less than 3 liters
of hydrogen at any time.
Switching carrier gases, particularly in large facilities, can be a complicated decision. A new carrier gas must make logistical and financial sense
to the lab manager, and the GC practitioner needs to be sure a new carrier
gas will be producing equal or superior results.
Hydrogen gas will satisfy all parties involved in this decision. Lab managers
who currently spend time dealing with irregular, expensive helium cylinder
deliveries will sleep a little better knowing their new carrier gas supply is abundant and cost-effective (especially if it’s produced on-site). And practitioners
will be happy with improved GC results and faster run times.