John V. Hinshaw
|
Technologies for laboratory analysis advance continuously, just as do computer technologies or transportation technologies.
Small advances tend to occur fairly often while major new technologies appear less frequently. As new capabilities become
available, laboratories must decide whether to acquire them or to defer and continue to use what they already have. Such decisions
are reached by considering the roles and requirements of the laboratory, the short and long-term costs of the new technologies,
new skills that laboratory workers may have to acquire, and the relative benefits and drawbacks of all of the changes.
Reasons and justifications for technology upgrades depend upon laboratories' current and future needs. While the benefits
of new capabilities are easy to describe, what may not be so evident are the collateral requirements of implementing new technologies
in the laboratory. For example, switching to hydrogen carrier gas generation eliminates the costs of carrier-gas cylinders
and can yield faster speeds of analysis if hydrogen is not already in use, but the change also invokes some new safety requirements
and procedures. This installation of "GC Connections" discusses two related gas chromatography (GC) technologies and their
impact on laboratory equipment and procedures.
Generate Your Own Gases
Installing carrier- and flame-gas generators is relatively easy, although there are some special considerations for hydrogen.
The benefits of zero future gas cylinder costs plus no cylinder transport or demurrage charges yield an attractive return
on investment (ROI), especially given the current high cost of carrier-grade helium. Zero-grade air generators are effective
as well. The cost of detector-quality cylinder air is much lower than carrier-grade helium, but its "burn rate" is much higher
at over 400 mL/min compared to a range of 50–250 mL/min for carrier gas with a split inlet system. Carrier-gas consumption
can be reduced by up to 80–90% if a gas-saver pressure-control mode turns off split flow while the inlet is not actively in
use. There is no corresponding saver mode for flame ionization detection (FID) air. A flame detector needs to stay lit and
stable as long as there are pending analyses. The result is that much more FID air is used in the average laboratory than
carrier gas.
Gas generators have limited flow and pressure ranges that cannot be exceeded. It's a good idea to acquire gas generators that
exceed current flow requirements by 25–50% to allow for future expansion. Also, installing gas generators will create a new
requirement for regular generator maintenance, although arguably this is less effort than it takes to haul cylinders in and
out of the laboratory.
Hydrogen: Generation of hydrogen for carrier and fuel gas invokes some additional concerns. For the majority of GC applications, hydrogen
carrier gas can be substituted for helium. The exceptions are for certain fixed-gas separations as well as for some detection
methods, such as helium ionization detection (HID) and electron-capture detection (ECD), in which helium actively participates
in the detection chemistry. Even in these cases, it is sometimes possible to apply helium as the makeup gas while using hydrogen
as carrier gas, which at least will reduce helium consumption. As an alternative, most ECD systems will work with a 5% methane
in argon makeup gas mixture, although sensitivities and relative responses will change compared to helium. Mass spectrometry
(MS) detectors are generally compatible with hydrogen carrier; some reduced pumping efficiency as well as lower background
ionization levels can be expected. Also, some extra attention to proper detector venting is called for when shutting down
to avoid hydrogen accumulation inside the detector's vacuum chamber. MS detector manufacturers can provide detailed information
about their specific products. For standard GC separations with FID, hydrogen carrier is an attractive choice because the
same hydrogen source also can be used for the FID fuel gas. See reference 1 for some additional frequently asked questions
about hydrogen carrier gas.
Switching to generated hydrogen carrier gas is a two-step process. First, if not already using hydrogen, install a tank of
high-purity hydrogen, or use the existing FID hydrogen tank if it's pure enough, and validate performance with the new carrier.
The column pressure settings will be different. Lower inlet pressures are required for the same average carrier-gas linear
velocities, while the optimum velocity for hydrogen is 10–20% higher than for helium. A flame ionization detector requires
a constant flow of hydrogen fuel, which means electronic pneumatics will be needed to maintain flow when the column is temperature
programmed. Run the carrier pneumatics in constant-flow mode if possible, and establish a constant total FID hydrogen flow.
Once the new carrier-gas and detector settings have been validated, then consider switching from cylinders to a gas generator.
From a cost point of view, it might be easier to justify a new hydrogen generator if several GC systems can be converted to
hydrogen carrier at once.
Beyond considerations for method parameters, using hydrogen carrier gas will invoke some concerns for the potential burning
or explosion hazards. A cylinder of flammable gas represents three distinct hazards. First, the very high pressure in any
gas cylinder is a physical endangerment to personnel if not well understood and handled correctly. Second, the cylinder is
very heavy and can present a lifting or falling hazard. Third, hydrogen is flammable and becomes explosive when mixed with
air at concentrations between the lower and upper explosive limits of 4–74 % by volume. A fully pressurized A-size cylinder
at 2600 psig (18 kPa) contains nearly 8 m3 of gas when expanded to room pressure. In a small 20 X 30 X 10 ft (6 X 9 X 3 m) laboratory, the lower explosive limit (LEL)
of hydrogen could be reached if the entire contents of a cylinder were released at once. However, this extreme occurrence
is very unlikely to take place by accident. Using a hydrogen generator to produce carrier and fuel gas relieves concerns for
the release of a tankful of hydrogen — the generators store only a small amount of hydrogen at any time. Small amounts of
hydrogen can be released into the laboratory from split vents or during column installation. For peace of mind, it might be
a good idea to install a hydrogen sensor near the ceiling of the laboratory. I have one such sensor in my laboratory that
gets tested — loudly — once in a while when I change the carrier gas to hydrogen and purge the carrier-gas lines. But I experiment
with different carrier gases much more often than would a production laboratory.
Modern GC systems include some safety features that address hydrogen concerns as well. Any laboratory that is considering
hydrogen carrier is strongly urged to use an instrument with an electronic pressure control system that limits the flow of
hydrogen carrier gas and detects and shuts off the flow under leakage or out-of-bounds conditions. Today's GC systems feature
explosion-safe ovens that, upon the extremely rare occasion of hydrogen accumulation and ignition, will contain the overpressure
safely inside the oven. Hydrogen leak detector accessories are available for gas chromatography ovens as well.
RSS
Bookstore
