Technology Advances

Technologies for laboratory analysis advance continuously, just as computer technologies or transportation technologies do. 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 lab workers may have to acquire and the relative benefits and drawbacks of all of the changes.

Reasons and justifications for technology upgrades depend on 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 lab. 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 instalment of GC Connections discusses two related GC technologies and their impact on laboratory equipment and procedures.

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 FID air — a flame detector needs to stay lit and stable as long as there are pending analyses. Result — much more FID air is used in the average lab 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 lab.

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 detectors, such as helium ionization (HID) and electron capture (ECD), in which helium actively participates in the detection chemistry. Even in these cases it is sometimes possible to apply helium as the make-up gas while using hydrogen as carrier gas, which will at least reduce helium consumption. As an alternative, most ECDs will work with a 5% methane in argon make-up gas mixture, although sensitivities and relative responses will change compared to helium. Mass spectrometric 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. Mass spectrometric 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 can also 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 may be easier to justify a new hydrogen generator if several GCs 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 may 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 × 30 ×10 foot (6 × 9 × 3 m) lab 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 may be released into the lab from split vents or during column installation. For peace of mind, it may be a good idea to install a hydrogen sensor near the ceiling of the lab. I have one such sensor in my lab 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 lab.

Modern GCs include some safety features that address hydrogen concerns as well. Any lab 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 GCs 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 GC ovens, as well.