Capability with tradeoffs
Field-worthy GC must be accessible at the technician level and eminently portable. Units tend to be battery powered with adequate data systems and good communication capabilities. Detection and carrier gas options are limited due to size and weight restrictions. “Operators will likely be carrying other equipment, and may be wearing protective clothing” explains Coen Duvekot, product manager at Agilent Technologies (Amstelveen, Netherlands). “The GC’s form factor must allow that.”
Duvekot differentiates between pGC and transportable systems in mobile laboratories with the same features as benchtop instruments, including broader carrier gas and detector options that include mass spectrometry. Mobile labs also provide the opportunity for sample preparation.
Although limited to gas analysis, pGC has developed a devoted following in the energy, environmental, and workplace hygiene markets. Natural gas is by far the largest market segment, particularly for inline and at-line applications where pGCs measure hydrocarbon composition and calculate calorific value continuously and, if desirable, unattended. At natural gas exploration sites, pGC sniffs out hydrocarbons and provides quality reports. Similarly, natural gas transportation and distribution sites employ pGC to monitor for leaks.
Related is pGC analysis of natural gas safety odorants, which consist most commonly of a mixture of thiols. “Odorants permitted in one country may not be allowed in others,” Duvekot notes. When used in a quality or safety setting, pGCs sold to the natural gas industry must be capable of discriminating among odorant blends.
Sensors respond instantly to analytes, but for an added 15 to 20 seconds of analysis time, pGC provides much more information. “Sensors usually quantify only one component,” Duvekot says. Multiple sensors are expensive, require a power source, and provide no separation capability. “Due to timing issues, individual sensors in arrays do not always measure the same sample, and may have difficulty with analytes with different response factors.”
One approach to miniaturizing instruments is to cram as many features as possible into the smallest possible box. It is possible as well to start with components of a desktop system—injector, column, oven, detector(s), data system, and carrier gas—and work backward toward some combination of absolute essentials.
Following this strategy to its logical conclusion, one might consider a more rugged column, room temperature elution, a simple detector, and air as the carrier gas. This would limit the analyte set and performance, but it could be enough for certain markets.
“That’s about as primitive a GC design as you can get,” says Hugh Goldsmith, president of SRI Instruments (Torrance, CA). SRI manufactures GCs of all types, including pGCs, to about half a dozen brands.
Non-heated, air-carrier pGCs are uniquely portable but limited to analysis of small-molecule gases. With a thin-film capillary column, a non-heated pGC handles hydrocarbons up to about C10, but the instruments obviously sacrifice analytical capability for portability. Hydrocarbon gases must be present at concentrations above 1000 ppm, for example, which is fine for natural gas field work. SRI’s line of pGCs employs a dry electrolytic conductivity detector, a simple design that requires a dry air carrier.
“The trick for manufacturers is to locate the sweet spot between portability and functionality,” Goldsmith says. “At any given time there are probably a hundred research groups attempting to miniaturize GC even further, but all they’re doing is essentially reinventing the wheel. If you wanted to sacrifice even more you could get these devices down to wallet size. But under no circumstances do you want to lug around a gas cylinder.”
Goldsmith sees pGC as a niche market ideally suited to narrow applications. The potential for innovation exists but not at current sales levels: corporate customers might purchase three or four such instruments; international organizations perhaps a dozen. He shrugs off the notion that handheld pGCs might be employed as simple detectors even though that has already come to pass for handheld spectroscopy, particularly Raman.
The resolving power of GC, he says, is unique among analytical instruments, even in ultra-miniaturized formats. “GCs have not disappeared into history because they separate molecules from each other and from matrix before presenting them to a sensor. Sensors can’t detect at ppm levels in a matrix,” he says.