Before getting to the detectors, let’s consider GC. Stephanie L. Smith, scientific and technical advisor for the Security & Crime Prevention Group of the US Postal Inspection Service, says, “Gas chromatography is the quintessential chemical separation technique in the forensic science laboratory, offering incredibly simple analysis of extremely small samples.” These features make GC useful in many other applications as well.
The first question is: Does the application require a universal or specific detector? The thermalconductivity detector (TCD) was the first GC detector, and a universal one—meaning that it detects about anything that gets through the chromatography, but not very specifically. Other universal detectors include helium ionization detectors and barrier ionization detectors. Even mass spectrometry (MS) sits in this class.
A universal GC detector, such as a flame-ionization detector (FID), provides a collection of benefits. Overall, these detectors are easy to use and inexpensive and look for many compounds. For example, Eric Phillips, GC, GC-MS marketing manager at Agilent in Santa Clara, California, says that FIDs “are known for their linearity and dynamic range capabilities.”
Although MS is also universal, it comes in a variety of forms. For instance, a singlequadrupole MS uses one filter to separate ions based on the mass-to-change ratio. Phillips says, “These types of MS systems provide the ability to quantitate known lists of compounds and information on unknown compounds.”
Scientists turn to specific GC detectors when looking for a needle in a haystack. In these cases, the application doesn’t need to identify every compound in a sample, just certain ones. “These detectors are blind to almost everything else,” says Mark Taylor, chromatography marketing manager at Shimadzu Scientific Instruments in Columbia, Maryland. This category of GC detectors includes electron-capture detectors and sulfur-specific detectors.
Until recently, Smith served as the assistant laboratory director of the US Postal Service’s National Forensic Laboratory’s Physical Sciences Unit (PSU). There, she says, “Scientists routinely use gas chromatography in the analysis of evidentiary materials.” She adds, “The GC detector complement in the PSU includes FIDs and mass spectrometers, both traditional quadrupole and ion trap.”
The tools used at the PSU depend on the objective. For example, Smith says that GC/FID provides excellent separation of a sample and information about a molecule’s retention time, but it does not reveal what molecules are in a sample. “Due to its linearity and sensitivity,” Smith says, “GC/FID is the combination of choice for the quantification of controlled substances.” She adds, “It is also a great initial screening tool, especially using a temperature program, when the sample size permits a series of destructive tests.” GC/FID is also relatively inexpensive to purchase and maintain, and it is simple and rugged. As Smith concludes, “It provides low detection rates and a linear response.”
For much of the PSU’s work, though, most of the detection falls on MS, which Smith calls the workhorse in the analysis of controlled substances and poisons and for the analysis of various types of “trace evidence,” including fire debris, paint, explosives, and general chemical unknowns. With GC-MS, a sample gets separated very well and the MS provides a quantitative analysis of the components, even distinguishing compounds with very similar structures. Nonetheless, Smith adds, “While it is useful for quantitative analysis, the linearity is typically inferior to FID.” In addition, MS costs more—sometimes much more— than FID, and requires more maintenance. Still, Smith says, MS, “when carefully maintained, can provide a reliable working life of up to a decade.”
Picking your parts
The required detection level often determines the best detector for a specific application. For example, maybe someone needs to measure a gas, but not at a very low concentration. That probably calls for GC/TCD, which provides a sensitivity down to high parts per million.
For applications that need higher sensitivity, scientists need more sophisticated technology. For example, a pulsed discharge helium ionization detector (PDHID) can pick out components at concentrations in the high parts per billion.
For trace analysis of target analytes, scientists turn to GC-MS, which Taylor says can see down in the low parts per billion range. Along with sensitivity, GC-MS has the added benefit of positive compound identification via library spectral matching. “This is very important in, say, forensics or drugs of abuse analysis, where you need to unequivocally identify the drug,” says Taylor.
It’s not all about the concentration, though, because selectivity might also matter. A universal detector such as a TCD or a barrier discharge ionization detector (BID) has pros and cons. As Taylor says, “The good news is that you’ll see everything coming through the GC, and the bad news is that you’ll see everything.” Consequently, this approach typically requires some higher resolution chromatography, such as a multidimensional technique, or cleanup steps at some point to partition some of the components. Otherwise, co-elution would render the data useless.
Some of this can be resolved with triple-quadrupole MS, which consists of two mass filters with a collision cell between them. Only the ions of interest get through the first mass filter, then the collision cell dissociates the components, and then the second mass filter measures the partitioned pieces. “This technology provides the best low-level detection of a known list of compounds in matrix,” Phillips says. “However, it is not ideal to determine if there are compounds in your sample that are not in the list of compounds you are looking for.”
Not every application fits an available option. In those cases, researchers need extra help to build the right GC/detector system, and that often means turning to a vendor for advice. Getting the system performing properly probably requires some back and forth with a vendor. In the end, most scientists must make some compromises, but the range of options improves the odds of finding an affordable and effective system.
For additional resources on gas chromatography systems, including useful articles and a list of manufacturers, visit www.labmanager.com/GC