“Fun new tools,” particularly in mass detection, have encouraged a new conversation among separation scientists, says Nicholas Hall, national sales director at LECO (St. Joseph, MI). “Every time this occurs, the instrument vendors engage in the equivalent of an arms race, where the battles are fought over specifications— more resolution, greater fragmentation capability.” But the real discussion has recently involved the very nature of chromatography, Hall says. “Just as important as the tool used for detection on the back end is the time and optimization that goes on at the front end.” Thus the resurgence of basic chromatography optimization, the application of solid analytical chemistry, and a focus on chromatography as the optimization of mass spectrometers. “If you have good separation and good sample preparation, and that goes into the MS, then you’re really optimizing the mass spectrometer’s capabilities.”
One tool that combines GC and MS exquisitely is two-dimensional GC, or GCxGC. Through this method, a chromatogram is run in the first dimension, perhaps through a nonpolar phase, and then the entire eluent is collected and reinjected onto a second column operating orthogonally. What emerge are significantly greater peak capacity, separation capability, and lots of MS data points. “It works similarly to 2-D HPLC, but it’s a lot easier because we’re in the gas phase,” Hall notes.
Massimo Santoro, GC marketing manager at Thermo Fisher Scientific, believes that instrument modularity and mass detection are two emerging trends in GC.
Thermo Fisher Scientific recently introduced the Trace 1300 GC, which allows users to replace injectors and detectors on the fly. “Now users can modify their GC configuration, as they can with HPLC, without a service call.”
Injectors become contaminated by matrix components: through poor sample preparation, high concentrations of certain components in samples, or long use. Where a service call might take days or hours in the case of on-site technicians (an increasingly rare situation), users can now swap out components as necessary.
“Modularity is also a way for small laboratories to maximize their investment,” Santoro tells Lab Manager. “They can purchase the system they need today, and if their business grows, they can add a new injector or detector to accommodate changing workflows.” Other benefits include easier troubleshooting.
Terry Sheehan, GC-MS manager at Agilent Technologies (Santa Clara, CA), has noted a shift in some markets away from traditional GC detectors toward mass detection. The trend is most pronounced in environmental and food analysis. In addition, the need to run through samples quickly has been addressed by fast GC, low thermal mass GC, and the use of hydrogen carrier gas to replace helium.
Hydrogen carrier gas has been touted as a smart alternative to helium as a GC carrier gas. Hydrogen’s benefits are faster runs and lower cost. The U.S. helium shortage has made the gas particularly difficult to obtain in some geographic areas.
Replacing helium with hydrogen has been a boon for some analytic areas, Sheehan says, but the process is not as simple as changing tanks. “The two gases differ significantly in nature and chemistry.” This is, in fact, the basis of hydrogen’s advantages, but exploiting those benefits requires an understanding of the two gases.
Hydrogen is active, helium inert. While fears of a hydrogen explosion in a typical GC setup are irrational, the gas may interact with analytes in ways that helium cannot.
“Yes, you will benefit from switching, but there are things to pay attention to. You can’t simply swap hydrogen for helium and be set to go.” The method will probably need to be redeveloped and, in some instances, revalidated. “But if you go in recognizing that, in many cases you may be pleasantly surprised. At the worst it will take a bit more work to transfer the method.”
Shift toward MS detection
Sheehan says that the biggest issue in GC relevant to workflows is sample preparation. “That has been the huge bottleneck in many GC labs.” All instrument manufacturers, including Agilent, have worked to speed up analysis so samples do not pile up. The hurdle has therefore shifted to readying the samples for GC.
Sample preparation, including cleanup and derivatization, may be aided by automation tools, as is standards prep. Unfortunately, the wide diversity of GC samples makes a one-size-fits-all preparative cleanup elusive.
This has caused GC users and instrument vendors alike to rethink MS detection—specifically the relative value of a triple-quadrupole detector versus a far less expensive single quad. The latter demands thorough sample prep to remove matrix, while the former does not.
Sheehan mentioned a recent analysis demonstrating, on the basis of workflow compression and cost, the value of triple-quad MS. Even though a tandem triple quad might cost three times as much as a single quad, the return on investment could be less than six months for very high-throughput labs. “And with an average lifetime of ten years, a triple quad provides lots of opportunity to save a lot of money,” Sheehan adds.
Mass detectors add significant cost to GC systems. A GC with standard detectors may be purchased for less than $10,000, but most research-grade instruments cost between $20,000 and $30,000. A single-quadrupole mass detector easily adds $35,000 to the cost, while a triple-quad system plus GC will set you back as much as $180,000.
So why upgrade to GC-MS?
MS provides a dimension that flame ionization or thermal conductivity does not. “In addition to elution time, you now get confirmative identification of your compound through mass spectrometry,” Santoro says. Analysts who know what they are looking for can employ the selective ion mode, by which the MS looks specifically for ions of interest. “This provides greater sensitivity.” MS-MS triple quads provide even greater sensitivity, particularly if analytes are known and even in cases where two or more species have identical mass. The instrument can look for transitions from ions with a mass of, say, 180 to a mass of 130, which may be specific for one analyte but absent in the other. “That provides a lot of signal and a lot of sensitivity.
Like this article? Click here to subscribe to free newsletters from Lab Manager