Adding a Second Dimension to GC
Gas chromatography-mass spectrometry (GC-MS) is the fastest-growing GC method. Mass detection, which can take a variety of forms based on the MS component, provides a dimension that conventional thermal conductivity or flame ionization detectors cannot, namely selectivity and absolute identification of both known and unknown compounds.
GC-MS is suited to every organicchemical discipline where GC is found, including the chemical, pharmaceutical, environmental, and forensics industries, as well as basic research. But the limitations of GC-MS are the same as for GC alone: compounds must be volatilized and relatively nonpolar; molecular weights are therefore limited to about 800 Dalton.
GC-MS identifies compounds based on matching a mass spectrum from a run with entries in a database or spectral library generated with the same MS technique (hardware, ionization, detection).
Whereas co-elution is common in GC, MS distinguishes closely related compounds, for example structural isomers, on the basis of their fragmentation patterns. And while MS cannot tell mirror-image enantiomers and most diastereomers apart, GC columns can—in the case of enantiomers with a chiral stationary phase. This is one reason why GC and MS are considered complementary techniques, and why their combination is so powerful.
Environmental Protection Agency and U.S. Pharmacopoeia GC methods are bedrock techniques used and referred to for environmental and pharmaceutical analysis, respectively. These methods have benefitted tremendously from the adoption of mass detection, notes Trisa Robarge, GC and GC-MS product manager at Thermo Fisher Scientific (Austin, TX). “As regulations for both industries evolve toward lower detection limits and higher specificity, analysts have benefitted particularly from triple- quad MS, which facilitates analysis of low-level compounds from complex samples.”
Software and information technology supporting GC-MS have also greatly improved, Robarge says. Many systems today are supported by compound libraries and integrate with laboratory information management systems and/or electronic notebooks that allow archiving, managing, and sharing of analytical data.
A typical entry-level GC-MS system would likely incorporate a single-quadrupole (“quad”) detector or a more sophisticated ion trap, which allows extensive analysis of fragments and fragments of fragments—“MS/MS” experiments. Also popular are timeof- flight (TOF) instruments.
These detectors suffice for most applications but their resolution is only about one atomic mass unit. To analyze isotope ratios one would turn to either a triple-quad or magnetic-sector MS detector. “But as selectivity goes up so does your investment,” Robarge tells Lab Manager Magazine.
Not for every application
Despite the appeal of GC-MS and the broad range of cost and capability available, not every method demands its sensitivity and selectivity. Flame ionization is perfectly suited to quantifying blood-alcohol levels or analyzing low-molecular-weight hydrocarbons in the oil and gas industry, and electron capture is routinely used to screen pesticides in foods.
“While standard detectors serve many applications today, when selecting a detector, users should try to calculate the price of being wrong,” cautions Robarge. “The bottom line is to get the detector you need to do the job.”
Mark Taylor, GC and GC-MS product manager at Shimadzu Scientific Instruments (Columbia, MD), notes that GC, particularly GC-MS, has been losing market share to liquid chromatography-mass spectrometry (LC-MS). “GC-MS was the flagship analysis tool for organic compounds for many years. But when ionization issues were overcome with LC-MS, users realized that LC was easier to operate and required less sample preparation than GC-MS. Classic GC-MS applications like drug screening and drugs-of-abuse testing are becoming LC-MS oriented.” LCMS also suffers from fewer matrix effects than its GC counterpart.
Triple-quad GC-MS is particularly effective at breaking through difficult sample matrices. Taylor tells Lab Manager Magazine that the first quadrupole acts as a type of filter that allows only one ion at a time to enter, while the second functions as a collision chamber (breaking the molecule apart) and the third provides the fragmentation spectrum.
2D GC plus MS improves capabilities
During the past decade manufacturers have perfected a technique, GCxGC, or two-dimensional GC, which employs two orthogonal columns connected in series through a modulator. Effluent from the first column remains in the modulator before being released to the second column. GCxGC provides 10 times the resolution of conventional GC. During GCxGC eluents are chromatographed twice, which is useful in resolving co-eluting peaks. The chromatogram is rendered in two dimensions through special software.
GCxGC-MS was difficult to achieve using relatively inexpensive singlequad detectors due to the slow data acquisition of the MS component. Until recently, chromatographers used either TOF MS instruments or conventional GC detectors. That has changed, says Taylor, with the introduction of quad MS detectors operating at 100 Hz. Fast-quad detectors are also required for methods that have been ported to narrow-bore columns to increase speed and throughput.
Gas Chromatography-Mass Spectrometry:
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