Introduction

The coupling of gas chromatography (GC) with mass spectrometry (MS) for the separation of individual components from a complex mixture followed by a definitive identification and accurate quantitative detection is the most widespread combination technique and forms the backbone of the modern environmental laboratory. The quadrupole mass selective detector is small enough to fit on a bench space right next to the GC making GCMS a true bench top analyzer.

While the fundamental technology of gas chromatography or mass spectrometry has not changed much over the years, there have been significant improvements in speed, miniaturization, and particularly in data systems and control software. The main reason to replace an older GCMS is the significant increase in productivity and decrease in detection limits that is possible with the newer analyzers. 

Why use GCMS?

Gas chromatography is a relatively inexpensive laboratory tool capable of separation and detection of individual components in a mixture. These mixtures are injected into a gas stream and propelled through a very long capillary open tubular column lined with a high boiling liquid phase that selectively absorbs components based on polarity. A ramping temperature oven heats the column causing the components to separate as they are carried in the gas stream through the column towards a detector; the components separate as a function of polarity and temperature. Because the components are carried in the gas phase they must be volatile at, or below, the temperature of the GC oven exposing one of the weaknesses of gas chromatography. The major weakness of GC, however, lies with the non-selectivity of traditional GC detectors. A traditional GC detector, such as a Flame Ionization detector (FID or Electron Capture Detector (ECD) responds only to chemical characteristics and does not provide much qualitative information about the composition of the chromatographic peak it is detecting. Identification is made exclusively by the time it takes the compound to travel through the column, or its retention time (RT). Since it is possible for more than one compound to have the same retention time on that column and also respond to the GC detector a secondary analysis on another column of a dissimilar liquid phase must be used for confirmation. If a peak is detected at the expected retention time on both columns its identity is confirmed.

The mass spectrometer detector ionizes the sample peak as it emerges from the detector causing it to break into fragments. Under controlled conditions, the relative intensity of each mass fragment and the combination of mass fragments is highly reproducible. This mass spectra (called a fragmentation spectra) is specific per compound and searchable using large databases (called Mass Spectra Libraries) available from organizations such as the National Institute of Standards and Technology (NIST) providing positive confirmation about the identity of the chromatographic peaks. An analyst runs a series of standards solutions, obtains fragmentation spectra of the components that elute at the defined and reproducible retention times and uses his searchable library to confirm the identity of the peak that eluted at that time. The software creates its own library based on the conditions of that instrument and sets up quantitation parameters for each component. A calibration curve is prepared that will be used to compare the response of unknowns to the response of the target masses at each retention time. When the analyst runs unknown samples, the software identifies the compounds detected at each retention time by comparing the fragmentation spectra and it quantitates the concentration of the unknowns from the calibration curve.

GCMS and Environmental Laboratories

The commonly used USEPA approved GCMS methods are 524 and 525 for volatiles and semi-volatiles in drinking water, 624 and 625 for volatiles and semi-volatiles in wastewater, and 8260 and 8270 for volatiles and semi-volatiles in solid waste and groundwater.