GC [Gas Chromatography]

Gas chromatography is used to separate the initial injection aliquot of the sample into a sequence of peaks spread out at retention times which are characteristic for each compound under the operating conditions. This is accomplished with a 20-50 meter glass column of perhaps 200um internal diameter and an internal coating a few microns thick. A carrier gas moves the sample through the column. The carrier gas is typically helium or hydrogen for most work but nitrogen or argon can also be used. The column inlet is temperature, pressure, and flow controlled to deliver the sample into the column. The column is coiled inside an oven for which both the flow and the oven temperature are precisely ramped. The mixture of compounds in the original injection becomes a series of peaks exiting the column. A detector is used to measure each peak.

FID [Flame Ionization Detector]

Flame ionization detectors route the end of the GC column through a hydrogen flame which actually burns and ionizes each peak as it exits the column along with the carrier gas. The chimney of the detector is a sensor which detects the ionization of the hydrogen flame caused by the burning of the peaks. This is a sensitive detector, with a very large dynamic response range. The output of a GC-FID instrument consists of the elution time of each peak and the amount of each peak. Compound identification depends on knowing the retention times for each compound and the uniqueness or separation from other compounds by the chromatography.

LAL uses GC-FID, by modified SW846 method 8015, to analyze the following:

TEPH DRO [Diesel Range Organics C10-C28]

MS [Mass Spectroscopy Detector]

The MS detector depends on fragmenting the compound in the chromatogram peaks with a hot plasma emission filament in a vacuum chamber. A repeller and charged lens stack directs the beam through a quadrapole mass spectrometer which rapidly scans and filters the mass/charge ratio allowed through the spectrometer. An ion cascade detector sequentially counts the whole range of mass/charge fragments many times a second. The result for each peak is measurement of the elution time and a spectrum of masses whose relative abundances are characteristic for each compound. One or more of the masses is used as the quantitation mass for that compound. This method is the best available for positive identification quantification of compounds even in the presence of overlapping peaks. A spectral library can be used to identify peaks that are not in the calibration.

LAL uses GC-MS by modified SW846 method 8260, to analyze the following:

BTEX [Benzene, Toluene, Ethylbenzene, Xylenes]
LAL uses the WI BTEX list which also includes the 1,3,5 and 1,2,4-trimethylbenzenes and napthalene

MTBE [Methyl Tert Butyl Ether]

TVH GRO [Gasoline Range Organics C6-C10]

8260 VOC [69 compounds, 8260 volatile list]