INFICON Explorer Portable Gas Chromatograph - Manual (Page 327)

Appendices
325
Technical Description
Explorer uses gas chromatography to separate and detect volatile
organic compounds (VOCs) present in an air sample.
In a gas chromatograph (GC), the sample is carried from the point of introduction,
through a separating column, and then out through a detector, by a continuous
stream of relatively inert gas called carrier gas. The column in a GC is a long tube
with sticky or sorptive material coating the inside walls or packed into the tube. The
sample components become separated from one another as they are carried through
the column because of differences in their rates of interaction with the sorptive
material. Each different sample component is therefore retained for a different
length of time within the column, and elutes from the column into the detector at a
characteristic retention time.
The detector responds as each component flows through the detector cell. If the
detector response is displayed against the elapsed time, then a series of peaks
separated in time is seen, one peak for each sample component. The retention time
of the tip of each peak indicates what the sample component is, and the area of the
peak indicates how much is present.
Chromatography is a comparative technique of analysis. The identities and
concentrations reported by
Explorer after a sample is analyzed are based on the
retention times and peak sizes of previously analyzed standards. When an assay
is set up in
Explorer, a standard of each compound of interest is stored in
Explorer's memory by retention time, peak area, and concentration. The ratio of
peak area to concentration is the sensitivity (the response to concentration ratio,
measured in mVS/ppm) for the compound. This information is saved in a part of
Explorer's memory called the library, and is automatically updated during a
calibration.
In each sample analysis, peak retention times are compared to retention times of
library compounds. If they match (within the limits of the peak recognition
window), then the peak is identified as the corresponding library compound. To
calculate the concentration of the identified peak, its area is divided by the
library sensitivity for the compound.