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IPN 074-397-P1G
HAPSITE Smart Operating Manual
1.4 Theory of Operation
The HAPSITE combines two analytical techniques, gas chromatography and mass
spectrometry, to separate, identify, and measure the organic components in a gas
phase sample. Using a flow of inert nitrogen carrier gas, the gas chromatograph
(GC) performs a time separation (Retention Time) of the sample compounds. The
separation order is primarily based on increasing compound boiling point. The
mass spectrometer (MS) detects and identifies the eluting compounds by breaking
the molecules apart and detecting the fragments. The resulting mass spectrum is
compared to a library of mass spectra to identify the compound.
The gas chromatography technique cannot always separate compound mixtures
into individually eluting compounds. Some of the eluting responses or peaks may
contain two, three, or more compounds which have taken the same time to
progress (elute) through the gas chromatograph. GC identification of compounds
is limited to matching the retention time of the unknown compound to that of a
known standard. See section 1.6.1, Gas Chromatograph, on page 1-4 for more
information on how the GC works. In order to further identify and measure the
individual components of such mixtures, the gas stream is directed into the mass
spectrometer.
In the mass spectrometer, the gas stream of eluting compounds is bombarded with
electrons. The electrons fracture the molecules into a characteristic combination of
smaller molecules or mass fragments. The mass spectrometer measures and plots
the response of these mass fragments to display a mass spectrum. See section
1.6.2, Mass Spectrometer, on page 1-6 for more information on how the MS works.
The introduction of a mixture of many compounds directly to the MS would produce
a very complex and uncharacteristic mass spectrum. However, because the GC
has largely separated the gases, the MS can usually differentiate between the few
co-eluting compounds and provide very precise identification and measurement of
the quantity of each compound. Qualitative identification can be made by
comparing the unknown compound spectrum to the NIST mass spectral library
(included with the Smart IQ software). See section 9.8, NIST Library Searches, on
page 9-23 for more information. Quantitative identifications can be made by
analyzing standards of known concentration and creating a target compound
library of concentration response curves. See Chapter 12, Target Compound
Methods for more information.
In summary, the GC first separates the gaseous compounds in time, then the MS
identifies and measures the gases contained in each of the time-separated peaks.
This enables the GC-MS system to report the specific identity and concentration of
each of the compounds present in the initial pulse of gas analyzed.
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