Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

9 SOLID PHASE MICRO-EXTRACTION AND HEADSPACE TRAPPING EXTRACTION
Figure 4: Dynamic headspace trapping technique
This technique is being used more generally after the introduction
of Tenax (poly(2,6-diphenyl-p-phenylene oxide)) as a universal adsorbent
for dynamic headspace GC by Zlatkis and his group, at the University of
Houston, in 1973. They used the technique for the investigation of biological
fl uids and demonstrated the reproducibility of the purge-and-trap method.
9.6 Principles of Static Headspace-GC Systems
Gas from the headspace of a closed vessel can be sampled
simply with a gas-tight syringe. However, with such a manual method, it is
diffi cult to reproduce all the conditions necessary for reliable quantitative
analysis. Therefore, today, headspace-gas chromatography (HS-GC) is carried
out almost exclusively with automated instruments, in which thermosetting,
aliquoting the headspace and introducing it into the gas chromatograph
are fully automated. In this way and using the proper calibration methods,
the required precision, accuracy and reliability are assured.
Present-day HS-GC instruments are of two types. In the fi rst,
the headspace aliquot is taken by an automated syringe which then is
moved above the injection port of the gas chromatograph and the sample
is injected. In essence, such systems are similar to the autosamplers used
in GC. In the second case, the aliquot from the vial’s headspace is not withdrawn
by suction as in the case of a syringe: instead, after equilibrium is
reached, the vial is pressurized by the carrier gas. After pressurization there
are two possibilities. The carrier gas fl ow can be temporary interrupted while
the pressurized gas in the vial is allowed to expand onto the column; the
transferred volume of headspace can be accurately controlled by controlling
the time of transfer and the pressure. The second possibility is to have a
gas introduced between the sample vial and the column, and fi ll the sample
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