Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

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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 In the second case, the aliquot from the vial’s headspace is not withdrawn by suction as in the case of a syringe: instead, after equilibrium is reached, the vial is pressurized by the carrier gas.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 162
EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS loop of the valve by the pressurized headspace gas. Today, automated instruments based on these principles are commercially available. 9.6.1 Trace Analysis by HS-GC HS-GC in both its dynamic and static versions permits the determination of analytes at low concentrations. Usually the dynamic technique is considered to be more sensitive; however, this is not necessarily true. For example, trace impurities in a water sample, at the parts-per-billion level, can be determined relatively easily by static HS-GC. 9.7 Headspace Trapping Techniques 9.7.1 Static Headspace Trapping Using the static method (Figure 5), a food sample is normally placed in a heated vessel, which is sealed gas-tight by a septum. The food sample stays inside the vessel for a certain period of time, so that the volatile compounds evaporate to a certain concentration in the air or to certain equilibrium. In order to determine the best conditions for the experiment, the odor of the headspace can be checked by sniffi ng the vessel. Subsequently, a distinct volume is taken out of the vessel by a gas-tight syringe and directly injected into a gas chromatographic column, with or sometimes without prior concentration (e.g. cryofocussing). The advantage of this method is that it accurately assesses the composition of the odorants. An application of this technique, called GC olfactometry of static headspace samples, has been widely used to identify the highly volatile compounds causing the fi rst odor impression of foods. Figure 5: Static headspace trapping technique 163
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