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 vola- tile 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
However, static headspace samples are normally too small to quantify odorants that are present only at low concentrations in the vapor phase. In other words, one can smell them, but in many cases it is not pos- sible to obtain a signal in a mass spectrometer.
9.7.2 Dynamic Headspace Trapping
To overcome the disadvantages of headspace trapping method, dynamic headspace trapping can be used (Figure 6). Again, the food sample is placed in a heated vessel but the evaporating compounds are continu- ously swept by a stream of inert gas into a trap containing a porous polymer, which adsorbs more or less the organic constituents. This method yields a much higher amount of trapped volatiles so that, after desorption, it is no longer problematic to obtain an MS signal.
Figure 6: Dynamic headspace trapping technique
However, the disadvantage of this procedure is the strong de- pendence on the yield of the odorants, on the velocity of the carrier gas and on the selectivity of the adsorption and desorption process for different compounds. It is very diffi cult to control these parameters precisely and therefore, the results of such quantitative measurements might be inac- curate.
9.7.3 Recovering the Adsorbed Volatiles by Thermal or Liquid Solvent Desorption
Several studies have reported methods of desorption using or- ganic solvents. Drawbacks of the use of solvent desorption include the loss of volatile compounds during removal of excess solvent before GC analysis, solvent selectivity and solvent impurities. We recently developed a sensi- tive and highly reproducible dynamic headspace (DHS) protocol with thermal
desorption (using injector glass liners packed with Tenax-TA as adsorbent traps for aroma collection at ambient room temperature) and desorption at the interior of a GC injector. This DHS-type protocol was used to characterize fresh tomato fl avor compounds; the results were compared with published data from a static headspace method (Table 1).
Table 1: Concentration of selected tomato aromas from heat-processed tomato juice by static headspace trapping (SHT) and dynamic headspace trapping (DHT),
expressed in parts per billion (ppb)
Compound SHT, ppb DHT, ppb
(E)-2-hexanal 5 340
1-Penten-3-one 61 100
2-Isobutylthiazole 2 450
2-Methylfuran 97 1,060
2-Pentylfuran 26 700
3-Methybutanal 17 750
3-Methylfuran 717 3,200
6-Methyl-5-hepten-2-one 21 1,330 Acetone 325 -
Benzaldehyde 3 30
Dimethyl disulfi de 16 630 Dimethyl sulfi de 5,205 2,974
Ethanol 311 -
Geranial 2 130
Hexanal 188 6,210
Pentanal 48 470
In the present study, this DHT-type protocol was used to charac- terize fresh tomato fl avour compounds for comparison with related literature methods.
9.7.4 Some Practical Examples of Headspace Technique Use
9.7.4.1 Tomato Juice
Fresh tomato juice was made from vine-ripe fruit by Campbell Soup Company’s R&D centre in Davis, USA. Chemicals were reagent grade, supplied from reliable sources.
9.7.4.1.1 Preparation of Traps
Traps were prepared using silane-treated glass tubing (79 mm x 6 mm) packed with 13 mg 60/80 mesh Tenax-TA (2,6-diphenyl-p-phenylene oxide) polymers held in place by silanized glass wool. The traps were initially conditioned at 330° C for 2 h under nitrogen gas at a 20 ml/min fl ow rate.
The traps were regenerated at 250° C for 1 h immediately before each purge-and-trap experiment.
9.7.4.1.2 Thermal Desorption
Adsorbed volatile compounds were recovered from the trap di- rectly inside the GC injector. The desorption time and temperature were pre- viously determined. The injector temperature was 200° C and a loop of the analytical column at the injector end was immersed in a liquid nitrogen-fi lled Dewar fl ask to cryogenically trap the desorbed volatiles. Subsequently, the injector glass liner (insert) was replaced with the trap to desorb volatiles.
Thermal desorption was carried out for 5 min with the split vent and septum purge closed.
9.7.4.2 Headspace of Hedychium coronium
Hedychium coronium fl ower has a delicate, pleasant fragrance, but the essential oil and concrete extracted by traditional methods usually lose this fragrance. Thus, the headspace of the H. coronium fl ower was ana- lyzed. The essential oil of H. coronium fl owers, which was absorbed by XAD-4 resin, eluted by organic solvent and concentrated, had a fragrance similar to the natural fragrance of H. coronarium fl owers.
9.7.4.3 Volatiles of White Hyacinths Isolated by Dynamic Headspace Trapping
More than 70 constituents of white hyacinths can be identi- fi ed by GC and GC-MS. The principal constituents are benzyl acetate and (E,E)-α-farnesene. Beside these, sensorily important substances like indole, oct-1-en-3-ol and phenylacetaldehyde were identifi ed. Minor traces of three substituted pyrazines were detected by GC-sniffi ng. The advantages of the si- multaneous closed-loop stripping technique using various adsorbing agents at the same time were demonstrated. By this method, artifact formation and discrimination of individual components can be determined
9.7.4.4 Medical Materials Testing by Headspace Trapping-GC-MS The new technology provided by the HS-40 Trap coupled with a sensitive detection method such as GC-MS allows volatile organic com- pounds in medical sutures to be analyzed easily at trace levels. Individual compounds present in the sutures can be analyzed by GC-MS and identifi ed
by a NIST library search of the acquired mass spectral data. The innovative, patent-pending, headspace trapping technology used in this application pro- vides sensitivity beyond the capability of traditional static headspace. This presents a new level of detection capability for the evaluation of materials used in medical applications, as well as in other types of material testing, including pharmaceutical formulations and food-packaging fi lm.