Headspace sampling variables

A mouth simulator is a valuable tool when determining what volatiles contribute to the flavor sensation during consumption of a food. This includes determining potency (e.g., Char-mAnalysis), intensity (e.g., OSME Acree and Barnard, 1994), contribution (e.g., omission tests), and effect of a compound on the flavor. Sample preparation with a mouth simulator gives a close representation of the human experience, without the expense and variability of using humans. The limitations of headspace sampling and detection sensitivity define the limits of the use of mouth simulators. 

The inlet for the neutrals may have a fixed or a variable position at the flow reactor. The sample inlet may be a controlled flow of a trace gas, a breath sample, or a headspace sample, introduced via a heated sampling line (Fig. 4.7). Variation of temperature, by external heating of the flow tube, and ion kinetic energy may be applied as well. 

The SPME process, adapted for solid or viscous matrix, is shown in Figure 10.1. A fused silica fibre, coated with a polymer, is installed inside a stainless steel hollow needle. In the first step, the needle is introduced in the sample vial through the septum. The fibre is then exposed to the headspace above the sample and the organic analytes adsorb to the coating of the fibre. After a variable sampling time, the fibre is drawn into the needle and the needle is withdrawn from the sample vial. Finally, in the same way, the fibre is introduced into the chromatograph injector where the analytes are thermally desorbed. 

This section deals not only with the variables pertaining to the headspace process (i.e. those affecting the removal of volatile species) but also with those involved in the passage of such species to the gas phase and the insertion of the compounds removed from the sample into the chromatographic column. 

Automated Procedures At least four commercial automated instruments exist which will concentrate headspace volatiles on adsorbents and thermally desorb them into a gas chromatograph. Times, temperatures, and gas flow rates are accurately controlled. Manual handling steps which can introduce variability are eliminated. Since these instruments are automated, sample throughput is enhanced they can be interfaced to high resolution gas chromatographs. 

External standard calibration in static headspace gas chromatography is best for analytes in liquid samples where the analytes are soluble in the matrix and the matrix has no effect on the analyte response. In these type of calibration is important to match the standard and sample matrix as closely as possible and to demonstrate equivalence in the response between the standards and the samples. The main difficulty with external standard calibration is that is does not compensate for any variability due to the gas chromatograph injection or due to variation in the analyte matrix. 

Solid phase microextraction (SPME) involves extraction onto a thin fiber and the technique has become more prevalent recently and additionally provides a preconcentration of analytes prior to analysis. The fibers used in the technique can be coated with a range of stationary phases and a nonpolar phase such as polydimethylsiloxane (PDMS) is typically used for the extraction of derivatized organotin species. An equilibrium is established between the sample extract solution (or the headspace above the solution) and the stationary phase coating the fiber. The analytes are then typically desorbed from the fiber for analysis, for example, using thermal desorption during GC analysis. The technique allows rapid and solvent-free extraction of the analytes. Very good extraction has been achieved for water samples however, the technique has been shown to be more variable with more complex matrices. 

Internal standard calibration can be used with headspace to compensate for variation in analyte recovery and absolute peak areas due to matrix effects, variability that results from sample preparation, and chromatographic injection variability. Prior to sample preparation, a known quantity of a known additional analyte is added to each sample and standard. Ideally, this compound, called an internal... 

Experimental variables (sampling volume, desorption time, desorption temperature and headspace pressure) were mathematically modelled and quantitatively assessed with a view to optimisation. Van Eldik et al. [985] applied solid polymer PT analysis for the screening of the outgassing behaviour of 61 styrenic samples (ABS, HIPS, PPO/PS), containing a variety of BFRs (such as DBBP, DBDPO, OBB, TBBP-A, TBPE, etc.), taken from used TV sets, computer housings and printers, and 13 polyamides (PA6, PA6.6) for electrotechnical applications in relation to health and safety for the user. The analytes were collected on a Tenax tube, cryofocused and analysed by means of TD-GC-MS. 

Variable quantity injected. can be controlled (programmed) by the length of the injection process, injection volume = injection time X flow rate of the GC column, no drop in pressure to atmospheric pressure, depressurizing to initial pressure of column. Change in pressure in the headspace bottle reproducible pressure build-up with carrier gas, mixing, initial pressure in column maintained during sample injection. Sample losses none known. 

Quantity injected on the instrument side fixed volume by loop, changes in the volume injected requires change of the sample loop, drop in pressure to atmospheric or variable backpressure on filling the sample loop. Pressure change in the headspace bottle compensation by pressurization with carrier... 

Cucumber fruits were studied by using the same analytical approach as they contain only five key volatiles and their concentration in cucumber tissue seems to be less variable than in tomato (see, for example. Ref. 12). Table 4 shows the El and API correlations for the five compounds each one could be attributed to a single ion mass on the API, and calibration with authentic standards allowed conversion of the ion signal into concentration units (parts per billion by volume). The amount of the Cg volatiles present in the macerated tissue was estimated by microwaving a sample of cucumber to inactivate the enzyme system that produces Cg and Cg volatile compounds. Inactivation was confirmed by APIMS analysis of the headspace above the treated samples. Microwaved samples were macerated after spiking them with known amounts of the Cg volatiles, then measuring volatile compound release in the blender apparatus. The values obtained from the spiked standards were then compared with the release traces from cucumber samples and the amounts of nonenal and nona-2,4-dienal estimated as 5 and 8mg/kg fresh tissue, respectively. These values compare well with the... 

Another factor complicating use of SPME headspace measurement to quantify the amount of components in juice is the variability of fiber equilibration times for the different compounds. Samples of the standard mixture in water (discussed previously) were allowed to equilibrate 16 hr (26°C) and then sampled for fiber contact times up to 120 min. The amounts of the various compounds and their equilibration times with the PDMS fiber... 

These headspace techniques using stationary phases of variable selectivity are especially useful for the extraction of volatile compounds emitted by the sample [BIC 04]. Indeed, they show themselves to be increasingly simple, practical and easily automatable, as they do not require large quantities of samples and they limit the occurrence of artifacts. 

The variables that affect SBSE extraction performance are similar to the ones affecting SPME, namely, extraction time, extraction temperature (mainly for headspace mode, it should be noted however that the sorption process on the PDMS coating is not favored at high temperatures), selection of modifiers to increase the extraction efficiency (sueh as small amounts of organic solvents or high contents of salts), pH, stirring, and sample volume. 

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