Headspace modes

The advantage of headspace mode is that only volatile components that will not contaminate the GC are injected. InvolatUes do not partition into the headspace and so never enter the injector. Effectively, the analyte is decoupled from the influence of the drug (but see the discussion on validation below). However, many analytes that are amenable to GC by direct injection are not sufficiently volatile to give a high-enough vapour pressure to be detected by conventional headspace injection. These semi-volatile components can sometimes be successfully analysed using a variant of the headspace technique known as total vaporisation headspace injection. In this instance, a few microlitres of the sample solution are injected into the headspace vial, which is then incubated at a temperature that vaporises the solvent completely into the headspace. 

Figure 4.1 Comparison of conventional headspace and total vaporisation headspace modes, (a) Conventional headspace mode during equihbration phase, (b) total vaporisation mode during sample loading and (c) total vaporisation mode during equihbration phase.

A fourth example (P17) is from the Introduction section of the article that examines PCBs in full-fat milk. For background information, the authors outline the general four-step procedure used to determine PCBs in full-fat milk. Conventional methods used to accomplish two of these steps, extraction and cleanup, are also described. In a new paragraph, the authors introduce solid-phase microextraction (SPME), a technique that greatly simplifies this four-step process. But SPME is not recommended for complex matrixes hence, the authors motivate the topic of their current paper, headspace mode SPME (HSSPME). 

Arthur and Pawliszyn introduced solid-phase microextraction (SPME) in 1990 as a solvent-free sampling technique that reduces the steps of extraction, cleanup, and concentration to a unique step. SPME utilizes a small segment of fused-silica fiber coated with a polymeric phase to extract the analytes from the sample and to introduce them into a chromatographic system. Initially, SPME was used to analyze pollutants in water - via direct extraction. Subsequently, SPME was applied to more complex matrixes, such as solid samples or biological fluids. With these types of samples, direct SPME is not recommended nevertheless, the headspace mode (HSSPME) is an effective alternative to extracting volatile and semivolatile compounds from complex matrixes. (Adapted from Llompart et ah, 2001)... 

The techniques discussed in this chapter vary in automatability and frequency of use. Thus, while automatic hydride and cold mercury vapour generation are implemented in laboratory-constructed or commercially available dynamic equipment that is straightforward, easy to operate and inexpensive, automating laboratory headspace modes and solid-phase microextraction is rather complicated and commercially available automated equipment for their implementation is sophisticated and expensive. Because of its fairly recent inception, analytical pervaporation lacks commercially available equipment for any type of sample however, its high potential and the interest it has aroused among manufacturers is bound to result in fast development of instrumentation for both solid and liquid samples. This technique, which is always applied under dynamic conditions, has invariably been implemented in a semi-automatic manner to date also, its complete automatization is very simple. 

Fig. 4.4. The three basic steps (A-C) of the pressurized headspace technique and two additional steps (D and E) involved in the multiple-headspace mode (A) equilibration, (B) pressurization, (C) transfer of an aliquot of headspace gas to the chromatographic column, (D) venting to the atmosphere and (E) re-equilibration of the vial for the next sample. (Reproduced with permission of Springer-Verlag.)... 

Although the static and multiple headspace modes use similar equipment, the two rely on rather different principles. On the other hand, purge and trap, and dynamic headspace, possess the same foundation, the only difference between them being the location of the tubing used to transfer the carrier gas to the sample container. 

Recently, a new headspace mode was developed that allows complete or partial sampling of the vapour phase from a pre-equilibrated headspace vial, using a dual needle system. The theoretical treatment of this system has shown that the maximum mass of an analyte in the gaseous phase (M is generated when the gas-condensed phase ratio (Kg/K. ) in the equilibrated headspace vial is equal to the square root of the distribution coefficient [61]. 

Soil spiked with trichloroethylene and toluene was analysed using a gas chromatograph equipped with a PT concentrator that was found to be replaceable by a headspace unit in order to simplify the overall assembly. The headspace analysis of soil samples was found to be restricted by incomplete desorption of the contaminants in soil-water mixtures this shortcoming, however, was effectively overcome by the addition of methanol. Henry s law constants for volatile organics in methanol must previously be determined if the method is to be applied to soils [142]. A comparison of the performance of static and dynamic (PT) headspace modes in the determination of nine VOCs in five different soils revealed poor PT recoveries in relation to those of static headspace (which ranged from 68 to 88%) the latter, however, required longer development times [143],... 

The use of this technique in the headspace mode was more recently shown by Zhang and Pawliszyn (37) to reduce equilibration time, reportedly because the equilibration of the analyte with a gas phase is about four orders of magnitude faster than equilibration with an aqueous phase. This is due to the faster diffusion rate of a molecule in a gas than in water. 

Fig.l Headspace analysis and microextraction methods (here only the headspace mode is shown) used for the determination of fuel oxygenates, (a) Headspace analysis, (b) headspace solid-phase microextraction (HS-SPME) redrawn after [60], (c) solid-phase dynamic extraction (SPDE) redrawn after [61] and (d) liquid-phase microextraction (LPME) redrawn after [62]... 

The stationary phases PDMS, PA, and PDMS-DVB are efficient using the immersion extraction mode (the headspace mode being used in Ref. [49]. Addition of NaCl enabled better partitioning. Optimization of the parameters shows an extraction time ranging from 20 to 60 min, a desorption temperature varying from 220 to 250°C, and a desorption time of 4 to 5 min. 

Fiber 85 (xm polyacrylate used in headspace mode Desorption 250°C for 2 min Analysis GC-FID and GC-MS. 

It should be stressed that the use of a general pharmacopeial method is not a reason not to validate the latter when analyzing a particular substance. The matrix effect, in particular, has to be investigated when using the static headspace mode of injection. 

More than one hundred papers reporting diverse applications to analyse wines were published until know. The majority of the referred methodologies use the headspace mode (HS-SPME) instead of the direct immersion mode (DI-SPME). In terms of performance, SPME showed comparable results to LLE or SPE. However, SPME is simpler and solvent-free, and uses smaller volumes of sample nevertheless, on the other hand, LLE had the possibility of carrying out simultaneously the extraction of several samples (Bohlscheid et al., 2006 Castro et al, 2008). When the interest is to obtain the maximum information about the volatile fraction of a wine, the coating DVB/CAR/PDMS seem to be the most suitable (Tat et al., 2005). On the other hand, for specific applications, the choice of a suitable solid-phase, depends on the class of compounds be analyzed, e.g. CAR/PDMS for volatile sulphides and disulphides (Mestres et al., 1999), on-fibre derivatization (PA) for the determination of haloanisoles and halophenols (Pizarro et al, 2007). 

Since SPME extraction procedure is performed in the headspace mode, verify that the fiber is not dipped into the liquid phase. 

Solid-phase microextraction (SPME) is a recent sample preparation technique for trace analysis by GC (12). It is a simple, solvent-free method that uses a polar or nonpolar coated fused-silica fiber to directly extract analytes from various matrices (usually aqueous). It can be used in a headspace mode as well. After the fiber is removed from the sample, it is transferred to the heated inlet of a chromatographic system and the analytes are thermally desorbed for analysis. The technique works well for the analysis of trace analytes in water or urine. It has been applied in the field of forensic science in the analysis of fire debris, explosives, and drugs in biological fluids (13-15). 

In headspace mode, /( determines the absolute SPME response and can... 

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. 

Few applications of SDME have been reported for the monitoring of PAHs in water samples. Two of them have been carried out in headspace mode (HSDME) [184,221], thus... 

Solid-phase microextraction (SPME) devices can be employed with solid samples as passive samplers, but this approach must be conducted in headspace mode and so it is limited to the more volatile PAHs [269], Furthermore, it appears that SPME is a better approach to extract primarily PAHs present in sediment porewater rather than in the colloidal phase [270], EPA method 8272 describes the determination of parent and alkyl polycyclic aromatics in sediment pore water by SPME-GC-MS. In a similar way, SPMDs have been mainly used for the determination of PAHs contained in sediment porewater [271],... 

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