Headspace techniques, trace analysis

Trace environmental quantitative analysis (TEQA) utilizes various determinative teehniques (Chapter 4) in combination with various sample prep techniques (Chapter 3). In this appendix, one specific trace analysis using static headspace sampling automatically coupled to capillary gas chromatography with element specific detection is described. LSQUARES is a computer program developed by the author in BASIC and is used in the quantitative analysis discussed below. The actual program written in GWBASIC is also listed after illustrating its use in TEQA. 

When analytes are under the limit of detection (LOD) of the technique is necessary to use enrichment techniques. In headspace analysis, for this purpose the target analytes must be separated from the headspace gas either by absorption into a liquid or by adsorption onto a solid adsorbent and also by condensation in a cold trap. (Kolb, 1999). Solvent free techniques are particularly desirable in case of trace analysis to avoid problems with solvent impurities. Consequently, cryogenic trapping is the preferred choice to improved detection limits in static headspace analysis... 

SPME is a recent sample prep technique for trace analysis by GC [18]. It is a simple, solvent-free method which uses a nonpolar fiber (usually dimethylpolysiloxane) to extract analytes from a polar matrix (usually aqueous). A fused silica fiber is coated with a thin film (7, 30, or 100 /im) of stationary phase. The small size and cylindrical geometry allow easy incorporation of the coated fiber into an ordinary GC syringe. The coated fiber is exposed to the sample matrix or to the headspace, and analytes are adsorbed (extracted) from the sample matrix. After the fiber is removed from the sample, it is transferred to the heated inlet of a GC system and the analytes are thermally desorbed for analysis. The technique works well with trace amounts of nonpolar and semipolar analytes in water. 

Considerable effort has been made to examine the volatiles and trace components that contribute to food flavors. Sone early techniques for measuring the volatile components in food products by gas chromatography consisted of analyzing headspace vapors to detect vegetable and fruit aromas (5) and volatiles associated with other food materials ( ). AlTo, sample enrichment has been used in the analysis of Tome food products. However, these techniques require steam distillation or extraction and concentration, or both, before the volatile mixture can be introduced into a gas chromatograph (, 9, 10). Besides being... 

The isolation and concentration of petroleum products can be performed in several ways. The most efficient method is passive adsorption. In this method, the sample along with a tube filled with Tenax TA adsorbent is placed in a thermostated (60-70 °C) tightly closed container, such as a glass jar, for over 10 h. Under these conditions, a balance between compounds present in the headspace of the sample and the sample adsorbed on the polymer adsorbent is established. Adsorbed compounds are subjected to thermodesorbtion then, the desorbed compounds together with the carrier gas are injected onto a GC column, where they are separated and then identified. This approach has enabled easy detection and identification of trace amounts of petroleum products. Headspace analysis with passive adsorption on Tenax TA is normally used for separation and concentration of analytes. Gas chromatography coupled with an autothermal desorber and a mass spectrometer (ATD-GC-MS) is the best technique for separation of multicomponent mixtures... 

In the early 90s, a new technique called solid-phase-micro extraction (SPME), was developed (Arthur and Pawliszyn, 1990). The key-part component of the SPME device is a fused silica fiber coated with an adsorbent material such as polydimethylsiloxane (PDMS), polyacrylate (PA) and carbowax (CW), or mixed phases such as polydimethylsiloxane-divinylbenzene (PDMS-DVB), carboxen-polydimethylsiloxane (CAR-PDMS) and carboxen-polydimethyl-siloxane-divinylbenzene (CAR-PDMS-DVB). The sampling can be made either in the headspace (Vas et al., 1998) or in the liquid phase (De la Calle et al., 1996) of the samples. The headspace sampling in wine analyses is mainly useful for quantifying trace compounds with a particular affinity to the fiber phase, not easily measurable with other techniques. Exhaustive overviews on materials used for the extraction-concentration of aroma compounds were published by Ferreira et al. (1996), Eberler (2001), Cabredo-Pinillos et al. (2004) and Nongonierma et al. (2006). Analysis of the volatile compounds is usually performed by gas chromatography (GC) coupled with either a flame ionization (FID) or mass spectrometry (MS) detector. 

For analysis of free compounds, the headspace analysis with a multiphase fiber is even more interesting and less time-consuming. This approach can replace the majority of the quantification of free compounds measured by SPE techniques, considering also the possibility of the sampling automation using a GC-MS system which can be coupled to a statistical treatment of fragments abundance (Kinton et al., 2003 Cozzolino et al., 2006). Moreover, HS-SPME/GC-MS is a very effective and efficient method to analyze specific compounds present in trace levels at about ppt level, because they can be better and selectively enriched in the headspace. This method is employed nowadays to quantify some important and peculiar sensory compounds such as ethyl and vinylphenols, pyrazines, cork off-flavour substances (TCA, etc.) and other contaminants such as geosmine (Riu et al., 2002 Chatonnet et al., 2006) and, as shown below, sulphur volatiles. 

Identified by Wang et al. (1983). The discovery of this heterocyclic compound was the result of a performant analysis of trace constituents, a technique combining headspace sampling, heart cutting, collection, sensory evaluation and two-dimensional GC. There are no comments on the reference compound or on its organoleptic properties. 

Wang T.H. (1981) Analysis of trace volatile organic compounds in coffee by means of headspace concentration technique and GC-MS. Thesis, US, University microfilms, 8208698 (Chem. Abstr. 97, 37615),... 

Gas chromatographic headspace analysis is a trace chemistry technique. Volatile organic substances can be detected even in the picogram range by this method. 

GC was introduced very early as the technique of choice for the detection and identification of accelerants in debris from arson cases because of its high selectivity and sensitivity. But to use the full potential of the technique the methods of recovery of traces of common accelerants from fire debris had to be developed and adjusted. The used methods include solvent extraction, direct headspace analysis, and enrichment by adsorbent-based techniques. In the past, the most common concentration steps prior to the analysis have been (heated) headspace direct injection using a gastight syringe for analyte collection and GC injection or headspace adsorption techniques, mostly using charcoal followed by carbon disulfide (CSi) elution. Some of these procedures have been quite effective and are standardized by... 

The headspace from a macerated tomato was collected onto Tenax and analyzed by the combined GC-EI-APIMS technique to yield the traces in Fig. 4. Again there were similarities between the two traces, and 13 compounds were identified by conventional analysis of the El trace (Table 3). There were two pairs of compounds that nominally had the same molecular weight and could have interfered with quantification. However, in both cases it was possible to obtain a unique ion (Table 3) when... 

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