Headspace procedure

Zero-headspace procedures involve the collection of a soil sample with immediate transfer to a container into which the sample fits exactly. The only space for gases is that within the soil pores. The volume of sample collected depends on the concentration of volatiles in the soil. It is imperative that the container employed can be interfaced directly with the gas chromatograph. Several commercial versions of zero-headspace sampling devices are available. The sample is transported to the laboratory at 4°C, where it is analyzed directly by purge-and-trap gas chromatography (EPA 5035) or other appropriate techniques, such as vacuum distillation (EPA 5032) or headspace (EPA 5021). 

In general, zero-headspace procedures are employed when the concentrations of volatiles in the soil are relatively low, and solvent extraction methods are used for more polluted soils. Irrespective of which procedure is used, quantitation of volatiles in soil is subject to serious errors if sufficient care is not taken with the sampling operation. Although direct purge-and-trap methods are frequently advocated for the determination of volatiles in samples collected by zero-headspace procedures, there are certain problems associated with this technique. Caution is advised since the procedure really collects only that fraction of the volatile that exists in a free form within the soil pore spaces or is at least in a facile equilibrium with this fraction. 

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

Several analytical approaches are employed to quantify sulphur volatiles in wine among them, the headspace procedure, such as the Purge and Trap, and solid-phase microextraction methods, combined with GC coupled to different detectors, was shown to be quite effective (Mestres et al., 2000 Rauhut et al., 1998). Recent papers suggest that the HS-SPME technique, with an improved choice of fiber coating phases, would be one of the more promising approaches for the concurrent measurement of compounds with different boiling points (Mestres et al., 2002 Fang and Qian, 2005). 

The classical methods like extraction and distillation often yield, particularly in the case of flowers, an extract that does not reflect the odour of the flower. More appropriate are the so-called headspace procedures that attempt to trap the fragrant volatiles of flowers directly. However, these methods produce such small amounts of odour concentrates that the analysis of the constituents has only become possible over the last 15 years with the aid of modern analytical techniques. 

Gas chromatographic (GC) methods have been used for determining volatile oxidation products. Static headspace, dynamic headspace or direct injection methods are the three commonly used approaches. These methods were compared in an analysis of volatile compounds in an oxidized soybean oil. It was found that each method produced significantly different GC profiles (Frankel 1985). The dynamic headspace and direct injection methods gave similar results, but the static headspace is more sensitive to low molecular weight compounds. Lee and co-workers (1995) developed a dynamic headspace procedure for isolating and analyzing the volatiles from oxidized soybean oil, and equations were derived from theoretical considerations that allowed the actual concentration of each flavor component to be calculated. 

The static headspace procedure is the simplest and can be applied to organic compovmds with high vapor pressure and low solubiHty in water. Only simple equipment is necessary and there are no great problems with impurities. However, the method is relatively insensitive because only a part of the vapor... 

Kolb, B. Application of an automated headspace procedure for trace analysis by gas chromatography. Journal of Chromatography 1976, 122,553-568. 

Samples collected on adsorbents can be desorbed by heat (thermal desorption) or by solvent extraction. Thermal desorption of samples from charcoal is not efficient however, because of the high temperature needed (950°C) to remove hydrocarbons from the charcoal (192). For this reason, most ACS passive headspace procedures use carbon disulfide to extract the adsorbed liquid residues. In 1967 Jennings and Nursten (193) reported concentrating analytes from a large volume of aqueous solution using activated charcoal as the adsorbent and extracting with carbon disulfide. Since then many adaptations of this method have been used to detect accelerants in fire debris, but currently dynamic headspace methods are seldom used because of the inconvenience of sampling and possible contamination issues with equipment. 

A SPME headspace procedure was recently described for the screening of urine for 21 amphetamine-related compounds, including amphetamine, methamphetamine, and related designer drugs [35]. Three deuterated amphetamines... 

One of the most elegant possibilities for instrumental sample preparation and sample transfer for GC-MS systems is the use of the headspace technique (Figure 2.9). Here all the frequently expensive steps, such as extraction of the sample, clean-up and concentration are dispensed with. Using the headspace technique, the volatile substances in the sample are separated from the matrix. The latter is not volatile under the conditions of the analysis. The tightly closed sample vessels, which, for example, are used for the static headspace procedure. 

Purge-and-trap sampling, dynamic headspace procedure, a concentration technique for volatile solutes. The sample is purged with an inert gas that entrains volatile components onto an adsorptive trap. The trap is then heated to desorb trapped components into a GC column. 

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