Sample application headspace

This method is based on the partitioning of compounds between a sample and a coated fibre immersed in it [16-18]. The volatiles and other compounds are first adsorbed onto the fibre immersed in a liquid sample, an extract, or in the headspace above a sample for a certain period of time. After adsorption is complete, the compounds are thermally desorbed into a GC injector block for further analysis. Particularly in food applications, headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components [16]. 

It is obvious that this method of odor measurement is a very time-consuming and expensive method. However, Stuetz et al. (2000) were able to correlate the global parameters BOD, COD and TOC of sewage samples by headspace measurements of odor using a commercial sensor array system. Thus, the electronic nose seems to have several interesting applications in wastewater treatment technologies. 

New techniques in application of capillary columns are also of great interest and very helpful in GC analyses. Direct injection of aqueous samples onto the column is an appealing and simple method which does not require sample preparation. Headspace injection onto a column, with or without cryofocusing, is a very promising technique for determination of low concentrations (submicrogram per... 

Tsoukali H, Raikos N, Theodoridis G, Psaroulis D. Headspace solid phase microextraction for the gas chromatographic analysis of methyl-parathion in post-mortem human samples. Application in a suicide case by intravenous injection. Forensic Sci Int 2004 143 127-32. 

Djozan, D. and Assadi, Y., A new porous-layer activated-cbarcoal-coated fused silica fiber application for determination of BTEX compounds in water samples using headspace sofid-phase microextraction and capillary gas chromatography, Chromatographia, 45(SuppL), S183-S189, 1997. 

It should be emphasized that the above discussion is valid only for direct extraction when the sample matrix passes through the capillary. This approach is limited to particulate-free gas and clean water samples. The headspace SPME approach can broaden the application of in-tube SPME. In that case, careful consideration of the mass transfer between sample and headspace should be given in order to describe the process properly. Also, if the flow is very rapid... 

A wider range of equipment that has particular features for special applications is described in the hterature. For example, special apparatus has been designed and reported for sampling the headspace of caimed foods and whole cheeses, and from single cigarettes. 

The solution headspace approach is applicable to a much wider range of samples than the solid approach. When working with sample solutions, headspace equilibrium is more readily attained and the calibration procedure is simplified. The sensitivity of the solution method depends upon the vapor pressure of the constituent to be analysed and its solubility in the solvent phase. Vinyl chloride, butadiene, and acrylonitrile, are readily transferred from polymer solutions into the headspace by heating to 90 °C. The headspace/solution partitioning for these constituents is not appreciably affected by changes in the solvent phase (namely, addition of water) since the more volatile materials favonr the headspace at 90 °C. Less volatile monomers such as styrene (bp = 145 "C) and 2-ethylhexyl acrylate (bp = 214 °C) may not be determined using headspace techniques with the same sensitivities realised for the more volatile monomers. By altering the composition of the solvent phase to decrease the monomer solubility, the equilibrium monomer concentration in the headspace can be increased. This resulted in a dramatic increase in the detection sensitivity for styrene and 2-ethylhexyl acrylate. 

The discussion of headspace methods for blood alcohol and solid-phase micro extraction (SPME) in Section 4.2 introduced the concept of creating an enriched head-space above a sample. Headspace methods may be passive or active and may involve heating the sample. Dynamic headspace (DHS) methods, used in arson analyses, exploit the equilibrium at the liquid-sample interface by sweeping tire headspace with a constant stream of gas, usually helium. DHS is also referred to as purge-and-trap (FT), allhough the latter can also mean a specific t) of sample preconcentrator used in environmental analysis. The trap material can be thermally desorbed or desorbed wifii a solvent. The thermal method is preferred, but is not always possible. The choice of trapping or sorbent materials depends on fire application arson typically requires charcoal or charcoal combinations. 

Pawliszyn and the author first reported on the technique of solid-phase microextraction (SPME) in 1990 (48). Since that time SPME has been commercialized (Supelco, Bellefonte, PA) and the number of applications of SPME to environmental analysis has exploded (7,49,50). When used for gas chromatography the technique requires no solvent. Solid-phase microextraction can be used to extract analytes from an aqueous phase or the gas phase. The technique can be used to sample the headspace over water and soil samples. 

Particularly in food applications headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components. Also SPME analysis is quite sensitive to experimental conditions. In addition to the stationary phase, sample, volume concentration of odorants, sample matrix and uniformity as well as temperature and time of the adsorption and desorption processes influence the yield. In quantitative SPME analysis these influences are eliminated by the use of labelled internal standards (cf. 5.2.6.1). 

The principles behind MAP liquid-phase and gas-phase extractions are fundamentally similar and rely on the use of microwaves to selectively apply energy to a matrix rather than to the environment surrounding it. MAP gas-phase extractions (MAP-HS) give better sensitivity than the conventional static headspace extraction method. MAP-HS may also be applied in dynamic applications. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix [477]. 

With regard to the solution approach, it is imperative that the solvent used be of the highest possible purity. Solution headspace is applicable to a much wider range of samples than the solid approach. When working with... 

Applications The potential of a variety of direct solid sampling methods for in-polymer additive analysis by GC has been reviewed and critically evaluated, in particular, static and dynamic headspace, solid-phase microextraction and thermal desorption [33]. It has been reported that many more products were identified after SPME-GC-MS than after DHS-GC-MS [35], Off-line use of an amino SPE cartridge for sample cleanup and enrichment, followed by TLC, has allowed detection of 11 synthetic colours in beverage products at sub-ppm level [36], SFE-TLC was also used for the analysis of a vitamin oil mixture [16]. 

Corwen [58] used this method for the analysis of ketones and aldehydes in seawater. Halocarbons were similarly separated from environmental samples by Kaiser and Oliver [59]. There have been many other applications of the technique [60-69]. The major advantage of the headspace method is simplicity in handling the materials. At most, only one chemical, the salt used in the salting-out procedure, needs to be added and in most cases the headspace gas can be injected directly into a gas chromatograph or carbon analyser. On the other hand the concentration of organic materials present is limited by the volume of seawater in the sample bottle. This is very much a batch process. 

Fig. 21.9. Flame ionization gas chromatograms of the headspace of the acceptable and unacceptable flavor samples. (Reprinted/redrawn from J. Chromatogr., 351, R.A. Sanders, and T.R. Morsch, Ion profiling approach to detailed mixture comparison. Application to a polypropylene off-odor problem, 525-531, Copyright (1986) with permission from Elsevier.)...

Several sampling procedures are applicable to volatile compounds but method application often depends on the compound(s) to be sampled (Dean, 2003). Part of the issue of sampling volatile compounds arises because some volatile substances sublime rather than boil, whereas other volatile substances emit significant quantities of vapor well below their boiling point. For sampling volatile hydrocarbons in the field, two procedures are generally recommended zero headspace and solvent extraction. However, these two procedures do not necessarily give equivalent results. 

In most cases the concentrations of the compounds detected by GCOH are too small for the identification experiments however, this disadvantage can be overcome when the odorants present in food are first detected in the extract by GC-O and then identified. Some of these odorants are also found by GCOH. As their odour quality, GC properties and chemical structures are known, they are easily identified in the headspace sample. In the case of parsley, a comparison of Fig. 16.2 with Table 16.5 indicates that odorant nos. 4, 6, 9, 11, 12 and 15 (Table 16.5) were known from AEDA. Further applications of GCOH are reviewed in [1]. 

Automatic headspace samplers are available from manufacturers of gas chromatographs. These devices are based on the technique of sampling an amount of vapor above the sample itself. Samples are sealed, neat or in a suitable solvent, in containers, and hold at a preset temperature in a thermostatted liquid bath. The headspace vapor results as a partition equilibrium is established between the liquid or solid and the gaseous phase of the volatiles. As each sample is presented to the analyzer, the vessel is punctured and a portion of the headspace gas is withdrawn by a pneumatic injection technique and forced into the column. The main application for those samplers is in the routine analysis of low-boiling fractions in samples containing nonvolatile solids or high-boiling components. Some of the more popular applications today are ... 

As an alternative to headspace extraction, analytes can be extracted by submersion of an SPME fiber in a liquid sample such as a beverage. While the ratio of analytes in the liquid phase is different from that which would be observed in the corresponding headspace gases, the concentration of most analytes is much higher in the liquid phase. Submersion SPME is most applicable when the basic composition in the food is desired. It is often used as a replacement for solvent extraction. 

The results obtained by application of this method to the flavors of two different olive oils are summarized in Table 5. In the oil sample A, fruity, green apple-like odor notes predominated, while the overall odor of oil B was characterized as fatty, stale. The SHA reflected these flavor differences. Only 0.1 mL or 1 mL, respectively, of the headspace of oil A were necessary to detect the odors of the fruity smelling esters ethyl 2-methylbutanoate and ethyl 2-methyl propanoate as well as the green smelling (Z)-3-hexenal indicating high odor activities of these odorants in oil A (Table 5). On the contrary, the fatty, soapy smelling octanal which was detectable in only 0.2 mL of the headspace of oil B, followed by... 

This technique is, of course, only applicable to organic compounds in soil that are sufficiently volatile at room temperature or slightly above that they exist in the headspace above the samples. For such samples, the technique is elegant in that it is solventless, i.e., there is no solvent interference, is amenable to automation, and can be directly coupled to a gas chromatograph and/or alternate techniques such as mass spectrometry to ensure equivocal identification of the organics. 

HWGs have successfully been applied to a wide variety of gas-sensing applications [44-52]. Micheels et al. [46] coupled a coiled MIR HWG to a FT-IR spectrometer measuring VOCs in the headspace of water samples. Yang et al. [47,48] partitioned organics from water or the headspace above a soil sample into the coating of a HWG. The waveguide was then inserted into the sample compartment of the FT-IR. 

The methods that generally are used to remove volatile organic chemicals (VOCs) from biological samples for analysis are applicable to chlorobenzene. These include headspace analysis, purge-and-trap (gas stripping) collection from aqueous solutions or slurry samples, solvent extraction, and direct collection on resins. Headspace analysis offers speed, simplicity, and good reproducibility for a particular type of sample. However, partitioning of the analyte between the headspace and the sample matrix is dependent upon the nature of the matrix and must be determined separately for different kinds of matrices (Walters 1986). 

Zwiener, C. andF.H. Frimmel. 1998. Application of headspace GC/MS screening and general parameters for the analysis of polycyclic aromatic hydrocarbons in groundwater samples. Fresenius J. Anal. Chem. 360 820-823. 

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