Headspace reproducibility

Figure 1, The production of hydroxyl radical ( OH) In llgnlno-lytic cultures of P, chrysosporlum as detected by ethylene formation. The 0H was detected by the OH-dependent formation of ethylene following the addition of a-keto-Y-methlolbutyrlc acid (KTBA) (3.3 mM final concentration) to cultures grown for 14 days In nutrient nitrogen deficient media. The ethylene produced was detected using gas chromatography and is expressed as nanomoles of ethylene/ml of headspace. Reproduced with permission from Ref. 9.

Figure 10.2 Total ion current chromatograms obtained for modern resins and gum resins after headspace SPME. Peak labels correspond to compound identification given in Table 10.1. Reproduced from S. Hamm, j. Bleton, A. Tchapla, j. Sep. Sci., 27, 235 243 (2004). Copyright Wiley VCH Verlag GmbH Co. KgaA. Reproduced with permission...

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

Figure 4.6 Headspace GC-ECD of some short-chain alkyl iodides. Reproduced with permission from S. Hamilton [27].

A new, fast, sensitive, and solventless extraction technique was developed in order to analyze beer carbonyl compounds. The method was based on solid-phase microextraction with on-fiber derivatization. A derivatization agent, 0-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBOA), was absorbed onto a divinyl benzene/poly(dimethylsiloxane) 65- xm fiber and exposed to the headspace of a vial with a beer sample. Carbonyl compounds selectively reacted with PFBOA, and the oximes formed were desorbed into a gas chromatograph injection port and quantified by mass spectrometry. This method provided very high reproducibility and linearity When it was used for the analysis of aged beers, nine aldehydes were detected 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, pentanal, hexanal, furfural, methional, phenylacetaldehyde, and (E)-2-nonenal. (107 words)... 

Separation of carbon tetrachloride from biological samples may achieved by headspace analysis, purge-and-trap collection from aqueous solution or slurry samples, solvent extraction, or direct collection on resins. Headspace analysis offers speed, simplicity, and good reproducibility, but 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 each different kind of matrix (Walters 1986). 

Figure 20.4—Static mode of headspace sample analysis. The sampling phial is pressurised with the carrier gas of the chromatograph. After equilibrium, a small volume of the gas containing the volatile compounds is inserted into a sample loop. Rotation of the six-way valve allows introduction of the sample into the injector of the chromatograph. Consequently, this set-up combines sample preparation with sample introduction into the chromatographic column. (Reproduced by permission of Tekmar.)...

Water activity (aw) is the ratio of the partial vapor pressure of water above a solution to that of pure water at the same specific temperature. It plays an important role in evaluating the microbial, chemical, and physical stability of foods during storage and processing. The vapor pressure in the headspace of a food sample can be measured directly by a manometer. A manometer has one or two transparent tubes and two liquid surfaces where pressure applied to the surface of one tube causes an elevation of the liquid surface in the other tube. The amount of elevation is read from a scale that is usually calibrated to read directly in pressure units. Makower and Myers (1943) were the first to use this method to measure vapor pressure exerted by food. Later, the method was improved, in terms of design features of the apparatus, by various scientists (Taylor, 1961 Labuza et al., 1972 Lewicki, 1987). Trailer (1983), Lewicki (1989), and Zanoni et al. (1999) used a capacitance manometer instead of a U-tube manometer for the measurement of vapor pressure. Lewicki et al. (1978) showed that the precision and reproducibility of the method can be improved by the simultaneous measurement of the water vapor pressure and temperature of the food sample. The method is reviewed in detail by Rizvi (1995) and Rahman (1995). 

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. 

SPME can be >95% reproducible. However, the following conditions must be carefully controlled to obtain reproducible results sample temperature, exposure time to the headspace, sample equilibration time (if using a closed container), sample flow rate (if using a dynamic system), sample size (both food sample and container), stirring speed (if stirred), and composition of the sample. 

Stuart et al. [127] studied the analysis of volatile organic compounds using an automated static headspace method. Recoveries decreased in the following order water, pure sand, sandy soil, clay and topsoil. A full evaporation technique that uses little or no aqueous phase and higher equilibration temperature gave the most reproducible analyte recoveries. 

There are many factors involved in optimizing static headspace extraction for extraction efficiency, sensitivity, quantitation, and reproducibility. These include vial and sample volume, temperature, pressure, and the form of the matrix itself, as described above. The appropriate choice of physical conditions may be both analyte and matrix dependent, and when there are multiple analytes, compromises may be necessary. 

Fyhr et al. [201] reviewed several commercially available oxygen analyzers intended for the analysis of oxygen in the headspace of vials. However, preliminary validation revealed insufficient reproducibility and linearity. The authors developed headspace analysis systems. Sample volumes down to about 2.5 ml could be used without significant errors. Sample recovery was in the range 100-102%. It was necessary to measure the head-space pressure and volume in order to be able to present the assay in partial oxygen pressure or in millimoles of oxygen. Up to 40 vials per hour could be analyzed using this technique. 

Figure 14 Volatile compounds identified from headspace of infested corn leaves but not (or in trace amount) in uninfested leaves. UD undamaged leaves ADL artificially damaged leaves 1st/2nd, 3rd, 6th host instars infesting corn leaves. Reproduced from J. Takabayashi S. Takahashi M. Dicke M. A. Posthumus, J. Chem. Ecol. 1995, 21, 273-287.

Figure 1 Coexistence of the past and present techniques in F F industry classical enfleurage process (photo on the left) and a supercritical carbon dioxide extraction facility as modern factory equipment (on the right). The photo on the left shows a stock of jasmine flowers in the basket (center) that are spread upon a wooden frame (chassis) that secures a glass plate coated with fat. The chassis is then piled to allow diffusion of fragrant components (note that the fat is applied on both sides of the glass plate to gain access to the headspace volume made by the chassis underneath). Enfleurage process photo reproduced from E. Guenther, The Essential Oils with permission from Krieger Publishing Company Melbourne, FL, USA, 1948 (reprinted 2006) Vol. 1, p 192.

A method for the automated analysis of volatile flavor compounds in foods is described. Volatile compounds are removed from the sample and concentrated via the dynamic headspace technique, with subsequent separation and detection by capillary column gas chromatography. With this method, detection limits of low ppb levels are obtainable with good reproducibility. This method has experienced rapid growth in recent years, and is now in routine use in a number of laboratories. 

Figure 7 GC - ICP-MS chromatogram of Se and S in Brassica jrmcea headspace after solid phase microextraction. The photograph shows Brassica plant grown for the experiments. (Montes-Bayon, Grant, Meija and Caruso. Reproduced by permission of The Royal Society of Chemistry)...

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