Headspace technique

There are limitations to the coupling of headspace analysis with GC-MS systems if moisture is driven out of the sample as well. In certain cases, water can impair the focussing of volatile components at the beginning of the GC column. Impairment of the G C resolution can be counteracted by choosing suitable polar and thick film G C columns, by removing the water before injection of the sample. 

It is also known that water affects the stability of the ion source of the mass spectrometer detector, which nowadays is becoming ever smaller. In the case of repeated analyses, the effects are manifested by a marked response loss and poor reproducibility. In such cases, special precautions must be taken, in particular in the choice of ion source parameters. 

The headspace technique is very flexible and can be applied to the most widely differing sample qualities. Liquid or solid sample matrices are generally used, but gaseous samples can also be analysed readily and precisely using this method. Even the water content in food or pharmaceutical products can be determined by headspace GC (Kolb, 1993). Both qualitative and, in particular, quantitative determinations are carried out coupled to GC-MS systems. 

There are two methods of analysing the volatiles of a sample which have very different requirements concerning the instrumentation the static and dynamic (purge and trap, P T) headspace techniques. The areas of use overlap partially but the strengths of the two methods are demonstrated in the different types of applications. 

GC separations can be carried out on packed or open tubular columns (OTCs). The column is connected directly with the injector and the detector by nuts and ferrules. Typical column dimensions are given in Table 2.2. 

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Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

Headspace analysis (EPA 3810, 5021) also works well for analyzing volatile petroleum constituents in soil. In the test method, the soil is placed in a headspace vial and heated to drive out the volatiles from the sample into the headspace of the sample container. Salts can be added for more efficient release of the volatile compounds into the headspace. Similar to water headspace analysis, the soil headspace technique is useful when heavy oils and high analyte concentrations are present, which can severely contaminate purge-and-trap instrumentation. Detection limits are generally higher for headspace analysis than for purge-and-trap analysis. 

J.C. Oxley, J.L. Smith, J. Moran, K. Shinde, Determination of the vapour density of triacetone triperoxide (TATP) using a gas chromatography headspace technique . Propellants Explos. Pyrotech. 127—130. 

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. 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

Faldt, J., Eriksson, M Valterova, I. and Borg-Karlson, A.-K. (2000). Comparison of headspace techniques for sampling volatile natural products in a dynamic system. Verlag der Zeitschrift flir Naturforschung 55c 180-188. 

The extent of oxidative deterioration will determine the acceptability of a food product. Because of this, methods for determining the degree of oxidation are very useful to the food industry. There are many possible methods that can be utilized (see Commentary) however, due to the stability of some of the end products, and their direct relationship with rancidity, headspace GC provides a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic, and solid-phase microextraction (SPME) methods. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

Roberts, D.D. and Acree, T.E. 1995. Simulation of retronasal aroma using a modified headspace technique Investigating the effects of saliva, temperature, shearing, and oil on flavor release. J. Agric. Food Chem. 43 2179-2186. 

For aroma extracts, the blank sample is a mixture of the solvents used in the extraction, and are concentrated in the same way as the aroma isolate. Some volatiles in aroma extracts may derive from trace impurities of the solvents. For headspace techniques, a blank run is also recommended to check impurities coming from the tubings and/or adsorbents used. 

Vapor Pressure of 2,4,6-Trinitrotoluene by a Gas Chromatographic Headspace Technique , JChromatogr 133 (1), 83-90 (1977)... 

Boyko, A. Morgan, M. Libbey, L. "Analysis of Food and Beverages Headspace Techniques," Charalambous, G. (Ed.), Academic Press New York, 1978 pp. 57-79. 

Holscher, W. and Steinhart, H. (1992) Investigation of roasted coffee freshness with an improved headspace technique. Zeitschriftjur Lehensmittel-Untersuchung und -Forschung, 195, 33-8. 

SDBS concentrations were determined by HPLC as described previously. Dodecane concentrations in both the aqueous surfactant and squalane were determined by a GC/static Headspace technique. 

The stir bar technique has been applied to headspace sorptive extraction (HSSE) [142-144], However, headspace techniques are discussed elsewhere, as they are more applicable to volatile organic compounds than to the semivolatile organic compounds that comprise the focus of this chapter. 

The United States Pharmacopeia (USP) test (467) describes three different approaches to measuring organic volatile impurities in pharmaceuticals. Method I uses a wide-bore coated open tubular column (G-27, 5% phenyl-95 % methylpolysiloxane) with a silica guard column deactivated with phe-nylmethyl siloxane and a flame-ionization detector. The samples are dissolved in water and about 1 p is injected. Limits are set for benzene, chloroform, 1,4-dioxane, methylene chloride, and trichloroethylene. Methods V and VI are nearly identical to method I except for varying the chromatographic conditions. For the measurement of methylene chloride in coated tablets, the headspace techniques described above are recommended. 

Charalambous, G. Analysis of Foods and Beverages, Headspace Techniques Academic Press New York 1978. 

For this purpose, we developed a modified dynamic headspace technique for the analysis of truffle flesh and the aromatic liquid released during cooking. 

The headspace technique developed in the present study to isolate volatiles from canned black truffles performed satisfactorily. The aroma isolate obtained was described as typical, and 36 compounds were identified for the first time as canned clack truffle constituents. The formation of the major part of them could be correlated to the thermal treatment applied. 

KNUDSEN, J. T., TOLLSTEN, L BERGSTROM, L. G., Floral scents - a checklist of volatile compounds isolated by headspace techniques., Phytochemistry., 1993,33, 253-280. 

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