Sample headspace techniques

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. 

Isolation of the products from complex matrixes (e.g. polymer and water, air, or soil) is often a demanding task. In the process of stability testing (10 days at 40 °C, 1 h at reflux temperature) of selected plastic additives (DEHA, DEHP and Irganox 1076) in EU aqueous simulants, the additive samples after exposure were simply extracted from the aqueous simulants with hexane [63]. A sonication step was necessary to ensure maximum extraction of control samples. Albertsson et al. developed several sample preparation techniques using headspace-GC-MS [64], LLE [65] and SPE [66-68]. A practical guide to LLE is available [3]. 

Solid-phase microextraction eliminates many of the drawbacks of other sample preparation techniques, such as headspace, purge and trap, LLE, SPE, or simultaneous distillation/extraction techniques, including excessive preparation time or extravagant use of high-purity organic solvents. SPME ranks amongst other solvent-free sample preparation methods, notably SBSE (Section 3.5.3) and PT (Section 4.2.2) which essentially operate at room temperature, and DHS (Section 4.2.2),... 

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. 

Miniaturisation of scientific instruments, following on from size reduction of electronic devices, has recently been hyped up in analytical chemistry (Tables 10.19 and 10.20). Typical examples of miniaturisation in sample preparation techniques are micro liquid-liquid extraction (in-vial extraction), ambient static headspace and disc cartridge SPE, solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). A main driving force for miniaturisation is the possibility to use MS detection. Also, standard laboratory instrumentation such as GC, HPLC [88] and MS is being miniaturised. Miniaturisation of the LC system is compulsory, because the pressure to decrease solvent usage continues. Quite obviously, compact detectors, such as ECD, LIF, UV (and preferably also MS), are welcome. 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

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. 

In 2003, Smith reviewed newer sample preparation techniques, including pressurized liquid extraction, solid phase microextractions, membrane extraction, and headspace analysis. Most of these techniques aim to reduce the amount of sample and solvent required for efficient extraction. 

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. 

In addition to the extraction of organic analytes from aqueous samples, the PDMS stir bars are also suitable for headspace and in vivo headspace sampling. Headspace sampling is a technique widely used to characterise the volatile frac-... 

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. 

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). 

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. 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

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. 

Bagheri, H. and A. Salemi. 2006. Headspace solvent microextraction as a simple and highly sensitive sample pretreatment technique for ultra trace determination of geosmin in aquatic media. J. Sep. Sci. 29 57-65. 

Multiple headspace extraction (MHE) is used to find the total peak area of an analyte in an exhaustive headspace extraction, which allows the analyst to determine the total amount of analyte present in the sample. This technique, along with the mathematical models behind it, was originally presented by McAuliffe [17] and Suzuki et al. [18]. Kolb and Ettre have an in-depth presentation of the mathematics of MHE in their book [15], and the reader is encouraged to reference that work for further information on the mathematical model. 

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. 

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. 

Headspace analysis is a very effective technique for the analyses of volatile compounds, and is particularly valuable when direct injection would ruin the column due to corrosive or highly non-volatile components present in the sample. Headspace analysis obviates extensive sample preparation, eliminates the possibility of unwanted component interference, and avoids degradation of susceptible components in the injection port or on the colunm. ... 

---