Headspace analysis solvents

R.M. Black, R.J. Clarke, D.B. Cooper, R.W. Read and D. Utley, Application of headspace analysis, solvent extraction, thermal desorption and gas chromatography-mass spectrometry to the analysis of chemical warfare samples containing sulfur mustard and related compounds, J. Chromatogr., 637, 71-80 (1993). 

Principles and Characteristics Pare et al. [475] have patented another approach to extraction, the Microwave-Assisted Process (MAP ). In MAP the microwaves (2.45 GHz, 500 W) directly heat the material to be extracted, which is immersed in a microwave transparent solvent (such as hexane, benzene or iso-octane). MAP offers a radical change from conventional sample preparation work in the analytical laboratory. The technology was first introduced for liquid-phase extraction but has been extended to gas-phase extraction (headspace analysis). MAP constitutes a relatively new series of technologies that relate to novel methods of enhancing chemistry using microwave energy [476]. 

Advantages and disadvantages of HS-GC over regular GC are summarised in Table. 4.26. HS-GC fingerprinting chromatograms obviously include only the volatile components present and do not provide a complete picture of sample composition on the other hand, when solvent extraction is used, all the soluble sample constituents are removed, including also those having no appreciable vapour pressure at the equilibration temperature. Headspace analysis enhances the peaks of volatile trace components. 

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 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. 

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. 

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). 

Besides classical headspace analysis, simultaneous distillation-extraction and solvent extraction, new sampling and enrichment developments include solvent-assisted flavour evaporation (SAFE) [3] and sorptive techniques like SPME solid-phase microextraction (SPME) [4,5] and stir-bar sorptive extraction (SBSE) [6], which are treated in a dedicated chapter in this book. This contribution will deal with advanced developments of GC techniques for improvement of separation and identification (classical multidimensional GC, or... 

Volatile organic compounds Comparison of solvent extraction, headspace analysis and vapour partitioning, methanol extraction [88]... 

In another investigation, ethylene oxide in polyvinylchloride was determined by dissolving 65 mg of sample in 1 ml of dimethylacetamide [189]. Headspace analysis was conducted on a glass column packed with Porapak T under isothermal conditions. The solvent was removed by back-flushing. An external standard was used for calibration. A vinylchloride monomer was also detected in this analysis (Figure 4.3). 

Miscellaneous organics Miscellaneous solvents Comparison of direct solvent extraction GLC and headspace analysis [474]... 

Sensory Evaluation of the contribution of identified compounds to the aroma In order to identify those compounds mainly contributing to the characteristic flavor of the Black Truffle, the odor of the individual components of the headspace analysis were tested by a panel of eight judges (trained in sensory evaluation of truffles). The compounds tested were diluted in vegetable oil, in a range of concentrations from 30 to 300 ppm. 5 point scales (5 = exceptionally good full truffle aroma, 1 = not different from solvent) were used for flavor imitation and intensity. 

Partition coefficients can be determined by vapour-phase calibration (VPC) [54], by the phase-ratio variation (PRV) method [also known as the vapour-liquid equilibrium (VLE) method] [57] for many solvents in their aqueous solutions, and by VLE for ethanol in water. If two sample vials of different volume are both filled with the same sample, the partition coefficient, K, will be the same. In order to determine the solute s partition coefficient, K, each vial, at equilibrium, is subjected to headspace analysis in order to derive the slope of the linear equation (4.1). The concentrations of a solute in the two vials can be written as... 

Dills RL. Kent SD, Checkoway H. et al. 1991. Quantification of volatile solvents in blood by static headspace analysis. Talanta 38 365-374. 

Rastogi SC. 1992. Headspace analysis of chlorinated organig solvents in aerosol cans by gas chromatography. Chromatographia 33(3-4) 117-121. 

The detection of low level concentrations of volatile petroleum hydrocarbons in either soil or water can be performed by static headspace analysis. In this technique, the gas phase in thermodynamic equilibrium with the matrix is analysed. The soil is placed in a headspace vial to which water and soluble salts such as sodium chloride are added to aid the transfer of hydrocarbons into the headspace. Internal standards and surrogate spikes can also be introduced. The vial is heated and an aliquot of the static headspace vapour is directly injected onto the column of the gas chromatograph. The advantages of this technique for volatiles such as gasoline range organics are less sample handling which minimises losses, no introduction of solvents which can interfere with the compounds of interest (MTBE), and the technique can be easily automated. 

Headspace analysis avoids the need for solvent extraction for volatile analytes. 

Analytes extracted from environmental samples are concentrated prior to measurement by evaporating the solvent, using low temperature and reduced pressure for rapid solvent removal. Volatiles in soils and sediments may be sampled directly using headspace analysis for gas chromatography. 

Often interference effects from either solvents [74] or other components in sample matrices can cause significant problems especially with direct injection of such solutions. Headspace analysis has been shown to be of great value for residual solvent analysis in drug substance [75] and dmg product [76] because the drag itself is not introduced into the system. Similarly, residual solvent analysis in pharmaceuticals using thermal desorption [77] and solid phase microexttaction (SPME) [78] has been shown to be of benefit. For more con ilex matrices such as... 

The analysis of volatile sulphur compounds is difficult as additional compounds may be formed if the sample is heated or exposed to light or oxygen. Headspace analysis by gas-liquid chromatography using a flame photometric detector is the most satisfactory technique although solvent extraction may be necessary for the less volatile compounds. The sulphur compounds which have been identified in beer are listed in Table 22.19. 

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