Analytes volatile

A technique for separating volatile analytes from liquid samples in which the analytes are subsequently trapped on a solid adsorbent. 

Volatile analytes can be separated from a nonvolatile matrix using any of the extraction techniques described in Ghapter 7. Fiquid-liquid extractions, in which analytes are extracted from an aqueous matrix into methylene chloride or other organic solvent, are commonly used. Solid-phase extractions also are used to remove unwanted matrix constituents. 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

And finally, the further improvement of high seleetivity of analytieal IR speetrometry is the using the ehemieal modifieation. There is possible to realize the fast and total transformation unvolatile and volatile analytes to the vapour phase. In those eases there is also essentially inereased the number of analyzed eompounds. 

Alternatively, LC is used for the separation and quantification of PAHs using both UV and fluorescence detection. The analytes are identified based on their relative retention times and UV and/or fluorescence emission spectra. For UV detection an efficient cleanup is a prerequisite since this detection method is not very selective (almost universal for PAHs), and hence it also responds to many coeluting compounds. Due to the high specificity of fluorescence detection for most PAHs, this LC detection method is less susceptible to potential interferences. As in the case of GC the apphcation of internal standard(s) is mandatory since solvents have to be evaporated during the cleanup, which may result in partial losses of some of the more volatile analytes. 

Volatile analytes. As residue analysis is also trace analysis in the lower ppm (mg kg ) to ppb ( ug kg ) range, concentration steps usually involve evaporation of solvents (sometimes with traces of water present) to near dryness. The volatility of analytes can be deduced from their elution temperatures in GC, and thus whenever an analyte elutes from a nonpolar GC phase of film thickness <0.25 qm below approximately 150 °C, losses due to co-evaporation during concentration by the rotary evaporator or by a stream of nitrogen need to be avoided. 

Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

Not traumatic (extraction of thermally labile and volatile analytes at low temperature)... 

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]. 

When the analyte is present in the polymer at very low concentrations some special precautions are needed to enhance the sensitivity of the extraction process, i.e. to lower the detection limit. The sample may be concentrated prior to analysis by SCF or solvent evaporation (at as low a temperature as possible to avoid degradation or partial loss of volatile analytes). Alternatively, a larger amount of polymer sample may be extracted (followed by LVI). Samples may also be concentrated or matrix effects minimised by using SPE [573,574],... 

The latter system allows quantitation [82]. Cryotrapping requires very low temperatures, because volatile analytes... 

Ashing is routinely used in the sample preparation of transition metals, but losses of volatile analytes such as halogens, S, P, Si, As, Se, Cd or Zn can occur. 

Unsuitable for volatile analytes. b Stability of analyte must be established. cCheck specific interference effects. 

Why are desorption ionization techniques well suited to the analysis of biomolecules (they do not depend on thermal energy to volatilize analytes). 

A purge-and-trap procedure is one in which a volatile analyte is purged from solvent by helium sparging and trapped on a sorbent held in a cartridge through which the helium then passes. 

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