Gas chromatography requirements

Gas chromatography requires volatility of the compound to be determined, so EDTA is most commonly converted into its methyl, ethyl, propyl, or butyl ester. Methyl esterification is most commonly used method of derivitization. Silylaltion has also reported, but salts (as in case of sea water) can interfere. 

The use of gas chromatography requires volatile compounds, of course, and derivatives must be made of the hydrolyzed monosaccharides. Many investigators have used methylation or acetylation to produce these derivatives (10,16). However, both of these methods require more steps than are desirable with a small sample. In addition, the reaction time required for quantitative conversion to the derivative may be a problem. The use of trimethylsilyl ethers makes the derivatization exceedingly simple and relatively fast. Formation of these derivatives in the absence of water has been shown to occur without anomerization and to proceed quantitatively (30,31). The reaction mixture is chromatographed directly (Figure 9). 

Gas chromatography requires that samples remain stable when volatilized in a stream of helium, first in the injector used to introduce the analytes onto the GC column, and then during the time that the sample components traverse the column as it is heated progressively inside an oven. The requiranent for volatility means that only nonpolar or moderately polar compounds can be analyzed by GC-MS. For capillary GC columns the flow rate of the gas that moves compounds through the column (the carrier gas) is low, about 1 ml/min. Such a quantity of gas can be introduced into the ion source directly, without compromising the vacuum in the instrument. This simplifies the interfacing of GC with MS so that a heated interface tube can be used to link the two instruments and through which the GC column is run so that it abuts the ion source. 

As we already know that GC is used to separate mixtures which vapourise at the operating temperatures. The main p>arts of gas chromatography are injector, column oven and detector. Rimning gas chromatography requires accessories one of which is a column which is the crucial pert of the separation. 

Tetrabromobisphenol A (4,4 -isopropylidenebis(2,6-dibromophenol) TBBPA) is the most widely used BFR in terms of production quantities [2]. It is used as both a reactive and an additive BFR in a variety of polymers, epoxy resins, and adhesives and, in particular, is a constituent in printed circuit boards at levels up to 34% (mass fraction). The acute toxicity of TBBPA is relatively low however, concern for its potential as an endocrine disrupter exists. TBBPA shares structural similarities to thyroxine (T4), and the compoimd competitively binds to the thyroid hormone transport protein transthyretin [135]. Analysis by gas chromatography requires derivitization, and LC methods are preferred to reduce sample processing. Biotransformation of TBBPA... 

Gas chromatography requires a certain level of volatility and thermal robustness of the analyte. Both injection block of the gas chromatograph and interface to the mass spectrometer ion source are always at high temperature even while the column oven is not. In order to adapt an analyte to these needs, derivatization is well... 

In some cases, analytes should be transformed (by means of derivatization) to compounds with better analytical features for the analytical technique to be used, for instance, gas chromatography requires that low-volatile analytes be transformed into volatile derivatives. Also, the formation of a complex sometimes enables coloured or fluorescent derivatives to be obtained before determination with spectrophotometric or fluorimetric detectors. Other reactions, like hydrolysis, saponification or redox are seldom applied (Figure 2.2.5). 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

An alternative to TBP distillation is simulated distillation by gas chromatography. As described by Green, Schmauch, and Worman [Anal. Chem., 36, 1512 (1965)] and Worman and Green [Anal. Chem., 37, 1620 (1965)], the method is equivalent to a 100-theoretical-plate TBP distillation, is veiy rapid, reproducible, and easily automated, requires only a small microliter sample, and can better... 

Identification of stmctures of toxic chemicals in environmental samples requires to use modern analytical methods, such as gas chromatography (GC) with element selective detectors (NPD, FPD, AED), capillary electrophoresis (CE) for screening purposes, gas chromatography/mass-spectrometry (GC/MS), gas chromatography / Fourier transform infra red spectrometry (GC/FTIR), nucleai magnetic resonance (NMR), etc. 

Fig. 17. A schematic of the alkane line obtained by inverse gas chromatography (IGC) measurements. The relative retention volume of carrier gas required to elute a series of alkane probe gases is plotted against the molar area of the probe times the. square root of its surface tension. The slope of the plot is yielding the dispersion component of the surface energy of...

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