Chromatographic figures

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. 

Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple and robust in construction and may be employed as static or flow detectors. Flow studies have received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas chromatography column. To achieve... 

J. P. Foley and J. G. Dorsey, Equations for Calculation of Chromatographic Figures of Merit for Ideal and Skewed Peaks, Anal. Chem. 1983,55, 730 ... 

A thermodesorption unit that will accept the PDMS-coated stir bar is used to transfer the analytes into a gas chromatograph (Figure 2.54). The analyte is desorbed from the stir bar and cryofocused on a precolumn. Subsequent flash heating transfers analytes into the gas chromatograph. After desorption, the stir bar can be reused. 

Care must be taken in the use and interpretation of literature efficiency values. Efficiency values are likely the most incorrectly calculated chromatographic figure of merit. The commonly used equations based on peak width at the base or at half the height of the peak are valid only for perfectly Gaussian shaped peaks. This problem has been realized by chromatographers for some time, but the popularity of these methods continues because until recently the only alternative was computer based moment calculations. Kirkland et al. addressed this problem and recommended that peak symmetry values be reported along with efficiency values (8 ). They further showed that the calculation... 

Advantages of this technique are the efficiency of development of methods, structured development profiles, and effective reporting of what was performed during the different method development iterations. In addition, it is possible to model the effect of parameter variation on the robustness of methods in addition to general chromatographic figures of merit apparent efficiency, tailing, resolution of critical pairs, backpressure of system, total run time. 

Foley, J.P. and Dorsey, J.G. (1983) Equations for calculation of chromatographic figures of merit for ideal and skewed peaks. Anal. Chem. 55, 730-737. 

Procedure Pass 10 ml of urine through a cation-exchange resin column (Dowex 60-W-X4, 20-50 mesh, in the hydrogen form) and wash the column with 30 ml of distilled water. Elute the amino acids with 20 ml of concentrated ammonia solution. Evaporate this eluate to dryness in vacuo. To the residue add 1 ml of trifluoroacetic anhydride and 0.5 ml of trifluoroacetic acid, and allow the mixture to stand at room temperature for 20 minutes. Remove excess reagents with a stream of air and treat the oily residue with an ethereal solution of distilled diazomethane (DANGER Poisonous-explosive) for 15 minutes. Remove excess diazomethane (CAUTION ) with a stream of air. Dissolve the residue in methanol and inject a suitable aliquot part into the gas chromatograph. Figures 25 and 26 show results obtained by this procedure. 

To the residue in the flask containing 6/10 of extract, add 100 /il of ethanol. Stopper the flask and rotate it to ensure complete dissolution of the residue. Inject an aliquot part (usually 2 or 3 jal) of this solution into the gas chromatograph. Figure 32 shows the results obtained by this procedure using a postmortem sample of blood which was found to contain amobarbital and secobarbital. The chromatogram of a mixture of nine common barbiturates is illustrated in Fig. 33. 

To avoid these inconveniences, an eluent generator can be used (either acidic or basic) which is inserted, as a supplementary module, between the pump and the injector of the ion chromatograph (Figure 4.7). If the flow rate of water and the electrolytic current are known then the concentration of the eluent can be determined with precision and concentration gradients can be effected, a procedure seldom used in ion chromatography. 

Static mode the sample (solid or liquid matrix) is placed in a glass vial capped with a septum such that the sample occupies only a part of the vial s volume. After thermodynamic equilibrium between the phases present (1/2 to 1 h), a sample of the vapour is taken. Under these conditions, the quantity of each volatile compound present in the headspace (volume above the liquid) will be proportional to its concentration in the matrix. After calibration (using methods of internal or external standards), it is possible to match the real concentrations in the sample with those of the vapours injected in the gas chromatograph (Figure 21.6). 

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