Gas chromatographic optimization

Optimization of gas chromatographic separations requires careful attention to a number of important variables and their interactions. In this chapter we will approach gas chromatographic optimization from the top down. First we will consider major options that have profound effects on a given separation, and which limit subsequent choices for many of the column variables, both physical—length, inner diameter, stationary phase—and parametric—temperature and flow or velocity. Then we will examine in more detail the questions of how to... 

The ensuing discussion of gas chromatographic optimization primarily addresses goal-oriented modification of column dimensions and operating conditions. Column selection, particularly of the stationary phase, has been addressed in Chapter 3. Here we focus on the effects of changing the column operating conditions and the column dimensions. 

Mathematical models for isothermal and for programmed-temperature gas chromatographic optimization are similar many programmed temperature models build on thermodynamic information obtained from isothermal chromatographic data, although some use programmed-temperatme input data to characterize the separation thermodynamically. 

Figure 4.14 shows a screen dump of an isothermal gas chromatographic simulation from a commercial gas chromatographic optimization program. The Temperature, Pressure, and Column tabs in the display permit the user to set elution conditions, including multiramp temperature and pressure programming, which were not exercised for this example. The Auto-Optimize tab carries out a minimum-resolution-oriented optimization calculation, which determines a set of conditions that lie within specified limits and meet the minimum resolution criterion. 

This equation and similar formulations are often used as the basis for commercial gas chromatographic optimization programs. Figure 4.15 illustrates a screen dump of a programmed-temperature simulation from a commercial computer program. Here, one chromatogram from a set of chromatograms that satisfy minimum... 

A variable-size simplex optimization of a gas chromatographic separation using oven temperature and carrier gas flow rate as factors is described in this experiment. 

It was found, that at standard gas-chromatograph sampling of 1 pL of analyte solution the limit of detection for different amines was measured as 0.1-3 ng/ml, or of about 1 femtomole of analyte in the probe. This detection limit is better of published data, obtained by conventional GC-MS technique. Evidently, that both the increasing of the laser spot size and the optimization of GC-capillary position can strongly improve the detection limit. 

D. Zuba, A. Parczewski, M. Reichenbacher, Optimization of solid phase microextraction conditions for gas chromatographic determination of ethanol and other volatile compounds in blood, J. Chromatogr. B, 773, 75 82 (2002). 

Colenut and Thorburn [51,52] have also described the procedure using gas stripping of the aqueous sample followed by adsorption onto active carbon from which surface they are taken up in an organic solvent for gas chromatographic analysis. They optimized conditions for the determination of parts per billion of pesticides and polychlorinated biphenyls. 

Reliable flame photometric detector quantification of organosulphur compounds requires careful optimization of the gas chromatograph parameters. Although the relative response of the flame photometric detector to various sulphur compounds remains somewhat controversial [7], analysis of organosulphur compounds by flame photometric detector is now relatively straightforward. 

Catalysts were tested for oxidations of carbon monoxide and toluene. The tests were carried out in a differential reactor shown in Fig. 12.7-1 and analyzed by an online gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors. Gases including dry air and carbon monoxide were feed to the reactor by mass flow controllers, while the liquid reactant, toluene was delivered by a syringe pump. Thermocouple was used to monitor the catalyst temperature. Catalyst screening and optimization identified the best catalyst formulation with a conversion rate for carbon monoxide and toluene at room temperature of 1 and 0.25 mmolc g min1. Carbon monoxide and water were the only products of the reactions. 

Vallejo-Cordoba B, Nakai S. 1993. Using a simultaneous factor optimization approach for detection of volatiles in milk by dynamic headspace gas chromatographic analysis. J Agric Food Chem 41(12) 2378-2384. 

The system shown in Fig. 4.11 is designed to dehver optimal performance when coupled to sample-preparation, sample-introduction and data-handhng products available. The HP 6890 series GC offers a smooth transition for users of HP 5890 series gas chromatographs through methods compatibihty, extremely useful for modern laboratories whose methodology is costly to develop. 

Fig. 1. Continuous-flow-apparatus for the optimization of homogeneous catalytic processes. A, catalyst solution B, starting compounds C, thermostated reactor D, trap E, gas-chromatograph F, data evaluation.

A final point about factors. They need not be continuous random variables. A factor might be the detector used on a gas chromatograph, with values flame ionization or electron capture. The effect of changing the factor no longer has quite the same interpretation, but it can be optimized— in this case simply by choosing the best detector. 

We acknowledge the assistance of Nikhil S. Dodhiwala who performed most of the SFE work, of Janet Benedicto and Lisa Balch, who performed all gas chromatographic and gas chromatographic/mass spectrometric analyses reported in this manuscript, and of Karin Bauer of Midwest Research Institute, who provided input on the statistical analysis of the preliminary results from our method optimization study. We would also like to acknowledge the assistance of Ashok Shah and Carl Stadler of Suprex Corporation who helped with the design of the dual-extraction setup on the Suprex SE-50 system, and of Hewlett-Packard Company who made available to us a Hewlett Packard extractor. 

An optimized version of the enantioselective SMB-GC unit was subsequently presented for enflurane enantiomers (chemical structure cf. insert in Figure 24) (Biressi et al., 2002b). It consisted of eight 80 cm x 15 mm (i.d.) stainless steel columns assembled in a home-made SMB-GC unit operated at 35°C (Scheme, cf. Figure 24). Each column with an adsorption bed volume of 140 ml each contained 20 % unpurified Lipodex E in the polysiloxane SE-54 and coated (17 %, w/w) on Chromosorb A (NAW, 20-30 mesh) 0.6 mm). This set-up represented the first gas-chromatographic SMB-GC unit for the preparative-scale separation of enantiomers. 

The following table gives the properties of common gas chromatographic carrier gases. These properties are those used most often in designing separation and optimizing detector performance. The density values are determined at CPC and 0.101 MPa (760 torr).1 The thermal conductivity values, X, are determined at 48.9°C (120°F).1 The viscosity values are determined at the temperatures listed and at 0.101 MPa (760 torr).1 The heat capacity (constant pressure) values are determined at 15°C and 0.101 MPa (750 torr).2... 

S92a) Kaiser, G., Grallath, E., Tschopel, P., Tolg, G. Contribution to the optimization of the chelate gas chromatographic determination of beryllium in limited amounts of organic materials. Z. Anal. Chem. 259, 257 (1972). 

J.F.K. Huber, E. Kenndler, G. Reich, W. Hack and J. Wolf, Optimal selection of gas chromatographic columns for the analytical control of chemical warfare agents by application of information theory to retention data, Anal. Chem., 65, 2903-2906 (1993). 

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