Ontwikkeling van instrumentasie vir omvattende tweedimensionele gaschromatografie

[摘要] ENGLISH ABSTRACT:Even capillary gas chromatography does not always give complete separation of thecomponents of complex mixtures. During the last few decades several two-dimensionalgas chromatographic techniques were developed to circumvent this problem and towardsthe end of the previous century, a technique that became known as comprehensive twodimensionalgas chromatography, was introduced with which the peak capacity ofcapillary gas chromatography could be increased by at least two orders of magnitude.This technique is based on utilizing different separation mechanisms of two coupledchromatographic columns to get a better separation of complex mixtures than would bepossible with the individual columns. To be classified as comprehensive twodimensionalgas chromatography, the analytes eluted from the first or primary columnmust al be transferred to the second column as sharp sample pulses by, for example,focusing of analytes. Focusing of the analytes can be achieved by trapping orimmobilizing the analytes in a short capillary tube that serves as a connection betweenthe two columns, after which the trapped material is released as a sharp pulse into thesecondary column by rapidly, i.e. within a fraction of a second, heating this capillarywhich therefore serves to modulate the effluent from the primary column. This ensuresoptimum separation on the secondary column and the independence of retention times ofthe analytes on the two columns.A modulator consisting of a capillary (modulator capillary) coated with a thick film of anapolar stationary phase was used in the present project to immobilize or trap the analytes.This capillary was housed in a stainless-steel tube (heater) which was subdivided into anumber of segments of equal lengths (maximum of 10). These segments were heatedsequentially to desorb the analytes from the inlet end of the modulator to its outlet end atsuch a rate as to generate and transfer a sharply focused analyte pulse into the secondarycolumn. In a typical analysis each of the 10 segments of a lO-segment heater would, forexample, be heated to a temperature 50°C higher than that of the gas chromatograph'soven (50°C temperature increment) within 200 milliseconds, after which each segmentwould be allowed to immediately cool down to the temperature of the oven. After thelast segment had been heated, a pause of, for example, two seconds followed to allowanalytes to be trapped in the modulator capillary after which the cycle was repeated untilthe analysis had been completed.For several reasons, heating the segments resistively by using a current of between 1 and20 Ampere was preferred to the application of high voltages. A computer controlledpower supply was developed with which any combination of duration of the energizingpulses of the segments from 10 to 2500 milliseconds, pause times from 100 millisecondsto 100 seconds and temperature increments of 100°C or higher could be used withacceptable precision and high reproducibility in comprehensive two-dimensional gaschromatographic analyses.The effectivity of the focusing that can be achieved with heaters having differentnumbers of segments, modulator capillaries with different inside diameters, differentheating increments, as well as different rates at which the modulators are heated, wereinvestigated. The best results were obtained with heaters having 8 and 10 segments, amodulator capillary with an inside diameter of 0.2 mm, a heating increment between50°C and 10Goe, and a heating cycle composed of a total heating time of two secondsfollowed by a pause time of two to three seconds before the next cycle is started.A light petroleum oil fraction was used in a preliminary evaluation of the comprehensivetwo-dimensional system that was developed. At this stage of the project the influence ofvarious parameters such as the average carrier gas velocity, the temperature program andthe length of the secondary column was investigated. It was found that changing oneparameter required the re-optimization of the other parameters. The concentrations of thesample also had a marked influence on the parameters that had to be used to achieveoptimum results. A low sample concentration appeared to require a higher carrier gasvelocity, a higher temperature-programming rate or considerably longer pause times toachieve satisfactory focusing of analytes, whereas too high a concentration resulted inbreakthrough of the analytes from the modulator capillary.The two-dimensional gas chromatographic device was also interfaced to a quadrupolemass spectrometer. A GC-MS analysis of a petroleum oil sample gave mass spectra ofsurprisingly good quality in spite of the high scanning speed that was required by thesharp constituent peaks produced by the gas chromatographic component of the system.The two-dimensional system that was developed therefore appears to offer a costeffectivealternative to other systems that have been developed elsewhere in which othermodulation mechanisms are used.One remaining problem that still has to be solved is the unsatisfactory synchronization ofthe timing device of the power supply with that of the computer on which data areaccumulated. Although the difference in timing may seem negligible, the result is thatcertain software packages cannot be used for the two-dimensional visualization of thedata Of several possible solutions to the problem, redesigning the control circuitry of thepower supply will be the first option to be explored.An important consideration in the development of the system was to avoid havingmoving parts so that the modulator could be installed in any gas chromatograph withoutrequiring structural alterations to the instrument. No provision was therefore made toinstall the two columns and the modulator in separate temperature-programmablecompartments in the oven of the gas chromatograph. During the evaluation of the presentsystem it was, however, found that the parameters which gave acceptable results wereconfined to rather narrow limits. Not being able to cool the modulator to temperaturesbelow that of the oven was found to be the most important limiting factor. A simplesolution to this problem is to cool the modulator to a selected suitable temperature belowthe oven temperature with compressed air, the flow of which is regulated by a computercontrolled mass flow regulator to maintain the same increment below the oventemperature right through an analysis. As this development was considered to be outsidethe scope of the present project, this idea was not implemented and evaluated. However,successful exploratory experiments were done in which the flow was mechanicallyregulated. A prototype of the component in which the modulator can be cooled was builtand the mass flow regulator, control unit and software will be commissioned shortly.

[发布机构] Stellenbosch University