Ontwikkeling van instrumentasie vir omvattende tweedimensionele gaschromatografie

Snyman, Tertia
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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: Even capillary gas chromatography does not always give complete separation of the components of complex mixtures. During the last few decades several two-dimensional gas chromatographic techniques were developed to circumvent this problem and towards the end of the previous century, a technique that became known as comprehensive twodimensional gas chromatography, was introduced with which the peak capacity of capillary gas chromatography could be increased by at least two orders of magnitude. This technique is based on utilizing different separation mechanisms of two coupled chromatographic columns to get a better separation of complex mixtures than would be possible with the individual columns. To be classified as comprehensive twodimensional gas chromatography, the analytes eluted from the first or primary column must 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 or immobilizing the analytes in a short capillary tube that serves as a connection between the two columns, after which the trapped material is released as a sharp pulse into the secondary column by rapidly, i.e. within a fraction of a second, heating this capillary which therefore serves to modulate the effluent from the primary column. This ensures optimum separation on the secondary column and the independence of retention times of the analytes on the two columns. A modulator consisting of a capillary (modulator capillary) coated with a thick film of an apolar 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 a number of segments of equal lengths (maximum of 10). These segments were heated sequentially to desorb the analytes from the inlet end of the modulator to its outlet end at such a rate as to generate and transfer a sharply focused analyte pulse into the secondary column. In a typical analysis each of the 10 segments of a lO-segment heater would, for example, be heated to a temperature 50°C higher than that of the gas chromatograph's oven (50°C temperature increment) within 200 milliseconds, after which each segment would be allowed to immediately cool down to the temperature of the oven. After the last segment had been heated, a pause of, for example, two seconds followed to allow analytes to be trapped in the modulator capillary after which the cycle was repeated until the analysis had been completed. For several reasons, heating the segments resistively by using a current of between 1 and 20 Ampere was preferred to the application of high voltages. A computer controlled power supply was developed with which any combination of duration of the energizing pulses of the segments from 10 to 2500 milliseconds, pause times from 100 milliseconds to 100 seconds and temperature increments of 100°C or higher could be used with acceptable precision and high reproducibility in comprehensive two-dimensional gas chromatographic analyses. The effectivity of the focusing that can be achieved with heaters having different numbers of segments, modulator capillaries with different inside diameters, different heating increments, as well as different rates at which the modulators are heated, were investigated. The best results were obtained with heaters having 8 and 10 segments, a modulator capillary with an inside diameter of 0.2 mm, a heating increment between 50°C and 10Goe, and a heating cycle composed of a total heating time of two seconds followed 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 comprehensive two-dimensional system that was developed. At this stage of the project the influence of various parameters such as the average carrier gas velocity, the temperature program and the length of the secondary column was investigated. It was found that changing one parameter required the re-optimization of the other parameters. The concentrations of the sample also had a marked influence on the parameters that had to be used to achieve optimum results. A low sample concentration appeared to require a higher carrier gas velocity, a higher temperature-programming rate or considerably longer pause times to achieve satisfactory focusing of analytes, whereas too high a concentration resulted in breakthrough of the analytes from the modulator capillary. The two-dimensional gas chromatographic device was also interfaced to a quadrupole mass spectrometer. A GC-MS analysis of a petroleum oil sample gave mass spectra of surprisingly good quality in spite of the high scanning speed that was required by the sharp constituent peaks produced by the gas chromatographic component of the system. The two-dimensional system that was developed therefore appears to offer a costeffective alternative to other systems that have been developed elsewhere in which other modulation mechanisms are used. One remaining problem that still has to be solved is the unsatisfactory synchronization of the timing device of the power supply with that of the computer on which data are accumulated. Although the difference in timing may seem negligible, the result is that certain software packages cannot be used for the two-dimensional visualization of the data Of several possible solutions to the problem, redesigning the control circuitry of the power supply will be the first option to be explored. An important consideration in the development of the system was to avoid having moving parts so that the modulator could be installed in any gas chromatograph without requiring structural alterations to the instrument. No provision was therefore made to install the two columns and the modulator in separate temperature-programmable compartments in the oven of the gas chromatograph. During the evaluation of the present system it was, however, found that the parameters which gave acceptable results were confined to rather narrow limits. Not being able to cool the modulator to temperatures below that of the oven was found to be the most important limiting factor. A simple solution to this problem is to cool the modulator to a selected suitable temperature below the oven temperature with compressed air, the flow of which is regulated by a computer controlled mass flow regulator to maintain the same increment below the oven temperature right through an analysis. As this development was considered to be outside the scope of the present project, this idea was not implemented and evaluated. However, successful exploratory experiments were done in which the flow was mechanically regulated. A prototype of the component in which the modulator can be cooled was built and the mass flow regulator, control unit and software will be commissioned shortly.
AFRIKAANSE OPSOMMING: Volledige skeiding van die verbindings in 'n komplekse mengsel is selfs met kapillêre gaschromatografie nie altyd moontlik nie. Oor die afgelope paar dekades is verskillende tweedimensionele gaschromatografiese tegnieke ontwikkel om hierdie probleem te bowe te kom en redelik onlangs is 'n tegniek bekend gestel wat as omvattende tweedimensionele gaschromatografie bekend staan en wat die piekkapasiteit van 'n kapillêre gaschromatografiese skeiding teoreties met sowat twee ordegroottes kan verhoog. Hierdie tegniek berus op die aanwending van die verskillende skeidingsmeganismes van twee chromatografiese kolomme wat aan mekaar gekoppel word en waardeur verbeterde skeiding van komplekse mengsels verkry kan word. Hierdie tegniek kan egter slegs as omvattende tweedimensionele gaschromatografie beskou word as die analiete wat van die primêre kolom elueer volledig na die sekondêre kolom oorgedra word in die vorm van skerp gedefinieerde monsterpulse, byvoorbeeld deur fokussering van analiete. Fokussering van die analiete kan verkry word deur die eluaat van die primêre kolom te immobiliseer in 'n kort kapillêr tussen die twee kolomme waarna dit as 'n skerp puls op die sekondêre kolom ingelaat kan word deur die kapillêr, wat dus as modulator dien, vinnig, dit wil sê binne breuke van 'n sekonde, te verhit. Hierdeur word optimum skeiding in die sekondêre kolom verseker asook die onafhanklikheid van die retensietye van verbindings op die twee kolomme. In hierdie projek is gebruik gemaak van 'n modulator wat bestaan uit 'n kapillêr (modulatorkapillêr) belaag met 'n dik laag van 'n apolêre stasionêre fase om die analiete te immobiliseer. Die kapillêr is in 'n vlekvrye staalbuis (verhitter) geplaas wat in 'n aantal gelyke segmente verdeel is (maksimum van 10) en die segmente is vinnig opeenvolgend verhit om die analiete vanaf die inlaat- na die uitlaatkant van die modulator uit die stasionêre fase in die kapillêr te desorbeer en as 'n skerp gefokusseerde puls na die sekondêre kolom oor te dra In 'n tipiese analise sou elkeen van die segmente van 'n 10- segment verhitter, byvoorbeeld, binne 200 millisekondes tot 'n temperatuur 500e hoër as die temperatuur van die gaschromatograafoond (500e verhittingsinkrement) verhit word, waarna die betrokke segment toegelaat word om oombliklik tot die oondtemperatuur af te koel. Na verhitting van die laaste segment volg 'n wagtyd van, byvoorbeeld, twee sekondes om analiete wat uit die primêre kolom elueer toe te laat om in die modulatorkapillêr gesorbeer te word waarna die siklus herhaal word tot die einde van die analise. Om verskeie redes is besluit om die segmente elektries te verhit deur verkieslik hoë stroomsterktes van tussen 1 en 20 Ampere te gebruik in plaas daarvan om van hoë spannings gebruik te maak. 'n Gerekenariseerde kragbron is ontwikkel wat dit moontlik gemaak het om enige kombinasie van verhittingstyd per segment tussen 10 en 2500 millisekondes, wagtye van 100 millisekondes tot 100 sekondes tussen verhittings en temperatuurinkremente van 100°C en selfs hoër met bevredigende presisie en hoë reproduseerbaarheid vir omvattende tweedimensionele gaschromatografiese skeidings te gebruik. Ondersoek is ingestel na die effektiwiteit van fokussering wat verkry word deur verhitters met verskillende getalle segmente, modulatorkapillêre met verskillende binnedeursnitte, verskillende verhittingsinkremente, asook verskillende tempo's waarteen die modulatorkapillêr verhit word, te gebruik. Die beste resultate is verkry met 8- en 10- segment verhitters, 'n modulatorkapillêr met 0.2 mm binnedeursnit, 'n verhittingsinkrement tussen 50°C en 100°C en 'n verhittingsiklus bestaande uit 'n twee sekonde totale verhittingstyd gevolg deur twee tot drie sekondes wagtyd voor die volgende siklus begin. 'n Ligte aardolie-fraksie is gekies as die eerste monster vir die evaluering van die omvattende tweedimensionele sisteem wat ontwikkel is. In hierdie stadium van die projek is die invloed van verskillende parameters soos die gemiddelde vloeisnelheid van die draergas, die temperatuurprogram, asook die lengte van die sekondêre kolom ondersoek. Daar is gevind dat verandering van een parameter, byvoorbeeld die lengte van die sekondêre kolom, die heroptimering van die ander parameters vereis. Die konsentrasie van die monster het ook 'n bepalende invloed op die parameters wat 'n optimum skeiding van verbindings lewer. 'n Lae konsentrasie mag 'n hoër vloei, vinniger temperatuurprogram of heelwat langer wagtyd vereis om bevredigende fokussering van die analiete te verkry, terwyl 'n te hoë konsentrasie tot deurbraak van die analiete in die modulatorkapillêr kan lei. Die tweedimensionele gaschromatografiese sisteem IS ook aan 'n kwadrupoolmassaspektrometer gekoppel en analise van 'n aardolie-monster het massaspektra van verbasende goeie kwaliteit gelewer ten spyte van die hoë skandeerspoed wat deur die baie smal gaschromatografiese pieke wat deur die sisteem geproduseer word, vereis word. Die tweedimensionele sisteem wat ontwikkel is, stel klaarblyklik 'n kosteeffektiewe alternatief daar vir die enkele ander sisteme wat deur ander navorsingsgroepe ontwikkel is en waarin van ander modulatortipes gebruik gemaak word. Een oorblywende probleem is egter die onbevredigende sinchronisasie van die tydhousisteme van die kragbron en die rekenaar waarmee die data versamel word. Hoewel uiters gering, bring die verskil in tydsmeting van die twee komponente mee dat sekere sagtewarepakette nie vir die tweedimensionele voorstelling van die data gebruik kan word nie. Vanverskillende moontlike oplossings vir hierdie probleem, sal die herontwerp van die beheerstelsel van die kragbron eerste ondersoek word. 'n Belangrike oorweging ill die ontwikkeling van die huidige sisteem was om 'n modulator sonder bewegende dele te ontwikkel wat sonder moeite in enige gaschromatograaf geïnstalleer sou kon word. Daar is dus nie voorsiening gemaak vir die installering van die twee kapillêre kolomme en die modulator in afsonderlik temperatuurprogrammeerbare kompartemente in die gaschromatograafoond rue. Gedurende die ondersoek het dit egter geblyk dat die parameters wat bevredigende resultate lewer, as gevolg van hierdie ontwerpskriterium, tussen redelike nou grense lê. Die mees beperkende faktor is die feit dat die modulator nie benede die oondtemperatuur afgekoel kan word nie. 'n Eenvoudige oplossing vir hierdie probleem is afkoeling van dié modulator tot 'n geskikte selekteerbare temperatuur benede dié van die gaschromatograafoond met druklug waarvan die vloei met behulp van 'n rekenaarbeheerde klep geprogrammeer kan word. Aangesien hierdie ontwikkeling buite die bestek van die huidige ondersoek geval het, is die idee nie geïmplementeer en volledig geëvalueer nie. Voorlopige eksperimente waarin die lugvloei meganies beheer is, is wel suksesvol uitgevoer. 'n Prototipe van 'n geskikte onderdeel waarin die modulator afgekoel word, is vervaardig en die massavloeiregulerende klep, beheereenheid en sagteware sal eersdaags in gebruik geneem word.
Thesis (PhD)--Stellenbosch University, 2001.
Gas chromatography, Dissertations -- Chemistry, Theses -- Chemistry