Thesis (PhD)--Stellenbosch University, 2001. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52382 |
| Date | 12 1900 |
| Creators | Snyman, Tertia |
| Contributors | Burger, B. V., Le Roux, M., Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | af_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 164 p. : ill. |
| Rights | Stellenbosch University |
Page generated in 0.0043 seconds