Chromatographic Detectors and Column Holdup for Organics PDMS Systems

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1 Irnaional Conference on Innovaions in Chemical Engiering and edical Sciences (ICICES'01) December 6-7, 01 Dubai (UAE) Chromaographic Deecors and Column Holdup for Organics PDS Sysems Edison uzenda Absrac chromaographic deecor ypes and applicaion, as well carrier gas flow rae and column hold-up measuremens are discussed in his paper. The flame ionizaion and ho wire deecors are discussed in deail. The measuremen of carrier gas flow rae using a soap film bubble flow meer is explaid. Column hold-ups for 4 poly(dimehylsiloxa) packed columns were measured using he homologous series mehod. Keywords Column hold-up, deecor, homologous, poly(dimehylsiloxa), reenion, hermodynamic funcion I. DETECTOS ETECTOS may be divided ino main classes (i) DThose which respond o he oal mass of maerial emerging from he column for example he auomaic iraion sysem and hese are called igral deecors (ii) Those, which respond o he concenraion of he vapour in he carrier gas a any insan for example he Kaharomeer and hese are called differenial deecors. Deecors of he ype have obvious advanage in quaniaive analysis since his only requires he measuremen of sep heighs. However he wo ypes of deecors are limied in heir applicaion. Several ypes of deecors are used in gas chromaography such as he ho wire deecors (HWDs), flame ionisaion deecors (FIDs), flame phoomeric deecors (FPDs) and he elecron capure deecors (ECDs). The main purpose of he deecor is o idenify he compons by means of heir reenion imes and o measure heir concenraions. Selecion of deecors depends on many facors, for example, sensiiviy (minimum deecion), sabiliy (liariy), seleciviy (signal o noise raio), response ime and he compons involved. In his work, he flame ionisaion and he ho wire deecors are discussed in deail while oher deecors are discussed briefly. Thermal conduciviy deecors have been widely used in he chromaographic deerminaion of hermodynamic daa. They are simple in consrucion and permi direc deerminaion of he gas hold-up of he chromaographic column by he iming Edison uzenda is wih he Deparmen of Chemical Engiering, Faculy of Engiering and he Buil Environmen, Universiy of Johansburg, Doornfoin, Johansburg 08, Tel: , Fax: , ( emuzenda@uj.ac.za) of an air peak. However, since heir response depends on he difference beween he hermal conduciviy of he organic solue and ha of he carrier gas, his response is reduced considerably when here is a consan bleed of pure volaile organic solvens ino he deecor. For volaile solvens, hermal conduciviy deecors are expeced o be limied in applicabiliy o sysems where hermal conduciviies of solven and solue vapours are differen. A more suiable deecor would be he flame ionisaion deecor. O drawback in he use of flame ionisaion deecors in he accurae deerminaion of reenion daa has been he elaborae and indirec mehod of measuring he column gas hold-up. This is because he deecor is insensiive o air under he condiions prevailing in a convenional column. The oher drawback associaed wih flame ionisaion deecors when volaile solvens are used is he decrease in sensiiviy of he deecor due o he increase in ion concenraion beween he elecrodes. This evenually leads o complee insensiiviy when he condiion of ion sauraion is reached. However, since a non-volaile solven (PDS) was used in his work, his drawback was eliminaed. FIDs are very useful in he deecion of volaile organic compounds in very low concenraions, say down o he 1ppm range. Therefore, he flame ionisaion deecor was used in his work as he focus is in very dilue regions. A. The Flame Ionizaion Deecor (FID) - Principle of operaion This deecor was developed by cwilliam and Dewar (1958) [1] and i is based on he principle ha he elecrical conduciviy of a flame burning hydrogen and oxygen mixure increases when he organic molecules er he flame. This is mainly as a resul of he difference in he ionisaion poenials of he gas ( ev) and organic molecules (9-1ev). However, he ionisaion of he organic molecules ha akes place in he flame is no well undersood. The increase in conduciviy of he flame is much more han would be expeced from he normal ionisaion poenial of he organic compounds. The poenials of organic molecules are in he range 9-1ev while he effecive ionisaion poenial is abou 5ev. O explanaion as posulaed by Sern (1951) [] in [3] could be he formaion of carbon aggregaes wihin he flame and heir subsequen ionisaion. This explanaion is srenghed by he fac ha he ionisaion poenial of solid carbon is 4.6ev and he deecor response is essenially proporional o he number of carbon aoms in he organic 0

2 Irnaional Conference on Innovaions in Chemical Engiering and edical Sciences (ICICES'01) December 6-7, 01 Dubai (UAE) molecules. Under he condiions prevailing in he convenional chromaographic column, he flame ionisaion deecor is insensiive o air, carbon dioxide, waer vapour and hydrogen sulphide. - Lighing he deecor flame No aemp was made o ligh he deecor flame unil he deecor emperaure was a leas 100 o C, below his emperaure, waer condensaion can ake place inside he deecor producing a noise signal. When lighing he deecor flame, he following procedure was followed. The firs sep was o ensure ha he combusion gases (air and hydrogen) were urd on a he cylinders. The cylinder pressure regulaors for air and hydrogen were se o 80kPa and 84kPa respecively. Sufficien ime was allowed for he hydrogen o purge he plumbing and he supply a pumaics pal was urd off. This was followed by seing he air pressure o 138kPa and he opimum air pressure was found by experimen. The x sep was o remove he lower oven door cover. Then, he ignier was held agains he FID chimy, he hydrogen supply was urd on and he flame was ligh. A disinc pop sound was heard as he flame ignies. A precauion was aken o make sure ha he ignie filamen was glowing before urning he hydrogen; his ensured ha he combusion akes place a he je and no above he collecor. The filamen lighing was checked by holding a cool shiny objec agains he deecor end and waer from he flame should condense ono he cold surface. - FID opimisaion The hydrogen and air flow raes were se o values wihin he ranges in Table I. TABLE I FID OPTIIZATION FLOW ATES Flow Approximae inle pressure (ml/min) kpa PSIG Air Hydrogen Invesigaion of he opimum pressure was do by measuring he flow rae a he manifold a he boom of he oven door wih a soap film bubble flow meer. B. The Ho Wire Deecor (Kaharomeer) - Principle of operaion The word kaharomeer meaning pure in Greek, was originally suggesed by Shakespeare (191) [3] o an insrumen he devised for measuring he puriy of hydrogen gas by hermal conduciviy. The kaharomeer as used in he GLC is a ype of deecor, which works on he principle ha he resisance of a heaed wire placed in a flowing gas changes according o he hermal conduciviy of he gas compons. When heaed filamens are placed in a flowing carrier gas hey mainain a definie consan emperaure and resisance. If he resisance of he carrier gas changes due o elued solue sample, is hermal conduciviy also changes. This will aler he rae of hea loss from he heaed filamens. Thus he emperaure and resisance will also change. Usually a pair of mached filamens are used, o being exposed o he carrier gas, reference filamen and he oher o he column, called he sample filamen. The reference and sample filamens are incorporaed in Wheaso bridge wih he ou of balance poenial passing o he recorder. The schemaic view of he ho wire deecor is shown in Fig. 1. Fig. 1 Schemaic view of he ho wire deecor As shown in Fig. 1, four ungsen rhenium filamens are conced as a bridge circui, wo filamens being locaed in each chanl. The presence of a sample compon in he carrier gas flowing chanl changes he resisance of he filamen in ha chanl, unbalancing he bridge and producing a signal relaed o he sample concenraion from which he chromaogram is ploed. The deecor is moud on a sandard cylindrical deecor module and is moud in he lower par of he oven door wih he amplifier board posiiod below i. The deecor has wo heaed zos, he deecor block and he irface which are separaely conrolled bu are always se a he same emperaure. - Choice of carrier gas Helium is he recommended carrier gas for use wih a ho wire deecor because of is high hermal conduciviy. The use of helium is avoided when helium or hydrogen con of he sample is being analysed. The use of hydrogen is a possibiliy, bu grea care mus be aken because of is high flammabiliy. Therefore, i is very cessary o make sure ha all he concions inside he oven are leak free. In he even of he helium or hydrogen con of he sample being analysed, nirogen or argon may be used as he carrier gases. When nirogen or argon is used, posiive or gaive peaks are obaid depending on wheher he hermal conduciviy of he paricular sample con is smaller or larger han he hermal conduciviy of argon. Someimes samples conaining hydrogen may show anomalous behaviour resuling in - shaped peaks when helium is used as he carrier gas. This can be eliminaed if a helium / hydrogen mixure conaining 1

3 Irnaional Conference on Innovaions in Chemical Engiering and edical Sciences (ICICES'01) December 6-7, 01 Dubai (UAE) beween 8 and 9 % by volume of hydrogen is used as he carrier gas. C. The Flame Phoomeric Deecor (FPD) This is similar o he flame ionisaion deecor. I is a combinaion of he FID and an opical sysem, which has a very high seleciviy wih adequae sensiiviy owards phosphorus and sulphur. I can deec sulphur compounds down o ppb levels. D. Elecron Capure Deecor (ECD) This is he mos sensiive deecor available for he nondesrucive deecion of elecrophilic subsances, such as chlorinaed hydrocarbons. I is very useful for pesicide residue deerminaions. I depends on he abiliy of he compounds o capure elecrons. any compounds have high elecron affiniies hus allowing heir deecion wih his sysem. E. Nirogen Phosphorus deecor (NPD) This is a hermionic deecor ha is sensiive o organically bound nirogen and phosphorus. I can be operaed in eiher wo modes, NP mode in which boh nirogen and phosphorus are deeced and quanified as well as he P mode in which he phosphorus is he only deeced and quanified compon. The NPD desroys he sample being analysed. F. Choice of Deecor In summary he ideal deecor should possess he following properies (i) A high sensiiviy which allows small amouns of impuriies o be deeced, small samples o be analysed, and low column emperaure o be used (ii) A small irnal volume, which ensures ha he resoluion of he compons, which are only jus separaed by he column is no los, and he shape of he peaks is no disored by he deecor (iii) An operaing emperaure such ha no appreciable amoun of he elued vapours condenses in i (iv) Insensiiviy he variaion of carrier gas flow rae and column emperaure (v) A rapid and reproducible response, produced by simple ancillary elecrical equipmen, accompanied by sabiliy of he baseli (vi) For qualiaive analysis a response which can be adjused so ha idenificaion is assised (vii) For quaniaive analysis a liar response wih concenraion or amoun of compon so ha he composiions may be calculaed (viii) A response o all subsances independen of heir chemical naure which depends on a simple law reducing calibraion o a minimum (ix) Low cos and reliabiliy I is imporan o noe ha no deecor fulfils all he desirable characerisics, herefore selecion is based on he ask o be performed. For some problems however here will be reasons for choosing o deecor raher han anoher. For insance, ionisaion deecors will almos always be preferred for work wih open ubular and packed columns conaining only a small loading of saionary phase because of he ed for high sensiiviy. However, excep for measuremen wih open ubular columns, i is probably no a big drawback as he effluen from a large column can be spli allowing only a small par o he deecor. An imporan limiaion of he FID is is lack of response o perman gases. The Ho wire deecor which can be made o respond o any subsance is ofenly used for gas analysis. The drawback of he ho wire deecor is ha is sensiiviy falls when he emperaure is raised above 100 o C. II. FLOW ATE EASUEENT The carrier gas flow rae is o imporan quaniy which has o be measured wih grea accuracy for reliable reenion daa. Several mehods of measuring flow raes are convenionally available. The mehod mus mee wo essenial requiremens, namely, he mehod mus be non-desrucive and accurae for small flow raes wihou causing any disurbance o he flow. In his sudy, he soap film bubble flow meer was seleced as i me he wo requiremens. In his work, he soap film bubble flow meer was made of a 100ml calibraed buree wih a small glass je aached o is base. The buree was horoughly clead wih chromic acid and rinsed wih disilled waer before assembling. When he carrier gas from he GC ered he je, i creaed a hin soap film ravelling verically up he buree. The rae of ravel of he soap film was imed wih a sopwach. The walls of he soap film flow meer were well weed by sending many bubbles compleely hrough he buree before measuring. Any dryss of he walls will lead o irreproducible flow rae measuremens. The flow rae was measured a room emperaure and hen correced o ha of column emperaure. Accurae flow rae measuremens were easily achieved wih he flame ionisaion deecor as i dilued he gas sreams. A schemaic diagram of he soap film bubble flow meer used in his work is shown in Fig.. Fig. Soap film bubble flow meer III. EASUEENT OF GAS HOLD-UP The gas hold-up ime, is a basic parameer in gas liquid chromaography (GLC). I is imporan for he deerminaion of soluion hermodynamic funcions derived from solue

4 Irnaional Conference on Innovaions in Chemical Engiering and edical Sciences (ICICES'01) December 6-7, 01 Dubai (UAE) reenion [4]. easuremen of he absolue and he relaive reenion daa requires he knowledge of he column gas holdup volume. Errors in he measuremen of he gas hold-up volume can inroduce serious errors in he reenion daa especially when he laer are comparaively small. In gas liquid columns, he gas hold-up conribues 1 o 10 percen of he oal reenion volume. The accuracy of he measuremen required is abou up o 10 percen which ofen leads o he reenion volume accuracy of 0.1 o 1 percen which is adequae for many applicaions. The air peak, homologous series and geomerical calculaion mehods can be used o measure he gas hold-up. Alhough, he air peak mehod is he simples and mos common, he homologous series procedure was used in his work because of he desirable high sensiiviy of he flame ionizaion deecor a dilue concenraions of organic compounds. When using deecors such as Kaharomeers, which respond o perman gases, he gas hold-up can be direcly measured from he air peak corresponding o he eluion of a non-absorbed air sample. The ionisaion deecors used in his work are insensiive o perman gases and hence he direc air peak mehod was no applicable. Therefore an indirec mehod was proposed based on he liar relaionship of he logarihm of he correced reenion ime and he carbon number of a homologous series. Using his liar relaionship he gas holdup was evaluaed as a mahemaical air peak and as calculaed dead ime from he reenion imes of hree members of a homologous series. No aemp was made o use he indirec meha peak procedure as i is an unreliable due o he small solubiliy of meha in organic solvens. A. Air Peak ehod The air peak or ir gas peak mehod is he mos ofen used because is simpliciy. I involves injecing he sample of air or oher non-sorbed gas whose reenion volume is considered as he gas hold-up. The selecion of a non-sorbed gas depends on he deecion sysem. In he case of Ho Wire Deecors, any gas such as air, hydrogen, helium or meha maybe used. When hydrogen or helium is used as a carrier gas, a small volume of air should be added wih he sorbed solue while hydrogen can be used o provide he ir gas peak when nirogen is he carrier gas. Air may also be used o provide he ir air peak bu i has o be injeced in a separae run in large volumes (100µl o 1ml) because of he small hermal conduciviies of nirogen and oxygen. B. Geomerical Calculaion ehod The gas hold-up is obaid from he empy column volume and he rue (absolue) solid densiy deermid under a liquid such as waer or kerose. This mehod depends on he assumpion ha he liquid occupies all he inraparicle pore space. There is a considerable variaion in hold-ups obaid in his manr compared o oher procedures. C. Homologous Series ehod In lieraure, he deerminaion of m has been derived from a relaionship for he reenion of n-alkas and oher homologous series [5]. This relies on he acceped geralisaion ha he logarihm of he adjused reenion ime ( ) is a liar funcion of he carbon number in any homologous series. Thus log ( ) = an b + (p and T =consans) (1) In (1) a and b are consans for he solue class and saionary phase. A known gas hold-up ime can be deermid as he value ha will liarise a plo of his equaion. This simpliciy glecs exra-column effecs and ( n) irfacial phenomena [4]. The reenion imes, of differen n-alkas are measured under he same condiions (p and T) and preferably on he same chromaogram, making he carbon number (n) he only unique variable. The gas hold-up is calculaed from () derived from (1). 1 3 = () The relaionship expressed in () is also valid for homologous members of carbon numbers n, (n + k) and (n + k) where n and k igers. Also any hree members no cessarily successive members of a homologous series may be used and he gas hold-up is calculaed from he ieraive soluion of (3). 3 log log 1 1 = n n n n (3) The success of he homologous series mehod depends on he relaionship (1) being obeyed and in some cases he plos are no liar as he lowes homologues depar from liar behaviour. IV. ESULTS AND DISCUSSION The gas hold-up is a measure of he oal volume of space available o he mobile phase in he sysem, ha is column dead void volume, injecor and deecor volumes as well as he volumes of any concing ubing for example from column o V deecor [6]. The gas hold-up volume ( m ) is defid in (4), where F and m are carrier gas flow rae and reenion ime respecively. V = F m (4) A sample conaining pena, hexa and hepa was injeced ino he GC a seleced flow rae and emperaure a consan pressure. The reenion imes of he hree solues were hen used o calculae he gas hold-up volumes using (). The gas hold-up volumes for he 4 poly(dimehylsiloxa) packed columns described in Table II are presed in Tables 3 o 6. 3

5 Irnaional Conference on Innovaions in Chemical Engiering and edical Sciences (ICICES'01) December 6-7, 01 Dubai (UAE) TABLE II DESCIPTION OF COLUNS Code V Lengh, m Chromosorb W or P, g PDS W % PDS Viscosiy of PDS, cs w of PDS TABLE III GAS HOLD-UP VOLUE COLUN I ean reenion ime ahemaic hold-up V Pen Hex Hepa al air reenion a a ime (min) ACKNOWLEDGENT The auhor is graeful o he School of ining, eallurgy and Chemical Engiering and he Faculy of Engiering and he Buil Environmen of he Universiy of Johansburg for conference suppor. EFEENCES [1] L. G. cwilliams, and. A. Dewar, Chromaography, D. H. Desy, Ed. London, Buerworhs, London: Buerworhs, 1958, pp. 14. [] B. Lewis, and G. von Elbe, Combusion, Flames and Explosion of es, Unied Kingdom, Oxford, Elsevier, [3] E. uzenda, easuremen and Compuaion of Phase Equilibrium a Infinie Diluion, PhD Thesis, Universiy of Birmingham, 000. [4] J.. Conder, and C. L. Young, Physicochemical easuremen by Chromaography, New York, Wiley, [5] F.. Gonzalez, Consisency of gas-holdup deerminaions, J. Chromaography A, vol. 83, pp , [6] [E. uzenda, A revis and review of experimenal variables affecing chromaographic organics polydimehylsiloxa iracions, IJBEES, vol. 1, no. 4, pp , 01. TABLE IV GAS HOLD-UP VOLUE COLUN II ean reenion ime Pena Hexa Hepa ahemaica l air reenion ime (min) hold-up V TABLE V GAS HOLD-UP VOLUE COLUN III ean reenion ime Pena Hex a Hepa ahemaical air reenion ime (min) hold-up V Tem p TABLE VI GAS HOLD-UP VOLUE COLUN IV ean reenion ime Pena Hex Hepa a ahemaica l air reenion ime (min) hold-up V The resuls show ha gas hold-up volumes were consan wih emperaure suggesing a rue measure of column void volume. This work has shown ha he homologous series mehod can be reliably used o measure gas hold-up ime,, an imporan parameer in he derivaion of hermodynamic funcions of mixing such infinie diluion aciviy coefficiens, enhalpies, enropies and parial molar free ergy from solue reenion. 4

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