Etyka psychologiczna - artykuł - Pomoc psychologiczna czy psychoterapia(1), Psychologia, Pomoc psychologiczna
Etiologiczna klasyfikacja cukrzycy, Pielęgniarstwo, Interna i pielęgniarstwo internistyczne, Diabetologia, Artykuły
ewangelizacja(1), Dokumenty (w tym Dokumenty Kościoła), Artykuły teologiczne, o ewangelizacji znalezione w internecie
Evolution, Filozofia, Filozofia - Artykuły
Evolution and the Problem of Other Minds(1), Filozofia, Filozofia - Artykuły
Evolution and the Problem of Other Minds, Filozofia, Filozofia - Artykuły
Ewangelie - omówienie, Artykuły różne, biblia i nie biblia
Evaluation of ethanol-chlorhexidine gluconate scrub surgical, Artykuły z zakresu dezynfekcji, Dezynfekcja - rodzaje baz, środków
EVALUAT REVIT HAOUSING, Architektura Krajobrazu, Artykuły różne - rewitalizacja, planowanie, krajobraz
Escherichia, SUM Zabrze, Mikrobiologia
Evidence for selectivity of absorption of volatile organic, Artykuły naukowe, SPME i HS-SPME
[ Pobierz całość w formacie PDF ]//-->Journal of Chromatography A, 885 (2000) 457–464www.elsevier.com / locate / chromaEvidence for selectivity of absorption of volatile organiccompounds by a polydimethylsiloxane solid-phase microextractionfibreStefan Niedziella , Susan Rudkin, Michael Cooke*The Toxic Gases Research Group,Centre for Chemical Sciences,Royal Holloway University of London,Egham,Surrey TW20 0EX,UK1AbstractSolid-phase microextraction using a 30mmpolydimethylsiloxane fibre has been used to sample the volatile organiccompounds from standard mixtures and from mixtures produced by the decomposition of organic compounds. This methodof sampling has been compared with the direct injection of an aliquot of headspace gas and shows an enrichment factor ofapproximately 100 over a 1 ml gas injection for organosulphur gases such as dimethyldisulphide. The performance of thefibre has been evaluated with respect to accuracy and precision at several concentrations in representing the composition ofmulticomponent mixtures. It was found that the presence of a second component in a gas sample reduced the capacity of thefibre to absorb the primary component. The selectivity of the fibre for various volatile compounds with differingfunctionality was also studied. It was found that the non-polar polydimethylsiloxane fibre preferentially absorbed thenon-polar components of a mixture, e.g nonane and, correspondingly, under reported the more polar components, e.g.ethanol. Hence, the fibre discriminates in favour of non-polar and against polar components in a mixture in comparison withdirect analysis of a headspace sample. Thus, quantitation of a component in a multi-component mixture is liable to errorfrom competitive interference from other components. A major advantage of the technique, however, is that it does notabsorb, and therefore introduce, water into the analytical system.©2000 Elsevier Science B.V. All rights reserved.Keywords:Solid-phase microextraction; Selectivity; Polydimethylsiloxane fibres; Volatile organic compounds1. IntroductionThe determination of low concentrations of or-ganic compounds in the gas phase in air remains achallenge for the analytical chemist. Low concen-tration and large volume means that introduction into*Corresponding author. Tel.:144-178-444-3414;fax:144-178-444-3386.E-mail address:m.cooke@rhbnc.ac.uk (M. Cooke)1Present address: Europa Fachhochschule Fresenius, LimburgerStrasse 2, D-65510 Idstein, Germany.a capillary gas chromatograph is a complex pro-cedure [1]. Traditional packed columns made theprocess simpler with the use of a gas loop of fixedvolume in line with the carrier gas to deliver aknown volume of gaseous sample into the carrier gasflow immediately ahead of the column [2]. Providingthe gas volume was small in comparison with thecarrier gas flow then chromatographic performancewas not seriously degraded. However, the ability topre-concentrate samples is limited by this method.When a capillary column is used then one of severalprocedures has been adopted. Firstly, the gas sample0021-9673 / 00 / $ – see front matter©2000 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 00 )00109-6458S.Niedziella et al./J.Chromatogr.A885 (2000) 457–464(typically of the order of 1 ml) is introduced into theinjector in the split mode. In this case the majority ofthe sample exits via the split valve and sensitivity isthus restricted by the volume of sample introducedonto the column [3]. In an effort to reduce this lossmicro-valve systems introducing small (ca. 20ml)ofsample directly into the capillary column have beendeveloped [4], but they are of limited value becauseof the small initial volumes used.To overcome this problem techniques such aspurge and trap and cryofocussing have been de-veloped to concentrate gaseous compounds at thehead of the capillary column or in the injection port[5–7]. Whilst the desired increase in sensitivity isachieved these techniques are complex and are notwell suited to certain types of sample. Specificallywhen the gas sample to be analysed is contained inthe headspace above an aqueous sample then themajor component of the headspace (apart fromoxygen and / or nitrogen) is water vapour. Trappingor cryofocussing water either harms the column anddisrupts detection or necessitates the use of a de-siccant in the injection port area [7] which furthercomplicates the analysis procedure.Our research group interest in the biomobilisationof elements [8,9] is centred on the study of organiccompounds present in the headspace above mi-crobiological cultures but extends to the study oflandfill gas [10] where biomobilisation of a range ofelements is suspected. Such samples are usuallysaturated with water vapour. Some compounds wewish to study are only produced under anaerobicconditions [8,9], are thermally labile and / or arereactive with oxygen. Minimal sample manipulationis thus desirable.Solid-phase microextraction (SPME) is a simpletechnique which has been developed to extract lowconcentrations of organic compounds such as pes-ticides from water [11–13], and has been extensivelyused for food aroma analysis [14–18]. Examples ofmedical applications are few but examples of its useinclude urine headspace sampling and blood anddrugs analysis [19–21]. The use of a hydrophobicpolymer as the extracting phase means that thesample introduced is effectively water free. A sam-pling method which pre-concentrates organic com-pounds in situ in the sample but which rejects themajor component (water), and also does not intro-duce either oxygen or nitrogen into the chromato-graph, is well suited to headspace sampling whenwater vapour is present at high concentrations.The aims of this study were (a) to evaluate theSPME system for the pre-concentration of organosul-phur and similar compounds from the headspacegases generated by biocultures without interferencefrom air or water, (b) to attempt quantitation ofspecific organosulphur compounds, (c) to trap phos-phine present in the headspace, and (d) to evaluatethe ability of the fibre to accurately reflect thecomposition of a complex mixture of volatile organiccompounds.2. ExperimentalChromatographic analysis of bioculture headspacesamples was performed on a Finnegan ITS-40 gaschromatograph–mass spectrometer fitted with anOptic 1 injection system (ATAS, Cambridge, UK)and equipped with a DB 1701 capillary column (30m30.32 mm I.D.,df1.0mm),carrier gas; helium at1.0 ml / min. Gas injections were made with a 1 mlgas-tight syringe (SGE, Milton Keynes, UK).Standard gas samples were contained in suitableglass containers, (unsilanized), fitted with smallsurface area septa (teflon coated) to minimise loss ofvolatile components to the septum or contaminationof the sample by the septum. The injector pro-gramme was: trapping temperature 308C; desorptiontemperature 2408C; rate of increase 168 / s; splitlesstime, 1.0 min. The SPME system (Supelco, Belle-fonte, PA, USA) was fitted with 30mmpolydi-methylsiloxane fibres.Solvent samples were chromatographed on aHewlett-Packard 5890 Series II gas chromatographequipped with electronic pressure control, split / split-less injector, flame ionisation detector and a capillarycolumn (Rtx-1, Thames Restek, Maidenhead, UK)30 m30.32 mm I.D.,df, 0.5mm),nitrogen carrier at1.0 ml / min. Dimethyldisulphide and di-isopropylsulphide and other volatile organic compounds weresupplied by Aldrich (Gillingham, UK).3. Results and discussionOur interests in the biomobilisation of elementsS.Niedziella et al./J.Chromatogr.A885 (2000) 457–464459such as sulphur, arsenic, phosphorus and antimonyhas led us to develop analytical techniques suitablefor the introduction and separation of volatile com-pounds containing these heteroatoms. For simplicitywe have developed a method which uses a 1 ml gassample introduced directly into the injection port ofthe GC–MS system with the split shut. At 1 ml / minthe transfer time onto the column is of the order of 1min and most of the volatile components are trappedat the front of the column which is held at 308C. Anyintroduced oxygen and nitrogen together with morevolatile components such as phosphine and di-methylsulphide are not retained. Information on thepresence of these compounds is thus lost as theyelute in the delay time before the detector is switchedon. By using a relatively thick film column (1.0mm)and a relatively polar phase (DB 1701, 50% phenyl /50% methyl) we have been able to obtain retentionof dimethyldisulphide and later eluting species. Thisapproach has been used to study a variety ofsamples, usually anaerobic biodegradation reactions,where a range of volatile compounds includingorganosulphides, short chain organic acids (C3–C7)and alcohols are present. However, because of theintroduction of both water vapour and air com-ponents chromatographic quality in the early part ofthe chromatogram is poor and column attrition isfairly rapid. Nonetheless a range of compounds canbe detected including organosulphur compounds,organo-oxygen compounds and saturated and unsatu-rated hydrocarbons.Although solid-phase micro-fibre extraction hasbeen extensively used for headspace sampling offlavours and aromas in food science its applicationhas been more limited as a headspace samplingmethod for environmental studies. Polydi-methylsiloxane fibres are non-polar and are thushydrophobic because of the nature of the polymerand thus offer the potential of not absorbing waterfrom high humidity headspace samples such as existsover a bioculture. To evaluate the fibres for samplingwater saturated headspace above a culture variouscultures were set up containing either cooked meatmedium (CMM), Schaedler Anaerobe Broth (SAB)or Tryptone Soya Broth (TSB). Inorganic phosphatewas added to each in an attempt to stimulatephosphine generation, and an anaerobic mud wasused as inoculant. These were incubated for severaldays and the headspace gases sampled both directlyas a 1 ml injection (splitless) and via the fibre with a60 min exposure time (splitless).After sampling for 60 min the fibre was thermallydesorbed in the injection port of the GC–MS systemusing an Optic 1 injection system equipped with anopen liner. Typically for a 1 ml gas injection onlythree major components were observed (di-methyldisulphide, dimethyltrisulphide and, tentative-ly, methoxyethanol:tR9.21 min) but for the fibreinjection a complex series of components was ob-served with several major components being iden-tified (Table 1). Although peak shape for the car-boxylic acids was satisfactory on this relatively polarcolumn the possibility of derivatisation was brieflyinvestigated. In situ derivatisation of formaldehydein the fibre has been reported previously using adinitrophenylhydrazine impregnated fibre [22].Exposure of the fibre impregnated with organic acidsto hexamethyldisilazane in the vapour phase for 30min followed by thermal desorption and study byGC–MS indicated that trimethylsilyl esters had beenproduced. Full details of this methodology will bereported elsewhere.Comparison of the peak intensities of the twosulphur-containing compounds common to bothchromatograms suggests that the fibre yielded ap-proximately a 100-fold increase in response over the1 ml injection. Thus, the fibre produced a responseequal to approximately 100 ml of headspace. In viewof the range of polymers available in fibre form andthe range of polarities thus available it should bepossible to optimise the polymer character toproduce a maximum efficiency concentration step fora particular compound. Thus, the polydimethylsilox-Table 1Identification of the typical major components of a cultureheadspace sampled by SPMERetentiontime (min)9.2111.1315.1217.1118.2520.1620.6721.0924.0626.01CompoundUnknownDimethyldisulphidel-Threonine1-MethoxyethanolButylpropanoate3-Methylbutylpropanoate2-Methylhexanoic acidDimethyltrisulphideAn aminobutylcarboxylic acidUndecanal or dodecanal460S.Niedziella et al./J.Chromatogr.A885 (2000) 457–464ane polymer used has proved to be capable oftrapping a wide range of compounds includingorganosulphur compounds and there are indicationsthat it is also suitable for use with medium chainfatty acids.Profiling of saturated headspaces over biocultureswas thus shown to be simple and free from theintroduction of oxygen and nitrogen into the GC–MSsystem. Water vapour was not trapped by the fibreand so did not interfere with the mass spectrometrynor damage the column. Attempts to trap phosphineon the fibre proved unsuccessful and pre-concen-tration of phosphine prior to analysis remains anobjective.Quantitation of the organosulphur compounds wasattempted through generation of a samples contain-ing known concentrations of dimethyldisulphide andthen exposing the fibre to them for different periodsof time from 1 to 60 min.Standard mixtures of dimethyldisulphide (DMDS)in nitrogen were made by filling a 1 l conical flask,fitted with a small rubber seal, with nitrogen andthen introducing a known volume of DMDS using amicrolitre syringe. From a knowledge of the densityof DMDS, the concentration inmg/ ml was calcu-lated. By serial dilution, using a gas-tight syringe [9],more dilute standards were made giving a range ofstandards from 1900 to 2.7mg/ ml. Standards wereallowed to equilibrate at room temperature overnightand the glass vessels used were not deactivated.Hence, it was assumed that some of the DMDSwould adsorb to the glass and that an equilibriumconcentration would be established in the gas phase.Thus, all concentrations quoted are nominal and arenot corrected for possible losses through adsorption.A total of six standards was thus made (Table 2).From Table 2 the following conclusions can bedrawn. Firstly the amount of DMDS delivered by thefibre is independent of the time of exposure. This isreflected in both the RSDs with respect to timesampled, which are essentially constant at approxi-mately 10%, and the constancy of the regressioncoefficientsRfor each plot of area vs. concentrationat various times. These results differ somewhat fromthose of Ai [23] who found a concentration–expo-sure time relationship. Two factors may explain thedissimilarity. Firstly Ai used 1-octanol with a non-polar fibre (100% dimethylsiloxane) and it is pos-sible that the larger molecule with a polar end groupis slower to reach equilibrium in the non-polar fibre.Secondly we used a 30mmpolymer thicknesswhereas Ai used 100mmgiving some 15 times morevolume of polymer (assuming constant fibre lengthand silica core diameter). Thus, our reduced phasevolume (and thus reduced sample capacity) willfacilitate rapid saturation of the phase. Hence aheadspace concentration vs. fibre concentration rela-tionship which is independent of exposure time isobserved at these relatively high concentrations.Clearly in our experiments an equilibrium concen-tration in the fibre was rapidly attained. An inciden-tal observation was that, over a sequence of measure-ments, particularly at higher concentrations thecapacity of the fibre decreased slightly. After aTable 2Peak area measurement against variation of time and concentration for dimethyldisulphideConcentration of DMDS (mg / ml)Order of measuringPeak area fort(min)t51t52t55t510t530t560TotalAverage area counts,n56Standard deviationRelative standard deviation (%)a19001118 500127 400117 600110 000116 90093 400683 800113 40010 5009.25900624 80025 30026 70026 40032 90032 900169 00028 100285010.1270222 70021 30019 50019 10017 50017 500117 60019 60017308.98273a225a2.7421528016025029023014252384318.6aRa44504360352041503410341023 300388041010.610701060870970120012006370106013012.30.9580.9570.9660.9680.9730.990By serial dilution.R5correlationcoefficient for timet.S.Niedziella et al./J.Chromatogr.A885 (2000) 457–464461period of non-use capacity recovered. It wouldappear that repeated heating of the fibre during aworking day causes some temporary loss of capacitywhich is reversible. A longer term, progressive lossof capacity over a duration of this study was alsoobserved (see below).The rapid attainment of equilibrium mass ofdimethyldisulphide in the polymer highlighted thefact that the amount of polymer available is fixed andso its capacity to absorb / desorb analytes must alsobe limited. By implication therefore, in a two (ormore) component mixture each substance must com-pete with the other component(s) for space in thefibre. To test this hypothesis three standard gasmixtures were prepared in nitrogen. These were 1850mg/ ml of DMDS, 868mg/ ml of di-iso-propylsulphide (DIPS) and a mixed standard of 1850mg/ ml of DMDS and 868mg/ ml of DIPS. Eachstandard was sampled for 1, 5, 10, 30 and 60 minand the areas of the resultant peaks recorded. Plots ofarea against time for all three standards were con-structed (Fig. 1). For DMDS alone line A wasplotted, for DIPS alone line B resulted. For themixed standard DMDS produced line C and DIPSFig. 1. Response vs. time plots for (A) dimethyl disulphide(DMDS) only at 1850mg/ ml, (B) di-isopropylsulphide (DIPS)only at 868mg/ ml, (C) for DMDS at 1850mg/ ml when bothcompounds are present and (D) for DIPS at 868mg/ ml when bothcomponents are present in the same headspace.gave line D. Clearly there is a competitive effect forthe fibre when the two components are present. Thisresults in a reduction in the equilibrium amounts ofeach in the fibre even though the concentration in thegas phase remains the same. Thus, for a singlecomponent mixture the total mass in the fibre will beproportional to the concentration in the headspace.However, for a multi-component mixture the totalmass of analytes in the fibre is made up of theequilibrium concentrations of the individual com-ponents moderated by the competitive effects of theother analytes. In other words the fibre is no longerpolydimethylsiloxane (PDMS) but PDMS modifiedby absorbed analytes which change its properties. Wesuggest a further complication in that the relativeaffinity of each analyte for the polymer will alsoinfluence the equilibrium mass achieved as occursbetween analytes and stationary phase in a gaschromatography capillary column with the differencebeing that in a capillary column they are presentsequentially in the phase whereas in the fibre theyare present simultaneously. Clearly this must makequantitation difficult because the mass of analytewhich partitions into the fibre will depend not onlyon the partial pressure of that analyte in the samplematrix but also the chemical character of the othercomponents and their relative concentrations. In anymulti-component sample, therefore, there must exista degree of mutual interference. Thus, standardaddition would appear to be the most promisingquantitative procedure but even then the addition of aknown amount of target compound to a fixed volumegas sample will necessarily change the relativeconcentrations of the other components present.Hence, the relationship between mass of analyte inthe fibre and the concentration of the analyte in thesample matrix is complex. Goreki et al. [24] hasrecently reported a similar conclusion for the ex-traction of a two component mixture from water.To test the influence of analyte structure andpolarity on relative fibre concentration a mixture ofeight solvents (ethanol, propan-1-ol, methyl isobutylketone (MIBK), butan-1-ol, cyclohexane, toluene,nonane anda-pinene)was prepared in a 2 l flask byaddition of 10mlof each to the flask and thenallowing the mixture to equilibrate overnight. Theconcentration range achieved was thus of the orderof 5 ng / l. Aliquots of the mixed standard were then [ Pobierz całość w formacie PDF ]
