DSpace at VNU: Analysis of fuel oxygenates in the environment

View Article Online / Journal Homepage / Table of Contents for this issue Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 a Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), Ueberlandstr 133, P.O Box 611, CH-8600, Duebendorf, Switzerland E-mail: torsten.schmidt@eawag.ch; Fax: +41 823 5210; Tel: +41 823 5076 b Center for Environmental Chemistry (CEC), Vietnam National University, 334 Nguyen Trai Street, Hanoi, Vietnam CRITICAL REVIEW THE Torsten C Schmidt,*a Hong-Anh Duong,b Michael Berga and Stefan B Haderleina ANALYST Analysis of fuel oxygenates in the environment www.rsc.org/analyst Received 19th October 2000, Accepted 23rd January 2001 First published as an Advance Article on the web 20th February 2001 Summary of Contents Introduction 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 3.3 3.4 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5 Fuel oxygenates are oxygen-containing substances, mainly dialkyl ethers and alcohols, used as blending components in order to increase the octane number of gasoline.1 The demand for fuel oxygenates has increased rapidly due to the phase-out of tetraalkyl lead compounds as octane enhancers and the regulation of gasoline composition during the 1990s in order to improve air quality In the USA, the 1990 Amendments to the Clean Air Act require a minimum oxygen content of 2.7% (w/ w) for oxyfuels and 2.0% (w/w) for reformulated gasoline in CO and ozone non-attainment areas, respectively In the European Union, there is no minimum requirement but, depending on the type of fuel oxygenate, the addition of up to 15% (v/v) is allowed Table lists the common fuel oxygenates, their abbreviations used throughout the text and the physicochemical properties relevant for their analysis or environmental behaviour Because the variability of the available data often is quite large, we critically evaluated the data and reported the most plausible values For instance, the Henry’s law constants of methyl tertbutyl ether (MTBE) range between 5.3 1024 and 3 1023 atm m3 mol21.2,3 The chemical structures of the fuel oxygenates are shown in Fig All fuel oxygenates in Table are Organisation for Economic Cooperation and Development (OECD) High Production Volume Chemicals.4 The most important fuel oxygenates today are MTBE and ethanol The production amounts of MTBE and fuel ethanol in the USA in 1999 were 9.3 and 4.4 million tons, respectively.5 tert-Butyl alcohol (TBA) is also of importance because it is the major degradation product of MTBE in aqueous systems Other dialkyl ethers, such as ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and diisopropyl ether (DIPE), are currently considered as substitutes for MTBE However, apart from TAME in Finland and ETBE in France, these substances are not yet used in large amounts because they cannot compete economically with MTBE Alcohols other than ethanol are at the moment not considered as fuel oxygenates on a large scale Methanol-based fuels [e.g M85 with 85% (v/v) methanol and 15% (v/v) conventional gasoline6] are still in use but their market share is very limited The physicochemical properties in Table imply that fuel oxygenates released into the environment will predominantly reside in air and water compartments rather than in soil and biota Therefore, the focus of this review is the analysis of fuel oxygenates in air and water phases Compared with classical fuel-related contaminants, such as benzene and other aromatics, alcohols and ethers have higher water solubilities, lower Henry’s law constants and lower sorption constants These Introduction Occurrence in water and air Analytical methods Sampling and enrichment: water Water sampling Direct aqueous injection Headspace analysis Purge and trap enrichment Solid phase microextraction Sampling and enrichment: air Separation Preparation of standards and calibration Detection Flame ionisation detection Photoionisation detection Mass spectrometry Atomic emission detection Fourier transform infrared spectroscopy Conclusions and outlook Acknowledgements Appendix: Abbreviations References Torsten C Schmidt obtained his PhD in analytical chemistry at Philipps-University Marburg, Germany, in 1997 The subjects of his dissertation were the development of new analytical methods for the determination of aromatic amines in water and site investigations at former ammunition plants After a postdoctoral stay with the mass spectrometry unit at the same university, he has been working as a Research Associate at the Swiss Federal Institute for Environmental Science and Technology (EAWAG) His current research interests are environmental forensics, the environmental assessment of polar fuel constituents such as MTBE and the study of the long-term behaviour of persistent organic compounds in the subsurface, in particular, with single compound isotope ratio mass spectrometry (see also the URL http://www.eawag.ch/ ~ schmidto) DOI: 10.1039/b008442p Analyst, 2001, 126, 405–413 This journal is © The Royal Society of Chemistry 2001 405 Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online properties make them difficult to enrich from aqueous samples, rendering their analysis in water at trace levels (µg L21 range) particularly demanding Because the monitoring of fuel oxygenates in surface and groundwater will be frequently required in the future, methods for water analysis are emphasised Although the literature concentrates mainly on MTBE, most analytical methods are also applicable to other dialkyl ethers Methods for the trace analysis of alcohols in water are generally scarce, and only a few might be suitable for the simultaneous analysis of ethers and alcohols With this review, we intend: (i) to provide an overview of the analytical methods available today; (ii) to evaluate critically the advantages and disadvantages of the different methods; and (iii) to point out needs for future developments in the environmental analysis of fuel oxygenates Occurrence in water and air MTBE is by far the most important fuel oxygenate and its use has been a matter of controversy for the last few years Numerous studies on environmental behaviour,2,7–9 toxicity3,7,10 and human exposure11,12 have therefore been carried out for MTBE A comprehensive overview on the environmental impact of MTBE was prepared at the University of California at Davis in 1998.7 Other important reports on fuel oxygenates in water have been published by US agencies.13,14 However, these studies almost exclusively deal with MTBE, for which a regularly updated information resource (498 entries by December 2000) on water quality issues is maintained by the US Geological Survey (USGS).15 Apart from MTBE, there is only little information on the environmental occurrence of fuel oxygenates Because the concentrations of fuel oxygenates differ by orders of magnitude from environmental background to sites affected by point sources, different analytical strategies are required Leaking underground storage tanks are frequent point sources of MTBE into groundwater, which have led to numerous research and remediation efforts, particularly in the USA.7,13,14 Possible sources of non-point or diffuse input of fuel oxygenates include precipitation, stormwater, runoff water and small watercraft.16–18 Recent studies on the occurrence of volatile organic compounds (VOCs) in groundwater have shown that MTBE is one of the most frequently detected substances today.19,20 In samples only affected by non-point sources, concentrations of MTBE in groundwater and surface water are in the low µg L21 range.18–20 In the vicinity of point sources, high mg L21 levels may be reached in ground- water.21–25 Schirmer and Barker26 and Landmeyer et al.22 reported that, after a fuel release, the highest concentration of MTBE (and benzene) was not found near the water table but at deeper sampling points The authors attributed this to recharge on top of the aquifer by rainwater Their results imply that samples should be taken at several depths when investigating contaminated sites Extensive information on gasoline spill sites as point sources is available from the American Petroleum Institute.27 In groundwater, MTBE is more recalcitrant to degradation than other fuel constituents of environmental concern, in particular under anaerobic conditions Although laboratory studies have shown that microorganisms can degrade MTBE to TBA under various conditions,9,23–30 the transformation rates are low and thus difficult to measure in the field The long-term behaviour of MTBE in groundwater therefore remains one of the most important areas for further research Compared with groundwater data, there are rather few studies on fuel oxygenates in air Background concentrations of MTBE range from 0.15 to ppbv,31,32 and for ethanol and methanol up to two times higher.33 In urban areas, MTBE concentrations can be substantially higher, ranging from 0.5 to ppbv.31,34,35 For methanol and ethanol, concentrations in urban air of 17 and 4–12 ppbv, respectively, have been reported.32,34 ETBE, TAME, DIPE and TBA were not detected in urban air in the only study available.31 Close to emission sources (i.e gasoline service stations), Vainiotalo et al.36 found MTBE concentrations ranging from 69 to 370 ppbv at the centre of the pump island and 0.14 to 34 ppbv at 50 m distance MTBE partitions readily into atmospheric water, which may lead to the diffuse input of MTBE with rain into soil and Fig Chemical structures of fuel oxygenates (except methanol and ethanol) Table Environmentally relevant physicochemical parameters of fuel oxygenatesa Etherb Molecular weight/g mol21 Boiling point, Tb /°C Density/kg L21 Vapour pressure, p0/mbar 21 Water solubility, csat w /g L Henry’s law constant, KH/atm m3 mol21 log KOW log KOC Max IR absorption frequency/cm21 Alcoholb Methyl tertbutyl ether (MTBE, 1634-04-4) Ethyl tertbutyl ether (ETBE, 637-92-3) tert-Amyl methyl ether (TAME, 994-05-8) Diisopropyl ether Methanol (DIPE, (MeOH, 108-20-3) 67-56-1) Ethanol (EtOH, 64-17-5) Isopropyl alcohol (IPA, 67-63-0) Isobutyl alcohol (IBA, 78-83-1) tert-Butyl alcohol (TBA, 75-65-0) 88.15 55.2 0.744 332 48 5.9 1024 102.18 72.2 0.73 203 12 2.7 1023 102.18 86.3 0.77 91 12 1.3 1023 102.18 32.04 68.2 64.6 0.73 (20 °C) 0.796 200 168 Complete 4.77 1023 4.6 1026 46.07 78.3 0.794 79 Complete 5.2 1026 60.1 82.2 0.789 61 Complete 7.9 1026 74.12 107.9 0.802 14 68.2 1.2 1025 74.12 82.4 0.791 56 Complete 1.4 1025 1.24 1.74 1.55 1.52 20.77 20.31 0.05 0.76 0.35 1.05 0.95 1.27 1.13 0.44 0.20 1.4 0.95 1.57 1205–1213 1199–1207 1185–1193 1122–1130 1055–1063 1052–1060 1141–1149 1037–1045 1207–1215 a Values are at 25 °C unless otherwise stated Data sources (references given therein): Houben,1 Zogorski et al.,13 US EPA,14 Diehl et al.,79 Sablji´ c et al.81 Environmental Fate Database by Syracuse Research Corporation (http://esc_plaza.syrres.com/efdb.htm) b Abbreviation and CAS no given in parentheses 406 Analyst, 2001, 126, 405–413 View Article Online groundwater The partitioning is strongly temperature dependent (see Fig 2) Pankow et al.17 have shown that, for shallow aquifers, only a few years might be required for groundwater to equilibrate with atmospheric MTBE Experimental data on rainwater concentrations of MTBE are scarce Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 Analytical methods 3.1 Sampling and enrichment: water Several enrichment and injection techniques are described in the literature, including purge and trap (P&T), headspace analysis (HS), direct aqueous injection (DAI) and solid phase microextraction (SPME) An overview of the enrichment and injection techniques described below is given in Table A compilation of reported method detection limits (MDLs) achieved with the discussed methods is given in Table All of the enrichment techniques are exclusively combined with gas chromatography (GC) as described in Section 3.3 3.1.1 Water sampling When sampling water for subsequent analysis of the fuel oxygenates, the same precautions as for sampling other volatile compounds have to be made.37 This includes the gentle filling of sample bottles until overflow to prevent volatilisation during sampling and storage The choice of sample bottles depends on the enrichment technique used Field blanks with analyte-free water passed through the whole sampling procedure are recommended For MTBE analysis, aqueous samples not have to be preserved as biodegradation is very slow Cool storage and analysis within a week is generally recommended for all fuel oxygenates Koester et al.38 recovered 80% of MTBE after 28 Fig Temperature dependence of MTBE partitioning between air and rainwater days of storage at °C Special precautions are necessary in highly reactive media, such as bacterial cultures, and for the analysis of the atmospheric oxidation product of MTBE, tertbutyl formate (TBF), which rapidly hydrolyses under acidic or alkaline conditions.39 For TBF, the use of acid as preservative, common in VOC analysis, must therefore be avoided Some standard preservation methods may also interfere with analytical techniques, for instance with DAI, where acids (e.g formate or hydrochloric acid) and/or involatiles (e.g mercury chloride) in rather high concentrations may damage the separation column 3.1.2 Direct aqueous injection DAI allows an aqueous sample aliquot to be injected directly onto the chromatographic column of a GC This technique is nowadays a common approach in water analysis with GC-flame ionisation detection (GC-FID) and GC-electron capture detection (GC-ECD) A prerequisite in all DAI applications is the use of a wide-bore precolumn that has to be shortened and exchanged regularly The combination of DAI with GC-mass spectrometry (GCMS) is more difficult because of the large amount of water vapour generated upon aqueous injection: due to the small molar volume of water, evaporation leads to five to eight times higher vapour volumes than for typical organic solvents Very efficient pumps are therefore required to maintain a stable vacuum in the ion source Otherwise, a breakdown of the vacuum causes a shutdown of the instrument Advantages of the DAI approach are the simplicity and speed of analysis as essentially no sample handling is necessary apart from the dilution of highly polluted samples Duplicate or triplicate analyses of the same sample can be easily carried out, and very low sample volumes are required The lack of sample preparation minimises the possibility of analyte losses The high polarity of all fuel oxygenates makes their enrichment from the aqueous phase or partitioning to the gas phase difficult Thus, these compounds are ideal candidates for DAI, which may allow the simultaneous analysis of alcohols and ethers in water Despite these advantages, there are only a few reports on the use of DAI for fuel oxygenate analysis Reasons for this include the above-mentioned necessity for appropriate GC-MS systems and the fact that chemical bonding of stationary phases has only recently been improved to an extent that tolerates multiple water injections without phase deterioration Furthermore, sensitivity may not be sufficient for ultratrace (ng L21 range) analysis and precolumns need to be replaced frequently due to the accumulation of non-volatiles Potter40 described the use of DAI-GC-FID for the analysis of water-soluble fractions from fuels, including a series of alcohols from methanol to hexanol and MTBE He used hot on-column Table Comparison of injection and enrichment techniques for water analysis Technique Sample volume/mL Sensitivity: alcohol/ ethera Selectivity criteria P&T 5–40 2/++ HS 5–20 22/2 DAI 0.001–0.1 +/+ HS-SPME 5–20 0/+ SPME 1.5–5 +/++ Purgeable compounds High Henry’s constant Elution earlier than water (polar column) Sorption to polymer Matrix effects Contamination of apparatus at high concentrations of a volatile compound critical Applicable to all kinds of water samples; addition of high concentrations of salts (salting out) compensates for different matrices 60b ++/0 Involatiles or aggressive media may lead to contamination, use of guard column essential Medium to high Henry’s constant, sorption to polymer Applicable to all kinds of water samples; addition of high concentrations of salts (salting out) compensates for different matrices 60b +/2 Time required/min 30b Cost:a automated/non- 2/22 ++/0 automated a Very good/inexpensive (++), good/inexpensive (+), fair (0), poor/expensive (2), very poor/expensive (22) sample can usually be processed during the current chromatographic run b Addition of high concentrations of salts (salting out) compensates for different matrices; lifetime of fibre limited 60b +/2 With the use of an autosampler, the next Analyst, 2001, 126, 405–413 407 Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online injection of 1–5 µL at 165 °C and found a much better performance than with split or splitless injection Using a DB624 (J&W Scientific) as a stationary phase, water is eluted before the analytes MDLs were only 5–100 µg L21, but were sufficient for the analysis of aqueous extracts of fuels MDLs as low as 0.1 µg L21 were reported by Church et al.41 using a mass spectrometer with a large vacuum capacity (Finnigan 4000) This study was the first to include the major dialkyl ethers, alcohols and three carbonyl compounds found as atmospheric oxidation products in one analytical method A polar polyethylene glycol column was used for the separation of the analytes This column retains water more strongly than the analytes, which opens up a retention window of several minutes between injection and the water vapour flush An important feature of this method was the protrusion of the glass wool packed liner from the hot splitless injector (130 °C) into the cold oven (30 °C) This approach was used for soil column studies,41 as well as for field investigations.22,26 Hong et al.42 have recently shown that a benchtop GC-MS (GC HP 5890, MS HP 5971A) can also be used for the DAI technique, and they extended the range of analytes to small organic acids proposed as oxidation products of MTBE in the atmosphere In order to avoid contamination of the column by the high salt content of the samples, a reverse-cup liner filled with Carbofrit (Restek) was used in a split/splitless injector and replaced every 30 injections A nitroterephthalic acid-modified polyethylene glycol (FFAP) stationary phase was found to be superior to a 6% cyanopropylphenyl type column (DB-624 equivalent) The system was not optimised for sensitivity; the MDLs ranged from 30 to 100 µg L21 Our laboratory also developed a DAI method using a benchtop GC-MS (Fisons GC 8000, MD800, 250 L pumps) for the investigation of groundwater at fuel spill sites.43 Using a polar polyethylene glycol column [Stabilwax, 60 m 0.25 mm, µm film (Restek)], the separation of fuel oxygenates, their major degradation products and BTEX (benzene, toluene, ethylbenzene, xylenes) was achieved in a single chromatographic run The 60 m column results in a better separation of MTBE and ETBE than reported by Church et al.41 and retains water sufficiently to prevent a breakdown of the vacuum in the ion source 3.1.3 Headspace analysis In this section, direct sampling of the headspace is discussed, whereas sampling of the headspace Table Method detection limits for trace analysis of fuel oxygenates in water (mg L21)a Reference Method Detection MTBE ETBE TBA 45 HS FID 50 44 HS FID 4.9 55 HS-SPME FID 56 HS-SPME FID 0.27 0.44 54 HS-SPME FID 0.7 57 HS-SPME MS 0.69 58 SPME MS 0.01 SPME MS 0.008 0.025 1.8 59b DAI MS 0.1 0.1 0.1 41c 23, 25, 45 P&T FID 1–2 47 P&T MS 0.09 50d P&T MS 0.06 0.02 18 P&T MS 0.1 52 P&Tmod MS 0.05 0.1 0.01 0.06 51 P&T HRMSe 54 CLSA FID 0.02 a Different approaches were used to calculate MDLs, which also depend on the equipment actually being employed Therefore these values have to be validated in each laboratory b This method is also suitable for TAME and ethanol with MDLs of 0.038 and 15 µg L21, respectively c This method is also suitable for TAME, tert-amyl alcohol and TBF with MDLs of 0.1, 0.1 and µg L21, respectively d MDL for TAME: 0.02 µg L21 e High resolution mass spectrometry 408 Analyst, 2001, 126, 405–413 with SPME is covered in Section 3.1.5 HS utilises the partitioning of compounds from water to air in a closed system As in P&T analysis, sufficiently high Henry’s law constants of the analytes are necessary HS is a rather robust technique which can easily be automated, requires little sample preparation and can be used with all kinds of water sample Elevated temperatures and addition of salt (‘salting out’) are used to enhance partitioning of the solutes into the gas phase HS is a static technique and not very sensitive, especially for the alcohols (compare KH values in Table 1) Thus, HS is well suited for the analysis of highly polluted samples which otherwise might cause matrix and carryover problems Robbins et al.44 used HS to determine Henry’s law constants for BTEX and MTBE; 200 mL of the headspace were withdrawn from the vials and injected; a DB-1 column (J&W Scientific) was used for separation and a flame ionisation detector for detection, although only single analyte samples were used Nouri et al.45 reported an HS method for MTBE in environmental samples using GC-FID Although the transfer of MTBE to the gas phase was enhanced by thermostatic control of the samples to 60 °C, a ten times higher detection limit than reported by Robbins et al.44 was obtained However, this method was specifically developed for screening aqueous samples at a contaminated site after experiencing problems with highly contaminated samples in a P&T system The authors reported less interferences in HS analysis of the original samples than in P&T analysis after several dilution steps Shaffer and Uchrin46 used HS-GC-FID with an HP-1 column (Hewlett Packard, dimensions not given) for the analysis of MTBE in soil adsorption studies, which were performed with 15 g of soil and 30 mL of water in 60 mL vials suitable for HS After addition of µL MTBE to the system, the concentration of MTBE was measured by consecutively analysing 50 µL gas phase over a 72 h period 3.1.4 Purge and trap enrichment P&T enrichment can be used for analytes having a sufficiently high Henry’s law constant (see Table 1) enabling an efficient stripping from the aqueous phase For the dialkyl ethers, P&T is very sensitive and can be used for analysis in the ng L21 to µg L21 range in water Cryofocusing of the analytes on top of the GC column is essential in order to obtain good chromatographic separation One of the disadvantages of P&T is its susceptibility to contamination from highly polluted samples, in particular when using an autosampler If contamination of the system occurs, it often takes a long time until acceptable baseline levels are achieved again Some laboratories therefore never use P&T for the analysis of unknown samples without previously checking the VOC content with a less sensitive method (e.g HS) Other points to consider in the choice of a suitable enrichment method are the complexity of P&T systems compared with other methods, and the low sensitivity for alcohols A standard analytical method for purgeable organic compounds by a P&T technique is US Environmental Protection Agency (EPA) method 524.2.47 When using MS detection, more than 60 VOCs can be analysed simultaneously with this method, but MTBE is the only fuel oxygenate included The EPA method is fully validated and often used in routine VOC monitoring of groundwater and drinking water In this method, a or 25 mL aliquot of water is introduced into the purge vessel The sample is purged with helium for 11 at a flow rate of 40 mL min21 Purged compounds are collected on a three-stage trap containing Tenax®, silica gel and charcoal The trap is then heated rapidly to 180 °C The desorbed compounds are cryofocused on the head of the GC capillary column at 210 °C The chromatographic separation is performed on a semi-polar capillary column (6% cyanopropylphenyl, 75 m 0.53 mm id, µm film thickness) A purging efficiency of 74% and an MDL of 0.09 µg L21 for MTBE in water samples were reported Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online Other EPA approved methods for VOCs, often adapted to fuel oxygenates, are method 8021B with photoionisation detection (PID)48 and method 8260B with MS detection.49 Raese et al.50 and Reuter et al.18 used a trap based on Carbopack B/Carboxen 1000 and 1001 sorbents (VOCARB 3000, Supelco) Desorption was carried out at 250 °C, cryofocusing at 220 °C The MDL of this method for MTBE was 0.06 µg L21, evaluated by the USGS The time dependence (1 day to year) of the method performance was investigated on three different GC-MS instruments using low concentration spike samples of MTBE (0.1–5 µg L21) in distilled water Typical results showed a recovery of 98% and a relative standard deviation of 8–13% Although traps containing a mixture of different sorbents appear to be more effective than those with Tenax® as sole sorbent, the use of Tenax® traps was described in the literature.45,51,52 Nouri et al.45 reported a recovery of 90% for MTBE at room temperature under optimised conditions (purge volume, 440 mL; purge flow rate, 40 mL min21; purge time, 11 min) and desorption at 200 °C for 2.5 followed by cryofocusing at 260 °C TBA analysis using P&T has been reported for blood and urine analysis,51,52 but not yet for environmental samples Lee and Weisel52 described a method for the determination of MTBE and TBA in a urine matrix with a non-commercial P&T system At a purging temperature of 90 °C, recoveries of 97.0% (TBA) and 97.6% (MTBE) were achieved using a high flow rate of 140 mL min21 for 15 A condensation trap was placed in an ice bath between the purge and the adsorbent tube in order to remove the large amount of water vapour in the hot purge gas Bonin et al.51 reported improved recovery, reproducibility and sensitivity of MTBE and TBA determination in blood and urine matrices by using an isotope dilution method with [2H12]methyl tert-butyl ether and [2H9]tert-butyl alcohol and a double focusing magnetic sector mass spectrometer The recoveries of 87–100% for MTBE and 94–107% for TBA were obtained with conditions of the P&T system similar to the EPA method 524.2 Due to the use of an internal standard (4-[2H3]2-butanone), the MDLs in blood matrix were reduced from 0.05 and 0.25 µg L21 to 0.01 and 0.06 µg L21 for MTBE and TBA, respectively The costs of instrumentation and deuterated standards prevent the adoption of this interesting method for routine analysis In closed loop stripping analysis (CLSA),53 a method related to P&T, a closed cycle system is used during the purge interval Activated carbon filters (20 µg) are used to trap the analytes, which are subsequently desorbed manually with an appropriate solvent For MTBE, very low MDLs (20 ng L21) were achieved even with a flame ionisation detector,54 thus providing the lowest MDLs of all reported enrichment techniques However, CLSA has several drawbacks: it is very time consuming, difficult to automatise and the organic solvent used for filter extraction often is carbon disulfide, which is highly toxic 3.1.5 Solid phase microextraction SPME has become increasingly popular for the analysis of volatile and semivolatile compounds, whereas attempts to extract polar, nonvolatile compounds from aqueous environments have only recently been reported.55 The advantages of SPME are its simplicity, low costs, ease of automation and the commercial availability of a wide range of fibre coatings with different properties SPME is either performed directly in aqueous solution or in the headspace above the sample In the direct mode, partitioning of the analytes between the fibre coating and water determines the extraction efficiency In the headspace mode, two-phase transition processes (partitioning between water and air and between air and the fibre coating) are involved The disadvantages of SPME are the often poor reproducibility due to sensitivity to matrix effects, which requires the use of appropriate internal standards for quantification, the slow phase transfer kinetics and the limited lifetime of the fibre, in particular in the direct mode In both modes, and regardless of the type of coating, salting out is used to enhance the partitioning from water either to the coating or to air Mostly used for that purpose is sodium chloride at concentrations of 25–35% (w/v) Adding salt improved the amount extracted with SPME for MTBE56 and alcohols55 by a factor of three Kadokami et al.57 studied the effect of different salts on extraction efficiencies for alcohols from water with a 85 mm polyacrylate phase (Supelco) They reported a 50 times higher peak area for TBA from a saturated potassium carbonate solution (approximately mol L21) in comparison with a saturated sodium chloride solution (approximately mol L21) without providing a rationale for this surprising finding The recovery and the MDL for TBA in water samples were excellent for their method (90–104% and 0.63 µg L21, respectively) The most common polymer coating in SPME is polydimethylsiloxane (PDMS) Sjöberg54 used a 100 mm PDMS coating (Supelco) for MTBE analysis MTBE was extracted in the headspace of a water sample for 10 at room temperature and subsequently desorbed in a splitless injector for at 190 °C The limit of quantification for MTBE was approximately 0.7 µg L21 with a relative standard deviation of 13.3% Carboxen/PDMS (Supelco) is a relatively new coating used for SPME of low molecular weight analytes, which has shown a ten times higher affinity for MTBE than PDMS56 and a higher efficiency than PDMS/divinyl benzene (PDMS/DVB) and Carbowax/DVB (both from Supelco).58 Achten and Püttmann58 used direct extraction for 60 and a Carboxen/PDMS fibre at °C With this set-up they achieved an MDL of 10 ng L21 for MTBE, which is even lower than with most P&T systems Cassada et al.59 used SPME with DVB/Carboxen/PDMS (Supelco) for the analysis of MTBE, ETBE, TAME, ethanol and TBA and achieved an MDL of ng L21 for MTBE For the other analytes, MDLs were 25 and 38 ng L21 for the ethers and 15 and 1.8 µg L21 for the alcohols, respectively (see also Table 3) Górecki et al.55 compared the extraction efficiency of several fibre coatings (polyacrylate, PDMS/DVB, Carbowax/DVB, all from Supelco, and Nafion custom-made fibres) for polar analytes, including TBA and isopropyl alcohol (IPA) Of the investigated coatings, the 65 µm PDMS/DVB provided the highest affinity for the adsorption of alcohols and ketones from water Estimated MDLs for TBA and IPA in water were and µg L21, respectively Combining an uncoated (deactivated) precolumn with a DB-WAX analytical column (J&W Scientific) improved the peak shape of the alcohols With the polyacrylate phase, insufficient enrichment was found Some of the analytes could not be detected even at 500 µg L21, which contradicts the results of Kadokami et al.57 When analysing mixtures of polar and non-polar analytes, competitive displacement of polar analytes by less polar compounds with higher affinities for the fibre but smaller diffusion constants occurs with time Short extraction times under vigorous stirring conditions or extraction of non-stirred samples under non-equilibrium conditions regarding phase transfer are recommended to cope with this problem.55 However, shortened extraction times also lower the sensitivity of the method 3.2 Sampling and enrichment: air Trace analysis for fuel oxygenates in air generally requires the preconcentration of analytes from large sample volumes (1–10 L air) Using solid sorbent traps,31,36,60,61 this can be done Analyst, 2001, 126, 405–413 409 Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online directly at the sampling site Otherwise, air samples are collected in stainless steel canisters32,34,62–64 or Tedlar® bags65 and concentrated in the laboratory, where cryotrapping34,62–64 or solid sorbents32,65 are used for analyte enrichment Solid sorbents (on-site or in the laboratory) can be extracted with a solvent or, more commonly, thermally desorbed at 220–360 °C Brymer et al.64 carried out a thorough study of sample collection and storage of VOCs in polished steel canisters They found stable concentrations of methanol, ethanol, IPA and MTBE after a storage time of 30 days In contrast, Kelly et al.32 reported stable concentrations days after sampling, but a significant loss after 12 days During air sampling, humidity is an important parameter and should be measured simultaneously A broader discussion of the different sampling methods can be found in Pankow et al.31 and references cited therein The US National Institute for Occupational Safety and Health (NIOSH) provides sampling guidelines for regulated substances at workplaces, which include the alcohols listed in Table and MTBE.60 The NIOSH methods are summarised in Table Although these guidelines provide important information, their focus is on occupational hygiene with threshold limit values in the mg m23 range, and therefore they may not necessarily be suited for trace analysis Ethanol and methanol are also included in the NIOSH screening method 2549 for VOCs, which uses a Carbopack/Carboxen (Supelco) multibed sorbent tube for enrichment, followed by thermodesorption and GC-MS Harper et al.61 described the use of passive sampling for MTBE, ETBE and TAME Pankow et al.31 used traps filled with Carbotrap B and Carboxen 1000 (Supelco) and concentrated analytes on-site from L air A rather new approach is the use of SPME fibres as passive samplers for air analysis A non-polar PDMS coating was used by Quigley et al.66 for the sampling of MTBE and other gasoline vapour constituents in the gas phase It remains unclear at the moment whether this method is also suited for trace analysis Nguyen et al.67 recently described a method for air analysis of alcohols that uses the sampling of 200 mL air in glass bottles and subsequent derivatisation with nitrogen dioxide After 30 min, the alkyl nitrites formed were analysed with GC-ECD With an injection volume of 500 µL of air, MDLs for methanol, ethanol and IPA were 0.9, 0.7 and 1.8 ppbv, respectively Due to the derivatisation step, this approach is limited to alcohols 3.3 Separation GC is the principal method employed to separate fuel oxygenates as other chromatographic techniques require precolumn derivatisation of the analytes for efficient separation A few exceptions are spectroscopic methods, in particular Fourier transform infrared (FTIR), which not necessitate any separation step (see below) For the separation with GC, a wide variety of columns may be used, ranging from non-polar PDMS to polar polyethylene glycol phases The appropriate choice depends on the enrichment and injection technique used as well as on the sample matrix For P&T analysis, a semi-polar megabore column (DB-624 like) with a thick film is often used The use of such a column is described in EPA method 524.2 (see above) and allows the separation of a wide variety of volatile compounds Non-polar columns (DB-1 and DB-5 like) are specified in most HS and SPME applications Polar columns (DB-WAX like, DB-FFAP like) are sometimes used with DAI to retain water strongly For air analysis, long non-polar columns (DB-1 like) with a thick film are used GC ovens are often held at subambient temperatures in order to improve the retention of volatiles Fuel oxygenates in environmental matrices may have to be analysed in the presence of other fuel-related compounds of similar volatility that tend to coelute A comprehensive list of retention indices on a non-polar DB-1 column (J&W Scientific) is available,65 from which possible interfering compounds can be identified These are n-butane and trans-but-2-ene (coeluting with methanol), 3-methylbut-1-ene (ethanol), and 2,3-dimethylbutane and 2-methylpentane (MTBE) J&W Scientific recently introduced a special column for MTBE analysis, called DB-MTBE, which is claimed to be less polar than a 100% PDMS, and was shown to resolve MTBE and 2-methylpentane.68 On a 75 m 0.53 mm DB-624 column (J&W Scientific) with a µm film as used in the P&T EPA method 524.2, MTBE coelutes with trans-1,2-dichloroethene.47 Lacy et al.69 suggested sequential purging and HS of aqueous samples to remove interfering alkanes with higher Henry’s law constants than the fuel oxygenates After six sequential purge steps, HS was carried out and the response for MTBE was backcalculated to the theoretical response without purge steps The results agreed well with P&T measurements In contrast, the headspace measurements without prior purging always overestimated MTBE concentrations However, the method is rather time consuming and not very sensitive Gaines et al.56 obtained baseline separation of MTBE from both 2,3-dimethylbutane and 2-methylpentane with thermally modulated two-dimensional chromatography using the following columns: m 0.1 mm, µm Quadrex 007-2 (DB-5 equivalent) and (as second column) m 0.1 mm, 0.14 µm Quadrex 007-1701 (DB-1701 equivalent) Another two-dimensional system without thermal modulation was used by Poore et al.35 for ambient air measurements with a DB-WAX precolumn and a DB-1 (both J&W Scientific) analytical column 3.4 Preparation of standards and calibration Because some fuel oxygenates are rather sensitive to volatilisation losses, stock solutions have to be prepared with special care Typically, a volumetric glass flask is 90% filled with the Table NIOSH methods for fuel oxygenates in air Parameter NIOSH method Sorbent material Methanol 2000 Solid sorbent tube: silica gel (50/100 mga) Flow rate/L h21 Sample volume/L 0.02–2 1–5 Ethanol, IPA, IBA, TBA 1400/1401 Solid sorbent tube: coconut shell charcoal 50/100 mg MTBE 1615 Solid sorbent tube: two charcoal tubes in series, front 400 mg, back 200 mg 0.1–0.2 2–96 0.01–0.05 EtOH: 0.1–1 IPA: 0.3–3 IBA: 2–10 TBA: 1–10 Range per sample/mg 0.02–6 165–3300 0.06–125 mL CS2, 30 Desorption mL water/propan-2-ol 95/5 mL 1% butan-2-ol in CS2b Analytical method GC-FID GC-FID GC-FID MDL per sample/µg 0.7 10 20 a At high humidity, 700 mg sorbent should be used b For desorption of IBA, 1% isopropanol in CS is suggested in NIOSH method 1401 410 Analyst, 2001, 126, 405–413 Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online solvent (e.g methanol or acetonitrile) A known amount of pure liquid compound is then added with a microsyringe below the surface of the solvent, the flask is filled up to the mark and immediately stoppered The mass concentration is calculated from the compound’s density Some researchers44,55,56 have used a gravimetric–volumetric method for the preparation of stock solutions A stoppered flask is weighed after solvent addition and 10 to allow drying of the wetted flask neck After addition of the analyte, the flask is stoppered and weighed again prior to being filled up to the mark The mass concentration is calculated from the mass difference However, according to our experience, the measurement of a small difference between two rather large masses may not be very accurate and this method is therefore not used in our laboratory Calibration standards are prepared by dilution of an aliquot of the stock solution in double distilled or Nanopure water In our experience, it is of the utmost importance to check the purity of the water used for the preparation of standards as MTBE contamination is frequently observed in Nanopure water Water exposed to laboratory air also becomes rapidly contaminated with MTBE.38 Stock solutions in organic solvents can be used up to month;69 diluted calibration solutions in water should be replaced daily.56 For the analysis of fuel oxygenates in air, calibration standards are often prepared in the solvent used to elute the trap (see above) Trapping and desorption efficiencies are determined separately However, calibration with gaseous standards is generally necessary The preparation of gaseous standard mixtures has been described for fuel oxygenates32 and for an internal standard mixture used in fuel oxygenate analysis.31 External calibration is common in both water and air analysis However, addition of an internal standard considerably improves the data reliability The physicochemical behaviour of the internal standard should closely resemble that of the analytes Thus the best choice is an isotopically labelled analyte if the detector can resolve the two isomers (usually possible only with a mass spectrometer) For methanol, ethanol, IPA, TBA and MTBE, deuterated and/or 13C-labelled isomers are commercially available, e.g from Fluka (Buchs, Switzerland), Cambridge Isotope Laboratories (Woburn, MA, USA) and Isotec (Miamisburg, OH, USA) The use of MTBE-d358 and MTBE-d1238 has been described in the literature If labelled compounds cannot be used, one should choose a physically and chemically similar ether (e.g tert-amyl ethyl ether) or alcohol (e.g tert-amyl alcohol), which is absent in the target samples, rather than internal standards common in VOC analysis (fluorobenzene31,47,50 or 1,2-dichlorobenzene-d431,50) In this case, complete chromatographic separation of the internal standard from all analytes must be achieved and often a combined internal/external external calibration is used accelerated in an electric field and the resulting ion current amplified PID allows a more selective detection of fuel oxygenates in the presence of alkanes which have a much weaker response.73 Although it might be a low price alternative to MS detection, the use of PID in the reviewed literature has rarely been described.64,69 3.5.3 Mass spectrometry MS is the universal detection principle in GC Correspondingly, it is also the predominant detector in fuel oxygenate analysis Its use is mandatory in P&T analysis according to EPA method 524.2 The selectivity of MS is very good, and thus interferences due to other fuel constituents are unlikely The on-line acquisition of mass spectra in combination with retention times allows the unequivocal identification of compounds In scan mode the sensitivity is comparable to FID in many cases If the target compounds are known, selected ion monitoring (SIM) can be used, which provides 100–1000 times lower MDLs than FID, thus rendering the mass spectrometer the most sensitive GC detector for fuel oxygenates (see also Table 3) Electron impact ionisation is the only ionisation mode described Due to the rather high energy transfer in this ionisation mode, fuel oxygenates, except for methanol, not yield molecular ions Instead, after a-cleavage, (M2CH3)+ or (M2C2H5)+ fragments are obtained as base peaks in the mass spectra In Table 5, the main fragment ions of the fuel oxygenates, which are commonly used for quantification and confirmation in SIM, are given Drawbacks of MS are its cost, both in acquisition and maintenance, and limitations in sample size due to the vacuum requirements (see discussion above for DAI-GC-MS) 3.5.4 Atomic emission detection The use of atomic emission detection (AED) has not yet been described for environmental analysis of fuel oxygenates However, there are two reports on gasoline analysis with GC-AED, which have both utilised atomic oxygen emission at 777.3 nm.74,75 At this wavelength, a selectivity of about 5000 : over carbon is obtained, but the low sensitivity does not allow trace analysis In general, trace oxygen measurements are complicated by the background signal from air oxygen diffusing into the detector 3.5.5 Fourier transform infrared spectroscopy FTIR spectroscopy has frequently been used in laboratory studies for monitoring the degradation of fuel oxygenates in air.76,77 In these cases, very long absorption path lengths are used (up to 60 m) in order to enhance sensitivity Remote sensing with FTIR has recently been described for the determination of methanol in the ppmv range.78 Comparison of this range with typical ambient air concentrations (see above) shows that the sensitivity 3.5 Detection 3.5.1 Flame ionisation detection Next to MS detection, FID is most frequently used for fuel oxygenate analysis FID is inexpensive, easy to use, and all fuel oxygenates are detected with a similar response; however, it lacks selectivity and sensitivity Therefore, FID is the ideal choice for laboratory studies with a limited number of compounds (coelution usually not a problem) and rather high concentrations (high sensitivity not necessary) For example, in the majority of laboratory studies on MTBE phase partitioning,70 sorption46 and degradation,28,71,72 FID was used For environmental trace analysis, FID is often not sufficient 3.5.2 Photoionisation detection In PID, the column effluent is irradiated with intense UV light in the 100–150 nm region, which causes the ionisation of molecules The ions are Table Ions used in GC-MS-SIM with corresponding fragments Compound Quantification Corresponding fragment ion ion (m/z) MTBE 73 (CH3)2COCH3+ a-cleavage ETBE 59 (CH3)2COH+ a-cleavage, onium reaction TAME 73 (CH3)2COCH3+ a-cleavage DIPE 45 CH3CHOH+ a-cleavage, onium reaction Methanol 31 CH2OH+ hydrogen radical loss Ethanol 31 CH2OH+ a-cleavage IPA 45 CH3CHOH+ a-cleavage IBA 43 (CH3)2CH+ a-cleavage TBA 59 (CH3)2COH+ a-cleavage TBF 59 (CH3)2COH+ a-cleavage, CO loss Confirmation ion (m/z) 43, 57 57, 87 43, 55 43, 87 32 45 59, 43 74, 31 41 57, 41 Analyst, 2001, 126, 405–413 411 Published on 20 February 2001 Downloaded by Universitat Politècnica de València on 25/10/2014 07:26:03 View Article Online still needs to be improved by at least three orders of magnitude In aqueous systems, FTIR detection has not yet been used However, FTIR has been applied to the determination of ethers and alcohols in gasolines.79,80 Reconstructed IR absorption frequencies from Diehl et al.79 are reported in Table Due to the overlap of analyte signals, Choquette et al.80 employed multivariate calibration to obtain single compound concentrations This technique may also be helpful for the resolution of other overlapping signals In both cases, determination of fuel oxygenates in gasoline was limited to the per mille range, and it seems rather difficult to improve the sensitivity to environmentally relevant concentrations Conclusions and outlook A wide variety of analytical methods have been published on the analysis of fuel oxygenates in environmental matrices The choice of an appropriate method depends on the compounds and matrix to be investigated, the concentration ranges to be analysed, the available laboratory equipment and compliance with regulations In general, the enrichment of fuel oxygenates is the crucial step in all methods, whereas the separation and detection of fuel oxygenates and their degradation products are less critical For the analysis of fuel oxygenates in water, it is necessary to distinguish between alcohol and ether analysis Enrichment techniques that allow the sensitive analysis of low molecular weight alcohols in water are still lacking For this purpose, SPME with the recently introduced coatings Carboxen/PDMS or DVB/Carboxen/PDMS may be useful, as shown for ethanol.59 DAI also seems to be promising Both methods need to be further evaluated for a variety of alcohols For ether analysis, all enrichment techniques described are principally suited The concentration range to be analysed will often determine the choice of a particular method For laboratory studies, HS-GC-FID will mostly be sufficient At spill sites, HS, SPME or DAI might be used, preferably with GC-MS DAI allows the simultaneous determination of alcohol degradation products of dialkyl ethers For measurements of background concentrations, P&T-GC-MS is the only approved method However, new developments in SPME have significantly improved MDLs and it may thus complement P&T methods in ultratrace analysis (ng L21 range) of ethers To date, methods for the in situ determination of alcohols and ethers in aqueous systems at the trace level are not available The development of such methods should be further pursued because they would allow real-time measurements at a high spatial resolution For the analysis of fuel oxygenates in air, validated sampling procedures exist, which involve trapping and either solvent or thermal desorption of the analytes FTIR allows real-time, in situ measurements of most analytes, but is still limited to laboratory systems due to poor sensitivity However, with the advent of new lasers in the IR range, it may become a versatile tool in air monitoring Appendix: Abbreviations Abbreviations used for the fuel oxygenates are given in Table AED atomic emission detection BTEX benzene, toluene, ethylbenzene, xylenes CLSA closed loop stripping analysis DAI direct aqueous injection EPAUS Environmental Protection Agency FID flame ionisation detection FTIR Fourier transform infrared GC gas chromatography HS headspace analysis MDL method detection limit MS mass spectrometry P&T purge and trap PID photoionisation detection SPME solid phase microextraction VOC volatile organic compounds References 10 11 12 13 14 15 16 Acknowledgements 17 18 We gratefully acknowledge financial support by Compagnie Générale des Eaux (CGE) and the Swiss Agency for Environment, Forests and Landscape (BUWAL) H.-A D acknowledges the Swiss Agency for Development and Cooperation (SDC) for support of her stay at EAWAG We thank 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J Hermens, Chemosphere, 1995, 31, 4489 Analyst, 2001, 126, 405–413 413 ... directly in aqueous solution or in the headspace above the sample In the direct mode, partitioning of the analytes between the fibre coating and water determines the extraction efficiency In the headspace... included The EPA method is fully validated and often used in routine VOC monitoring of groundwater and drinking water In this method, a or 25 mL aliquot of water is introduced into the purge vessel The. .. been published on the analysis of fuel oxygenates in environmental matrices The choice of an appropriate method depends on the compounds and matrix to be investigated, the concentration ranges to