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ÿþtrends in analytical chemistry, vol. 21, nos. 9+10, 2002 637 AnaIysis of voIatiIe organic compounds using gas chromatography Jo DewuIf, Herman Van Langenhove* Research Group Environmental Chemistry and Technology, Department of Organic Chemistry, Faculty of Agricultural & Applied Biological Sciences, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium GyuIa Wittmann Department of Inorganic and Analytical Chemistry, Faculty of Science, University of Szeged, DómTér 7, H-6701 Szeged, Hungary The focus of this review is the anaIysis of voIatiIe organic solvents. An effect-oriented definition, organic compounds (VOCs) by gas chromatography mainly used in the USA, states that VOCs are (GC) in the fieId of environmentaI, food, flavour and all organic compounds contributing to photo- fragrance, medicaI and forensic sciences. New trends chemical ozone creation. More general defini- in sampIe injection, separation and detection are tions are based on physical and chemical covered, incIuding muIti-dimensionaI and high-speed properties of the compounds, such as chemical GC. Attention is drawn to a growing interest in structure, boiling point, air/water partitioning, quaIity assessment. From the review, it is cIear that and vapour pressure. it remains a chaIIenge to generate muIti-component Frequently used are definitions based on gaseous standards of VOCs at ppbv and pptv. # vapour pressure. In the USA, VOCs are defined 2002 PubIished by EIsevier Science B.V. AII rights as organic compounds that have a vapour pres- reserved. sure more than 13.3 Pa at 25 C, according to ASTN test method D3960 90. In the European Keywords: Gas chromatography (GC); Volatile organic com- Union, a common definition is that VOCs are pounds (VOCs) organic compounds with a vapour pressure above 10 Pa at 20 C (European VOC Solvents Directive 1999/13/EC). The Australian National Pollutant Inventory defines a VOC as 1. Introduction a chemical compound based on carbon chains VOCs are a topic of interest in many disciplines, or rings (and also containing hydrogen) with a such as food, flavour and fragrance, medical and vapour pressure greater than 2 mm of mercury forensic sciences. The main area dealing with (0.27kPa) at 25 C, excluding methane. VOCs may be environmental chemistry, because Whatever the definition taken, it is obvious VOCs contribute to stratospheric ozone deple- that all research areas dealing with VOCs have tion, tropospheric ozone formation, toxic and made progress based on analysis by GC. Ana- carcinogenic human health effects, etc. lysis of VOCs by means of other techniques is It is worth mentioning that there is no agree- rather limited. Examples of these techniques ment about the definition of VOCs. In spoken include DOAS (differential optical absorption language, VOCs are often used as synonyms for spectroscopy) for gaseous BTEX (benzene, tol- uene, ethylbenzene and xylenes) and formalde- hyde, and MIMS (membrane introduction mass *Corresponding author. Tel.: +32 9 264 59 53; Fax: +32 9 264 62 43. spectrometry) for gaseous and liquid samples. 0165-9936/02/$ - see front matter # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S0165-9936(02)00804-X 638 trends in analytical chemistry, vol. 21, nos. 9+10, 2002 The objective of this review is to focus on 2. SampIe injection analytical procedures for determination of VOCs by GC. VOCs analysis, whether in a Sampling and pre-treatment end up with tar- gaseous, liquid or solid matrix, usually starts get VOCs in different physical states. Being with pre-concentration of VOCs. A number of gaseous, dissolved in a liquid, trapped cryogeni- recent review papers can be found dealing with cally or adsorbed on a solid material, VOCs pre-concentration through solid sorbents, cryo- have to be brought onto the GC column. Some genic pre-concentration, membrane devices, sol- recent reviews include a discussion of VOCs vent extraction, static and dynamic headspace, sample injection [6 9,12 18,25]. solid phase micro-extraction (SPME), stir-bar Injection of liquid or gaseous samples is typi- sorptive extraction (SBSE), supercritical fluid cally carried out by syringes and sample loops, extraction (SFE) and distillation and sublima- employing on-column, split and split less injec- tion techniques. General reviews are from tion. For solvents, large volume injection has Majors [1], environmental-oriented papers are been developed in order to improve limits of from Clement et al. [2,3], Fox [4], Richardson detections. A major difficulty with large-volume [5], Dewulf and Van Langenhove [6 8] and liquid injections of VOCs is the limited differ- Helmig [9]. In the field of flavour analysis, ence in volatility between solvent and analytes. reviews on wine aroma [10], volatiles from cer- Recent strategies to overcome this difficulty are eals [11] and food flavours [12] have to be reported [26 29]. Boselli et al. [26] inserted mentioned. Finally, a number of reviews focus restrictions between the uncoated precolumn and on specific pre-concentration techniques, such the vapour exit so that solute accumulation at the as headspace techniques [13], sorbent trapping front of the flooded zone was avoided. The [14] and SPME [15 18]. accumulation at the front was the result of a Pre-concentration techniques have to take strong pressure drop over the flooded zone owing into account the polarity of the target analytes, to liquid plug formation. Hankemeier et al. [27] especially sorbent-based techniques. Water and Adahchour et al. [28] describe a strategy in interference is a special issue that requires which the evaporation rate is determined by attention. Recent papers focusing on water increasing the injection time at a fixed injection removal [3,6 8,13] discuss techniques based on speed, injection temperature and head pressure. selecting hydrophobic sorbents and/or introduc- The measurement of the flow rate in the carrier- ing hygroscopic salts, water-sorbing polymers gas supply line to the on-column injector allowed a (Nafion) and dry purge stages. rapid optimisation: some five injections proved to In this review, emphasis will be put on the be sufficient. Another possibility is called   inverse GC-analysis itself, i.e. sample injection, separa- large-volume injection  , where a semi-volatile sol- tion, detection and analytical quality assurance vent elutes after the target compounds [29]. and control. Attention will mainly be paid to Pocurull et al. [30] designed a special swing developments of the last two to three years. The injection system for introducing large volumes basis of the work is three-fold: of water-containing samples. Sample evapora- tion and solvent-solute separation were per- formed separately by a set-up consisting of two . a thorough screening of Analytical Abstracts (Silver PTV (programmed temperature vaporization) Platter International) for the period January 1999- injectors filled with different sorbent materials August 2001; and kept at different temperatures. The design . study of the review issues of Analytical Chemistry allows analysis of target compounds with a wide of 1999-2001 [2-5,19-24]; and, range of volatilities present in water-containing . use of an in-house database on environmental liquid samples (Fig. 1). VOC analysis, built up during the last 10 years and Whereas cryogenically pre-concentrated comprising 979 articles. VOCs can be injected in a narrow band, release trends in analytical chemistry, vol. 21, nos. 9+10, 2002 639 3. Separation In the past, VOCs have typically been sepa- rated on capillary columns, mainly silicone-type Wall Coated Open Tubular (WCOT) and alu- mina-based Porous Layer Open Tubular (PLOT) columns for highly volatile compounds. More recently, there have been some new trends in the separation technology. 3.1. Recent column developments Silicone-based columns are commercialised under different names, e.g. AT-1, EC-1, CP- Sil5CB, DB-1, BP-1, HP-1, OV-1, RSL-150, RTX-1, SPB-1 and MXT-1, for 100% poly dimethylsiloxane. Over and above these multi- purpose columns, manufacturers offer columns specially designed for VOCs analysis according to EPA methods, e.g. VOCOL, RTX-VMS, RTX-VGC, RTX-VRX and DB-VRX. The design, with aid of computer modelling [37], is Fig. 1. The sample-introduction and the desorption modes driven to shorten analysis times through higher of the swing system designed to introduce large water-con- taining liquid samples with a wide range of volatilities peak capacities; analysis of 66 targets in 30 min- (From [30]). utes on a WCOT column [38] and 15 targets in 16 minutes on a PLOT column [39] have been reported. Preparation, applications and future directions of PLOT capillary columns have been reviewed by Ji et al. [40]; extension of of targets from a solid sorbent by means of PLOT applications by improving the thermal, thermal desorption results in band broadening. mechanical and chemical stability of the column Traditionally, cryogenic refocusing after thermal has been reported [41]. PLOT column applica- desorption, carried out in front of the injector, tions for oxygenated compounds in air have overcomes this problem. This solution compli- been published [42]. Zeng et al. [43] fabricated a cates the analytical procedure and equipment, capillary column with crown-ether-based sta- contributing to higher variances and risks of tionary phases, through a sol-gel process and errors. Attempts to simplify solid-sorbent desorp- coating onto the inner wall of a fused silica tion are found in recent literature, simulta- capillary. The columns showed high selectivity neously speeding up the pre-concentration and for the separation of positional isomers of aro- offering on-line analysis capabilities. In a first matic compounds. Recently, Mangani et al. [44] set-up, refocusing is done within the GC used a Graphite-Lined Open Tubular (GLOT) injector on a cold column head. A second for BTEX analysis. approach involves thermal desorption of min- In the analysis of biogenic VOCs (especially iaturized traps. Typical examples here are des- terpenoids), flavour and fragrances, there have orption of SPME-fibers [13,15 18] and PTV- been reviews of the use of chiral cyclodextrin- injectors [31 34] and narrow-bore capillaries based stationary phases [45,46]. [35,36], the latter two both filled with sorbent In most applications, separation of VOCs is material. based mainly on the interaction with the stationary 640 trends in analytical chemistry, vol. 21, nos. 9+10, 2002 phase, since interactions with mobile phases, power of 2D GC for volatiles is impressive: such as He, N2 or H2, are negligible. Recently, about 550 individual peaks of compounds Berezkin et al. [47,48] reported a technique in within the C6-C14 range are isolated in one run which specific gases or vapours are used in the [54]. An illustration is given in Fig. 2 for an mobile phase. In this   acidic-basic GC  urban air sample. The huge number of species (ABGC), the apparent degree of dissociation of was here further classified using retention volatile acidic or basic VOCs in the stationary behaviour, indicating about 100 multi-sub- liquid phase is influenced by the gases or stituted aromatics and 50 carbonyls, along with vapours, e.g. amines and ammonia, introduced many hundreds of aliphatic hydrocarbons. to separate carboxylic acids. In a typical configuration, a polar second col- umn performs the separation of portions from a first, non-polar column with a periodicity of 3.2. Multi-dimensional separation seconds [19]. Although there are, in principle, Multi-dimensional  two-dimensional (2D)  valve and thermal modulation, only thermal separation has been developed in order to modulation is considered to be really compre- overcome the peak overlap experienced in one- hensive [55]. Thermal modulation can be car- dimensional chromatograms of complex mix- ried out by both heating and cooling, as tures [49]. The 2D concept has been reviewed represented schematically in Fig. 3. The mod- recently [50 52]. Applications of 2D analysis of ulator, passing effluents from the first column volatiles are mentioned for air analysis [9], petro- as sharp pulses suitable for high-speed chroma- chemicals [53] and flavours [45]. The separation tography onto the second column, is the critical Fig. 2. Sections of 1D and 2D chromatograms of an urban air sample. About 15 peaks can be observed in the single-column 1D chromatogram; the comprehensive 2D chromatogram shows around 120, with discrimination between aliphatics (band 1), carbonyls (band 2) and aromatics (band 3) (From [54]). trends in analytical chemistry, vol. 21, nos. 9+10, 2002 641 Fig. 3. Schematic representation of heated and cryogenic modulation. Heated modulation with (1) trapping of solutes; (2) remobilisation of solutes; (3) continuous refocusing of solutes and trapping of next fraction; and, (4) release of solutes. Cryogenic modulation with (1) trapping of solutes; (2) release of solutes; (3) separation and next trapping. (From [55]). 642 trends in analytical chemistry, vol. 21, nos. 9+10, 2002 device in 2D GC [22]. Leonard and Sacks [56] analysis, the capabilities of this technology have report that 2D fast GC with time-of-flight mass recently been investigated [67,68]; at the spectrometry (TOFMS) can collect 500 spectra moment, resistive heating systems for fast per minute, thus allowing very short analysis GC, with heating rates up to 70 C/min., are times, since overlapping peaks can be tolerated. available [69]. 2D comprehensive liquid chromatography-gas Van Deursen et al. [70] investigated the use of chromatography (LC-GC) for volatiles, with LC wide-bore columns for fast GC with vacuum separations in the 5 10 minute range and GC outlet, allowing a high sample capacity. A special separations in the 1 2 second range, has been feature of HSGC is its capability for use in on- reported [57]. This comprehensive LC GC line monitoring equipment in the field, when it instrument, with 100% water mobile phase is combined with fast sample introduction sys- in the LC stage, is an alternative to current tems. Typical applications are with SPME or methods that employ headspace analysis. membrane-based sample pre-concentration for Separations of compounds such as BTEX, as air and water analysis, including portable instru- well as chloroform and methylene chloride are ments [71 74]. Further on, the combination presented. with time-of-flight mass spectrometry (TOFMS) looks very promising [52,75,76]. 3.3. High-speed GC Since the late 1990s, reduction of GC analysis 4. Detection time has resulted in high-speed GC (HSGC), or fast, rapid or ultra-fast GC (for a report, see Capillary GC (CGC) analysis of VOCs typi- Sacks et al. [58], for reviews, see [22,59 62], and cally employs FID (flame-ionisation detection), for theory on HSGC, see Blumberg [63 65]). MSD (mass-spectrometry detection), ECD Hinshaw [62] distinguished four levels of speed (electron-capture detection), (D or H)ELCD in capillary GC (Table 1). HSGC is char- ((dry or Hall) electrolytic conductivity detec- acterised by relatively short columns, small col- tion), PID (photo-ionisation detection), FPD umn diameters and fast temperature (flame-photometry detection) and NPD (nitro- programming, typically 50 C/min., all resulting gen-phosphorus detection). Recent detector in analysis times of the order of seconds. The developments have been reviewed by Fox [5] high heat-up rates are based on resistive heating, and Eiceman et al. [22]. Although VOC detec- already described in the 1960s [66]. For fast-GC tion is still largely based on FID, MS and ECD TabIe 1 Four IeveIs of speed in capiIIary gas chromatography (CGC) [62] LeveI Description ReIative CoIumn dimensions InIet Injector Detector speed of pressure anaIysis (psig) 1 Conventional CGC 1 10 100 m 0.18 0.53 mm 2 100 Conventional Conventional 2 Rapid CGC on conventional 3 5 5 50 m 0.18 0.53 mm 2 100 Conventional, Conventional, instruments PTV* cold sampling rates trapping at 50 Hz 3 High-speed CGC on 5 10 2 25 m 0.10 0.25 mm 10 150 Rapid heating Conventional, modified conventional or higher sampling at instruments or components 200 Hz 4 Very-high-speed CGC 10 50 2 10 m 0.03 0.10 mm 20 200 High sensitivity, on specialized instruments or higher high speed *Programmed-temperature vaporizing. trends in analytical chemistry, vol. 21, nos. 9+10, 2002 643 Fig. 4. Fault tree showing the principal modes of failure for VOC analysis using active sampling onto an adsorbent trap (From [86]). (see e.g. [9]), new types are reported, such as 5. AnaIyticaI quaIity micro-ECD [77], HID (helium-ionisation detection, which is especially sensitive for oxy- In research papers, emphasis is mainly put on genates, when compared with FID) [78], MIP- novel developments. However, it is not common AES (microwave-induced plasma atomic-emis- practice to assess analytical performance by sys- sion spectrometry) for halogenated VOCs [79], tematically giving data on limits of detection SCD (sulphur-chemiluminescence detection) (LOD), limit of quantification (LOQ), blank [10] and RF-IMS (radio-frequency ion-mobility levels, reproducibility, repeatability, accuracy, spectrometry) [80]. calibration, specificity, and range of application. Taking into account their selectivity, their Nevertheless, some recent papers do focus on sensitivity and their ability to provide structural analytical performance [86 89]. Huxham and information, detectors are connected in series or Thomas [89] observed seven stages contributing in parallel at the outlet of the GC column in to errors and uncertainties in the determination of order to get the maximum information. In VOCs concentrations in air (Fig. 4). environmental applications, Koziel et al. [81], The ultimate test of performance is inter- for example, reported VOC analysis on a por- laboratory comparison. Such exercises have table GC equipped with PID, FID and DELCD been reported for the analysis of terpenes in air in series. In flavour analysis, where typically [89], chlorinated VOCs in surface waters [88], GC-olfactometry is utilized, sniffing ports and VOCs by SPME [90] and volatiles in beer [91] FID have been configured in parallel [82]. GC and spirits [92]. Jurvelin et al. [93] discussed analyses of flavours have been compared typical problems experienced in multi-centre recently to direct sensors used without any studies of gaseous VOCs analyses. This study separation, such as MS-based and semi- shows the necessity for carefully planned and conducting polymer sensor-based electronic realised quality control and quality assurance noses [83 85]. Structural information is typically (QC/QA) in multi-centre studies, where a provided by MS. Zhou et al. [11] have reported common sampling method and laboratory ana- the employment of FTIR and MS in series. lysis technique is not used. 644 trends in analytical chemistry, vol. 21, nos. 9+10, 2002 One essential aspect in achieving high quality ever, high-speed GC is shortening GC separa- in airborne VOCs analysis is the availability of a tion from minutes to seconds. calibration mixture [3,6,94]. Mixtures are usually Secondly, research is focussed on maximizing prepared in the laboratory itself by static or the information one can get out of a sample. dynamic dilution, whether starting from gaseous Novel developments in this field are new column mixtures or from liquids. Dynamic dilution is technologies, e.g. chiral separations, and columns typically done by diffusion through a membrane especially designed for VOCs analysis. However, (permeation) or through a capillary. Recently, progress might mainly be in the development of Gautrois and Koppmann [95] combined the use 2D GC. It will enable substantial progress in the of diffusion through a capillary with a second study of biogenic VOCs, the atmospheric fate of dynamic dilution with pure air, down to pptv VOCs, and flavour and fragrances. levels, comprising a large range of VOCs and Finally, quality assessment is a topic receiving showing a high linear dynamic range. Ethene growing attention, as can be noticed from recent calibration starting from thermal decomposition research papers and published interlaboratory of a suitable surface compound has been comparison results. reported [96]. A remaining challenge is the generation of McKinley and Majors [94] question the trace- easily available and traceable gaseous standards ability to standards in airborne VOCs analysis; with dozens of target compounds at relevant more than 400 VOCs of environmental and concentrations. industrial interest can be measured, but trace- level standards are available for only about 30 compounds. AcknowIedgements Calibration of VOCs analysis based on equilib- rium-based pre-concentration techniques, e.g. The authors acknowledge financial support of static headspace or SPME, requires information the Belgium-Central and Eastern Europe on the partitioning of the analytes between the Research Fellowships Programme of the Bel- matrix to be analysed and the matrix to be gian Federal Office for Scientific, Technical and injected in the GC [13,16,97 99]. Namiesnik et Cultural Affairs. al. [16] discussed two approaches to calibrate SPME-GC, based on knowledge of distribution coefficients or on preparation of standard mix- References tures. Calibration of SPME fibers for air analy- [1] R.E. Majors, LC.GC North Am. 17 (1999) S7. sis by means of mathematical modelling has [2] R.E. Clement, P.W. Yang, C.J. Koester, Anal. Chem. 71 been proposed [100,101]. Murray [102] investi- (1999) 257 R. [3] R.E. Clement, P.W. Yang, C.J. Koester, Anal. 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