Review
Sample preparation for the analysis of volatile organic compounds in air and water matrices

https://doi.org/10.1016/j.chroma.2007.01.012Get rights and content

Abstract

This review summarizes literature data from the past 5 years on new developments and/or applications of sample preparation methods for analysis of volatile organic compounds (VOC), mainly in air and water matrices. Novel trends in the optimization and application of well-established airborne VOC enrichment techniques are discussed, like the implementation of advanced cooling systems in cryogenic trapping and miniaturization in adsorptive enrichment techniques. Next, focus is put on current tendencies in integrated sampling–extraction–sample introduction methods such as solid phase microextraction (SPME) and novel in-needle trapping devices. Particular attention is paid to emerging membrane extraction techniques such as membrane inlet mass spectrometry (MIMS) and membrane extraction with a sorbent interface (MESI). For VOC enrichment out of water, recent evolutions in direct aqueous injection (DAI) and liquid–liquid extraction (LLE) are highlighted, with main focus on miniaturized solvent extraction methods such as single drop microextraction (SDME) and liquid phase microextraction (LPME). Next, solvent-free sorptive enrichment receives major attention, with particular interest for innovative techniques such as stir bar sorptive extraction (SBSE) and solid phase dynamic extraction (SPDE). Finally, recent trends in membrane extraction are reviewed. Applications in both immersion and headspace mode are discussed.

Introduction

Volatile organic compounds (VOC) are an issue of major concern for many scientists worldwide, being active in different disciplines such as (i) food, flavour and fragrances, (ii) medical, pharmaceutical and forensic sciences, and particularly (iii) environmental sciences. The latter is mainly because of the growing awareness of the impact of VOC on both human health and global environment. Both VOC and their degradation products may be important in the epidemiology of respiratory disorders and cancer [1], [2]. Next, VOC contribute to major environmental problems such as global warming, stratospheric ozone depletion, photochemical ozone formation and odour nuisance [3].

In the literature, a wide range of definitions of VOC can be found. Basically, two categories can be distinguished. First, there are effect-oriented definitions such as the one used by the US-EPA, defining VOC as organic compounds contributing to photochemical ozone creation [4]. Other definitions make use of physical–chemical properties such as pressure in combination with temperature. Kennes and Veiga [5] define VOC as organic chemicals (vapours) containing carbon atoms and having a normal boiling temperature below 373.15 K at 101 kPa. According to test method D3960 of the American Society for Testing and Materials, VOC are organic compounds having a vapour pressure larger than 13.3 Pa at 25 °C [6]. The EU Solvents Directive (1999/13/EC) defines VOC as organic compounds having a vapour pressure of at least 10 Pa at 20 °C [7]. Whatever the definition, VOC cover a broad range of organic compounds including paraffinic, olefinic and aromatic hydrocarbons, and various oxygen-, nitrogen-, sulfur-, and halogen-containing molecules [8]. Methane is very often considered separately because of its relative nonreactivity in the troposphere and the quite different concentration range at which it occurs in the atmosphere [9]. Therefore, the acronym VOC often means ‘non methane volatile organic compounds (NMVOC)’. This review is dealing also with NMVOC.

To gain knowledge on the occurrence, fate and behaviour of VOC in all fields of interest, precise and accurate analytical techniques are necessary. In view of the physical–chemical properties of these target compounds, most common analytical methods include separation by gas chromatography (GC) followed by on-line mass spectrometry (MS), flame ionisation detection (FID) or electron capture detection (ECD) [10], [11], [12], [13], [14]. More recently, atomic emission spectroscopy (AES) has been recognized, if applicable, as a sensitive and highly selective detection system for GC [15], [16]. In a limited number of cases, high performance liquid chromatography (HPLC) or ion chromatography (IC) is used, particularly for the analysis of carbonyl compounds after derivatization [17], [18], [19], [20], [21], [22]. Alternatively, immediate detection without separation is applied, e.g. by spectroscopic techniques [23], [24], [25], [26], [27], [28], [29], and direct MS techniques such as membrane inlet MS (MIMS) [30], [31], [32], [33], [34], [35], [36], atmospheric pressure chemical ionisation MS (APCI-MS) [37], [38], and proton transfer reaction MS (PTR-MS) [35], [39], [40], [41].

However, the analytical procedure does not only imply the analysis of the target compounds in sensu strictu, i.e. separation and detection. Particularly in environmental matrices, where VOC concentrations are low (mostly pg L−1 to μg L−1 levels), appropriate sampling and preconcentration techniques are necessary to comply with the sensitivity of the analytical instrument [42]. Matrix components disturbing the instrumental analysis also have to be removed through adequate sample preparation. The ultimate importance of sample preparation for VOC analysis is well illustrated by the recommended ECA-IAQ Working Group 13 definition of total VOC (TVOC) concentration, that is explicitely based on the analytes sorption behaviour on Tenax TA (Section 2.3.1), next to their chromatographic behaviour on a deactivated non-polar GC column [43]. Sample preparation is often the bottleneck and most time consuming task in the analytical scheme [11]. Growing interest in the development and optimisation of reliable and robust extraction, trapping and/or preconcentration techniques has resulted into intensive research in this field. Recent developments and applications of VOC sample preparation and analysis in food, flavour and fragrances are reviewed in [44], [45], [46], [47], [48], [49], [50], [51], whereas VOC sample preparation in medical, pharmaceutical and forensic sciences is reviewed in [45], [48], [52], [53], [54], [55], [56], [57], [58].

Since the major part of literature data can be found in the field of environmental chemistry, the scope of this review is to highlight the state-of-the art on sample preparation methods for VOC analysis in environmental matrices. Table 1 summarizes review articles published since 2001 on VOC analysis in the environment, including information on sample preparation.

Despite numerous literature data available, most articles discuss only one or few sample preparation methods of interest [30], [36], [44], [46], [48], [58], [59], [61], [63], [66], [67], [68], [70], [72], [74], [75], [76], [77], [78], [81], or focus on one particular compound or subgroup of VOC [14], [58], [62], [71], [80]. Other reviews deal with monitoring data [42], [64], [65], [69], [73] or focus on separation and detection techniques with only limited attention to sample preparation [10], [11], [14], [58], [72]. In this manuscript, a comprehensive overview is given of new developments and/or applications of sample preparation techniques, especially dedicated to VOC analysis in air (Section 2) and water (Section 3). Focus is put on these matrices since they represent the major field of interest, given the physical–chemical properties of the analytes. Main literature sources for this article consist of reviews from the past five years (Table 1) and peer-reviewed research papers, published between January 2003 and June 2006 and indexed by Web of Science. The outline of our paper is shown in Fig. 1, giving a schematic overview of all sample preparation methods discussed in a two-dimensional grid with the sample matrix on one axis and the extraction and/or preconcentration matrix on the other one.

Section snippets

Sample preparation methods for VOC analysis in air

Since air is a low density matrix (about 1.2 kg m−3 at standard temperature and pressure, STP), i.e. about three orders lower than liquid or solid matrices (>1000 kg m−3 at STP), and because of the low mixing ratios of VOC (pg L−1 to μg L−1), sample preparation for VOC in air primarily aims at preconcentration. At present, ultraviolet differential absorption spectroscopy (UV-DOAS) is the only technique that allows measurement of VOC in air without sample preparation but the set of analytes is limited

Sample preparation methods for VOC analysis in water

VOC concentration levels found in drinking and natural water samples are typically in the order of ppt (ng L−1) to ppb (μg L−1) [145], [146], [147]. Unless large volume injection (LVI) is applied, the injection volume into a capillary GC column is limited to microliter(s) [73]. Taking into account the sensitivity of most detectors, high preconcentration factors are usually necessary during sample preparation for trace analysis of VOC in water. Solvent extraction has long been the method of

Conclusions

Within the entire analytical scheme, sample preparation is often the most time consuming and challenging step, particularly for VOC present at trace concentration levels (pg L−1 to ng L−1) in air and water matrices. Main tasks to be fulfilled during sample preparation are (i) analyte preconcentration allowing low LODs, (ii) elimination of interferences, (iii) if needed, analyte conversion making it more suitable for separation and detection, and (iv) providing a robust and reproducible method not

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