Environmental Mass Spectrometry

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Russia; email: [email protected]. Annu. Rev. Anal. Chem. 2013. 6:163–89 ... February 28, 2013. The Annual Review of Analytical Chemistry is online.
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Environmental Mass Spectrometry Albert T. Lebedev Organic Chemistry Department, M.V. Lomonosov Moscow State University, Moscow 119991, Russia; email: [email protected]

Annu. Rev. Anal. Chem. 2013. 6:163–89

Keywords

First published online as a Review in Advance on February 28, 2013

tandem mass spectrometry, ambient ionization, accurate mass measurements, portable mass spectrometers, emerging contaminants, isotope ratio mass spectrometry

The Annual Review of Analytical Chemistry is online at anchem.annualreviews.org This article’s doi: 10.1146/annurev-anchem-062012-092604 c 2013 by Annual Reviews. Copyright  All rights reserved

Abstract Environmental mass spectrometry is an important branch of science because it provides many of the data that underlie policy decisions that can directly influence the health of people and ecosystems. Environmental mass spectrometry is currently undergoing rapid development. Among the most relevant directions are a significant broadening of the lists of formally targeted compounds; a parallel interest in nontarget chemicals; an increase in the reliability of analyses involving accurate mass measurements, tandem mass spectrometry, and isotopically labeled standards; and a shift toward faster high-throughput analysis, with minimal sample preparation, involving various approaches, including ambient ionization techniques and miniature instruments. A real revolution in analytical chemistry could be triggered with the appearance of robust, simple, and sensitive portable mass spectrometers that can utilize ambient ionization techniques. If the cost of such instruments is reduced to a reasonable level, mass spectrometers could become valuable household devices.

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1. INTRODUCTION

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GC/MS: instrumentation and/or method that combines gas chromatography and mass spectrometry LC/MS: instrumentation and/or method that combines liquid chromatography and mass spectrometry Perfluorinated compounds (PFCs): persistent pollutants that are not readily metabolized Disinfection by-products (DBPs): compounds that arise from, for example, treatment of water with chlorinated amines High-resolution mass spectrometry (HRMS): provides molecular formulae from accurate masses Tandem mass spectrometry (MS/MS): method to characterize individual ions (and their corresponding mixture components) by mass separation, fragmentation, and mass analysis of fragments Selected ion monitoring (SIM): a method used to detect and quantify a selected analyte, record one or several characteristic ions, and thereby improve signal in unit time TOF: time of flight

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Environmental problems first attracted serious interest in the mid 1970s; this attention coincided with the development of organic MS and the appearance of commercially available gas chromatography–MS (GC/MS) instruments based on quadrupole mass filters. The first list of priority pollutants created by the US Environmental Protection Agency (EPA) contained mainly organic compounds that are amenable to GC/MS for which the official methods were written for the use of quadrupole mass spectrometers. A decade later, the extensive implementation of liquid chromatography–MS (LC/MS) led to increasing interest in “new” natural or anthropogenic compounds. These chemicals, known as emerging contaminants, represent an extremely wide group containing pharmaceuticals and their metabolites, musks, nanomaterials, perfluorinated compounds (PFCs), hormones, disinfection by-products (DBPs), flame retardants, sunscreen filters, naphthenic acids, and so on. Human activities cause these compounds to enter the environment (usually at parts-per-billion or parts-per-trillion levels). Although much information about the dangers of many of these pollutants is available, most of these compounds are not yet regulated. Some rules and restrictions exist (1), but their safe values have not been determined. MS appears to be the most efficient method for their detection, identification, and quantification. A book titled Comprehensive Environmental Mass Spectrometry, containing 21 chapters and covering most various fields of environmental MS, was published in April 2012 (2). Numerous reviews on this subject have also been recently published (1, 3–36). Because of length restrictions, I cannot address publications dealing with the resolution of some environmental problems by use of classic MS approaches; instead, I discuss articles dealing with innovations in environmental MS. Thus, in focusing on publications that appeared in 2011 and 2012, I direct readers’ attention to certain trends that, in my opinion, are the most relevant for modern environmental MS: 1. A significant expansion of the list of target analytes, with parallel interest in nontarget ones. 2. An increase in the reliability of analyses involving high-resolution MS (HRMS), tandem MS (MS/MS), and isotopically labeled standards. 3. A shift toward faster high-throughput analyses, with minimal sample preparation, through various approaches, including ambient ionization techniques and miniature instruments.

2. TARGETED AND NONTARGETED ANALYSIS The great benefit of MS, even in its early stages, has been its ability to identify and quantify many different analytes in one run. Most MS methods aim to identify numerous target analytes. MS/MS or HRMS in the selected ion monitoring (SIM) mode can successfully identify and quantify selected contaminants at trace levels (2). However, targeted analysis is, by definition, selective and provides information about only certain preselected organic pollutants. The main risk may be associated with missing nontargeted components. On the contrary, rapid recording of full mass spectra [by, e.g., time-of-flight (TOF) instruments] can reveal large numbers of compounds in a sample (nontargeted analysis), and the data obtained remain available for retrospective analysis without the need to reinject the sample.

2.1. Gas Chromatography–Mass Spectrometry There are several new trends in GC/MS analysis. An important development involves the twodimensional GC (GC×GC) method, which has increased the number of analytes that can be recorded in one run to thousands. The history and modern applications of multidimensional GC, including the heart-cut version and the comprehensive GC×GC variant, are reviewed in Lebedev

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Rxi-200 (1.41 m × 0.18 mm × 0.2 μm)

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Masses: 322, 334, 356, 368, 390, 402, 424, 436, 460, 472, 306, 318, 340, 352, 374, 386, 408, 420, 444, 456

3

Octa Hepta Hexa

2

Penta Tetra

1

1, 2, 3, 4, 7, 8-HxCDD 1, 2, 3, 6, 7, 8-HxCDD 0 536

1,036

1,536

2,036

2,536

Rxi-XLB (30 m × 0.25 mm × 0.25 μm) Figure 1 Two-dimensional selected ion contour plot for 17 priority polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (Rxi-XLB and Rxt-200 columns). These compounds, including 1,2,3,4,7,8- and 1,2,3,6,7,8-HxCDD isomers, are well resolved. Reprinted with permission from Reference 41. Copyright 2011, Elsevier.

Reference 17. GC×GC/MS and fast GC/MS approaches for environmental analysis are discussed in Reference 37. GC×GC requires the use of TOF instruments with the highest acquisition rate (up to 500 spectra per second), which enables efficient analysis of extremely complex samples. The ability to separate closely related compounds is another benefit of GC×GC. This technique was recently used to analyze enantiomeric o,p -DDT (dichlorodiphenyltrichloroethane) and o,p DDD (dichlorodiphenyldichloroethane) in contaminated soil and air in a malaria-affected area of South Africa (38). Although GC×GC/TOF-MS instruments are more sophisticated and expensive than their GC/MS, their application is simpler, providing enhanced sensitivity and enabling fast enantioselective analysis of chiral analytes. GC×GC coupled with MS/MS or HRMS yields an extremely powerful analytical tool. Hashimoto et al. (39) used both techniques to analyze persistent organohalogen compounds. GC×GC/MS/MS, in constant neutral loss mode, allowed for the detection of halogenated compounds in fly ash samples, and GC×GC/HRMS was used to identify new compounds (40). The GC×GC/TOF-MS method has been refined for the analysis of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (41). In this study, the authors combined an Rxi-5 Sil MS column with an Rtx-200 column. The results agreed reasonably well with those obtained with GC/HRMS. Because full-range mass spectra are always acquired in GC×GC/TOFMS, the use of this approach is ideal when screening for multiple environmental pollutants in a single analysis; it is a viable tool for dioxin screening and quantification. All 17 PCDDs and PCDFs named in EPA Method 1613 (42) can be separated, ensuring that the final sample toxic equivalent value can be accurately determined (Figure 1). To this end, GC×GC/TOF-MS has been used to analyze samples from a hazardous waste treatment facility in South Africa. The samples were screened for PCDDs, PCDFs, and four www.annualreviews.org • Environmental Mass Spectrometry

PCDDs/PCDFs: polychlorinated dibenzo-p-dioxins/ polychlorinated dibenzofurans

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252,093

Intensity

405

305 252,185 205

252,280 252,149

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251,968 251.9

252,005 252.0

252,059

252,241 252.1

252.2

252.3

m/z Figure 2 Mass spectrum of a snow sample showing the presence of at least 12 ions with the same integral mass of m/z 252 but a different exact mass. Reprinted with permission from Reference 45.

dioxin-like polychlorinated biphenyls (PCBs) (43). The detection limits and values obtained with GC×GC/TOF-MS were similar to those obtained with GC/HRMS, which is the benchmark technology for such analyses. Heating of the soil containing these ecotoxicants can contribute substantially to the emission of toxicologically relevant PCDDs and PCDFs. Black et al. (44) showed that, when sufficient soil heating occurs in field fires, a portion of total PCDDs and PCDFs in the emissions may be due to volatilization and formation of new compounds of this class in combustion processes. Thus, the emission of 2,3,7,8-TCDD (tetrachlorodibenzo-p-dioxin) and 2,3,7,8-TCDF (tetrachlorodibenzofuran) was greater than expected on the basis of the emission of isotopically labeled congeners added to the soil; this finding indicated that these compounds formed because of soil heating. Additionally, the toxic equivalent value of the emissions increased with soil temperature. Accurate mass measurements that can establish the elemental composition of analytes make the overall analysis much more reliable (6). HRMS is almost compulsory for LC/MS when only one molecular ion or a few fragment ions are observed, although MS/MS scans are an alternative route to obtaining structural information. This characteristic also greatly benefits targeted analysis, nontargeted analysis with the use of spectral libraries, and manual structural elucidation in GC/MS studies (45). Figure 2 shows the presence of at least 12 ions with the same integral mass of m/z 252 (primary ion for the detection of benz[a]pyrene), but a different exact mass, in a snow sample. Without HRMS, the chance of a false-positive result would be high. The probability of falsepositive results increases with matrix complexity as the chance of the presence of a compound that has a retention time close to that of the target analyte and that contains an ion with the same nominal mass in its electron ionization (EI) spectrum becomes higher. Reference 6 illustrates the potential of GC coupled with high-resolution TOF-MS for environmental pollution analysis and related fields. More than 20% of the overall number of tentative identifications of nontarget analytes, based on the score provided by an NIST11 library search, were more correctly assigned with accurate mass data (46). Without exact mass data, correct hits appeared in the hit list but were far from the top position. Manual interpretation also benefits greatly when accurate mass information is available (45). Another important GC/MS development involves the recently demonstrated supersonic molecular beam (SMB) interface, which improves classic EI with new and impressive possibilities 166

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and expands the range of compounds that are amenable to GC/MS analysis (37). An SMB device greatly increases the analysis speed and simultaneously increases the abundance of molecular ions. The latter capability is important because observation of the molecular ions in a compound is crucial for correct identification of the compound. However, 24% of the compounds listed in the NIST08 library have no molecular ions in their EI spectra, or their intensity is lower than 1% (47). Li et al. (48) have proposed a way to identify 49 pesticides in one run by use of GC/multiphoton ionization/TOF-MS; this approach is based on the third-harmonic emission (267 nm) of a femtosecond Ti:sapphire laser (100 fs) as the ionization source. This method was tested on samples of fruits and vegetables; its detection limits were mostly better than or equal to those of the classical EI approach. Another advantage of multiphoton ionization is the presence of molecular ions in the spectra of the pesticides under investigation. The authors of this study detected more pesticides in vegetable samples due to the method’s high sensitivity and selectivity. These benefits arise from the suppression of signals of aliphatic compounds and reduced fragmentation of the target analytes. The combination of GC separation with surface-assisted laser desorption/ionization (SALDI), followed by TOF, can determine phenylalkylamines at low detection limits (49).

PBDEs: polybrominated diphenyl ethers

2.2. Liquid Chromatography–Mass Spectrometry Although GC/MS remains very popular for many environmental applications, LC/MS demonstrates definite advantages in many others. LC/MS/MS is a favorable alternative even for some classical GC/MS methods, such as analysis of fatty acids (50). GC/MS requires a tedious derivatization procedure, and high–boiling point fatty acids are not readily amenable to such analyses. Specifically, GC/MS exhibits peak tailing, long retention times, and high detection limits. By contrast, LC/MS/MS approaches may be applied without the need for derivatization. Investigators have drawn similar conclusions in favor of LC/MS versus GC/MS in the cases of certain flame retardants and N-nitrosamines (11, 26). Many LC/MS methods for the determination of emerging contaminants have recently been reported (1, 8, 9). These techniques demonstrate the utility of multiresidual analysis and apply more reliable high-resolution full-scan methods involving isotopically labeled internal standards. The main advantages of these LC/MS methods include increased selectivity and fewer falsepositive results. The lack of standardized criteria for identification and quantification of emerging contaminants in environmental matrices and the difficulty in comparing analytical methods are current drawbacks (9). There is a need to standardize methods to identify emerging contaminants. To this end, the European Union attempted standardizing analytical methods used by the member states to monitor a selected set of compounds (10). Polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons, and alkylphenols, compounds that are included in European Commission Directive 2008/105/EC (51), have been analyzed in standard solutions and river water samples in an investigation into variabilities in different steps of analysis. MS (mainly GC/MS) was the method of choice for the vast majority of the participating laboratories, but only the laboratories that used HRMS were successful in analyzing individual PBDEs at the required detection limits. Publications describing MS strategies for the analysis of pesticides and their metabolites in food and water matrices, covering the period 2005–2010, have been reviewed (18). That review emphasized applications of the orbitrap method, which is characterized by higher full-scan sensitivity, greater mass resolution and accuracy, higher sensitivity in the MS/MS mode, and a wider linear dynamic range. A comparison between existing techniques for analyzing pesticides and their transformation products in different matrices showed that HRMS, MS/MS, multiresidual analysis, and ambient ionization MS hold the most promise for future pesticide analyses. The European Commission also required that confidence in compound identification be increased www.annualreviews.org • Environmental Mass Spectrometry

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Single reaction monitoring (SRM): an MS/MS experiment in which a single transition (typically a fragmentation) between two ions is used to identify and quantify the analyte in an experiment that gives high sensitivity and adequate specificity Electrospray ionization (ESI): an MS method used to ionize analytes Atmosphericpressure chemical ionization (APCI): an MS method used to ionize analytes Multiple reaction monitoring (MRM): an MS/MS experiments in which more than one SRM transition is used; often one transition is used with the analyte and the other quantifies the internal standard HPLC (UPLC): (ultra)high-pressure liquid chromatography

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(52); this publication specified reliability points according to the type of MS analysis. The shift toward the use of HRMS and MS/MS obviously leads to more reliable identification. Despite the popularity of LC/MS/MS, it is often used “blindly” by measuring the intensity of the signal involving transition between two characteristic ions, that is, by doing a single reaction monitoring (SRM) experiment. For a more rigorous approach, a basic understanding of fragmentation of even-electron ions formed by electrospray ionization (ESI) and atmospheric-pressure chemical ionization (APCI) that is similar to that of odd-electron ions in EI mode is needed. Niessen (24) summarized the available MS/MS data on many classes of drugs, with an emphasis on class-specific fragmentation. The recognition of class-specific fragmentation may help identify a specific unknown compound and may help optimize the selectivity of target analyses of compounds of a particular class. A drawback of collision-induced dissociation (CID) of even-electron ions formed in ESI conditions is that an insufficient number of fragment ions might be available to carry out structural elucidation. Mosely et al. (53) studied the possibilities of electron-induced dissociation (EID) in structural studies of numerous pharmaceuticals. Their method is based on the interaction between protonated or cationized organic molecules and electrons that have energies greater than 10 eV. Instead of recombination, a process similar to classic EI takes place. EID can provide more product ions than does CID, and the two techniques are complementary.

3. EMERGING CONTAMINANTS 3.1. Pharmaceuticals The problem of pharmaceuticals in wastewater has recently become a major concern for both human health and the environment. In the European Union alone, approximately 3,000 different pharmaceutical ingredients are currently in use. Multiresidual methods involving the simultaneous extraction of all the analytes—followed by LC/MS/MS, preferably with HRMS—should be used for their determination in the environment. The retention time, full mass spectrum, and two SRM transitions with the correct intensity ratio guarantee the most reliable analytical results. The application of isotopically labeled standards may be important in counteracting matrix effects. A study describing the influence of matrix effects on solid-phase microextraction (SPME)/highperformance liquid chromatography (HPLC)/ESI/MS/MS analysis of pharmaceutical samples emphasized the benefits of isotopic internal standards (54). SPME/HPLC/ESI/MS/MS was used to estimate the levels and sources of 16 sulfonamides in samples from wastewater treatment plants (WWTPs) (55). This experiment assessed the biodegradability of these compounds and the efficiency of the plants in removing contaminants. HPLC/MS/MS has been used to study 73 pharmaceuticals in 12 classes; the investigators compared their levels in the effluents of hospitals with those in the corresponding WWTPs (56). The analysis revealed nine substances that posed a high risk at the concentrations detected in the hospital effluents. Five of them exhibited high ecotoxicity. Antibiotics caused the most concern because the treatment plants were unable to effectively remove them. Another group developed a very reliable method for the simultaneous determination of 47 pharmaceuticals (57). All the analytes were extracted in a single solid-phase extraction (SPE) step, followed by their simultaneous determination in positive and negative ESI modes (two SRM transitions per compound); the chromatographic run time was only 10 min. The authors emphasized that matrix effects may cause serious problems when dealing with real environmental samples and that, because isotopically labeled internal standards are not always available, it is Lebedev

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important to find a realistic compromise among time, cost, analytical efforts, and quality of the information obtained. ESI/MS/MS has been used to identify 55 pharmaceuticals and hormones, as well as their metabolites, in natural water used for the production of drinking water (58). This study used deuterated analogs as surrogates and internal standards. A total of 35 chemicals were detected in the natural water. The behavior of these pollutants was studied at each step of drinking water production. Only five compounds were detected in the final drinking water at approximately 5% of the original levels. Multiresidual SPE/ultrahigh-performance liquid chromatography (UPLC)/ESI/MS/MS in MRM mode (two transitions per analyte) has been used in environmental monitoring of 65 pharmaceuticals in water (59). This methodology enabled the analysis of all compounds in one extraction step, one reconstitution step, and one LC-MS/MS experiment. Of the targeted 65 compounds, 46 were detected in WWTP inflows and 36 in river water. An advanced MS method (SPE/HPLC/ESI/MS/MS) that records two transitions per compound and uses isotope dilution (60) for reliable quantification allowed investigators to resolve a problem posed by earlier results concerning the removal of diclofenac and sulfamethoxazole from subsurface environments. Both of these compounds can be temporarily and reversibly affected by denitrifying conditions. Their concentration decreases when nitrite builds up but rebounds when nitrite is reduced to nitrogen. Similar processes may lead to an incorrect estimation of WWTPs’ efficiency in removing these drugs, and perhaps other aromatic amines, because their nitro derivatives can transform back into the parent compounds when released in the environment (60). In a study of the photochemical degradation of trimethoprim (61), the authors used HPLC/ESI/MS to identify several oxidation products of trimethoprim and to suggest mechanisms underlying its transformation. UPLC/ESI/quadrupole TOF (QTOF) has been used to identify the transformation products of the solar photocatalytic oxidation of trimethoprim (62). More than 20 products were detected, and their structures were tentatively assigned through accurate mass measurements and fragmentation patterns via MS/MS. The degradation products of 1-diazo-2-naphthol-4-sulfonic acid with Fenton’s reagent were identified with UPLC/ESI/MS (63). The authors of this study found 19 aromatic intermediates. Barcelo´ and colleagues (64, 65) created and applied a powerful multiresidual method of determining illicit drugs in wastewater. Their technique involved the quantification of 21 drugs, including some metabolites, by online SPE/HPLC/ESI/MS/MS. Detection was performed in MRM mode (two transitions per analyte), and the quantification was based on isotope dilution. This example is typical of the trend toward multiresidual, highly reliable analysis. UPLC/MS/MS method, with recording two MS/MS transitions per compound, was used to analyze the efficiency of a drinking water treatment plant (66). The levels of 29 pharmaceuticals and 12 illicit drugs identified at intake were measured along the entire potabilization process. Despite the high efficiency of the procedures, several analytes were detected in the treated water at the nanogram-per-liter level. A new approach dealing with the measurements of illicit drugs in wastewater, known as sewage epidemiology, enables estimations of drug consumption in a given community through wastewater analysis (67). For example, most characteristic human urinary metabolites of cocaine were measured in untreated urban wastewater with SPE followed by HPLC/ESI/MS/MS performed with deuterated analogs as analytical standards. The profiles of these cocaine metabolites in wastewater matched those found in human urine, suggesting that wastewater reflects actual human excretion and that wastewater analysis is suitable for assessing drug consumption. The appearance and stability of benzoylecgonine in wastewater make this compound the best target for estimations of community drug use by wastewater analysis (67). Sewage epidemiology has also been successfully www.annualreviews.org • Environmental Mass Spectrometry

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PCPs: personal care products

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demonstrated in the study of illicit drug consumption in a prison in Spain (65) and in a one-year sampling campaign in the largest WWTP in Brussels, Belgium (68). Traditional target MS approaches rely on library searches, known masses, or multiple reaction monitoring transitions. Grabenauer et al. (69) proposed a method to detect both known and novel analogs of designer drugs. These authors studied a series of indole-derived synthetic cannabinoids by using a UPLC/QTOF instrument with a resolving power of approximately 20,000 in ESI mode. Although most of the structural modifications of synthetic drugs caused a shift in mass, the mass defect typically remained similar to that of the original compound. Following the application of a mass defect filter to an LC/MS data set, ions with mass defects that had significantly shifted from those of the original compounds were eliminated. Because most indole-derived synthetic cannabinoids have mass defects between 0.13 and 0.23 Da, a mass defect filter centered at 0.185 with a window of ± 50 mDa captured approximately 75% of all the currently known structures.

3.2. Personal Care Products PCPs are often found in higher concentrations in natural water, wastewater, and swimming pools than are pharmaceuticals. UV filters, preservatives, musk fragrances, siloxanes, and repellents are PCPs which are also emerging contaminants. PCPs, many of which are biologically active, are continually being released into the aquatic environment. Methods for PCP analysis in water have recently been reviewed (25); GC/MS/MS and LC/MS/MS were considered the most useful analytical tools. UV filters are used as sunscreen agents and in various products to prevent their photodegradation. They are also utilized as corrosion inhibitors in deicing fluids for aircraft and in automotive cooling systems. HRMS and MS/MS have been used to quantify known and new metabolites formed during bio- and photodegradation of the sunscreen active ingredients BP1 and BP3 (70). This study found that glycoconjugation is the first step in the metabolism of these compounds. Two research groups (71, 72) reported ways to determine UV filters in sediments and sludges. The first group (71) utilized pressurized liquid extraction followed by UPLC/ESI/MS/MS (two SRM transitions per compound) for the quantification of eight UV filters in sediments. This method is rapid; the chromatographic run time is 9.5 min. Surprisingly, the detection limits reported using the second group (72) for five similar benzophenones (HPLC/ESI/MS/MS) and six benzotriazols (GC/MS in SIM mode) in sludge were approximately one-hundredth of those reported by the first group. Another study (73) reported a rapid and sensitive method for the analysis of 11 PCPs, belonging to seven classes, by use of UPLC/ESI/MS/MS. The method not only shortened the analysis time to 8 min but also significantly reduced the analysis cost by omitting the SPE process. Pure methanol and acetonitrile were employed for positive and negative ESI/MS modes, respectively. The authors observed that the decrease in the flow rate increased the signal intensities of 10 out of 11 PCPs. Estrogens and progestogens are female-steroid hormones known to function as endocrine disrupters. The use of GC/MS and LC/MS to analyze estrogens in environmental samples has been reviewed (16); the authors concluded that LC/MS/MS is the most reliable technique for the analysis of steroid hormones. The recording of two SRM transitions per compound was sufficient to confirm the presence of analytes in the samples, and isotopically labeled internal standards were used for quantification.

3.3. Flame Retardants Flame retardants, used in various materials to reduce the risk of fire, constitute a significant group of emerging pollutants. The vast majority of them are polybrominated, although some are 170

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polychlorinated compounds. The best known are PBDEs, which are toxic, hydrophobic, persistent, and amenable to bioaccumulation and biomagnification. Besis & Samara (26) summarized results from studies on the appearance of PBDEs in both indoor and outdoor environments. They observed that highly brominated deca-BDE is frequently not included in published GC/MS reports due to its thermal degradation. Guerra et al. (11) summarized current analytical methods based on LC/MS/MS for selected halogenated flame retardants and their metabolites. They compared these methods’ characteristics with those of GC/MS techniques, which are still widely used. These authors observed that high injector temperatures cause the analytes to degrade to some extent and that to obtain diastereoisomer-specific data on hexabromocyclododecane (HBCD), one must use LC, given that diastereoisomers interconvert at temperatures above 160◦ C (11, 26). The transformation of PBDEs and other polyhalogenated flame retardants in the environment usually involves the substitution of halogen for hydroxyl with formation of polar and high–boiling point metabolites. GC/MS is unsuitable for studies of these compounds; LC/MS and LC/MS/MS appear to be the methods of choice. The latter technique provides the required sensitivity and reliability for analysis of environmental samples and has low detection limits. The use of isotopically labeled standards is always preferable (11). The efficiency of MS/MS compared with that of HRMS for the analysis of PBDEs (41 congeners) in fish has been studied (74). A triple-quadrupole (QQQ) instrument in SRM mode was employed for this purpose. The limits of detection and quantification for the MS/MS approach were lower than or comparable to those obtained from HRMS. The authors concluded that MS/MS offered an acceptable alternative to HRMS in terms of cost and technical requirements. This study did not discuss the specificity of the two methods, another important consideration when selecting a methodology. Zheng et al. (75) obtained valuable results by using GC/MS with accurate mass measurements for the qualitative determination and quantification of 25 PCBs and 13 PBDE congeners by isotope dilution. The soil samples were collected at Balang Mountain at increasingly high altitudes. The cold trapping effect (which involves a higher deposition rate of compounds from the atmosphere at sites with lower temperatures) was confirmed for PCBs and PBDEs in the alpine meadow area. GC/MS in negative ion chemical ionization (NICI) mode was used to estimate PBDE levels both in sewage sludge samples collected at WWTPs in Italy (76) and in air samples taken from around these plants (77). The quantification of congeners allowed the authors to conclude that WWTPs are sources of PBDEs in the atmosphere. In a similar study, Birgul et al. (78) measured PBDE levels at 71 sites in the United Kingdom during a period of 10 years. These authors observed a sharp decrease in the levels of these compounds after 2001–2003. GC/MS has been used to determine long-term trends in PBDE contamination in the Western Atlantic near the United Kingdom (79). The investigators worked with bird eggs collected between 1977 and 2007 at two Scottish colonies. The PBDE profiles and temporal trends of both sets of eggs were similar, reflecting the rise and then the restriction in use of penta-BDE formulations in Europe. Worldwide restrictions on the use of PBDEs resulted in the increased use of alternative flame retardants, namely HBCD and organophosphates. UPLC/ESI/MS/MS and GC/MS were used between 1999 and 2007 to measure the levels of HBCD and PBDEs in fish from US rivers (80). During that time, PBDE levels decreased to one-quarter of their initial levels and HBCD levels increased 300-fold. Despite these results and those from a few other studies (see the references in Reference 80), not enough information about the toxicity of HBCD is available. Thus, despite its high bioaccumulation potential, this compound is not included in the US EPA’s Section 313 list of toxic substances (81). Using GC/MS in NICI mode, investigators studied the eggs of four gull www.annualreviews.org • Environmental Mass Spectrometry

Negative ion chemical ionization (NICI): an MS method used to ionize analytes and register negative ions

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species from 26 colonies on the Pacific and Atlantic coasts of Canada (82) for the presence of 14 PBDEs and 20 other flame retardants. The higher levels of PBDEs found by this pan-Canadian study, in comparison to results from most studies conducted in the rest of the world, probably reflect the historically greater use of PBDEs in North America. The levels of various brominated flame retardants in air samples from around the Great Lakes, collected between 2005 and 2009 (83), demonstrated their maxima at two urban sites: Chicago and Cleveland. The atmospheric concentration of 15 PBDEs and 13 other brominated compounds was monitored with GC/HRMS for approximately 1 year at two remote stations, one on the Tibetan Plateau and the other in the Canadian High Arctic (84). Three new flame retardants were detected at relatively high levels. SIM-mode GC/MS was used to estimate the levels of various organophosphate flame retardants in indoor dust samples from Belgium (85). High-resolution GC/TOF-MS was used to measure the levels of these compounds, among other pollutants, in snow samples from Moscow, Russia (45). The levels appeared to be high enough to consider including them in the list of priority pollutants. GC/MS, LC/MS, HRMS, and MS/MS have been used to analyze the levels of PBDEs, PBBs, PCBs, DDTs, and PFCs in fish and in the eggs and livers of birds from the west coast of Sweden (86). Even though this region is more urban than the Arctic, much (100-fold) higher levels of pollutants (i.e., PBDEs, PCBs, and DDTs) were found in seabird species from the Arctic as a result of the so-called cold finger effect, which involves trapping of atmospheric aerosols and gas-phase species in the polar or mountainous regions of the Earth due to the lower temperatures there.

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3.4. Perfluorinated Compounds PFCs, their precursors, and their degradation products have been a focus of environmental monitoring studies, mainly because of their persistence, bioaccumulative potential, and global distribution. A comprehensive review (27) explores the biological, geographical, and environmental problems posed by PFCs. The authors of this review identify analytical difficulties arising from the complexity of analytes and matrices. They recommend that HRMS and MS/MS be used to combat these challenges. Well-defined standards and isotopic analogs are keystones to high-quality analysis. The global distribution and long-range transport of PFCs have been investigated with HPLC/ESI/MS/MS in seawater samples collected from the Greenland Sea, the eastern Atlantic Ocean, and the Southern Ocean between 2009 and 2010 (87). Cai et al. (88) used HPLC/ESI/MS/MS to study the spatial distribution of nine PFCs in coastal water samples from the eastern to southern China Sea. They emphasized the importance of using isotopic internal standards for each analyte. The same method was used to measure the levels of perfluorinated alkyl acids (PFAs) in drinking water from 34 locations around Australia (89). Five classes of PFCs were found in the air around WWTPs and two landfill sites by use of GC/MS and LC/MS/MS (90). Weinberg et al. (91) employed GC/MS and LC/MS/MS to demonstrate that WWTPs may be the sources of PFCs, PBDEs, and musk fragrances in ambient air. HPLC/ESI/MS/MS has been used to measure the levels of perfluorooctane sulfonate and perfluorooctanoate in samples from WWTPs in China (92). The levels of perfluorinated sulfonates were comparable to those found in other countries, whereas the levels of PFAs were much higher in China. Finally, target PFCs were reliably measured with UPLC/ESI/MS/MS in surface and groundwater samples in Decatur, Alabama, where contaminated biosolids from a local WWTP had been used as an agricultural soil treatment for 12 years (93). 172

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Gebbink et al. (94) studied the presence of perfluoroalkyl carboxylates and sulfonates in gull eggs from breeding sites on the Atlantic and Pacific coasts of Canada. They obtained reliable measurements of the levels of these pollutants by using LC/MS/MS with atmospheric-pressure photoionization (APPI) and ESI. HPLC/ESI/MS/MS was used to reliably measure the levels of PFCs in tissues from beluga whales (Delphinapterus leucas) from Alaska (95) and to examine spatial and long-term (9-year) temporal trends of PFCs in water, sediments, and fish from locations where an accidental release of firefighting foam containing these compounds occurred in 2000 (96). MS/MS and HRMS in full-scan mode were used to study the temporal trends and patterns of PFCs in tawny owl (Strix aluco) eggs from Norway, collected between 1986 and 2009 (97).

MIMS: membraneintroduction mass spectrometry

3.5. Disinfection By-Products Millions of people are exposed to DBPs not only by drinking chemically disinfected water, but also by showering, bathing, and swimming. The current state of the problem is well described in Reference 98, and the formation of nitrogenous DBPs is reviewed in Reference 28. MS is the most efficient tool for analysis of DBPs in drinking water. Approximately 600 DBPs are known (98), but only a few of them are regulated and have known safe-level values. However, some emerging DBPs may be more important than the regulated ones because of their high concentrations and toxicities. Thus, the mechanisms of transformation of various organic compounds should be studied, and both regulated and emerging DBPs should be better controlled. Brominated DBPs are generally more toxic than their chlorinated analogs (99). However, most of these products remain unexplored. To find new representatives of polar brominated DBPs, investigators applied UPLC/ESI/QQQ in precursor ion scan mode (99). Because brominecontaining compounds easily form Br− ions in negative ion mode, precursor ion scan mode identifies all the parent ions (deprotonated molecules) generated in ESI which undergo this characteristic fragmentation. A subsequent product ion scan of the detected precursors allows for structural elucidation. In studies of simulated drinking water samples, numerous known bromine-containing DBPs were detected. The authors observed the formation of new brominated DBPs and their decomposition during chlorination and proposed tentative chemical structures and corresponding reaction schemes (99). C- and N-DBPs form from the amino acid tyrosine during drinking water preparation (100). The authors proposed a detailed scheme of the transformation of tyrosine with formation of the most various DBPs on the basis of results from GC/MS analyses. Existing approaches for the analysis of inorganic chloramines in water are reviewed in Reference 29 and references therein. Kinani et al. (29) point out that there are only a few reports that describe the analysis of chloramines with GC/MS and one publication on the applicability of membraneintroduction MS (MIMS) for this purpose. Although LC/MS may be very efficient, this method has not yet been applied to the study of chloramines. Bromate (BrO3 − ) is not commonly found in natural water, but it does form as DBPs due to the interaction between ozone and naturally occurring bromides. Although it is an inorganic anion, BrO3 − can be detected and quantified in water by GC/MS. A method that derivatizes BrO3 − into volatile bromophenols or bromoanilines to measure the latter by headspace SPME/GC/MS has been developed (101). This approach can be applied to seawater samples in which the levels of 15 common inorganic ions are 1,000-fold higher than those of BrO3 − . Because emerging contaminants are often detected in surface waters, their presence in the inflows of drinking water treatment plants is inevitable. Several studies have described the aquatic chlorination of these compounds in disinfected water. A study of the transformation of the antiinflammatory drug diclofenac in the presence of excess active chlorine (102) used LC/MS and LC/MS/MS in APCI mode to identify several degradation products that form due to chlorine www.annualreviews.org • Environmental Mass Spectrometry

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electrophilic attack on the aromatic ring or the amine functional group of diclofenac. The chlorination products of amphetamine and five related compounds have also been studied (103); two previously unknown DBPs were identified and confirmed through synthesis. Both compounds were detected in samples collected at a drinking water treatment plant in Barcelona. Furthermore, Krkoˇsek et al. (104) studied the chlorination of an antihyperlipidemic agent, gemfibrozil. These authors identified four chlorination products by using GC/MS, ESI/MS/MS, and NMR. HRMS confirmed the proposed structures of these compounds. Triclosan is a widely used antimicrobial compound that is incorporated into an increasing number of consumer products. Because 96% of these products are disposed of in residential drains, large triclosan loads have been measured in WWTPs inflows (105); therefore, triclosan is an emerging contaminant. Following exposure to active chlorine at WWTPs, triclosan forms three chlorinated derivatives because of the introduction of chlorine into an activated phenol ring. Photolysis is one of the major pathways of degradation of triclosan and its chlorinated derivatives which are converted to chlorinated dioxins. 2,8-DCDD (dichlorinated dioxin) is generated from triclosan itself, and 2,3,7-triCDD (trichlorodibenzo-p-dioxin), 1,2,8-triCDD, and 1,2,3,8-TCDD (tetrachlorinated dioxin) from its chlorinated derivatives. These congeners are toxic; the general cell toxicity is approximately 1% that of 2,3,7,8-TCDD (105). N-nitrosamines are well-known environmental pollutants. For a long time, investigators paid a great deal of attention to the carcinogen N-nitrosodimethylamine (NDMA), which is a degradation product of asymmetric dimethylhydrazine, a rocket fuel. Later, NDMA and other N-nitrosamines were identified during the chloramination of drinking water. Using MS, Kemper et al. (106) studied various precursors of NDMA and the mechanisms of its formation. They showed that quaternary amines may be the precursors of this ecotoxicant. The EPA included five N-nitrosamines in its final drinking water contaminant candidate list (107) and added the six most widespread N-nitrosamines to its unregulated contaminant monitoring rule (108). Many MS procedures have been developed to identify and quantify various N-nitrosamines in water. Pozzi et al. (109) used several GC/MS approaches to determine nine N-nitrosamines in drinking water and swimming pools. Chemical ionization (CI), followed by selected ion storage, appeared to be more sensitive than either EI or CI/MS/MS. N-nitrosopyrrolidine was detected in all the samples from the swimming pools; this compound formed as a result of an interaction between proline, a main constituent of skin collagen, and disinfecting agents. A sensitive and reliable LC/APCI/MS/MS method for the detection and quantification of eight N-nitrosamines in drinking water has also been developed (110). Two transitions per compound were recorded (MS/MS), and the empirical formula of the product ions was confirmed by accurate mass measurements. Isotopically labeled N-nitrosamines were used as internal standards. The performance of LC/MS/MS equaled that of GC/MS in an analysis of nine N-nitrosamines in the EPA 521 standard mixture (111), whereas with regard to a broader range of such ecotoxicants, LC/MS/MS is better able to detect and identify new nitrosamines, as well as other DBPs. HRMS is likely to become important, especially in wastewater analysis, in ensuring lower detection limits while detecting multiple N-nitrosamines (111).

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3.6. Other Emerging Contaminants Nonylphenols, precursors to commercial detergents and surfactants, pose a major concern due to the high risk of human exposure and their toxicity. Rabouan et al. (30) reviewed various GC/MS and LC/MS approaches for the analysis of nonylphenols and their ethoxylated derivatives in environmental samples; they described the difficulties that arise when selecting reference material for such analyses because of the multiplicity of isomers and the variability in composition of batches. 174

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These authors proposed using 4-(1-ethyl-1,3-dimethylpentyl)phenol (353NP), in the form of a diastereomeric mixture, as a reference standard for LC/APCI/MS/MS. They emphasized that, although 353NP does not allow one to quantify all nonylphenols, evidence of its presence enables an estimation of nonylphenol exposure and the levels of the related compounds. Selenium is a trace element for which there is only a small difference between essential and toxic levels. It is released from both natural and anthropogenic sources (112), and its toxicity depends on its chemical form. Dimethylselenide and dimethyldiselenide are the main selenium forms released into the atmosphere. Headspace hollow-fiber protected liquid-phase microextraction, combined with GC/MS, has been used to determine these two compounds in environmental samples (water and soil extracts); the detection limits were approximately 60 ng liter−1 (112). Another study proposed the use of a real-time method of quantifying trace amounts of H2 Se, CH3 SeH, (CH3 )2 Se, and CH3 –Se–Se–CH3 vapors in ambient air with ion flow-tube MS to analyze these chemicals’ emissions from maize (113). A new, sensitive, and reliable laser diode thermal desorption/APCI/MS/MS method has been developed for the direct analysis of explosives in water (114). Nitroaromatic compounds were detected as anion radicals, and nitrate esters and nitramines were detected as adducts with chloride anions. The sample-to-sample run time of 15 s makes the method approximately 60 times faster than the classical approaches. The technique is solvent free and requires very little sample manipulation. Although organometallic compounds are increasingly being used for various purposes, despite their known adverse environmental effects, few studies have performed MS of these compounds. The most typical approach involves bulk measurements performed with inductively coupled plasma MS. The MS behavior of these ecotoxicants is peculiar in many ways. Reference 19 briefly describes the ionization and fragmentation processes of various organometallic compounds, mainly in ESI conditions. Some examples of such compounds are discussed in Reference 2. Organometallic compounds represent a class of chemicals for which MS has not yet demonstrated wide applicability.

4. SAMPLING AND AMBIENT IONIZATION MASS SPECTROMETRY 4.1. Sampling and Sample Preparation For many years, a drawback of the use of MS involved the need for often-complicated sample preparation procedures. Sample preparation in MS analysis has been reviewed elsewhere (3, 31). However, some innovations in this field have recently been reported. Dispersive liquid-liquid microextraction (DLLME), followed by SALDI, has been used (115) for the determination of antibiotics in water and urine matrices. In-syringe demulsified DLLME uses low-density extraction solvents for highly sensitive determination of fungicides in water samples with LC/MS (116). Investigators have developed a simple version of supported liquid extraction that employs diatomaceous earth in small cartridges to extract organic analytes from 1 ml of water for further MS analysis (117). A fast, inexpensive, and simple approach for GC/MS analysis of several priority pollutants has also been reported (118). The sensitivity of modern instruments allows one to modify EPA Method 8270D dealing with GC/MS analysis of priority pollutants in environmental samples (119) for use with 1 ml (rather than 1 liter) of water without compromising the detection limits. The method involves mixing 1 ml of a water sample, 1 ml of dichloromethane, and anhydrous sodium sulfate. After the mixture is shaken for several minutes, all the organic compounds appear in the organic phase with reasonable recoveries and no need for a concentration step. www.annualreviews.org • Environmental Mass Spectrometry

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DART: direct analysis in real time

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In a novel GC/MS approach for the rapid analysis of PFAs in water (120), an ion-pair reagent (tetrabutylammonium hydrogen sulfate), a dispersive reagent (ethanol), and an extraction solvent (chloroform) were added directly into a water sample for ion-pairing formation and dispersive extraction under violent agitation for 30 s. The extraction was complete within 6 min, making the method simpler and faster than traditional extractions. PFA salts and tetrabutylammonium hydrogen sulfate (in chloroform) transformed into the butyl esters of the corresponding PFAs inside a GC injector. The use of GC/MS/MS analysis in NICI mode allowed for sensitive, selective, and reliable quantification (two SRM transitions per compound) of the original analytes (120). MIMS has been employed in online, real-time chemical analysis for more than 35 years (see Reference 15 and references therein). Because MIMS requires no sample handling, cleanup, and separation steps, it provides a fast alternative to classic MS analysis. Analytes are usually removed from the membrane by desorption into the gas phase. However, analytes with vapor pressures less than 1–3 Pa are difficult to desorb into a carrier gas, even with thermal assistance. Duncan et al. (121) developed a capillary hollow-fiber membrane interface, using methanol as the acceptor phase, to deliver target analytes to the ESI source of a QQQ instrument. This approach enabled continuous online monitoring of polar and charged analytes in complex aqueous samples. The system can be operated in either continuous-flow mode or stopped acceptor-flow mode to increase sensitivity. The interface was successfully used to detect pharmaceuticals, phenols, and other contaminants in natural water and artificial urine. Online monitoring of phenol’s aquatic chlorination reaction has also been performed (121). Trends in MIMS during the past decade and many successful applications of MIMS coupled to portable instruments are described in Reference 15. The combination of portable instrumentation and MIMS is clearly an attractive tool for on-site, online measurements of environmental pollutants in various matrices (122).

4.2. Ambient Ionization Methods Although GC/MS/MS and LC/MS/MS (especially in HRMS mode) demonstrate excellent analytical performance, they often require time-consuming sample preparation prior to analysis. Ambient ionization avoids sample preparation. Harris et al. (12) classified the existing ambient ionization methods into seven categories related to the mechanism of ion formation. The main promise of ambient MS lies in the delivery of a greatly simplified way to perform analysis. Here, the term ambient means that the technique should operate in the open-air (ambient) environment; should be easily coupled with various types of mass spectrometers without requiring substantial MS modification; and should be a soft ionization method that provides direct ionization for rapid, real-time, and high-throughput analyses with minimal or no sample preparation. Several recent reviews, which may be considered tutorials, describe ambient MS methods (12, 13, 123, 124). So far, desorption ESI (DESI) and direct analysis in real time (DART) are the leading ambient ionization techniques (18). HPLC has been coupled with DART for the analysis of pharmaceuticals and PCPs in water (125). Important advantages of DART are its low tendency toward ion suppression and the ability to use phosphate-based buffers. A comparative study of the DART, ESI, APCI, and APPI methods with HPLC demonstrated the excellent performance of DART; the results showed that the rate of ion suppression was less than 11% for all the analytes studied in river water samples and for 9 out of 12 analytes in wastewater samples. All the other MS techniques demonstrated much more significant matrix suppression. Laskin et al. (126) recently reported promising applications of nano-DESI, a variant on the early DESI method. In addition to its chemical and biological applications, reactive nano-DESI enables chemical analyses of complex organic mixtures with selective identification and quantification 176

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of compounds containing particular functional groups. The same technique proved useful for petroleomics (127). When acetonitrile/toluene was used as a solvent, nano-DESI generated mass spectra of crude oil samples that were similar to those obtained with ESI or APPI. In contrast, both positive and negative mode spectra obtained with polar acetonitrile/water and methanol/water solvents were dominated by Ox and Ox Sy compounds. Minimal overlap was observed between the neutral species detected in different ionization modes, indicating their complementary nature. Because these compounds are water-soluble components of crude oil, the ability to detect them in crude oil without sample preparation is an important advantage of the nano-DESI (127), given that their detection using ESI is not easy. Easy ambient sonic-spray ionization MS (EASI/MS), a technique developed by the Eberlin group (128), is well suited for environmental analysis. Among ambient techniques, EASI/MS is one of the easiest to assemble. It provides ultrasoft ionization without the need for voltage, UV radiation, laser beams, or heat. This method has been successfully used to analyze triacylglycerols in fat and meat samples (129). The simplicity of sampling was impressive: First, a slice of fat or meat was placed on a paper surface. Second, a few drops of MeOH/CHCl3 solution were dripped onto the sample surface, which was heated for approximately 90 s. Third, the sample was removed while triacylglycerols imprinted on the paper surface were analyzed by EASI/MS with sonic spray of pure methanol with N2 nebulizing gas. No substantial alterations were observed in the fat profiles, even after 7 days. The paper could be sent to any laboratory for remote analysis. Venturi EASI/MS in both liquid (VL -EASI/MS) and solid sample (VS -EASI/MS) modes provided nearly immediate and specific identification of timber, as has been demonstrated for mahogany and six other, similar types of wood (130). Only a single wood chip is required to perform such an analysis within a few minutes. So-called Spartan V-EASI (131) represents a further development of earlier EASI and V-EASI techniques (123) that involves the use of inexpensive and readily available parts, including a surgical two-way catheter, an aerosol can of compressed air, a fused-silica capillary, and a hypodermic needle (Figure 3a). Simultaneous self-pumping and sonic-spray ionization are efficient because a constant gas flow is available from the can containing compressed air and chlorofluorocarbons as propellants. This matrix does not interfere in the experiments described, resulting in V-EASI mass spectra that are free from background in both positive and negative ion modes. For liquid samples, the end of the capillary is dipped directly into the analyte solution. For solid samples, methanol comes from the sample vial while the tip of the hypodermic needle is positioned close to the mass spectrometer entrance. The spray bombards the sample surface, causing desorption and ionization of the analyte molecules. This approach has been successfully used to analyze solutions containing various compounds (131). The authors of this study stated that this technique is valuable primarily for giving significantly improved signal-to-noise (S/N) ratios rather than increasing the absolute intensities of the signals. Such devices would be especially useful when coupled with a portable mass spectrometer. The contactless atmospheric-pressure ionization technique (132) includes a short capillary used to drive liquid samples from a microscale reservoir toward the outlet. Fine droplets form at the tapered tip of the capillary in the vicinity of the mass spectrometer entrance. This approach is applicable to various organic compounds in complex matrices. Cooks and colleagues (133) developed a simple and promising method termed paper spray. Paper-spray ionization mechanisms are described in Reference 134, and the effects of experimental variables on the various biomedical applications of this technique are discussed in Reference 135. Paper-spray ionization involves the release of ionized analytes into an electrospray when an electrical potential is applied to a solvent-wetted paper triangle bearing the sample. This approach has been successfully employed for rapid in situ detection of diphenylamine and thiabendazole on the surface and in the tissues of fruits through the use of a portable mass spectrometer equipped www.annualreviews.org • Environmental Mass Spectrometry

EASI/MS: easy ambient sonic-spray ionization mass spectrometry

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a MS

Sample Capillary

Needle

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Figure 3 (a) “Spartan” Venturi easy ambient sonic-spray ionization. Reproduced courtesy of M. Eberlin. Abbreviation: MS, mass spectrometer. (b) Leaf spray. Reproduced courtesy of R.G. Cooks.

with an ambient ionization source (136). Paper-spray ionization was performed by wiping the surface of an orange, then ionizing the chemicals directly from the wipe. The agrochemicals on the orange were identified immediately. Paper-spray ionization followed by MS/MS analysis enabled semiquantitative determination of plasticizers, Sudan red, melamine, and other contaminants in various foods (137). The simplicity of operation along with the advantages of using MS/MS make this approach an excellent high-throughput method of screening for the presence of various contaminants in foods and environmental samples. In 2011, Cooks and colleagues (138) developed a method derived from paper spray that they termed leaf spray (LS). This technique is convenient for direct, rapid screening of anthropogenic and biogenic compounds in the leaves, peel, and pulp of fruits and vegetables (Figure 3b). The emission of droplets containing various compounds can be studied by cutting a tip from the plant material and applying an electrical potential. If the plant is dry, a drop of solvent improves the process. The spray of charged droplets carries compounds from the plant to an adjacent mass spectrometer. LS operates continuously during a period of minutes, which is enough to perform MS/MS and make mass measurements. LS has been used to identify various pesticide residues; the detection limits were well below regulatory levels (139). The results were confirmed by MS/MS. Each analysis took approximately 100 s and required very small volumes of solvent. All these experiments (139) were conducted in the ambient environment, so this technique could be applied to in situ analyses with portable mass spectrometers in the future. 178

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Organic orange flavedo + 100 ng g–1 imazalil 100

Relative abundance

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m/z Figure 4 MS2 and (inset) MS3 spectra of imazalil by leaf spray. Reproduced courtesy of R.G. Cooks.

LS/MS has been used to instantaneously detect allergenic compounds in poison ivy leaves (140). In negative ion mode, the efficiency of ionization was enhanced following the addition of chloride anions to the spray solution. When coupled with a high-resolution tandem mass spectrometer, LS enabled the detection and structural elucidation of complex organic compounds, such as glycosides in fresh, untreated leaves (141). A comparison between these results and those obtained with other ambient methods (e.g., DESI, paper-spray ionization, and low-temperature plasma) demonstrated that LS provided the highest-quality spectra.

5. PORTABLE MASS SPECTROMETERS Another shortcoming of older MS techniques involved the size of the instruments. The development of field-portable devices for fast, on-site analysis is one of the greatest challenges in analytical chemistry. Instruments operated in situ are required to rapidly detect and identify a wide range of compounds, including those at trace levels. Such instruments should produce fast results and be able to tolerate various external conditions. They should also be able to distinguish the compounds of interest, including closely related compounds in complex matrices; should be portable for ease of transportation, simple to use, and robust; and should require minimal sample preparation. Essential information about the status of these devices is provided in References 32 and 122. The combination of MIMS and portable mass spectrometers is discussed in References 15 and 122. The main problem with miniaturization involves sensitivity. Recent publications have, however, demonstrated some success with miniature devices. Handheld mass spectrometers used to measure the levels of agrochemicals directly on the skin of fruits for sale can perform several stages of MS/MS (136). Protonated diphenylamine was analyzed in MS2 mode, and imazalil was identified in MS3 mode (Figure 4). A reduced-size low-temperature plasma ionization probe coupled with a portable Mini 11.5 mass spectrometer was used for the in situ detection of trace amounts of explosives (142). The analysis was conducted in less than 1 min, and detection limits of less than 10 ng were obtained for all the analytes. These examples demonstrate the power of in situ analyses of complex samples through the use of ambient ionization and handheld mass spectrometers. www.annualreviews.org • Environmental Mass Spectrometry

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6. ISOTOPE RATIO MASS SPECTROMETRY IRMS: isotope ratio mass spectrometry

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CSIA: compound-specific isotope analysis

Isotope ratio MS (IRMS) is a fascinating technique that allows one to draw reliable and elegant conclusions on complex systems. Investigators conducting experiments on the degradation of crude oil in soil demonstrated, on the basis of δ13 C measurements, that microbial transformation of oil hydrocarbons into products that are accessible as substrates for other living systems could be treated as a source of organic fertilizers, which could stimulate plant growth (143). Reference 144 describes the basic principles of this method. Multiple-isotope analysis provides information about the degradation pathways of various pollutants and enables identification of the contamination source. It is possible to perform bulk isotope ratio measurements of a certain element in a complex sample, or measurements of the isotopic composition of an individual compound [compound-specific isotope analysis (CSIA)]. The isotope pattern can provide additional information to unequivocally elucidate an environmental process (for example, it can differentiate biotic and abiotic processes or identify transformation reactions). Reference 22 describes the principles and applications of CSIA. The lack of available GC- and LC-amenable organic standards with known isotope compositions remains a significant problem for CSIA users. Although hydrogen, carbon, nitrogen, and oxygen isotope ratio measurements are widely used (144), there is a definite demand for the development of reliable approaches for the analogous analysis of other elements, namely environmentally relevant chlorine, bromine, and sulfur. Reference 23 reviews CSIA results for isotope ratios of bromine- and chlorine-containing compounds and describes recent applications of these techniques, including analyses of simple volatile organic compounds and complex mixtures of PCBs, PBDEs, and PCDDs. Bernstein et al. (145) showed that chlorine isotope values could be confidentially assessed if they differed by at least 0.4 in GC/IRMS measurements and by 2–4 in GC/MS measurements carried out with quadrupole instruments. Another promising report (146) describes an IRMS analysis of organic compounds containing chlorine, bromine, or sulfur atoms and discusses their quantitative conversion into HCl, HBr, and H2 S. McHugh et al. (147) applied CSIA to five residences and found that outdoor and indoor sources of trichloro- and tetrachloroethanes often exhibited distinct carbon and chlorine isotope ratios. Mundle et al. (148) determined the enrichment factors associated with microbial degradation of ethene in anaerobic and aerobic microcosms; these results allowed the authors to determine whether biotransformation of ethane and biodegradation of the chlorinated ethenes occurred.

7. IMAGING MASS SPECTROMETRY Imaging MS is another fascinating technique, which was developed at the very end of the twentieth century. This unique method allows the detection and mapping of thousands of chemical compounds in, for example, tissues, rocks, fruits, and artworks, enabling the determination of the spatial distribution of any pollutant or its metabolite inside the sample. A short, informative review (149) describes mainly the medical applications of imaging MS. Imaging techniques used to study environmental and biological problems have been reviewed elsewhere (150). The state of ambient MS techniques and their application for in vivo analysis and in situ molecular tissue imaging are described in Reference 33. The authors of this study compared the abilities of various recently reported ambient methods; they observed that molecular assignment protocols typically involve accurate mass measurements and searches against available spectral databases. Positive matches are elucidated on the basis of isotope distribution and molecular fragmentation behavior in tandem experiments and are ultimately confirmed by comparison with chemical standards. Because the correct identification of an isomer is often of primary importance 180

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ION MOBILITY SPECTROMETRY Ion mobility spectrometry (IMS), an independent analytical tool related to MS, is advantageous with respect to the size, weight, and power consumption of the instrumentation. Their fast and accurate measurement ability and portability, as well as the absence of vacuum requirements, make IMS instruments promising devices for infield analyses. The complexity of environmental matrices, the influence of humidity, and the achievable limits of detection are current challenges. These problems and various environmental applications of IMS are reviewed in Reference 21. The application of IMS and MS for the detection of explosives is discussed in Reference 156. The origin and principles of operation of selected ion flow-tube MS and proton-transfer reaction MS (PTR MS), as well as their application to analysis of volatile compounds, are reviewed in Reference 20. A modified instrument that can ionize compounds via charge-transfer reactions has recently been developed (157). In this instrument, all the advantages of PTR MS are preserved, with the added possibility of utilizing Kr+ as another ionizing agent, which allows for measurements of an increased number of analytes. PTR MS instruments could become universal gas analyzers with the advantages of exceptional sensitivity, low online detection limits, and the versatility of ambient ionization MS.

for environmental analyses, sample separation may be helpful as an orthogonal dimension for unambiguous identification. Some plants function as indoor-air purifiers. Secondary ion (Cs+ ) MS imaging has been used to locate atmospheric pollutants within such plants (151). The distribution of bromine in the leaves and roots of Hedera helix growing in an atmosphere with a high level of bromotoluene has been studied. The authors obtained lateral resolution down to 50 nm. However, because of the high ionization energy, the authors investigated only the distribution of Br− , rather than that of particular organobromines. Sample preparation for imaging may not be necessary. An analysis of tissue samples in the ambient environment with a spatial resolution of better than 12 μm and a high S/N ratio of each pixel demonstrated the efficiency of nano-DESI imaging (152). The main advantage of this method is that no sample pretreatment is needed prior to analysis.

8. CONCLUSION Modern environmental MS can handle any potential ecotoxicant. The absence of an MS method for a certain anthropogenic pollutant means that this compound has not yet attracted the attention of researchers. MS methods have demonstrated excellent results in studies of extremely complex mixtures. Their sensitivity is better than that of any available alternative. However, a real revolution in analytical chemistry could be triggered with the appearance of robust, simple, and sensitive portable mass spectrometers that can apply ambient ionization techniques. If the cost of such instruments is reasonable, then mass spectrometers may become valuable household devices, ushering in a new era in MS. Other future directions related to environmental MS [petroleomics (153), huminomics (154), ion mobility spectrometry (20, 21, 156, 157; also see the sidebar), and aerosol MS (155)] are important, but because of space constraints, they are not discussed herein.

DISCLOSURE STATEMENT The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. www.annualreviews.org • Environmental Mass Spectrometry

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Annual Review of Analytical Chemistry

Contents

Annual Review of Analytical Chemistry 2013.6:163-189. Downloaded from www.annualreviews.org by Moscow State University - Scientific Library of Lomonosov on 06/24/13. For personal use only.

Volume 6, 2013

Is the Focus on “Molecules” Obsolete? George M. Whitesides p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1 Synthetic Nanoelectronic Probes for Biological Cells and Tissues Bozhi Tian and Charles M. Lieber p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p31 Multiplexed Sensing and Imaging with Colloidal Nano- and Microparticles Susana Carregal-Romero, Encarnaci´on Caballero-D´ıaz, Lule Beqa, Abuelmagd M. Abdelmonem, Markus Ochs, Dominik Huhn, ¨ Bartolome Simonet Suau, Miguel Valcarcel, and Wolfgang J. Parak p p p p p p p p p p p p p p p p p p53 Nanobiodevices for Biomolecule Analysis and Imaging Takao Yasui, Noritada Kaji, and Yoshinobu Baba p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p83 Probing Molecular Solids with Low-Energy Ions Soumabha Bag, Radha Gobinda Bhuin, Ganapati Natarajan, and T. Pradeep p p p p p p p p p p97 Microfluidic Chips for Immunoassays Kwi Nam Han, Cheng Ai Li, and Gi Hun Seong p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 119 Semiconductor Quantum Dots for Bioimaging and Biodiagnostic Applications Brad A. Kairdolf, Andrew M. Smith, Todd H. Stokes, May D. Wang, Andrew N. Young, and Shuming Nie p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 143 Environmental Mass Spectrometry Albert T. Lebedev p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 163 Evidence-Based Point-of-Care Diagnostics: Current Status and Emerging Technologies Cangel Pui Yee Chan, Wing Cheung Mak, Kwan Yee Cheung, King Keung Sin, Cheuk Man Yu, Timothy H. Rainer, and Reinhard Renneberg p p p p p p p p p p p p p p p p p p p p p p p 191 Adsorption and Assembly of Ions and Organic Molecules at Electrochemical Interfaces: Nanoscale Aspects Soichiro Yoshimoto and Kingo Itaya p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 213

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Structural Glycomic Analyses at High Sensitivity: A Decade of Progress William R. Alley, Jr. and Milos V. Novotny p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 237 Structures of Biomolecular Ions in the Gas Phase Probed by Infrared Light Sources Corey N. Stedwell, Johan F. Galindo, Adrian E. Roitberg, and Nicolas C. Polfer p p p p p p 267

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Next-Generation Sequencing Platforms Elaine R. Mardis p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 287 Structure Determination of Membrane Proteins by Nuclear Magnetic Resonance Spectroscopy Stanley J. Opella p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 305 Scanning Electrochemical Cell Microscopy: A Versatile Technique for Nanoscale Electrochemistry and Functional Imaging Neil Ebejer, Aleix G. Guell, ¨ Stanley C.S. Lai, Kim McKelvey, Michael E. Snowden, and Patrick R. Unwin p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 329 Continuous Separation Principles Using External Microaction Forces Hitoshi Watarai p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 353 Modern Raman Imaging: Vibrational Spectroscopy on the Micrometer and Nanometer Scales Lothar Opilik, Thomas Schmid, and Renato Zenobi p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 379 The Use of Synchrotron Radiation for the Characterization of Artists’ Pigments and Paintings Koen Janssens, Matthias Alfeld, Geert Van der Snickt, Wout De Nolf, Frederik Vanmeert, Marie Radepont, Letizia Monico, Joris Dik, Marine Cotte, Gerald Falkenberg, Costanza Miliani, and Brunetto G. Brunetti p p p p p p p p p p p p p p p p p p p p p 399 Real-Time Clinical Monitoring of Biomolecules Michelle L. Rogers and Martyn G. Boutelle p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 427 Indexes Cumulative Index of Contributing Authors, Volumes 1–6 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 455 Cumulative Index of Article Titles, Volumes 1–6 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 459 Errata An online log of corrections to Annual Review of Analytical Chemistry articles may be found at http://arjournals.annualreviews.org/errata/anchem

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