Rapid Characterization of Naphthenic Acids Using Differential Mobility

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Rapid Characterization of Naphthenic Acids Using Differential Mobility Spectrometry and Mass Spectrometry Matthew R. Noestheden,†,∥ John V. Headley,‡ Kerry M. Peru,‡ Mark P. Barrow,§ Lyle L. Burton,† Takeo Sakuma,† Paul Winkler,† and J. Larry Campbell*,†,∥ †

AB SCIEX, Concord, Ontario, Canada L4K 4V8 Water Science and Technology Directorate, Environment Canada, Saskatoon, Saskatchewan, Canada S7N 3H5 § Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom ‡

S Supporting Information *

ABSTRACT: To analyze the naphthenic acid content of environmental waters quickly and efficiently, we have developed a method that employs differential mobility spectrometry (DMS) coupled to mass spectrometry (MS). This technique combines the benefits of infusion-based MS experiments (parallel, on-demand access to individual components) with DMS’s ability to provide liquid chromatography-like separations of isobaric and isomeric compounds in a fraction of the time. In this study, we have applied a DMS-MS workflow to the rapid gas-phase separation of naphthenic acids (NAs) within a technical standard and a real-world oil sands process-affected water (OSPW) extract. Among the findings provided by this workflow are the rapid characterization of isomeric NAs (i.e., same molecular formulas) in a complex OSPW sample, the ability to use DMS to isolate individual NA components (including isomeric NAs) for in-depth structural analyses, and a method by which NA analytes, background ions, and dimer species can be characterized by their distinct behaviors in DMS. Overall, the profiles of the NA content of the technical and OSPW samples were consistent with published values for similar samples, such that the benefits of DMS technology do not detract from the workflow’s accuracy or quality.

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INTRODUCTION Naphthenic acids (NA) from oil sands process-affected waters (OSPW) have been the subject of numerous mass spectrometrybased environmental studies.1,2 The classical definition of NAs (CnH2n+zO2, where z is an even negative integer representing hydrogen deficiency) has recently been expanded to the naphthenic acid fraction component (NAFC), which includes unsaturated and aromatic NA derivatives, with increased oxygen content and compounds containing nitrogen and/or sulfur.1,3,4 NAFCs are of particular concern in northern Alberta, Canada, where the caustic extraction of bitumen from surface mineable oil sands produces large volumes of OSPW.5 The documented toxicological properties of NAFCs in OSPWs are of significant interest to both regulatory agencies and industry.3,6−11 One common analytical workflow for NAFC analysis employs liquid chromatography (LC)12−17 or gas chromatography (GC)18 prior to sample analysis by mass spectrometry (MS). Variations on these themes include off-line fractionation19 and two-dimensional GC.20−22 The rationale for using chromatography involves (1) reduced sample complexity by separating NAFC analytes from background contaminants, (2) mitigation of ion suppression, and (3) improvement of the method resolving power. Unfortunately, the high complexity of OSPW extracts often necessitates relatively long run times, multiple sample handling steps, and multidimensional chromatography. © 2014 American Chemical Society

Moreover, LC and GC are disadvantaged by being inherently serial processes, delivering each analyte to the MS for only a few seconds. This results in limited time to perform MS analyses and/or deeper structural interrogation (e.g., fragmentation using tandem mass spectrometry or MS/MS). Another common workflow for NAFC analyses involves direct infusion of minimally processed liquid samples into a mass spectrometer.3,6,19−23 Ultrahigh resolution mass spectrometry (resolving power > 100 000) is critical to obtaining useful NAFC profiles with this approach.3,5,13,14,24 While these instruments can provide elemental compositions, additional steps are required to interrogate molecular structure (e.g., MS/MS). For example, if multiple isobaric analytes (i.e., same integer m/z value but different fractional m/z value) are present within a narrow mass window (e.g., m/z 425. This was unexpected given that the majority of reported NAFCs in

Figure 1. DMS-MS behavior of an OSPW extract with (red) and without (blue) MeOH (1.5%v/v) added to the N2 carrier gas. The addition of MeOH greatly improved method resolving power, as demonstrated by the total ion chromatograms (TICs; top). Extracted ion chromatograms (XICs; m/z 255.2 ± 0.1 Da) are shown as an example of typical peak widths using DMS-MS (bottom). Signal intensity was normalized for ease of data presentation. fwhm, full width at half-maximum height.

It was important to demonstrate with these complex OSPW extracts that DMS-MS could separate species without compromising dynamic range.13 Indeed, signal loss was minimal when operating under the DMS-MS conditions presented here. For example, the peak intensity of m/z 235.1804 was sustained at approximately 7.0 × 104 counts per second (cps) during an infusion experiment (no DMS employed), while the same ion had an intensity of 8.0 × 104 cps during optimal transmission through the DMS (CV = −25.7 V; Figure S3). In addition, there was no adverse effect on total integrated signal as a function of increasing SV (from 0 Vpp to 4000 Vpp, Figure S3). Also noteworthy was the finding that the sample solvent did not appear to have an appreciable effect on the NAFCs detected (data not shown). A global overview of the NAFC ionograms from the Merichem and OSPW samples reveals a bimodal distribution of ions

Figure 2. General behavior of NAFCs by DMS-MS. Evaluation of carbon and z series (left) demonstrated that DMS-MS behavior was not simply correlated to m/z. Trends in CV shifts for carbon number (top right) and z series (bottom right) are consistent with reported behavior of similar samples. Data were normalized to 100% within each ionogram and are shown for an OSPW extract. 10266

dx.doi.org/10.1021/es501821h | Environ. Sci. Technol. 2014, 48, 10264−10272

Environmental Science & Technology

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analysis of each m/z 143.1080 ion was easily performed by fixing the CV at the value corresponding to each isomer. Interestingly, the resulting MS/MS spectra for m/z 143.1080 yielded fragmentation patterns that are consistent with the presence of distinct isomeric species (Figure 3). Analysis of three authentic C8H16O2 isomers (n-octanoic acid, 2-ethylhexanoic acid, and 3propylpentanoic acid) revealed that the OSPW extract contained 2-ethylhexanoic acid and n-octanoic acid. Separation of such isomers by GC or LC generally requires minutes of elution time.47 Using DMS, these isomers were separated in seconds and could be analyzed on-demand. We also encountered several examples where DMS separated isobaric species within the NAFC samples. In one case, palmitic acid (C16H31O2−), which is a known contaminant in laboratory environments, was observed at m/z 255.2329 (Figure 4). Alongside the ionized palmitic acid was an isobaric ion at m/z 255.1405. ToF-MS alone would easily resolve these ions, but further interrogation by MS/MS would be complicated without DMS since both ion populations would be sampled by the quadrupole mass filter and a heavily convolved MS/MS spectrum would result. However, using DMS-MS the ions of m/z 255.1405 were transmitted at CV = −26 V, while those ions at m/z 255.2329 (palmitic acid) transmitted at CV = −21 V (Figure 4). Like the C8H16O2 isomer separation, selectively tuning the CV facilitated the interrogation of each isobaric ion in real time (MS and MS/MS) without complications from other interferences. The MS/MS of m/z 255.1407 (CV = −26 V) showed carbon dioxide loss (m/z 211.1520, 13.2 ppm) as the dominant product ion, suggesting a carboxylic acid (Figure 4). The elemental

OSPW extracts occur m/z 300 suggested incorrect identification. A detailed evaluation of the MS/MS spectra demonstrated the presence of sodium- and proton-bound heterodimers, as illustrated for one example in the included table.

composition C17H19O2− correlated with m/z 255.1405 (6.5 ppm). The anion at m/z 255.2329 (CV = −21 V) fragmented via loss of water (m/z 237.2234, 4.3 ppm) as the dominant product ion, which is consistent with palmitic acid.48,49 The mass spectra and DMS ionograms are inundated with similar isobaric pairings (Figure S5). DMS Mitigates the Effects of Interfering Species: Identification of Background Ions and Dimers. In this study, it was critical to obtain an accurate assessment of the potential interfering species present in the analytical samples. Such interferences come from background contamination and unintended variations in the analyte-derived signal. In both cases, DMS improves the analyses of NAFC-containing samples by sequestering such ions in discrete CV ranges, outside of which they will not appear in the analytical data. This is in contrast to LC-MS analyses of NAFCs, where interferences like palmitic acid, stearic acid, or dodecyl sulfate, often present in the LC system or solvents themselves, are ionized continuously throughout the chromatographic run. These ever-present background ions may deteriorate mass spectrometer perform-

ance by artificially increasing ion flux statistics for processes like automatic gain control, throttling the detection threshold, and potentially limiting the interscan linear dynamic range. To assess potential background contamination in our DMS workflow, we generated each of our working analytical solutions using a common protocol. This included the preparation of an analytical “blank” solution (i.e., 100% ACN) that made physical contact with the same analytical equipment as the NAFCcontaining samples (Merichem and OSPW). The analyses revealed 12 compounds that were present in the blank and in one or both of the sample types, including saturated and monounsaturated fatty acids, several of which are known background ions in laboratory solvents and analytical instrumentation (Table S2).50 One ion of interest was m/z 265.1477, which matched the formula C15H22O4 to within 12 ppm (Figure S6). This ion was present in all samples and displayed an interesting response in the presence of increasing concentration of Merichem standard (Figure S7). For example, we observed an increase in the response of m/z 265.2150 (an authentic NAFC signal) while the 10268



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Figure 6. Evaluation of Merichem and OSPW samples as a function of area response (left) and the number of database-identified compounds grouped by class (right). Results are further broken down by compounds class (top) and z value (bottom). Values shown above the homologue plots are the number of homologues identified during DB searching. Due to the large differences in absolute response between compound classes, these data were converted to a log10 scale.

signal intensity of m/z 265.1477 dropped as the Merichem concentration was increased. Unfortunately, the “C15H22O4” assignment for m/z 265.1477 was rejected as it was outside the ±10 ppm mass accuracy filter for DB searching. The Formula Finder feature of PeakView software suggested C12H26O4S as a more accurate formula assignment (0.8 ppm). This was supported by MS/MS spectra obtained for these DMS-isolated ions, which contained an intense product ion at m/z 96.9602 corresponding to HSO4− (1.0 ppm). These findings suggested that this compound was dodecyl sulfate, a widely used ionic surfactant.51,52 The NAFC DB used here would not find this compound since C12H26O4S is a z = +2 formula, which implies no double bonds are present (i.e., no carboxylic acid functionality). Incorrect assignment of this compound would have resulted in a 0.6% error in the total area response of the OSPW extract. We also used DMS to characterize other background ions, including an O3S series with z = −6. The series ranged from C16 to C19 with mass accuracies of 0.5−3.0 ppm. Unlike dodecyl sulfate, these formulas were present in the NAFC DB. With their high z value and O3S compound class, we suspected these components were benzylsulfonates, which are also well-known laboratory contaminants.52,53 MS/MS of these DMS-isolated ions verified the presence of alkyl-substituted benzylsulfonates (Figure S8).51 While this series exhibited relatively low responses, their inclusion in the Merichem and OSPW data could present a false indication of the compound classes present. These findings highlight the importance of carefully evaluating the results of DB searches, including routine examination of representative blanks. More specific to the analysis of OSPW extracts, the results suggest that sample preparation materials (including extraction solvents) should be assessed for background contamination since surfactants (ionic and nonionic) and fatty acids are known to be present in a variety of laboratory sources.50

Besides the aforementioned background ions, we also utilized the DMS to characterize unexpected forms of the NAFC molecules produced in an ESI-based experiment. While carboxylic acids deprotonate readily by negative-mode ESI, these analytes may also aggregate (e.g., form proton- or sodiumbound dimers, etc.), complicating DB searching. We employed the DMS to interrogate such a group of ions centered at CV ∼ 0 V that comprised larger m/z ions (>m/z 425). The original DB search led to assignment of O6N2, z = −4 compounds, spanning carbon number values of C24−C29 and m/z 455−526. However, subsequent MS/MS analyses of several individual ions demonstrated a conspicuous absence of product ions > m/z 300 (i.e., no loss of water or carbon dioxide identified; Figure 5). This suggested a possible misassignment by the DB search. Evaluating the MS/MS spectra revealed that the m/z values of the fragment ions between m/z 200−300 could represent monomeric species, where the precursor ions were not, in fact, O6N2 compounds but instead sodium- and proton-bound heterodimer species (Figure 5). Such heterodimers take the form [(C n H 2n+z O x ) a + (CnH2n+zOx)b − 2H + Na]−, where a and b represent different deprotonated NAFCs. They have been reported elsewhere54 and are produced during ESI and ion sampling.3 The presence of high levels of sodium hydroxide in the OSPW extract from the extraction method likely facilitates such heterodimer formation.38 As well, several sources make specific mention of adjusting MS parameters to mitigate dimer formation during NAFC analysis.3,54 For a technical Merichem standard, Grewer and coworkers reported 12% of the identified mass peaks (3.2% of the total ion current) as dimers.3 In the current study, the Merichem standard contained 28% of the identified peaks as dimers, or 11.3% of total ion current. (Note: these values are based on the assumption that all DB matches > C20 correspond to dimers.) These differences are likely accounted for by the known 10269



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and a small amount of higher oxygen content and heteroatomcontaining species. The OSPW extract showed a more widely distributed number of DB matches across the compound classes and z values evaluated (Figure 6). Interestingly, despite comprising 49% of the positive DB matches, O3−O8 compounds only accounted for 2.8% of the total area response. Similar to the Merichem results, this suggests that such compounds either have low response factors and/or are present at low levels. Conversely, the z-value results for the OSPW extract showed a relatively even distribution across the number of DB matches and the relative area responses of the different z classes. These results are consistent with the demonstrated composition of OSPW extracts, which are known to contain higher oxygen content and increased unsaturation/polycyclic compounds due to natural weathering and metabolic processes.1,3 A detailed breakdown of the compound classes (log10 scale) by area response showed that the OSPW extract was composed of mostly O2 species (86%), with significant amounts of O2S (6.6%), O3 (0.8%), and O3S (0.6%) compounds (Figure 6). In addition, several potentially interesting compound classes (e.g., O2NS, O5NS, and O4N2S2) were observed at very low abundances (m/z 450 (vide supra), we refined our selection of elemental parameters for the DB searches on the NAFC-containing samples. Initially, we chose to extend our DB beyond the commonly reported compositions of NAFCs in OSPW extracts to include larger m/z values6,23 using the following parameters: C5−30, z0−16, O2−8, N0−2, S0−2 (mass range: 96−640 Da). This DB was modified based upon the aforementioned workflow blank and dimer analyses to include a more broadly accepted mass range for NAFCs of 100−450 Da (or approximately C20).3,13,15,56 This new DB (C5−20, z0−16, O2−8, N0−2, S0−2) spanned a mass range of 96−500 Da and contained 8127 formulas. Compared with the C30 DB, a compound search using the C20 DB yielded a 28% reduction in the number of identified ions ( m/z 450 being recharacterized as dimers (vide supra). For comparison, we also analyzed the OSPW sample without using DMS; all of the DB identified ions were also found in the DMS-based data set, with similar mass assignments, S/N, and peak intensities (Figure S2). However, as stated previously, the DMS also provided evidence of additional multiple isomeric forms for several ions, leading to the additional identifications. Efforts to identify these isomers (e.g., as we did for C8H16O2, Figure 3) are underway. Characterization of the NAFC Content in a Technical Merichem Standard and OSPW Extract. All Merichem standard and OSPW extract DB matches were compiled as a function of compound class and z value. Results were broken down based on total area response and the number of homologues identified (Figure 6). Exact agreement between the distribution of ion classes observed in the current study and literature values was not expected given the heterogeneity of NAFCs.13,55 By area response (99%) and the number of identified homologues (97%), the Merichem standard consisted primarily of O2 species. Analysis of the Merichem results by z value showed a significant number (42%) of homologues z ≤ −6 (i.e., −8, −10, etc.). This result seemed contradictory to reported compositions, which state that the Merichem standard consists predominantly of compounds z ≥ −4.1,3 However, these compositions were all response comparisons, not an identification of the number of homologues present. The response data in the current study showed that those compounds z ≥ −4 accounted for 82% of the total area response, consistent with literature reports. The results by compound class (log10 scale) supported this comparison, with the Merichem standard containing almost exclusively O2 species 10270



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Figures S1−S10 and Tables S1−S4 are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 289-982-2863. E-mail: [email protected]. Author Contributions ∥

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. For Research Use Only. Not for use in diagnostic procedures. The trademarks mentioned herein are the property of AB Sciex Pte. Ltd. or their respective owners. AB SCIEX is being used under license. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Michael McDonell (AB SCIEX) for helpful discussions and motivation for this research, as well as Carmai Seto and Ashish Pradhan (AB SCIEX) for invaluable assistance with these experiments.

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Rapid Characterization of Naphthenic Acids Using Differential Mobility

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