Article pubs.acs.org/est
Comprehensive Analysis of Oil Sands Processed Water by DirectInfusion Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry with and without Offline UHPLC Sample Prefractionation Adrien Nyakas,† Jun Han,† Kerry M. Peru,‡ John V. Headley,‡ and Christoph H. Borchers*,†,§ †
University of Victoria - Genome British Columbia Proteomics Centre, Vancouver Island Technology Park, #3101 − 4464 Markham Street, Victoria, BC V8Z 7X8, Canada ‡ Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada § Department of Biochemistry and Microbiology, University of Victoria, Petch Building Room 207, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada S Supporting Information *
ABSTRACT: Oil sands processed water (OSPW) is the main byproduct of the large-scale bitumen extraction activity in the Athabasca oil sands region (Alberta, Canada). We have investigated the acid-extractable fraction (AEF) of OSPW by extraction-only (EO) direct infusion (DI) negative-ion mode electrospray ionization (ESI) on a 12T-Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS), as well as by offline ultrahigh performance liquid chromatography (UHPLC) followed by DI-FTICR-MS. A preliminary offline UHPLC separation into 8 fractions using a reversed-phase C4 column led to approximately twice as many detected peaks and identified compounds (973 peaks versus 2231 peaks, of which 856 and 1734 peaks, respectively, could be assigned to chemical formulas based on accurate mass measurements). Conversion of these masses to the Kendrick mass scale allowed the straightforward recognition of homologues. Naphthenic (CnH2n+zO2) and oxy-naphthenic (CnH2n+zOx) acids represented the largest group of molecules with assigned formulas (64%), followed by sulfur-containing compounds (23%) and nitrogencontaining compounds (8%). Pooling of corresponding fractions from two consecutive offline UHPLC runs prior to MS analysis resulted in ∼50% more assignments than a single injection, resulting in 3-fold increase of identifications compared to EO-DIFTICR-MS using the same volume of starting material. Liquid−liquid extraction followed by offline UHPLC fractionation thus holds enormous potential for a more comprehensive profiling of OSPW, which may provide a deeper understanding of its chemical nature and environmental impact.
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INTRODUCTION
The enormous complexity of OSPW has been shown in various studies, which in most cases have focused on the detection of organic acids, especially naphthenic acids (NAs).6 The general formula for NAs is CnH2n+zO2, where n is the number of carbon atoms and z is a negative, even integer representing the hydrogen deficiency of the NA.7 Small NAs have been shown to be degraded naturally by microorganisms, which leads to a considerable reduction in the acute toxicity of the OSPW.8,9 There is still a serious concern about the ecological influences of NAs in OSPW, given that the amount of OSPW stored in tailings ponds will increase dramatically in
In northern Alberta, Canada, large amounts of bitumen are stored in the form of oil sands, which has led to the formation of a rapidly growing industry specialized in mining the approximately 170 billion barrels of economically available bitumen.1,2 Oil sands are composed of clay, sand, water, and bitumen, of which only the latter is of commercial interest. Oil extraction is achieved by applying the Clark caustic hot-water method, in which hot alkaline water is used to release the bitumen of the surface-mined ore.3,4 For each barrel of recovered oil, approximately 2−4 barrels of water are used, generating a huge amount of slurry, which is stored in on-site tailing ponds due to its severe acute and chronic toxicity to the aquatic environment.5 Storage in the basins first leads to sedimentation of the solids, leaving a layer of water which is called oil sands processed water (OSPW).6 © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4471
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number resulted in an increased retention time, whereas a larger hydrogen deficiency (smaller z-value) decreased retention on a reversed-phased (RP) capillary HPLC column.24 In general, comprehensive analysis of on-site tailings ponds is of the utmost importance to provide high-standard quality control services for the surrounding aquatic environment, such as the Athabasca River in Alberta, Canada. In addition to providing compound lists that need to be monitored, knowledge about characteristic “chemical patterns” of the individual tailings ponds would allow tracing of the contaminants back to the precise source of pollution in a very short time frame. Consequently, the analysis of OSPW samples should be as comprehensive as possible, to facilitate recognition of characteristic compound profiles in different OSPW storage facilities. The purpose of this study was to investigate the effects of a preliminary RP-ultrahigh performance liquid chromatography (UHPLC) fractionation on the comprehensive characterization of OSPW by negative ion ESI-FTICR-MS. In this paper, we present an approach that combines FTICR-MS with offline LC prefractionation. Our studies show that the fractionation significantly increases the number of identified organic compounds in OSPW.
the next decade, without there being a solution for their safe release to the aquatic environment.10 The term NA is sometimes also used for further oxidized NAs or “oxy-naphthenic acids” (ONAs) CnH2n+zOx and, in some publications, even for the organic acid content in general.6,11 Grewer et al. stated that the initial definition for NAs is becoming obsolete, since more than 50% of the compounds in the acid extractable fraction (AEF) of OSPW correspond to either ONAs or heteroatom-containing molecules.6 In this manuscript, however, we used the term NAs for “classic” naphthenic acids (CnH2n+zO2) and ONAs for oxynaphthenic acids, unless stated otherwise. Compounds containing additional heteroatoms such as sulfur or nitrogen will be classified separately. In 1986, MacKinnon and Boerger12 showed that the FTIR spectrum of a commercial NA mixture was almost identical to the spectrum from an acidic extract of OSPW, leading to the assumption that NAs constitute the main toxic component in tailings pond water. Consequently, initial mass spectrometrybased investigations of NAs focused on the analysis of “classical” NAs (CnH2n+zO2). The initial studies were mainly performed using gas-chromatography mass spectrometry (GC/ MS).13,14 GC/MS, however, is not the ideal technique for the analysis of larger molecular weight NAs, which exhibit an inherently low volatility after chemical derivatization (usually to methyl- or t-butyl-dimethylsilyl esters).15 Thus, softer ionization methods were applied in later studies, such as fluoride ion chemical ionization MS16 and fast atom bombardment MS.17 With these soft ionization techniques, the interpretation of the mass spectra was enormously simplified since each compound generated one major peak without producing a large number of fragment ions upon gas-phase transfer. Due to its particularly soft ionization character and greater sensitivity, electrospray ionization (ESI)-MS is now the most commonly used technique for the analysis of NAs.18−23 Typically, the water samples are either acidified and extracted with a polar, organic solvent such as dichloromethane (DCM)21 or ethyl acetate (EA),24 or the NAs are concentrated from the water sample by solid-phase extraction, as shown by Headley et al.22 Prior to infusion into the mass spectrometer and analysis in the negative ion mode, the pH of the extract is raised by adding approximately 0.1% of ammonia to facilitate deprotonation of organic acids. At first, MS-based characterization of NA mixtures was performed at unit mass resolution.8,25−27 Martin et al. showed, however, that the use of high-resolution mass spectrometry significantly reduced false-positive detections as well as incorrect classifications.6,20 Due to the complexity of OSPW organic extracts, the use of Fourier transform ion cyclotron resonance (FTICR)-MS has been shown to significantly improve the integrity of profiling data, giving a more comprehensive overview of the organic acids that are present.11,28−30 FTICR-MS offers much higher mass resolution (>500 000) and mass accuracy ( 2) account for the vast majority of identified compounds (65%), followed by 4475
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Figure 4. Distribution of the detected homologues for the main sulfur- and nitrogen-oxide compound classes.
the relative peak intensity was not performed due to the fact that a variety of compounds were found to be distributed over multiple fractions. Figure 3 shows the distribution of all identified compounds belonging to the NA or ONA class
displaying the number of homologues within an entire compound class. Furthermore, it has been shown repeatedly that most NAs found in the AEF of OSPW cluster around a carbon number of 12−17.8,20 Data comparison on the basis of 4476
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why the overall number of detected homologues is rather small in comparison to the OxS- and Ox-classes. Compounds containing nitrogen as the only heteroatom (CnH2n+zNy) were not observed. Barrow et al.18 have shown, however, that although the detection of these molecules in the negative ion mode is poor, the MS response increased enormously when APPI in the positive ion mode was used. Thus, ESI in the negative ion mode would not be the ionization method of choice for facile detection of this compound class. In contrast to the CnH2n+zOxS compound class, a correlation of hydrogen deficiency (z) and oxidation state (x) was observed for the −NOx family, although the correlation is not as good as for NAs. The highest number of homologues was found in the region of x = 3 to 4 and for z-values of −7 to −11. In addition to the compounds belonging to the CnH2n+zOxS class, compounds containing two sulfurs were also detected (CnH2n+zOxS2). However, in contrast to all previously discussed compound classes, the largest number of homologues was detected by single-UHPLC fractionation and not by doubleUHPLC fractionation (Supporting Information Figure S4). With EO-DI, only four different series of homologues could be detected (C n H 2n−6 O 2 S 2 , C n H 2n−8 O 2 S 2 , C n H 2n−10 O 4 S 2 , CnH2n−12O2S2) with an overall total of 19 compounds. SingleUHPLC fractionation enabled the detection of 68 compounds belonging to multiple OxS2 classes. In contrast to the other compound classes, however, the use of the double-UHPLC method decreased that number to 27, a decrease of more than 50%. Given the small total number of compounds belonging to the CnH2n+zOxS2 class, detection of fewer molecules with double-UHPLC than single-UHPLC is likely to be caused by incipient suppression effects. The small overall amount of homologues for the OxS2-class is in good agreement with ESIFTICR-MS data from recent studies of OSPW, where these molecules on average contributed less than 3% to the total intensity count.2,18,37 The detailed nature and the structure of the CnH2n+zOxS2 species have not yet been revealed. Stanford et al.38 stated that, due to their water solubilities, OxSy species detected in the negative ion mode probably correspond to carboxythiophenes as well as polycyclic and polyaromatic thiophenes. The extent of aromaticity and cyclization can be represented by DBEs, which have been found to be mostly in the range of 1−10 for these types of molecules.38 In our study of OSPW, the DBEs of the homologues of the OxS2 species ranged from 4 to 7 for EO-DI and doubleUHPLC fractionation and from 3 to 7 for single-UHPLC fractionation. This indicates that mostly mono- and dicyclic compounds, some of which are likely to be carboxythiophenes, were extracted from the OSPW. Additionally, compounds containing sulfur as the only heteroatom (CnH2n+zSx, x = 1, 2) were detected with all three approaches (data not shown). EO-DI and double-UHPLC fractionation yielded the largest number of homologues, with the detection of 30 and 49 sets of homologues, respectively. Application of two consecutive runs had the main advantage in that more compounds with smaller z-values were detected. The number of detected homologues decreased with single-UHPLC fractionation, however, and comprised only 50% of the EO-DI identifications. DBE values ranged from 2 to 10, which indicates that the structures of the compounds ranged from more linear to more cyclic and aromatic. Tomczyck and Winans39 proposed that CnH2n+zS species with a DBE of 10 most likely represent
(CnH2n+zOx) for EO-DI, single-UHPLC fractionation, and double-UHPLC fractionation. Moving from EO-DI to singleUHPLC fractionation yielded more than twice the number of identifications (513 versus 1117, respectively). A second UHPLC fractionation increased the number by approximately another 50%, to 1636. The highest number of compound homologues detected by EO-DI corresponded to molecules containing two to four oxygen atoms and a hydrogen deficiency of four to twelve. In general, this compound class was detected up to an x-value of 8 (CnH2n+zO8) and a z-value of −22. Application of a single offline UHPLC fractionation immediately changed the overall compound profile in that more compounds with higher x- and smaller z-values were detected. The largest number of homologues was found for the CnH2n+zO6 class in the range of z = −8 to −12. However, while a larger amount of more highly oxidized NAs was found, in comparison to the EO-DI approach, fewer compounds were detected containing one to four oxygen atoms (CnH2n+zO1−4) over the whole z-range. The NA profile obtained for double-UHPLC fractionation was more comprehensive and more consistent with the findings of the EO-DI approach, especially with regard to the number of homologues containing fewer than four oxygen atoms. The trend for increased detection of compounds with higher x- and z-values continued with double-UHPLC fractionation. In general, a high hydrogen deficiency (low z-value) was frequently found for compounds with a high oxidation level. Compound classes exhibiting a hydrogen deficiency of z = −6 to −16 and containing 4 to 8 oxygen atoms showed the highest number of homologues with the highest value at x = 4. With two consecutive UHPLC fractionations, the compounds corresponding to the “classic” NAs (CnH2n+zO2) were underrepresented in comparison to higher oxidized compounds and accounted for less than 10% of the total compound content. This is a large difference from the DI approach where, due to a smaller number of NAs in general, “classic” NAs accounted for more than 20% of the total number of oxidized NAs present. The main sulfur-containing (CnH2n+zOxS) and nitrogencontaining (CnH2n+zNOx) compound classes are displayed in Figure 4. Both species show the same trend of enhanced oxidized compound detection using the offline UHPLC approach. With two consecutive fractionations, approximately twice as many compounds were detected as in the EO-DI approach, accounting for 433 and 206 homologues, respectively. Prefractionation had a particular impact on the detection of homologues containing four to seven oxygen atoms, which were detected much more frequently over the whole z-range. The characteristic correlation of higher hydrogen deficiency and increased number of oxygen atoms as shown in Figure 3 was not observed for CnH2n+zOxS compounds. For each of the three methods, a rather homogeneous distribution was observed. The distribution of the CnH2n+zNOx species is displayed in the lower part of Figure 4 and shows by far the largest increase in the number of detected homologues when comparing the EO-DI and the UHPLC fractionation approaches. EO-DI enabled detection of only 36 different compounds, whereas single-UHPLC fractionation increased that number more than 3-fold (131) and double-UHPLC fractionation more than 8fold (293). As shown by Barrow et al.,18 nitrogen-containing molecules ionize more readily in the positive ion mode, which explains why the NOx species is strongly influenced by matrix effects when EO-DI is applied in negative ion mode ESI, and 4477
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QTOF
aromatic thiols. Thus, the compounds found with smaller DBE values indicate the presence of cyclic, aromatic, and/or linear structures. In conclusion, the acid extractable fraction of an OSPW sample from the Athabasca oil sands region was investigated by EO-DI and offline UHPLC fractionation followed by DIFTICR-MS, which offers ultrahigh mass resolution and mass accuracy. We demonstrated that a preliminary separation of the extracted molecules greatly increased the number of detected peaks, as well as the number of compounds with defined chemical formulae, using the same volume of starting material. When the double-UHPLC procedure was used, the total number of identified NAs and oxidized NAs increased by a factor of 3, twice as many sulfur oxides were detected, and more than an 8-fold increase was observed in the number of detected NOx species. Therefore, we conclude that a preliminary UHPLC fractionation is highly beneficial for the comprehensive analysis of organic OSPW extracts and greatly facilitates the detailed profiling of this industrial waste.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1: Total ion current (TIC) of the OSPW extract with UHPLC-MS. Figure S2: FTICR mass spectra corresponding to EO-DI, as well as UHPLC fractions 4−7 and a corresponding mass spectrum acquired from the dried residue of UHPLC effluent. Figure S3: Negative ion DI-ESI-FTICR-MS spectrum of the AEF. Figure S4: Distribution of detected homologues for different CnH2n+zOxS2 classes using EO-DI, single-UHPLC fractionation, and double-UHPLC fractionation. This material is available free of charge via the Internet at http://pubs.acs.org.
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quadrupole time-of-flight
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; phone: (250) 4833221; fax: (250) 483-3238. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Genome Canada/Genome British Columbia Technology Development Grant and a platform grant from Genome Canada and Genome British Columbia. Authors are also grateful to the Western Economic Diversification of Canada for providing funding. We are also grateful to Dr. Carol E. Parker for critical reading of the manuscript.
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ABBREVIATIONS OSPW Oil sands processed water AEF acid extractable fraction DI direct infusion EO extraction only ESI electrospray ionization FTICR Fourier transform ion cyclotron resonance MS mass spectrometry UHPLC ultrahigh performance liquid chromatography NA naphthenic acid ONA oxy-naphthenic acids FTIR Fourier transform infrared GC gas chromatography RP reversed phase 4478
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