Ketones in Fossil Materials—A Mass Spectrometric Analysis of a

Article pubs.acs.org/EF

Ketones in Fossil MaterialsA Mass Spectrometric Analysis of a Crude Oil and a Coal Tar Ahmad Alhassan and Jan T. Andersson* Institute of Inorganic and Analytical Chemistry, University of Muenster, Corrensstrasse 30, 48149 Münster, Germany S Supporting Information *

ABSTRACT: Ketones in fossil materials are a group of reactive compounds that may take part in reactions leading to the material becoming instable. Since they have never been much studied, we devised an analytical method based on high-resolution mass spectrometry. Two commercial reagents are compared as derivatization reagents to selectively introduce a positive charge into the ketones for the detection. The reagents are Girard T and a quarternary aminoxy (QAO) compound, and their suitability was tested in the analysis of ketones in a Wilmington crude oil. Orbitrap spectra were recorded to obtain high sensitivity and mass resolution. The derivatives had to be separated from basic nitrogen heterocycles through a simple and rapid chromatographic step to avoid complete suppression of the signals of the derivatives. QAO was superior to Girard T since a better detectability and a larger number of ketone signals were found. As a second complex real-world sample, a coal tar was investigated. Like in the crude oil, the ketones belonged to different classes of compounds: n-alkanones and saturated one- or two-ring structures, ketones containing aromatic rings, and ketones with either one additional oxygen or a nitrogen atom. Unique to the coal tar was a series of aromatic ketones with double bond equivalents up to 23, suggesting large aromatic structures, and with only a few (≤4) carbon atoms in the side chains, while the corresponding ketones in the crude oil showed double bond equivalents up to 14 and a large number of carbon atoms in the side chains.

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compounds,7,8 and acidic compounds,9 one reason being that such compounds are easily ionized when used with electrospray ionization (ESI). Neutral aromatic compounds, including furans, are not efficiently ionized in ESI but can be investigated using chemical ionization,10 photoionization,11,6 or laser ionization.12 Such methods are nonselective and would also ionize other oxygen containing compounds that feature aromatic rings, such as the isomers shown in Figure 1. Here

INTRODUCTION Petroleum products sometimes show instability on storage that negatively influences their performance. An insoluble sediment or sludge can be produced that can cause manifold technical problems like the plugging of nozzles and filters. The exact mechanism of this formation is not known, although hydroperoxides are suspected of initiating such reactions.1 The compound classes involved are not known with certainty but compounds containing heteroatoms like sulfur and nitrogen heterocycles are suspected of being central to this instability.1−3 Oxygen compounds in the form of phenols have also been implicated in instability reactions.4 Oxygen containing compounds are present in all crude oils in small quantities. As is true for all heteroatoms, oxygen is mainly found in the heavier resin and asphaltene fractions. The element is found in ethers, phenols, carboxylic acids, ketones, and furans, leading to an oxygen content of a crude of usually below 2%. It is a central task in the molecular characterization of crudes to investigate how a heteroelement is distributed on the different kinds of functional groups. Carboxylic acids and phenols, because of their acidic properties, have been analyzed in detail, whereas other oxygen species have received much less attention. Given their reactivity, ketones should be a class of compounds of interest. They can enter acid or base catalyzed condensation reactions, and additions across the CO double bond are numerous in organic chemistry, including those with hydroperoxides.5 Despite this, not much is known about the presence of ketones in fossil materials, probably in part because of the lack of suitable analytical methods. Recent analytical developments have led to very powerful mass spectrometric (MS) methods for the study of fossil materials, but they have mainly been used for nitrogen,6,7 sulfur © 2013 American Chemical Society

Figure 1. Model structures illustrating three isomeric oxygen containing compounds of the formula C28H48O, mass 400.37052, and DBE 5.

compounds of the formula C28H48O are shown with different functional groups. A furan with two naphtheno groups represents the aromatic oxygen compounds; naphtheno rings condensed onto aromatic rings are common in crudes, and for sulfur aromatics up to five such rings have been demonstrated.13 The three compounds are isomeric, meaning that MS would bunch all of them into one signal and no information on the functionality of the oxygen would be gained. The isolation of the ketone fraction from the rest of the crude therefore seems to be necessary. There are only limited data on ketones in fossil materials available. An IR study on the resins fraction of several crudes Received: May 22, 2013 Revised: September 5, 2013 Published: September 15, 2013 5770

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showed a clear carbonyl band,14 but it was not assigned specifically to ketones. Published procedures for the separation of the ketones using liquid chromatography were used for west Siberian oils15 and a Proterozoic bitumen16 but are not convenient for a separation of all ketones since those that also contain a nitrogen atom are easily lost in other fractions. Gas chromatography (GC) and/or GC-mass spectrometry (GCMS) require the separation of a ketone-enriched fraction16,17 and are not applicable to the analysis of nonvolatile compounds. In this paper, we present a method for the analysis of the whole ketone fraction present in very complex mixtures that avoids the drawbacks mentioned. It is based on the introduction of a charged trimethyl ammonium moiety into the poorly ionizable carbonyl compounds through a derivatization step followed by a chromatographic separation of these charged compounds from the matrix on a short open column. ESI coupled with Orbitrap MS is employed for the universal profiling and trace analysis of ketones. The well-known Girard T (GirT) reagent reacts with ketones under weakly acidic conditions and yields derivatives containing a charged quaternary ammonium moiety. This reagent has been used to selectively derivatize steroids18,19 and oligosaccharides,20 and a very brief report has appeared for its use with crude oils.21 The resulting derivatives show an enhanced sensitivity in ESI or matrix-assisted laser desorption/ionization in the positive mode. We here compare this reagent with a novel quaternary reagent (QAO) that contains an aminooxy (−ONH2) functionality and that was introduced more recently. This reagent also forms derivatives that possess a trimethyl ammonium ion, and thus the MS sensitivity is strongly increased. QAO has been successfully used to derivatize and analyze testosterone at 97% for all ketones except that a considerable amount of 2,2,6-trimethylcyclohexanone did 5773

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Figure 5. High resolution mass spectra of deasphalted Wilmington crude oil after derivatization with the GirT reagent. (A) The sample was infused into the Orbitrap without fractionation. (B) The isolated fraction was infused after removal of matrix components on an alumina column.

Figure 6. Mass scale-expanded segments of the mass spectra of Figure 5. Before separation of matrix components (top) and after (bottom).

Following our introduction of fluoroaromatic compounds as convenient internal standards in chromatography,33,34 we decided to investigate their use in the analysis of ketones and therefore synthesized two fluoronaphthyl ketones with a pentyl or heptadecyl group. They are added to the samples in known amounts to provide a quantitative reference. The average of the intensities relative to the largest signal (4-fluoro-1-acetonaphthone) and the relative standard deviation (RSD) for this set of measurements were calculated for all signals in the mass range of 220−315 Da (Figure 3) except the signal at m/z 254.2226 that is known to reflect an incomplete derivatization. A similar calculation was carried out for the QAO signals (Figure 4) except for that at m/z 255.2426 (incomplete derivatization). For the QAO derivatives, the average was 87.7% and somewhat higher than for GirT (80.4%). The RSD of QAO was 7.6% and distinctly less than that for GirT (12.3%). As a result, the QAO derivatization led to derivatives which show less variation in their ESI response than the GirT derivatives so that the QAO reagent appears preferable to us.

The deasphalted Wilmington crude oil was investigated as an example of a very complex real-world sample. The ESI-MS spectrum from a derivatization with GirT and direct infusion into ESI-MS (without removal of matrix components) is illustrated in Figure 5 (top). The ESI-MS spectrum of the unfractionated sample contains an enormous number of signals of even and odd masses, so obviously not only GirT derivatives are recorded here. Most of the interferences could be easily removed. Figure 5 (bottom) represents the ESI-MS spectrum after removal of the matrix components on an Al2O3 column. As can be seen in the mass scale expanded spectrum in Figure 6 (top), the even mass signals are much more abundant than those of odd masses before the fractionation. The signals with even masses correspond to protonated compounds containing one nitrogen atom and those with odd masses are their 13C isotopic peaks. Only a few of the ions of very low abundance could be assigned as GirT derivatives, but many of the expected derivatives were not found. Presumably a signal suppression by the much more abundant nitrogen heterocycles is the cause. After the separation of the heterocycles, all the 5774

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Figure 7. Zoom mass segments (A, B) from Figure 6 (top) and zoom mass segments (C, D) from Figure 6 (bottom) showing that the two signals at m/z 404.36291 and 406.37852 in zoom mass segments C and D, respectively, are suppressed in the mass spectra of the unfractionated sample. In the top traces, only a small part of the right-hand slope of the very large peak is seen (note the scale on the y axis).

has been found in shale oil30 and coal tar.37 A DBE of 10 may correspond to the 9-fluorenone series. These ketones have been reported to occur in Western Siberian Jurassic oils,17 Wilmington oil,38 and shale oil.39 A DBE of 13 was suggested to represent compounds like benzofluorenones, also discovered in Western Siberian Jurassic oils.17 Some masses were recorded that fit ketones that contain one sulfur atom, but they were by far not as abundant and as numerous as the ketones containing one extra oxygen atom (Figure S3). The abundance (at DBE 7, the most abundant of the sulfur containg ketones) was only about one tenth of that of the not very abundant ketones at the same DBE (Figure 8a). Ketones possessing a nitrogen atom do not seem to have been reported to occur in petroleum previously, although the elemental composition CnHxNO has often been found in fossil materials.40,41 Figure 8c shows ketones CnHxNO with DBEs from 7 to 16. A DBE of 10 might correspond to compounds like carbazoles containing a carbonyl group in an alkyl chain (C3−C34). The most abundant ones are C14-substituted compounds. Ketones with a DBE of 11 probably are compounds like acridines substituted with alkyl chains containing between 5 and 42 carbon atoms and/or carbazoles with one naphtheno ring. A DBE of 14 most likely represents benzacridine ketones with 4−25 carbon atoms in the alkyl chains. Comparison between the GirT and the QAO Reagents. The QAO derivatization led to sufficient enhancement of sensitivity (Figures 8a,b) so that ketones with DBE 14, which were not observed when the sample was derivatized using GirT, could be identified. A large number of ketones of DBE 1 to 13 were visible when QAO was used. Also ketones

signals (Figure 6 bottom) now correspond to the elemental compositions of GirT derivatives. Most of these ions did not appear in the ESI-MS spectrum of the unfractionated sample. For example, the signals at m/z 404.36291 (Figure 7C) and 406.37852 (Figure 7D) represent GirT derivatives but were not detectable in the mass spectrum of the unfractionated sample as illustrated in Figure 7A and B. Note the huge difference in scaling on the y axis. Removing the matrix is therefore necessary to enhance the sensitivity of the MS analysis of these as well as the QAO derivatives. Three types of ketones were found in the deasphalted Wilmington oil. The first is a series of ketones of the general formula CnHxO. The second is a series of ketones containing one nitrogen atom, CnHxNO, and the third consists of ketones with one extra oxygen atom, CnHxO2. Ketones of the first group were more abundant than the others. The aliphatic ketones (alkanones) with DBEs of 1, 2, and 3 are the most prominent ones (Figure 8a,b). n-Alkanones (DBE 1) have been identified in a number of shale oils as major components of carbonyl containing compounds.30,35,36 These ketones are mainly 2alkanones and in small quantities 3-alkanones.30,36 A DBE of 2 represents cyclic ketones. Cyclopentanones (C0−C10) in reasonable quantities, cyclohexanones (C0−C2), and cycloheptanones have been found in shale oils.30 A DBE of 3 most likely represents ketones that contain two saturated rings (or one saturated ring and one carbon−carbon double bond). This type of ketone has been reported for shale oil 30 as cyclohexenones and cyclopentenones but is less likely here because this oil has not been heat-treated. We therefore assume that these ketones contain two saturated rings. Ketones with a DBE of 5 most likely represent the acetophenone series that 5775

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Figure 8. Kendrick plots of the ketones in the deasphalted Wilmington crude. Kendrick plots of ketones CnHxO identified using (a) QAO and (b) GirT; Kendrick plots of ketones CnHxNO identified using (c) QAO and (d) GirT.

Figure 9. High resolution mass spectrum of the isolated fraction obtained after derivatization of a coal tar with QAO and fractionation on Al2O3.

with DBEs up to 17 were visualized through this derivatization when the separated QAO derivative fraction was diluted with MeOH instead of DCM+EtOH (Figure S4). The QAO

derivatization made it possible to recognize nitrogen-containing ketones with DBEs from 13 to 16 (compare Figures 8c and d) that were not observed from the GirT derivatization. Also, a 5776

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CONCLUSIONS Ketones in fossil materials can conveniently be analyzed using the derivatization with QAO which is preferable to Girard T as the derivatization reagent. Since trimethylalkylammonium ions are formed, the ESI-MS detectability is excellent, but the signal intensities for ketones are not equimolar. In order to be detectable at all, the derivatives must be separated from interfering nitrogen heterocycles, but this separation is quite easy and rapid. This analytical protocol is remarkably simple and is now being extended to other materials as well as for the study of instability effects in fuels.

larger number of ketones with DBE from 7 to 12 were indicated using QAO. The QAO derivatization leads to products containing oxime bonds, and these products are more stable than the GirT hydrazones, as was confirmed in a recent paper.22 Taken together with the more homogeneous mass signal intensities, QAO is clearly to be preferred to GirT for the derivatization. Ketones in Coal Tar. Since the QAO reagent proved to be advantageous, it was employed rather than the GirT reagent to study ketones present in another complex fossil material, namely in a coal tar. Figure 9 displays two signals at m/z 359.24911 and 527.43702, corresponding to the QAO derivatives of FNH and FNO, respectively. The relative intensity of these peaks was 55:100, despite the use of an equimolar mixture, and this is ascribed to the variation in ESI response even among closely related analytes as previously described. A separate experiment showed that both ketones were quantitatively derivatized by the reagent. A reliable quantification using ESI-MS therefore seems out of the question. In this coal tar, the same three types of ketones were found as for the Wilmington crude, namely CnHxO, CnHxNO, and CnHxO2. The most abundant one was CnHxO. A Kendrick plot was generated for the CnHxO (Figure 10) and indicates a very

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of 1-(4-fluoronaphthyl)hexan-1-one (FNH) and 1(4-fluoronaphthyl)octadecan-1-one (FNO), GC-FID parameters, GC-MS parameters, reactions of carboxylic acids and esters with Girard T and QAO, GirT derivatization, QAO derivatization, and (−)-ESI-MS of the deasphalted Wilmington crude oil and the coal tar. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-251-8333102. Fax: +49-251-8336013. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Matthias Letzel and Heinrich Luftmann for the Orbitrap measurements, Stephen A. Wise for providing the Wilmington crude oil, and Winfried Boenigk for the coal tar.

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DEDICATION This paper is dedicated to Prof. Bernt Krebs on the occasion of his 75th birthday. REFERENCES

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