Energy & Fuels 2007, 21, 3369–3374
3369
A Positive-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Study of Russian and North Sea Crude Oils and Their Six Distillation Fractions Jaana M. H. Pakarinen,† Marjo J. Teräväinen,† Atte Pirskanen,† Kim Wickström,‡ and Pirjo Vainiotalo*,† Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, 80101 Joensuu, Finland, and Technology Centre, Neste Oil Oyj, P.O. Box 310, 06101 PorVoo, Finland ReceiVed June 20, 2007. ReVised Manuscript ReceiVed September 18, 2007
This paper discusses an FT-ICR mass spectrometry study of Russian and North Sea crude oils and their six distillation fractions (260–310, 310–360, 360–410, 410–460, 460–510, and 510–560 °C) by positive electrospray ionization (ESI) mode. Only one major heteroatom class, N class (pyridine benzologues), was found for both crude oils and for every distillation fraction. The relative abundance of N class was found to be somewhat higher for North Sea oil than the Russian oil, containing higher series (DBE values) and carbon distributions. On the other hand, the second most abundant class found for crude oils, the NS class, was found to be more abundant for Russian than North Sea oil samples, in accordance with experimentally measured sulfur values for crude oils. Correspondingly, Russian oil contained a higher series and also carbon distributions NS compounds, in comparison to North Sea oil. The oxygen containing sulfur compounds (OS class) were present only at a low distillation temperature (400 Da can be reliably assigned.17,18 From the elemental composition (CcHhNnOoSs), it is possible to directly define the compound “class” (NnOoSs), “type”/series (8) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. K. Org. Geochem. 2002, 33, 743–759. (9) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Mankiewicz, P. Org. Geochem. 2004, 35, 863–880. (10) Hughey, C. A.; Galasso, S. A.; Zumberge, J. E. Fuel 2007, 86, 758–768. (11) Kujawinski, E. B. EnViron. Forensics 2002, 3, 207–216. (12) Barrow, M. P.; McDonnel, L. A.; Feng, X.; Walker, J.; Derrick, P. J. Anal. Chem. 2003, 75, 860–866. (13) Miyabayashi, K.; Naito, Y.; Tsujimoto, K.; Miyake, M. J. Jpn. Petrol. Inst. 2004, 47, 326–334. (14) Crawford, K. E.; Campbell, J. L.; Fiddler, M. N.; Duan, P.; Qian, K.; Cobarty, M. L.; Kenttämaa, H. I. Anal. Chem. 2005, 77, 7916–7923. (15) Klein, G. C.; Rodgers, R. P.; Marshall, A. G. Fuel 2006, 85, 2071– 2080. (16) Klein, G. C.; Sunghwan, K.; Rodgers, R. P.; Marshall, A. G.; Yen, A. Energy Fuels 2006, 20, 1973–1979. (17) Kendrick, E. Anal. Chem. 1963, 35, 2146–2153. (18) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145–4149.
10.1021/ef700347d CCC: $37.00 2007 American Chemical Society Published on Web 10/25/2007
3370 Energy & Fuels, Vol. 21, No. 6, 2007
(hydrogen deficiency), and the carbon distribution (number of CH2 groups). The number of rings plus double bonds and therefore the type/series are described by either a double bond equivalent value (DBE) or a Z value. The DBE value (c - h/2 + n/2 + 1) for the elemental composition of the neutral molecule CcHhNnOoSs increases by 1 unit and Z value (h – 2c) decreases by 2 units for each additional ring and double bond. The relationship between the DBE and Z value is defined by Z ) -2(DBE) + n + 2. The suitability of ESI FT-ICR mass spectrometry to monitor the compositional changes in different crude oil distillation fractions has been presented earlier, but only in a few papers.19–21 Marshall et al. studied three different vacuum gas oil distillation fractions using both positive and negative ES ionization and found some characteristic behavior of different compound classes as a function of distillation temperature.20 Generally, aromaticity and carbon number increase with an increasing boiling point which, however, seems to have only small effects on chemical classes in the distillates. For example, oxygencontaining sulfur compounds were mostly present at low distillation temperature. Also, in the case of negative electrospray ionization the aggregation tendency of especially nonaromatic O2 species was found to decrease as the boiling point of the distillation cut increased.21 Naturally, the concentration of the oil sample is crucial for aggregation. Generally, concentrations above 1 mg/mL show evident aggregation for ESI FTICR mass analysis, but in the lowest distillation cuts concentrations as low as 0.05 mg/mL have shown clear dimer formation.21 Only a few studies exist concerning polar components of Russian and North Sea oils, in spite of their clearly significant roles in oil production. The history of the Russian oil industry began in the 1880s, and during recent years Russia has been the second leading oil producer in the world.22,23 However, in 2006 Russia extracted 9.236 million barrels of oil per day and overtook Saudi Arabia as the world’s leading oil producer. The history of North Sea oil is much younger; the first well came on line in 1971. There are five countries involved in North Sea oil production—the United Kingdom, Norway, Denmark, Germany, and The Netherlands—and the highest oil production was obtained in 1999, being nearly 6 million barrels per day. Brent blend, one of the earliest crude oils produced in the North Sea, is a light sweet crude oil that contains ∼0.37% sulfur. On the other hand, Russian oil is generally known as a sour crude oil, containing more than 0.5% sulfur. Although the impurities will need to be removed before the oil can be used for refining, thus increasing the cost of processing, the lower purchase price of Russian crude oil makes it attractive. In our previous study, a compositional analysis of acidic compounds of Russian and North Sea crude oils and their distillation fractions was performed by using negative ESI FTICR mass spectrometry.19 For both oils, the three most abundant compound classes were O (phenols)-, O2 (naphthenic acids)-, and N (pyrrole, indole, and carbazole types)-containing species. (19) Teräväinen, M. J.; Pakarinen, J. M. H.; Wickström, K.; Vainiotalo, P. Energy Fuels 2007, 21, 266–273. (20) Stanford, L. A.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2006, 20, 1664–1673. (21) Smith, D. F.; Schaub, T. M.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2007, 21, 1309–1316. (22) Krylov, N. A.; Zolotov, A. N.; Gogonenkov, G. N. In The Oil Industry of the Former SoViet Union. ReserVes and Prospects, Extraction, Transportation; Krylov, N. A., Bokserman, A. A., Stavrovsky, E. R., Eds.; Gordon and Breach Science Publishers: Amsterdam, 1998; pp 1–68. (23) Organization of the Petroleum Exporting Countries. Annual Statistical Bulletin 2005, Vienna, 2006.
Pakarinen et al. Table 1. Studied Fractions of Russian and North Sea Crude Oils and Their Yields yield (mass %) oil Russian North Sea
260–310 310–360 360–410 410–460 460–510 510–560 °C °C °C °C °C °C 9.59 10.13
9.00 8.97
9.16 8.15
6.96 6.15
7.03 7.75
6.16 5.65
Table 2. Composition of the Russian and North Sea Crude Oils
Russian North Sea
TANa (mg KOH/g)
basic Nb (mg/kg)
total Nc (mg/kg)
total Sd (mass %)
99%, Sigma) was added to give a final acid concentration of 0.5%. Mass Analysis. All mass spectra were obtained using a Bruker BioAPEX II 47e Fourier transform ion cyclotron resonance mass spectrometer (Bruker Daltonics, Billerica, MA). The instrument was equipped with a 4.7 T passively shielded superconducting magnet (Magnex Scientific Ltd., Abington, UK) and an Infinity ion source cell. Ions were produced in an external Apollo electrospray ion source (Bruker Daltonics). The vacuum system was maintained by rotary vacuum and turbo pumps (Edwards High Vacuum International, Crawley, UK), which yielded an ultrahigh vacuum (1 × 10-11 Torr). The operating conditions used were as follows. The sample solution was continuously injected by a syringe infusion pump at a flow rate of 90 µL/h, and nitrogen was used as the drying gas (6 psi) and the nebulizing gas (20 psi). Ions were collected at an end-plate electrode (-3.2 kV) and passed through a dielectric glass capillary (entrance and exit potentials -3.6 kV and -120 V, respectively). After that, ions were focused with skimmer 1 (14 V), prehexapole, and skimmer 2 (9 V) to a (rf) hexapole (600 Vp–p, 5.2 MHz) which was used to accumulate the ions for a predefined time (D1 ) 1 s). After the ions were extracted from the hexapole, the delay (P2) was set to 2200 µs to transfer the ions to an ICR cell by electrostatic focusing of transfer optics. In the ICR cell, ions were trapped using the Sidekick technique before conventional frequence sweep excitation and broadband detection. Data files consisted of 1M (1 048 576) data points and represent 16 scans. The mass range was set to m/z 100–1000; in this mass range, resolution was between 200 000 and 30 000. Measurements were performed using the Bruker XMASS 6.0.2 software, and data handling was carried out using Generate Molecular Formula (GMF) mode in DataAnalysis software. The signals which were counted as peaks in the mass spectra were 3 times higher than the baseline noise.
Russian and North Sea Crude Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3371
Figure 1. Positive-ion electrospray ionization FT-ICR mass spectra of (a) Russian and (b) North Sea crude oils.
Mass Calibration. Mass spectra were first calibrated externally using sodium trifluoroacetate ion clusters24 and internally using singly and doubly charged ions produced from synthetic pyridine– piperazine compounds (named L1 (M ) 268), L2 (M ) 298), and L3 (M ) 456) in ref 25). The externally calibrated mass spectra of oils were then recalibrated with respect to an internally defined and known homologous series of CnH2nN (DBE ) 9). The mass accuracy for recalibrated peaks was better than 0.1 ppm. Kendrick Mass Defect. The data were sorted and the homologous series were identified using the Kendrick mass defect method.17 Members of a homologous series have different numbers of CH2 groups and are 14.015 65 Da apart from each other. Converting the measured masses to Kendrick mass (CH2)14.000 00 rather than 14.01565) facilitates the identification of the homologous series: Kendrick mass ) IUPAC mass × (14 ⁄ 14.01565) (1) Members of a homologous series have an identical Kendrick mass defect (KMD), defined in the equation Kendrick mass defect ) nominal Kendrick mass – Kendrick mass (2) After the homologous series were identified, prediction of molecular formulas was carried out using the Generate Molecular Formula (GMF) program. The molecular formulas were limited to 100 C atoms, 200 H atoms, 6 N atoms, 10 O atoms, 4 S atoms, and 2 Na atoms. The mass accuracy for the major compound classes in the experiments were less than 3 ppm for the mass range m/z 100–450. Above the mass value m/z 450, ions were mostly classified on the basis of KMD values. Average Molecular Weights. The number- and weight-averaged molecular weights (Mn and Mw) were calculated using eqs 3 and 4, where Ai is the measured peak area (integral) of mass Mi: Mn )
∑ (A M ) ⁄ ∑ A i
i
Mw )
(3)
i
i
i
∑ (A M ) ⁄ ∑ A M 2
i
i
i
i
i
(4)
i
Both Mn and Mw values were calculated using the Polymer mode in the XMASS program.
Results and Discussion Crude Oils. The broadband positive electrospray ionization mass spectra of Russian and North Sea crude oils are presented in Figure 1. The amount of peaks and therefore the basic polar species was considerably higher in the mass spectrum of North Sea crude oil (2140) than in the mass spectrum of Russian crude oil (1430). The ions counted as peaks were 5-fold larger than the baseline noise. The smaller amount of peaks in the Russian crude oil mass spectrum compared to the North Sea crude oil mass spectrum may be due to a higher relative abundance of unknown peaks at m/z 301 and 337, which were clearly (24) Moini, M.; Jones, B. L.; Rogers, R. M.; Jiang, L. J. Am. Soc. Mass Spectrom. 1998, 9, 977–980. (25) Nuutinen, J. M. J.; Purmonen, M.; Ratilainen, J.; Rissanen, K.; Vainiotalo, P. Rapid Commun. Mass Spectrom. 2001, 15, 1374–1381.
Figure 2. Sums of relative abundances of different classes normalized to the total ion abundance in (a) Russian and (b) North Sea crude oil and in different distillation fractions. Zoomed inset shows the distribution for classes with a relative abundance less than 10%.
suppressing the signal of oil sample components. The origin of these ions is uncertain, and they are discovered frequently, especially in the case of poorly ionizable compounds. For example, the absence of these unknown peaks for the highest distillation fraction oil sample of Russian oil enables one to detect considerably more ions (1990) compared to the crude oil sample. Also, one would expect the number of the peaks in the two different crude oils to be roughly the same because the basic nitrogen concentrations (530 and 510 mg/kg) are almost equal. The number- and weight-averaged molecular weights (Mn/ Mw) as calculated are again almost the same for both oils as they were in the negative-ion mode but a little bit higher in the positive-ion (Russian, 338/412; North Sea, 341/417) than in the negative-ion (Russian, 316/366; North Sea, 300/349)19 mode. No aggregation tendency was noticed for either crude oil or distillation fraction samples, although the concentrations of oil samples were pretty high (1 mg/mL). On the other hand, to this point Marshall et al. have reported the greatest aggregation tendency using negative electrospray ionization, especially for organic acids.21 Also, they found that when using the FT-ICR instrument the formation of association products was not that common and required a higher sample concentration than when using linear quadrupole ion trap mass spectrometry. However, concentration-dependent aggregation tendency has been presented earlier also in positive-ion mode for unprocessed and processed diesel fuel samples.26 Only one major heteroatom class (62 and 84% of the total ion abundance), N class, containing the pyridinic compounds, was found for both the crude oils studied. Figure 2 shows the five most abundant heteroatom classes found from crude oils and distillation fractions. The relative abundance of the N class in the Russian and North Sea oil is in the range that has been (26) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1186–1193.
3372 Energy & Fuels, Vol. 21, No. 6, 2007
Pakarinen et al.
Figure 3. Sums of relative abundances of different N series normalized to the total ion abundance in Russian (blue) and North Sea (red) crude oils (left) and DBE ) 7, 10, and 13 of N series for Russian (right top) and North Sea (right below) crude oils.
observed earlier by other researchers. For example, the earlier study of nine different light, medium, and heavy crude oil samples showed the relative abundance of N class varying between 36 and 63%.27 On the other hand, the analysis of the heavy ends of six different crude oils showed a relative abundance of the N atom class as high as ∼80%.28 Figure 3 shows sums of relative abundances of different N series (DBE distribution) normalized to the total ion abundance for both crude oils. The N class appears for both crude oils from the DBE value of four, containing one (hetero)aromatic ring, most notably the pyridinic ring. The relative abundance of the N class raises to be the most abundant at the DBE value of eight and finally disappears at the values of 16 (Russian) and 21 (North Sea), the North Sea crude oil mass spectrum containing N compounds with notably higher aromaticity. However, this difference may again be due to a suppressing effect of the high-intensity peaks of m/z 301 and 337, leaving small higher mass ions undetectable. The more detailed inspection of DBE ) 7, 10, and 13 series shows that the carbon distributions of the series, the values ranging from 13 to 44 carbons for Russian crude oil and from 12 to 59 carbons for North Sea crude oil, behave rather similarly (Figure 3). These series correspond most probably to differently substituted basic quinoline, acridine, and benzoacridine compounds. As the aromaticity (DBE value) increases, the maximum relative abundance of carbon distribution moves to higher carbon numbers. For example, for DBE ) 7 the relative abundance of carbon distribution increases sharply to be highest in 20 carbons for Russian oil and 22 for North Sea oil. Instead, for DBE ) 10 and 13 the maximum relative abundance is reached at 23/ 24 (Russian/North Sea) and 26 carbons, respectively. The fourth most abundant class for Russian crude oil and the second most abundant for North Sea crude oil was the NS class, most notably containing the pyridinic benzothiophene compounds. The mass difference between isobaric ions of NS and N class is small (0.0034 Da, differing in elemental composition by SH4 vs C3), and the separation requires good mass resolving power. Although we have only 4.7 T magnet in our instrument, we have been able to separate these two ions routinely up to 430 Da. The peaks with mass values higher than 430 Da are recognized by the values of Kendrick’s mass defect method. This means that on high mass values only the more intense ion of these isobaric ions can be observed, the less intense ion being left underneath. (27) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A.; Taylor, S. Energy Fuels 2007, 21, 973–981. (28) Mullins, O. C.; Rodgers, R. P.; Weinheber, P.; Klein, G. C.; Venkataramanan, L.; Andrews, A. B.; Marshall, A. G. Energy Fuels 2006, 20, 2448–2456.
The relative abundance of the NS class was higher for Russian (∼5%) than for North Sea (∼3%) crude oil, which is in accordance with the higher amount of experimentally measured total sulfur of Russian crude oil (1.39 mass %) compared to the North Sea crude oil (0.25 mass %). Also, the distribution of different series was larger for Russian crude oil containing the DBE values from 6 to 14, 9 being the most abundant series and carbon numbers ranging from C14 to C40. Instead, the distribution was notably narrower for North Sea crude oil, having the DBE values 6–10, 9 being the most abundant and carbon numbers ranging from C12 to only C27. Other heteroatom classes found from the Russian crude oil were, for example, NO and OS (Figure 2). However, NO and OS classes were ionized and detected easier from North Sea oil distillation fraction samples, meaning that they are not totally missing from North Sea oil. Therefore, it is sometimes justifiable and necessary to also study the distillation fractions of crude oils in order to find some specific compound classes. Distillation Fractions. Six crude oil distillation fractions between 260 and 560 °C were studied to see how different classes distribute as a function of distillation temperature. The two lowest distillation fractions which were studied using negative ESI,19 namely 160–210 and 210–260 °C, were left out from the present study because of the small amount of ions with the same Kendrick mass defect values and therefore a short homologous series in positive-ion mode. Naturally, the lowboiling fractions include very small amounts of N class components because of their high boiling points. The mass spectra of three distillation temperature fractions, 310–360, 410–460, and 510–560 °C, are presented in Figure 4. There is a notably higher complexity and also a clear shift in the mass distribution to higher masses as the temperature of the distillation fraction rises, indicating the presence of higher molecular weight species at high distillation temperatures. The number- and weight-averaged molecular weights determined for Russian oil spectra from the above-mentioned distillation temperature fractions are 314/389, 378/416, and 375/454 and for North Sea oil spectra 311/388, 383/413, and 365/448. Both crude oils show similar behavior in the case of the N class; first the abundance rises, being highest in fraction 360–410 °C (Russian) and 410–460 °C (North Sea), after which the abundance decreases slightly in fraction 460–510 °C and continues to rise again passing toward the highest distillation fraction (Figure 5a). The results obtained from different fractions for two different oils of origin are remarkably similar. As the distillation temperature increases, the oil sample is found to contain a higher series and the most abundant series is passed
Russian and North Sea Crude Oils
Energy & Fuels, Vol. 21, No. 6, 2007 3373
Figure 6. N series for (a) Russian oil and (b) North Sea oil from three different distillation fractions: 310–360 °C (red), 410–460 °C (blue), and 510–560 °C (green). Figure 4. Positive-ion electrospray ionization FT-ICR mass spectra for different Russian and North Sea oil samples: (a, d) fraction 310–360 °C, (b, e) fraction 410–460 °C, and (c, f) fraction 510–560 °C.
Figure 7. DBE ) 10 of N series for (a) Russian oil and (b) North Sea oil from three different distillation fractions: 310–360 °C (red), 410–460 °C (blue), and 510–560 °C (green).
Figure 5. Relative abundances of compounds containing (a) N atoms and (b) NS atoms in Russian (blue) and North Sea crude oil (red) and in different distillation fractions.
on to higher DBE values (Figure 6). The distillation fraction 310–360 °C includes the N series (DBE) from 4 to 11, 6 being the most abundant for both oils studied. Instead, the fraction 410–460 °C includes series from 4 to 15/16, 10 being the most abundant, and the fraction 510–560 °C from 5 to 24/21, 14/13 being the most abundant for Russian and North Sea oils, respectively. Therefore, every 100 °C increase in distillation temperature raises the DBE value by 5.
Interestingly, the orders of the N series are comparable, at least in the case of series DBE ) 7, 10, 13, 16, and 19, and the series included even the same most abundant carbon distributions for each fraction for both the oils studied (not shown). For example, the most abundant number of carbons for DBE ) 10 was found to rise from 16 to 25 and finally to 37 carbons when going from fraction 310–360, 410–460 to 510–560 °C for both oils of origin studied (Figure 7). Also, the maximum number of carbons for three different distillation fractions was almost the same for both the oils studied and was found to be at its highest 32 (Russian oil, DBE ) 5), 46 (North Sea oil, DBE ) 9), and 58 (North Sea oil, DBE ) 13) carbons, respectively (not shown).
3374 Energy & Fuels, Vol. 21, No. 6, 2007
In the case of the NS class, there is a clear distinction between distillation fractions from the two origins (Figure 5b). The amount of high boiling NS compounds is considerably lower for North Sea oil than for Russian oil. In the two lowest distillation fractions, the abundance of the NS class is somewhat similar both in the case of Russian and North Sea oil. However, there is a clear difference when examining the last four distillation fractions. Namely, first the abundance of the NS class jumps up even to ∼7% in fraction 360–410 °C for North Sea oil. In the next fraction, Russian oil behaves similarly, reaching ∼9.7%, and remains high for the two last fractions, too. On the other hand, the abundance of the NS class for North Sea oil decreases when moving from the distillation fraction 360–410 °C to higher temperature distillation fraction spectra and is not found from the mass spectrum of the last distillation fraction. Also, Russian oil exhibited a higher series with higher DBE values and higher carbon distributions with a maximum at C51 (510–560 °C, DBE ) 16) compared to C27 in North Sea oil (460–510 °C, DBE ) 8). As was noticed in the case of crude oil spectra, distillation fraction spectra also state agreement with the higher amount of experimentally measured total sulfur of Russian crude oil (1.39 mass %) in comparison to the North Sea crude oil (0.25 mass %). Of the other studied classes, only the NO class was found at almost every distillation fraction, the abundance being, however, minor throughout (relative abundance
