Energy & Fuels 1993,7, 268-272
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High-Boiling Aromatic Hydrocarbons Characterized by Liquid Chromatography-Thermospray-Mass Spectrometry Chang S. HSU*and Kuangnan Qiant Exxon Research and Engineering Co.,P.O. Box 998, Annandale, New Jersey 08801 Received October 5, 1992. Revised Manuscript Received November 23, 1992
Thermospray (TSP)extends the hydrocarbon characterization by on-line liquid chromatographymass spectrometry (LC/MS) into petroleum fractions with boiling points greater than 1050 O F . Coupled with normal-phase liquid chromatography, TSP selectively ionizes aromatic hydrocarbons by protonation, leaving saturated hydrocarbons largely un-ionized. Thus, LC/TSP/MS facilitates the determination of molecular weight and, to some extent, compound type distributions of aromatic hydrocarbons in petroleum residua.
Introduction Our recent advances in liquid chromatography (LC) with moving belt (MB) interface-mass spectrometry (MS) resolve some of the difficulties encountered in the molecular-level characterization of heavy petroleum fractions.' However, LC/MB/MS is limited to fractions with boiling points below 1050 O F because the solute needs to be thermally desorbed from the surface of the belt made of a polymeric material. T h e r m ~ s p r a y ,which ~ , ~ sprays heated LC effluent containing high-boiling materials directly into an ionization chamber, requires no thermal desorption process and is a logical choice to extend hydrocarbon characterization by on-line LC/MS to petroleum fractions with boiling points higher than those amenable to LC/MB/MS.4 This paper describes our evaluation of liquid chromatography with thermospray interface-mass spectrometry (LC/TSP/MS) for the characterization of petroleum residua (boiling points greater than 1050 OF). High-boiling petroleum fractions have also been analyzed by mass spectrometry employing field ionization (FIP9and field desorption (FD)9J0techniques. However, with these techniques tedious and time-consuming offline separations by high-performance liquid chromatography (HPLC) are usually required to enhance mass spectral interpretations. FI requires samples to be heated + Current addreas: W.R. Grace & Co., Washington Research Center, 7379 Route 32, Columbia, MD 21044. (1)Qian, K.; Hsu, C. S. Anal. Chem. 1992,64, 2327-2333. (2) Blakley, C. B.; Vestal, M. L. Anal. Chem. 1983, 55, 75C-754. (3) Gartiez, D. A,; Vestal, M. L. LC Mag. 1985,3, 334-346. (4) McLean, M. A.; Hsu, C. S. Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Topics, Tucson, AZ, June 3-8,1990; American Society for Mass Spectrometry: Santa Fe, NM, 1990; pp 1077-1078. (5) McKay, J. F.; Latham, D. R.; Haines, W. E. Fuel 1981,60, 27-32. (6)Boduszynski, M. M. Energy Fuels 1987,1, 2-11. (7) Boduszynski, M. M. Energy Fuels 1988,2, 597-613. (8)Gallegos,E. J. Proceedings of the 37th ASMS Conference on Mass Spectrometry and Allied Topics, Miami Beach, FL, May 21-26, 1989; American Society for Mass Spectrometry: Santa Fe, NM, 1989; pp 296297. (9) Rechsteiner, C. E.; Attoe, T. H.; Boduazynski, M. M. Proceedings of the 33rd ASMS Conference on Mass Spectrometry and Allied Topics, San Diego,CA, May26-31,1985;AmericanSocietyfor Mass Spectrometry: Santa Fe, NM, 1985; pp 937-938. (lO)Reynolds, S. D.; Aczel, T. Proceedings of the 33rd ASMS Conference on Mass Spectrometry and Allied Topics, San Diego, CA, May 26-31, 1985; American Society for Mass Spectrometry: Santa Fe, NM, 1985; pp 656-657.
0887-0624/93/2507-0268$04.00/0
at high temperatures for vaporization, which could suffer from thermal decomposition. FD suffers from repeatability in peak height measurement and limitation in mass resolution due to the transient nature of signals.9 In addition, neither of these two techniques is suitable for on-line LC/MS applications primarily due to the incompatibility of the FI/FD emitter at a high potential (about 10 kV) with the LC/MS interface at ground potential.ll In normal-phase HPLC, an "external" ionization (i.e., the sample molecules are not ionized by an interaction with a buffer salt dissolved in the LC mobile phase) device, such as a discharge electrode or a heavy-duty filament, is used where the ionizationprocess take place in the presence of a large volume of solvent. In this mode, the solvent is ionized preferentially and then the sample molecules are ionized by solvent ions via ion-molecule reactions. Under these conditions, saturated hydrocarbons which have low proton affinity (for protonation) and higher ionization potentials than the solvent (for charge exchange) are not ionized. This can be a desirable feature especially when saturated hydrocarbons that would interfere with themass spectral measurement of aromatic compounds are not completely resolved from aromatic hydrocarbon by HPLC. This type of LC/MS characterization may also become very important in petroleum processing because most of the sulfur compounds that are of concern to refinery engineers and chemists designing catalysts are in aromatic hydrocarbon fractions. Only trace amounts of sulfur compounds are present in saturate hydrocarbon fractions. Although TSP utilizes heat to nebulize the LC effluent it does not deposit excess energy into the sample molecules. This results in the formation of intact molecular (or pseudomolecular) ions with little fragmentation thus simplifying determination of molecular weight distribution. Furthermore, since TSP was originally developed for on-line LC/MS analysis, it is readily implemented without extensive developmental research. The use of LC on-line for compound class separation facilitates mass (11)When TSP is placed on a magnetic sector instrument, the source and vaporizer are at high voltage and isolated from ground via standoffs and a length of fused silica capillary tubing.
0 1993 American Chemical Society
Energy & Fuels,Vol. 7, No. 2,1993 269
High Boiling Aromatic Hydrocarbons
spectral identification of complex hydrocarbon mixtures due to the separation of overlapping mass series based on polarity.12
100
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Experimental Section A vacuum residuum of a Middle East crude oil was further fractionated under reduced pressure of less than 0.002 Torr using a short-path distillation apparatus (DISTACT,Leybold-Heraeus GmbH).6J3The DISTACT fractionwith equivalentboiling points at atmospheric pressure of 1120-1305 "F (C, 84.13 wt %; H, 11.07wt %; S,4.32wt % ;N,0.24wt % ;0,0.24wt % by difference; H/C, 1.58) was chosen for our LC/TSP/MS studies. A model compound, 1-phenyltridecane, was purchased from Aldrich. Normal-phase liquid chromatographic separation was performed on a Varian 5560 ternary gradient HPLC system. A 25 cm X 4.6 mm column packed with 5-pm particle size and 300 8, pore diameter of 2,4-dinitroanilinopropyl(DNAP) silica (E&S Industries, Marlton, NJ) was used with a solvent gradient of hexane, methylene chloride, and isopropyl alcohol to separate the petroleumdistillate into saturates,monoaromatic,diaromatic, triaromatic, tetraaromatic, and polar fractions.l* A Vestec thermospray interface was used to transport LC effluent into a VG 70-VSE double focusing mass spectrometer, where solvent vapor along with sample molecules were ionized by a discharge electrode. The thermospray vaporizer's control temperature (TI) was set at 135 "C, tip temperature (Tz) at 285 "C, and block temperature at 310 "C during the LC/MS runs. An ISCO LC2600 syringe pump was used for postcolumn addition of methylene chloride. The flow rate of the mobile phase was at 1.5 cm3/min with no postcolumn addition, and at 1.0 cm3/min with a 0.33 cm3/min of postcolumn addition of methylene chloride. The mass resolution of VG-70 VSE was set at 2000 to ensure mass spectrometric sensitivity. A mixture of poly(ethy1ene glycol) (PEG) 300 and PEG 600 was used as a mass calibrant for accurate mass measurement with a mass range of 100-1200.4
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Figure 1. Thermospray spectrum of 1-phenyltridecane with hexane as a mobile phase.
MASS
Results and Discussion Due to the complexity of petroleum fractions, it is desirable to produce only molecular ions or pseudomolecdar ions (protonated, hydride-abstracted, and adduct molecular ions) from individual components to facilitate the determination of molecular weight and compound type distributions. Numerous soft (low energy) ionization techniques, including low-voltage electron-impact ionization,lJ2J4CSz charge exchange,15FI,5-9and FD,9J0 were developed for this purpose. Thermospray (TSP), which was originally developed for on-line LC/MS, is another soft ionization technique with ionization mechanisms similar to those of chemical ionization (CI).3 In CI, a variety of ion-molecule reactions, such as protonation, hydride abstraction, adduction, fragmentation, etc., can take place simultaneously. The presence of ionic species derived from more than one type of reactions can complicate the mass spectral interpretation and make the determination of molecular weight and compound type distributions difficult. TSP employed in normal-phase LC/MS is essentially a CI technique with solvent vapor functioning as a reagent gas.3 Under proper conditions, proton transfer can be a predominant ionization process for the sample molecules. A model compound, 1-phenyltridecane, was used to (12)Hsu, C. S.; McLean, M. A,; Qian, K.; Aczel, T.; Blum, S. C.; Olmstead, W. N.; Kaplan, L. H.; Robbins, W. K.; Schulz, W. W. Energy Fuels 1991, 5 , 395-398. (13) Vercier, P.;Mouton, M. Analusis 1982, 101, 57-70. (14) Field, F. H.; Hastings, S. H. Anal. Chem. 1956, 28, 1248-1255. (15) Hsu, C. S., Qian, K., submitted to Anal. Chem.
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Figure 2. Thermospray (upper trace, a) and field desorption (lower trace, b) spectra of a 1120-1305 "F DISTACT fraction of a vacuum residuum from a Middle East crude oil.
investigate the ionization of TSP when hexane being used as an HPLC mobile phase. Protonated molecular ions are essentially the only ionic species present in the TSP spectrum of 1-phenyltridecane,as shown in Figure 1.This result is consistent with our previous TSP result obtained from a 650-950 OF distillate where protonated molecular ions were found to be the predominant ionic species derived from the distillate components when hexane was used as a mobile phase? For petroleum residua, we chose a 1120-1305 OF DISTACT fraction of a vacuum residuum from a Middle East crude oil for the TSP studies. The upper trace of Figure 2 illustrates a TSP spectrum of the fraction which was introduced into the TSP ion source via a loop injection
Hsu and Qian
270 Energy & Fuels, Vol. 7, No. 2, 1993 tetraaromatics
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Table I. Comparison of the Relative Abundances (in wt % ) of the Monoaromatic, Diaromatic, Triaromatic, and Tetraaromatic HPLC Elution Regions Determined by Evaporative Mass Detector (EMD) and LC/Thermospray/ MS (LC/TSP/MS) with Postcolumn Addition
HPLC elution region saturates monoaromatics diaromatics triaromatics tetraaromatics polars
HPLC/EMD 9.9 10.0 18.5 13.6 20.2 27.7
TSP/LC/MS n.m.O 9.2b 17Bb 15.6b
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n.m. = not measured by LC/TSP/MS. Adjusted valuestomatch with the total concentration of aromatic fractions (monoaromatics through tetraaromatics) determined by HPLC/EMD.
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romatics
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Figure 3. Total ion chromatograms of the 1120-1305
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DISTACT fraction obtained from normal-phase liquid chromatography without (upper trace, a) and with (lower trace, b) postcolumn addition of methylene chloride.
with hexane as a mobile phase. Only protonated molecular ions are present essentially, with negligible amounts of fragment ions below mass 500. The molecular weights of the components range from 500 to 1200, with a median molar mass slightly greater than 800 and a full width at half-height of about 250 massunits. The molecular weight distribution determined by TSP compares favorably with that of field desorption (FD) mass spectrum of the same sample, as shown in the lower trace of Figure 2. These results are also consistent with molecular weight distributions of other high boiling petroleum fractions determined by field i o n i ~ a t i o n . ~ ~ J ~ The 112E1305 O F distillate was also analyzed by LC/ TSP/MS using a 2,4-dinitroanilinopropyl(DNAP) column using a gradient of hexane, methylene chloride, and isopropyl alcohol modeled on that suggested by Grizzle and Th0ms0n.l~ The upper trace of Figure 3 shows the total ion chromatogram (TIC, a total ion current trace as a function of elution time) of the monoaromatic, diaromatic, triaromatic, and tetraaromatic elution regions. There is an apparent signal enhancement when methylene chloride was introduced into the mobile phase at the beginning of triaromatic region. On the other hand, the introduction of isopropanol for eluting off polars (not shown) essentially stops proton transfer reactions to the sample molecules and greatly decreases the TSP signals.
This is one of the examples that a method developed for HPLC may not be suitable in use for on-line LC/MS. To enhance TSP signal for mono- and diaromatic regions,methylene chloride was added to the mobile phase after the HPLC column (postcolumnaddition) during the entire LC/MS run. The lower trace of Figure 3 shows the TIC of the same four aromatic elution regions shown above with postcolumn addition of methylene chloride. The signals in the monoaromatic and diaromatic regions are greatly enhanced by the postcolumn addition. The relative abundance5 of the monoaromatics, diaromatics, triaromatics, and tetraaromatics also compare favorably with those determined by HPLC evaporative mass detector, as shown in Table I. Figure 4 illustrates the mass spectra integrated over each of monoaromatic, diaromatic, triaromatic, and tetraaromatic HPLC elution regions. The average molecular weight decreases steadily with increasing number of aromatic rings. Similar shifts in molecular weight distribution toward lower masses from monoaromatic to tetraaromatic elution regions were also observed in our studies of a 650-1050 O F distillate by LC/MS with moving belt interface using low-voltage electron impact as an ionization means.' It can be concluded that in a finite distillation range, compound types of higher aromaticity will have a lower average molecular weight. High-resolution mass measurement at high masses is much more difficult than a t low masses for several reasons. The resolution required for resolving overlapping hydrocarbons and compounds containing heteroatoms increases with mass.18 Both the number of possible elemental combinations a t each accurate mass measured and the number of components in a given nominal mass also increase with mass. In addition, with linear scans the spacing between adjacent nominal masses narrows steadily with increasing mass when a mass measurement is performed using either a tandem sector double focusing (i.e., a combination of electrostatic sector and magnet) or an ion cyclotronresonance mass spectrometer. An optimal resolution at high masses can only be achieved in a rather narrow mass range (a few nominal masses) with slow scans and the aid of a high speed computer. Furthermore, the stability of the instrument also plays an important role in precisionto measure and in accuracy to calibrate individual mass peaks. With an average molecular weight above 800,a resolution of more than 200000 would be required to resolve overlapping sulfur compounds from hydrocarbons. With an instrument capable of only 10 OOO resolution or less, we
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(16)Boduszynski, M. M. Liq. Fuels Technol. 1984, 2, 211-232. (17) Grizzle, P. L.; Thomson, J. S. Anal. Chem. 1982,54, 1071-1078.
(18)Hsu, C. S.; Qian, K.; Chen, Y. C. Anal. Chim. Acta 1992, 264, 79-89.
Energy 6 Fuels, Vol. 7, No. 2, 1993 271
High Boiling Aromatic Hydrocarbons M 0NOA RO M AT I CS
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Figure 6. Hydrocarbon type distributions in the monoaromatic, diaromatic, triaromatic, and tetraaromatic HPLC elution regions. attempted to resolve overlapping compound series based on their corresponding hydrocarbon types. The Kendrick mass scale was used to facilitate compound type identifications because on this mass scale each compound type has a unique mass defect.l8 The distributions of overlapping hydrocarbon types in a nominal mass series are determined by comparing the average Kendrick mass defect (KMD) of the series with those of overlapping hydrocarbon types. For example, the average KMD of a nominal mass series from mass 500 to 1144 in the 11201305 OF fraction being studied was found to be -0.15 u, which was between the KMD of -0.12 u for the CnHzn-18
and -0.21 u for the CnH2n-32 hydrocarbons. The weight percenta of the CnH2-18 and C n H 2 n 3 2 hydrocarbons were therefore calculated to be 8.77% and 4.38% ,respectively, by splitting the 13.15% measured for the whole nominal mass series in inverse proportion to the differences of the measured KMD from the KMD's of the respective hydrocarbon types. Figure 5 illustrates the distributions of the hydrocarbon types, expressed by their z numbers, where z is the hydrogen deficiency as in CnH2n+z,l8 in the monoaromatic, diaromatic,triaromatic, and tetraaromatic LC elution regions. In the high-sulfur distillate we analyzed (S, 4.32% ) the presence of benzothiophenes
272 Energy & Fuels, Vol. 7, No. 2, 1993
(CnHzn-&3,which overlapswith CnH2,m) in the diaromatic region and dibenzothiophenes (CnH2n-l6S,which overlaps with CnHln-26) in the triaromatic region contributes to the highest abundances of the CnH2n-20 and CnH2n-26 series in their respective LC regions. These two sulfur compound types are commonly found in high abundances in highsulfur petroleum fractions. Small amounts of CnH2n-2 and CnH2n~series found in the monoaromatic region are possibly due to naphthenic sulfides. Confirmation of the presence of sulfur compound types might be achievable by using ultra-high-resolution Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry with an external ion source.
Conclusions Thermospray (TSP) extends on-line LC/MS characterization of petroleum and synthetic fuel fractions with atmospheric equivalent boiling points beyond 1050 O F . TSP is mainly a chemical ionization technique using
Hsu and Qian solvent vapor as reagent gas that selectively ionizes molecules with high proton affinity such as aromatic hydrocarbons. Coupled with normal-phase liquid chromatography, TSP can be used to determine molecular weight and, to some extent, compound type distributions of aromatic hydrocarbons in high-boiling petroleum and synthetic fuel fractions. To characterize saturated hydrocarbons in these fractions especially for molecular weight distribution, other sophisticated on-linetechniques such as the combination of a particle beam LC/MS interface with field ionization would need to be developed.
Acknowledgment. We thank Bill Olmstead for providing the 1120-1305 O F DISTACT fraction of a Middle East crude vacuum residuum, Swapan Chowdhury for field desorption mass spectrometric analysis of the fraction, and Win Robbins for the HPLC/EMD measurement. Valuable comments from Matt McLean of Vestec are also highly appreciated.
