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Energy & Fuels 1991,5, 356-360
by K. L. Chasey and Thomas A w l , using separations and quantitative high-resolution low-voltage mass spectrometry. Particular emphasis is placed on the fate of proximate carcinogens, such as polyaromatic hydrocarbons. The detailed analytical approach used focuses on both aromatic ring and related alkyl side chain distribution. The great potential of combined, on-line LC high-resolution MS is illustrated in the research carried out by C. S. Hsu, M. A. McLean et al. This approach is capable of separating and distinguishing between isomeric aromatic and naphthenoaromatic components of the same elemental formulae (naphthalenes vs trinaphthenobenzenes) and hard to resolve aromatic hydrocarbon and aromatic thiophene components (naphthenochrysenes/dibenzothiophenes) that cannot be distinguished by MS above. The resolving power required to separate this type of molecular ions at m / e 400 is approximately 118000,well
beyond the capabilities of existing commercial mass spectrometers when operated in a dynamic mode, even at very slow scanning speeds. As seen by this brief introduction, the work covered in the following papers is indeed a very good representation of the leading edge research that is being carried out by several organizations. This and similar research, as well as the ongoing and planned effort in many groups, will take us even closer to a fundamental, molecular level understanding of the composition of fossil fuels and of the relationship between composition, processability, and endproduct performance. That is our ultimate goal. Thomas Aczel Corporate Research, Analytical Sciences Laboratory, Exxon Research and Engineering Company
Petroleum Mass Spectral Hydrocarbon Compound Type Analysis Terrence R. Ashe* Imperial Oil Limited, P. 0. Box 3022, Sarnia, Ontario, Canada N7T 7M1
Steve G. Colgrove Exxon Research and Development Labs, P. 0. Box 2226, Baton Rouge, Louisiana 70821 Received December 10, 1990. Revised Manuscript Received February 13, 1991
A review of hydrocarbon "type" analysis by mass spectrometry is presented. Type analyses have been used in the petroleum industry since the 1940s. Methods were developed originally for use on magnetic sector mass spectrometers (CEC 100 series). Quadrupole mass spectrometers, such as the Hewlett Packard mass-selective detector (MSD), are now being used successfully to obtain hydrocarbon type analyses. Comparisons of results from the MSD and a CEC instrument are presented. Introduction Mass spectrometry is an analytical tool which plays an important role in the petroleum industry. Many of the initial advances in this technique were made by the petroleum industry itself. Researchers found that mass spectrometry was very useful for analyzing complex hydrocarbon mixtures. In fact, it is possible to introduce a petroleum distillate or even a whole crude oil directly into a mass spectrometer and obtain quantitative information about groups of hydrocarbons. Analyses done in this manner are referred to as hydrocarbon "type" analyses. Petroleum products are complex mixtures containing many isomers of different classes of compounds which are not easily separated and or identified, even with modern analytical instrumentation. Historically, petroleum chemists and engineers relied on correlative techniques such as density, refractive index, and elemental analysis to determine composition. Mass spectral hydrocarbon compound type analyses were developed to provide fast, reliable, and more informative data for unseparated complex mixtures. The earliest petroleum applications of mass spectral mixture analysis were for refinery gas streams. Components which could be quantified included H2, N2,02,CO, C02, C1 to Cs hydrocarbons,and some sulfur species. Later
Table I. Common MS Methods for the Analysis of Hydrocarbon Mixtures method no. amlication ASTM Methods D-2425 hydrocarbon types in middle distillates D-2625 chemical composition of gases D-2786 hydrocarbon types in gas oil saturates D-2789 hydrocarbon types in low olefinic gasoline D-3239 aromatic types in gas oil aromatic fractions other methods Shell sats hydrocarbon types in naphtha and middle distillate saturates Robinson total oil crude oils
on, methods were developed for liquids, including the analysis of petroleum naphthas, middle distillates, lube oils, and entire crude oils. Table I lists some of the type analysis which are commonly used in the petroleum industry. Most of the mass spectral hydrocarbon type analyses in use today were developed using low-resolution, single focusing, magnetic sector mass spectrometers manufactured by Consolidated Electrodynamics Corp. (CEC). The CEC 100 series instruments were widely used by the petroleum industry, and some of these instruments are still in use today. In the mid 19609, CEC was acquired by DuPont.
0887-0624f 91f 2505-0356$02.50f 0 0 1991 American Chemical Society
Petroleum MS Hydrocarbon Compound Type Analysis In 1976 DuPont decided to cease manufacturing this type of instrument (the model at the time was the DuPont 21-104) due to limited market. During the late 1970s and throughout the 198Os, many petroleum labs upgraded the electronics of their CEC instruments and did their best to keep the instruments running. Because of increasing difficulty of obtaining spare parts and increasing failure of electrical components, petroleum mass spectrometrists became faced with the task of replacing the CEC instruments with a new generation of mass spectrometers. In both of our labs, we now use a Hewlett Packard mass-selective detector (MSD) for doing type analyses of liquids, and in some cases, solids. The MSD was designed to be a detector for a gas chromatograph. We have found the MSD to be suitable as a stand-alone mass spectrometer as well.
Experimental Section In a mass spectral type analysis, a sample is usually introduced into the mass spectrometer source via a batch inlet. The older inlets, especially those used only to analyze gases, were made of stainless steel. Modern inlets are usually made of glass in order to minimize the chance of sample decomposition or alteration while in the inlet. A sample is typically introduced into the evacuated inlet via a syringe, a sample loop (mainly gases), a gallium covered glass frit, encapsulation in indium, or in a solids sample 'cup". For the analysis of liquids or solids, the inlet is kept hot in order to vaporize the sample. The inlet usually contains a 0.5-1-L 'expansion volume" which aids in sample mixing and for more easily controlled metering of the sample vapor via a "leak" line into the mass spectrometer source. The need to introudce the sample as a vapor to the mass spectrometer generally limits the type analyses to materials which boil below 600 "C (1050 OF). The inlets are attached to vacuum pumps and usually have at least one valve which can be opened to allow the sample to be pumped out after analysis. When samples are analyzed without separation, a repeatable set of conditions must be used. Analytical applications require molecular flow of the analyte to the m u spectrometer ion source, ensuring that the composition of the vapor in the inlet system is identical with that in the mass spectrometer. In addition, the leak rate of the inlet should be slow enough that the amount of analyte reaching the ion source not change appreciably during the analysis. Quantitative analyses require that pure components and mixtures yield repeatable spectra per unit amount of analyte introduced. Component analysis requires either daily calibration or long term stability of the instrument. The early work done in analytical mass spectrometry showed that repeatable results could be obtained with mass spectra acquired with 70-eV electron ionization and a fixed magnetic field to separate the ions. The ion source temperature needed to be controlled to within 0.1 O C to minimize thermal effects in the fragmentation patterns. This is still critical for component analysis and certain carbon number distribution measurements. Liquid analysis instruments were usually standardized by using the fragmentation pattem of hexadecane. Normally a parent ion (mlz 226) intensity to a fragment ion (e.g., m / z 127, mlz 57, or m / z 43) intensity ratio is used to determine where to set the source temperature. Discussion In order to do a type analysis it is necessary to obtain a mass spectrum (or the average of several spectra) of a component mixture, determine summations of peak heights, or intensities of groups of ions which are characteristic of each hydrocarbon type to be determined, then calculate concentrations of the hydrocarbon types using a calibration 'inverse matrix". In order to illustrate how a type analysis works, let us assume that we have a mixture of three components A, B, and C, whose "spectra" are shown in Figure 1. The problem is to determine the relative amounts of each component in the mixture. Es-
Energy & Fuels, Vol. 5, No. 3, 1991 357 Component
c
"Spectrum"
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I
.
Figure 1. 'Spectra" for components A, B, and C. sentially, the solution to the problem involves solving three equations in three unknowns. These equations are listed as follows: alA + blB + clC = # 0 u~A + b2B + cZC = # W a3A + b3B + c3C = # A The relative concentrations of the components are given by the values A, B, and C. The number (#) of a's,Ws, and A'S are measured by introducing the mixture into a mass spectrometer. Before the equations can be solved, however, one must determine the values of the nine coefficients. This is done by using the information supplied in Figure 1. For pure component A (relative concentration = l),the concentrations of B and C are 0, and therefore, the B and C terms in the equations drop out. The value of a, is equal to the number of 0 ' s in the pure spectrum of ,component A, Le., 5. Similarly, the values for a2 and a3are equal to the number of W s and A's, respectively, in the spectrum of component A, Le., 2 and 1, respectively. Using the spectra for pure component B and C, one finds that the values for bl, b2, b3, cl, c2, and c3 are 2, 4, 2, 1, 1, and 6, respectively. After substituting the actual values for the coefficients into the three equations, and writing the equations with matrix algebra, we obtain the following equation:
This equation can now be rearranged to give the component concentrations by multiplying each side of the equation by the inverse matrix of the coefficient matrix:
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Ashe and Colgrove
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For actual hydrocarbon analysis, the A, B, and C terms in the example might correspond to types such as paraffins, naphthenes, and aromatics. The geometric shapes might correspond to ion intensity summations such as the intensities of m/z 43 57 + 71 + 85 for paraffins, intensities of m/z 91 + 105 + 119 + 133,etc., for alkylbenzenes, etc. For each hydrocarbon type method one would need to determine the inverse matrix values by obtaining the mass spectra of pure compounds or blends of pure component types. In some cases, the inverse matrix values can be estimated either by interpolation or by extrapolation from known materials. In the early development of mass spectral hydrocarbon type analyses (using magnetic sector instruments), researchers determined the inverse matrices for a large number of petroleum analysis methods. These inverse matrices could be used for other mass spectrometers of the same type, as long as the other mass spectrometers were adjusted to give the same fragmentation patterns for a given hydrocarbon such as hexadecane. Many of the inverse matrices and mass spectrometer conditions needed to do specific hydrocarbon type analyses have been published by ASTM.' In 1984,the DuPont 21-104 mass spectrometer used at Imperial Oil's Petroleum Products Research Division in Sarnia, Ontario, reached a point where it needed to be replaced. A Hewlett Packard 5790 mass-selective detector was chosen as the replacement. The MSD is a bench top quadrupole mass spectrometer. A manual valve Brunfeldt batch inlet was interfaced to the MSD. In 1987,a decision was made to replace one of the CEC 103 mass spectrometers at the Exxon Research and Development Labs in Baton Rouge. That mass spectrometer was also replaced with an H P 5790 MSD instrument. An automated Brunfeldt batch inlet (ALIS 11)was interfaced to the MSD. The design of the MSD is very different from the CEC-type mass spectrometers. At both of our labs, we wanted to be sure that the MSD would give results similar to those obtained on the magnetic sector instruments. As was noted earlier, the mass spectrometer source temperature was an important parameter in maintaining re-
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(1) Annual Book of ASTM Standards; Volumes 05.01 to 05.03, published annually. ASTM: Philadelphia.
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,
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BENZENES
%AROM MSD
Figure 4. Lube oil aromatics repeatability comparison of DuPont 21-104 and MSD instruments.
peatability on the CEC-type instruments. The MSD instruments, however, do not have a heater in the ion source. Instead, a heating tape wrapped around the analyzer manifold heats the source indirectly. While it is possible to adjust the temperature of the heating tape by turning an adjustment screw accessed through the side of the MSD, we found that the fragmentation pattern obtained was mainly a function of the tuning of the instrument. The data station supplied with the MSD has a software program called AUTOTUNE which can be used to automatically adjust the mass spectrometer lens voltages and the quadrupole scanning voltages. Unfortunately, the repeatability of the AUTOTUNE instrument tuning is not very good. We often have to adjust the lens voltages manually in order to obtain a consistent mass spectrum for the perfluorotributylamine used to calibrate the instrument. With careful tuning, we found it is possible to obtain repeatabilities for hydrocarbon type analyses which are as good as, and often superior to,those obtained with the CEC-type instruments. Figure 3 shows a comparison of MSD and DuPont 21-104 repeatability data for a naphtha QC sample used at the Sarnia lab. For all four hydrocarbons types, the MSD data is more repeatable. The paraffin value for the QC sample is significantly lower than that obtained on the DuPont instrument, however. A comparison of the data with results obtained from a capillary gas chromatography analysis of the same sample showed that the MSD value was a closer match.
Petroleum MS Hydrocarbon Compound Type Analysis
Energy & Fuels, Vol. 5, No. 3, 1991 359
E G;
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t
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210
230
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M/Z
Figure 5. 70-eV MSD mass spectrum of a middle distillate.
Figure 6. 10-eV MSD mas8 spectrum of a middle distillate.
Figure 4 shows a comparison of the DuPont and MSD instruments for a lube oil aromatics analysis based on the ASTM D 3239 method. Again, the repeatability is better for the MSD data. In the ASTM D3239 method, the aromatic compounds are lumped into seven aromatic "types". For example, type I consists of the sum of benzenes + benzothiophenes + naphthenophenanthrenes, and type VI1 is the total amount of phenanthrenes. As can be seen in Figure 4, the MSD value for type VI1 is significantly higher than that obtained with the DuPont instrument. This is a result of the better high-mass transmission in the MSD. At the Baton Rouge lab, we found some significant differences for naphtha results between the MSD and the CEC 103. We decided to determine a new naphtha inverse matrix for the MSD rather than try to make the MSD give the same spectra as the CEC instrument. In addition, we now analyze known naphtha component blends whenever we analyze samples, and determine response factors for each of the component types. The response factors serve to correct for any differences in instrument tuning from day to day. As supplied by Hewlett Packard, the MSD operates with a fixed ionization voltage of 70 eV. At our request, however, HP supplied us with a modified circuit board which allows us to control the ionizing voltage via an external power supply. At 70 eV, the electron ionization mass spectra of petroleum hydrocarbons show that a large portion of the initial molecular ions formed in the mass spectrometer undergo fragmentation to produce ions of small mass. Figure 5 shows the 70-eV mass spectrum of a middle distillate. Most of the signal appearing below mass 140 is due to fragment ions. The amount of energy required to remove an electron from a hydrocarbon molecule is in the range of 9.5-12 eV for saturates, and about 7.5-9.5 eV for aromatics. Therefore, at 70 eV, a considerable amount of excess energy is available to lead to fragmentation. A technique which is very useful in providing quantitative information for aromatic hydrocarbons makes use of "low"-energy electron beams for ionization. At electron beam energies of about 10 eV, molecular ions of aromatic molecules predominate. Ion signals for saturates are very weak, and may not even be detected. The overall signal is generally 1-2 orders of magnitude lower than when the electron beam energy is kept a t 70 eV. Because of the greatly reduced amount of fragmentation, the "low"-voltage spectra are much simpler to analyze. Figure 6 shows the 10-eV MSD mass spectrum of the same middle distillate whose 70-eV spectrum appears in Figure 5. Note that almost all of the peaks in the low-voltage spectrum are due
to molecular ions of aromatics or carbon-13 isotope ions. Because of the greatly reduced amount of fragmentation, it is possible to obtain carbon number distributions of the aromatic hydrocarbons using the low-voltage spectrum along with low-voltage response factors as determined from pure compounds. At the Baton Rouge lab, we analyze a blend of aromatic hydrocarbons to obtain the response factors whenever we analyze samples. A complete analysis of a mixture of saturates and aromatics can be done by combining 70-eV-type analysis data with a detailed analysis of the aromatics using the low-voltage spectra.
Future Needs/Developments A technique which has been available for many years, but which has not been widely applied to the analysis of naphthas and middle distillates, is field ionization mass spectrometry (FIMS).2 This method uses a high electric field to ionize vaporized sample molecules. This ionization method gives molecular ions with little or no fragmentation for aromatic and saturate hydrocarbons. With this technique, it is possible to obtain carbon number distribution of the saturate and aromatic components of complex mixtures. The FIMS technique is available only on magnetic sector instruments at this time. One analysis which continues to be difficult is the determination of olefins in the presence of naphthenes. The mass spectra of the two types of compounds are very similar, and makes it virtually impossible to distinguish between the two using a typical type analysis. One way to deal with samples having high levels of olefins, e.g., catalytic cracker naphthas, is to analyze the sample, then remove the olefins (e.g., by acid washing), and then analyze the olefin-depleted sample and obtain the olefin amount by difference. Another challenge facing the oil industry is the need to be able to adjust type analyses to handle changes in fuels that result from environmental regulations. For example, oxygenates are becoming more prevalent as additives in gasoline. Type analyses that were developed for petroleum naphthas were not designed to be applied to samples with significant amounts of oxygenates. Commonly used oxygenates in gasoline give some characteristic mass spectral fragments which can be used for quantitation. A new inverse matrix would have to be determined, however. Conclusions The Hewlett Packard 5790 mass-selective detector can be used to do hydrocarbon type analyses. With the added (2) Lattimer, R
P.;Schulten, H.Anal. Chem. 1989,61,1201A-l216A.
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Energy & Fuels 1991,5, 360-370
capability of performing low ionizing voltage analyses, the MSD can provide carbon number distributions of aromatic comDounds. T& analyses are usually done using batch inlets, which allow the entire sample to flow into the mass spectrometer. also be as an inlet to a mass A gas smctrometer. With the amrorxiate choice of GC column, sample components can b;! separated roughly according to boiling points. With this type of separation, one can sum mass spectra for narrow boiling point ranges and
perform type analyses. Using low ionizing voltages, it is also possible to determine individual aromatic compounds rather than iust carbon number distribution^.^
Acknowledgment. We acknowledge Ron Nesbitt and Rodney Smith for obtaining the mass spectral data at the Sarnia and Baton Rouge labs, (3)Aczel, T.;Hsu,C. S.Znt. J . Mass Spectrom. Ion Processes 1989, 9~1-7.
Optimizing Performance of Double-FocusingMass Spectrometers. 1. Determination and Use of Total Aberrations Stuart E. Scheppele* Amoco Research Center, Naperville, Illinois 60566, and Bartlesville Project Office, US. Department of Energy, Bartlesville, Oklahoma 74003
Glenn L. Nutter and Domenic L. Parisif Amoco Research Center, Naperville, Illinois 60566 Received February 14, 1991. Revised Manuscript Received March 20, 1991
Ion intensity is an important factor in determining the repeatability of type analyses. Consideration of ion intensity is exceedingly important in methods of type analysis employing double-focusingmass spectrometers because the sensitivity of these instruments decreases with increasing resolving power. Furthermore, this decrease is essentially linear only over a moderate range of resolving powers and exceeds linearity at higher resolving powers. The ion intensity at a given resolving power is a function of the magnitude of the total aberrations, the width of the source slit, and the value of the transmission. The transmission is the ratio of the beam width at the collector slit to the width of the collector slit. Equations are given for calculating values of the total aberrations from resolving powers obtained at two or more seta of values for the ion source slit width (w,) and the transmission (7'). Values of the aberrations are used in calculating the optimum values of w, and T in order to implement a procedure described for obtaining a resolving power with maximum ion intensity. The reduction in ion intensity resulting from use of nonoptimum values of we and T i s considered for different ratios of the resolving power to the aberrations. The effect of the aberrations on analytical repeatability is discussed.
Introduction The complexity of fossil fuel chemistry reflects in part the large number of organic compounds present in these materials. Fortunately, many of these compounds exhibit similar chemical properties and a regular gradation of physical properties. A set of such compounds constitutes a chemical type. Thus, the chemistry of fossil fuel materials can often be adequately simplified by determining their compositions in terms of chemical types. Consequently, mass spectrometric methods have been and continued to be developed to obtain such analyses.'-% An essential requirement for all mass spectrometry based methods of type analysis is attainment of high repeatability. The standard deviations in the quantitative results obtained by these methods are ultimately limited by ion statistics. Thus, realization of high ion intensities Present address: Mobil Research and Development Corp., Paulsboro, NJ 08066.
is one prerequisite for attaining high analytical repeatability. Instrumental factors that affect ion intensities are (1)Field, F. H.;Hastinp, S . H. Anal. Chem. 1966, 28, 1248-1266. (2)Lumpkin, H. E.Anal. Chem. 1958,90,321-326. (3)Sharkey, Jr., A. G.;Wood, G.; Shultz, J. L.; Wender, I.; Friedel, R. A. Fuel 1959,38,315-340. (4)Kearns, G.L.;Mnranowski,N. C.; Norris,M. S. Anal. Chem. 1959, 31,1646-1661. (6)Crable, G.F.; Kearns, G. L.; Norris, M. S . Anal. Chem. 1960,32, 13-17. (6)Carbon, E. G.; Paulisaen, G. T.; Hunt, R. H.; O'Neal, Jr., M. J. Anal. Chem. 1960,32,1489-1493. (7)Sharkey, Jr., A. G.;Shultz, J. L.; Friedel, R. A. Fuel 1962,41, 369-371. (8) Lumpkin, H.E.; Aczel, T. AM^. Chem. 1964,36,181-184. (9)Gallegos, E.J. h o c . Seventh World Pet. Congr. 1967,4,249-260. (10)A w l , T.;Allan, D. E.; Harding, J. H.; Knipp, E. A. Anal. Chem. 1970,42,341-347. (11)Aczel, T.Reo. Anal. Chem. 1971,I , 226-261. (12)A w l , T.;Lumpkin, H. E. Proc. 19th Annu. Conf. Mass Spectrom. Allied Top., Alanta, Ga 1971,328-330. (13)Shultz, J. L.;Kesaler, T.; Friedel, R. A,; Sharkey,Jr., A. G.Fuel 1972,51,242-246.
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