Simplified Hydrocarbon Compound Type Analysis Using a Dynamic

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Energy & Fuels 1997, 11, 879-886

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Simplified Hydrocarbon Compound Type Analysis Using a Dynamic Batch Inlet System Coupled to a Mass Spectrometer Stilianos G. Roussis* and Andrew S. Cameron Research Department, Products and Chemicals Division, Imperial Oil, Sarnia, Ontario, Canada N7T 7M1 Received December 12, 1996. Revised Manuscript Received April 10, 1997X

A simple dynamic batch inlet system (DBIS) has been developed for hydrocarbon compound analysis by mass spectrometry. The system consists of a small expansion unit (2 cm3) that is easily installed into the GC oven of commercial GC/MS systems. The main advantages of the dynamic batch inlet system compared to conventional all-glass heated inlet systems (AGHIS) are (1) simplicity of operation, (2) low cost, (3) ease of maintenance, (4) rapidity of analysis, (5) automation, and (6) small sample size. In this work, the operation of the DBIS is described and typical results obtained by the analysis of representative petroleum fractions are given.

Introduction Knowledge of the composition of petroleum fractions is very important to the petroleum industry because it permits the determination of changes in process variables and changes in the quality of finished products. Analysis of petroleum fractions is very challenging due to the presence of large numbers of compounds with similar chemical composition. Compounds in a crude oil can extend over a boiling point range wider than 1000 °C, with the complexity of the sample increasing considerably with boiling point. The number of possible hydrocarbon isomers becomes extremely high with increasing hydrocarbon carbon number. For example, the C40 paraffin (atmospheric equivalent boiling point, AEBP, 522 °C) has more than 6 × 1013 possible isomers, and the C100 paraffin (AEBP, 708 °C) has more than 5 × 1039 possible isomers.1 Mass spectrometry was recognized early as a promising analytical method for the characterization of hydrocarbons. The method cannot directly analyze the different hydrocarbon isomers in samples containing compounds higher than C6, but it can be used to readily obtain compound type information. In compound type analysis, molecules are grouped together so that compositional changes can be readily observed. Classification of hydrocarbons by mass spectrometry is typically done using the formula CnH2n+z, where n is the number of carbon atoms and Z has values +2, 0, -2, -4, -6, etc. Paraffins are the most saturated hydrocarbons and their Z number is +2. The Z number for 1-ring naphthenes is 0. Homologous compounds with alkyl substituents of increasing chain length belong to the same Z series. The first hydrocarbon type analysis methods were developed to operate in the electron ionization (EI) mode * To whom correspondence should be addressed. Telephone: 519339-2441. FAX: 519-339-4436. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Boduszynski, M. M. Energy Fuels 1988, 2, 597-613.

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under high-energy2-7 (50-70 eV) or low-energy8-14 (∼10 eV) ionization regimes. More recently other ionization methods such as chemical ionization,15-17 field ionization/ desorption,18-21 and thermospray ionization,22 have been introduced for hydrocarbon type analysis. The various ionization techniques can either transfer considerable amounts of internal energy upon ionization, and hence lead to the generation of fragment ions,2-7 or transfer sufficient internal energy to form the molecular ion but insufficient energy to generate fragment ions8-22 (soft ionization). The generation of mass spectra consisting primarily of molecular ion species greatly simplifies the analysis due to the absence of fragment ions and provides direct information on the molecular weight distribution of the sample. Hydrocarbon compound type analysis by mass spectrometry is based on the use of compound mixtures with known compositions to deter(2) Brown, R. A. Anal. Chem. 1951, 23, 430-437. (3) Lumpkin, H. E.; Johnson, B. H. Anal. Chem. 1954, 26, 17491722 (4) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946-1948. (5) Hastings, S. H.; Johnson, B. H.; Lumpkin, H. E. Anal. Chem. 1956, 28, 1243-1247. (6) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833-1838. (7) Teeter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143. (8) Field, F. H.; Hastings, S. H. Anal. Chem. 1956, 28, 1248-1255. (9) Lumpkin, H. E. Anal. Chem. 1958, 30, 321-325. (10) Lumpkin, H. E.; Aczel, T. Anal. Chem. 1964, 36, 181-184. (11) Lumpkin, H. E. Anal. Chem. 1964, 36, 2399-2401. (12) Aczel, T.; Allan, D. E.; Harding, J. H.; Knipp, E. A. Anal. Chem. 1970, 42, 341-347. (13) Aczel, T. Rev. Anal. Chem. 1972, 1, 226-261. (14) Chasey, K. L.; Aczel, T. Energy Fuels 1991, 5, 386-394. (15) Dzidic, I.; Peterson, H. A.; Wadsworth, P. A.; Hart, H. V. Anal. Chem. 1992, 64, 2227-2232. (16) Allgood, C.; Ma, Y. C.; Munson, B. Anal. Chem. 1991, 63, 721725. (17) Hsu, C. S.; Qian, K. Anal. Chem. 1993, 65, 767-771. (18) Scheppele, S. E.; Hsu, C. S.; Marriott, T. D.; Benson, P. A.; Detwiler, K. N.; Perreira, N. B. Int. J. Mass Spectrom. Ion Phys. 1978, 28, 335-346. (19) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983, 55, 225-231. (20) Boduszynski, M. M.; Hurtubise, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983, 55, 232-241. (21) Boduszynski, M. M. Energy Fuels 1987, 1, 2-11. (22) Hsu, C. S.; Qian, K. Energy Fuels 1993, 7, 268-272.

© 1997 American Chemical Society

880 Energy & Fuels, Vol. 11, No. 4, 1997

mine the appropriate instrument response factors.5-13 These response factors are then used in conjunction with the measured mass spectrum to determine the composition of the unknown hydrocarbon mixture. The usefulness of hydrocarbon compound type analysis methods by mass spectrometry greatly depends on the capability to introduce petroleum fractions of widely varying boiling point ranges into the mass spectrometer. The entire sample has to be reproducibly introduced from ambient conditions into the vacuum (i.e., ∼10-410-6 Torr) of the mass spectrometer without any component loss or quantitative discrimination. The development of specialized sample introduction systems has been a major area of work since the early days of hydrocarbon type analysis by mass spectrometry. One of the most common introduction systems was the gallium frit which permits the analysis of hydrocarbons up to C40.23 Gallium (mp 30 °C, bp 2403 °C)24 was selected due to its high boiling point which does not permit vaporization of the molten metal at operating temperatures of 300-400 °C. The sample was introduced with a capillary pipet through the galliumcovered frit into the heated inlet system. Other galliumcovered orifice systems were later reported with improved capabilities for quantitative analysis.25,26 All-glass heated inlet systems (AGHIS) were subsequently developed that do not allow the sample to come in contact with metal surfaces and thus avoid possible catalytic reactions of the sample with the metal surface.27-30 The all-glass heated inlet systems can be heated to temperatures higher than 300 °C, permitting the analysis of high-boiling petroleum fractions. The use of all-glass heated inlet systems is quite common in petroleum mass spectrometry laboratories. Most standard ASTM (American Society for Testing and Materials) mass spectrometric hydrocarbon compound type analysis methods are performed using commercial or home-built AGHIS. Unfortunately, the operation and maintenance of the AGHIS are quite complex. A simpler, equivalent sample introduction inlet system would be highly desirable to alleviate most of the problems associated with the use of the AGHIS. In this work, the use of a simple dynamic batch inlet system (DBIS) has been examined for its capability to perform hydrocarbon compound type analysis. The DBIS consists of a small expansion unit (2 cm3) and can be easily installed into widely used commercial GC/MS systems. Sample introduction can be automated by using commercial autosamplers. The advantages of the DBIS are (1) low cost, (2) easy to operate and maintain, (3) little or no downtime for repairs, (4) can be automated, (5) its use does not require specially trained personnel, (6) shorter analysis time, and (7) smaller (23) O’Neal, M. J.; Wier, T. P. Anal. Chem. 1951, 23, 830-843. (24) CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, R. C., Ed.; CRC Press Inc.: Boca Raton, FL, 1981-1982; p B-17. (25) Genge, C. A. Anal. Chem. 1959, 31, 1747-1748. (26) Boyer, E. W.; Hamming, M. C.; Ford, H. T. Anal. Chem. 1963, 35, 1168-1171. (27) Lumpkin, H. E.; Taylor, G. R. Anal. Chem. 1961, 33, 476-477. (28) Peterson, L. Anal. Chem. 1962, 34, 1850-1851. (29) Teeter, R. M.; Doty, W. R. Rev. Sci. Instrum. 1966, 37, 792793. (30) Stafford, C.; Morgan, T. D.; Brunfeldt, R. J. Int. J. Mass Spectrom. Ion Phys. 1968, 1, 87-92.

Roussis and Cameron

Figure 1. Block diagram of a heated glass inlet system interfaced to a Hewlett-Packard 5970 quadrupole mass spectrometer.

sample size. Previous work by Gallegos and Pazzi31 indicated the potential of the DBIS to be used in petroleum applications. In the present work, the use of the DBIS is described and its suitability for hydrocarbon compound type analysis applications is evaluated by the analysis of typical petroleum samples. The results are compared to those obtained by a commercial AGHIS. Experimental Section All-Glass Heated Inlet System (AGHIS). A block diagram of the AGHIS (Brunfeldt Co., Bartlesville, OK) is shown in Figure 1. A gallium-covered frit is used to introduce lowboiling materials (naphthas, middle distillates, propylene tetramers, etc.). A capillary pipet filled with sample (∼1-3 µL) is introduced into the gallium-covered frit. The sample is drawn from the pipet through the frit and into the expansion volume (1 L) which is under vacuum (∼10-3 Torr). The liquid gallium serves as a vacuum lock that maintains the vacuum in the AGHIS and the mass spectrometer while permitting the introduction of volatile samples. Less volatile liquids (lube saturates, lube aromatics, etc.) and solids are introduced via an ampule inlet. Vacuum is maintained by mating of the optically flat surfaces of the ampule and lock. This alleviates the use of common vacuum grease to ensure the integrity of the vacuum. Such a grease would produce an interfering background signal and would have detrimental long-term effects on the reproducibility of the analysis. The glass manifold, seven magnetically coupled sapphire ball glass valves, and the molecular leak to the mass spectrometer are located in the main AGHIS oven. There are three independently controlled heating zones: (1) the main oven, (2) the vaporizing oven, and (3) the transfer line zone. The ampule and frit inlets are heated by the vaporizing oven. The glass manifold, the valves, and the transfer line are heated by the main oven. The capillary transfer line (length, 0.3 m; (31) Gallegos, E. J.; Pazzi, E. C. Proceedings of the 39th ASMS Annual Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Nashville, TN, 1991; pp 1091-1092.

Simplified Hydrocarbon Compound Type Analysis

Energy & Fuels, Vol. 11, No. 4, 1997 881 Table 1. Summary of Experimental Parameters for the Hydrocarbon Compound Type Analysis Experiments Using an All-Glass Heated Inlet System (AGHIS) and a Dynamic Batch Inlet System (DBIS)

Figure 2. Block diagram of a dynamic batch inlet system installed in the GC oven of a Hewlett-Packard Series-II 5890 gas chromatograph. The GC oven is interfaced to a 5970 Hewlett-Packard quadrupole mass spectrometer. i.d., 0.25 µm) is also heated by the transfer line heating zone. The three heating zones are typically maintained at 300 °C. Dynamic Batch Inlet System (DBIS). A simple 2 cm3 bulb is used as expansion volume (Brunfeldt Co., Bartlesville, OK). The borosilicate glass expansion unit is bonded on each end to stainless steel 1/16 in. fittings that permit the interfacing with capillary transfer lines. Capillary lines permit the transfer of the sample into the bulb and from there to the mass spectrometer. The dimensions of the capillary lines connected to the DBIS can be changed depending on the desired turnaround time for the analysis. Typical dimensions of capillary lines are given in Figure 2. Longer capillary lines with wider internal diameters can also be used resulting in approximately the same overall analysis time (e.g., 0.25 mm × 100 cm, and 0.25 mm × 10 m). The most suitable dimensions of the capillary transfer lines are empirically determined and depend on the nature and pressure of the carrier gas (if used), the GC oven temperature, and the pumping capacity of the mass spectrometer. The carrier gas used for the experiments was helium. Hydrogen produced results similar to those of helium with improved analysis times but was not used for the experiments due to safety considerations. Experimentation using no carrier gas showed that it is possible to introduce the sample into the bulb and from there to the mass spectrometer, provided that the differential pressure due to the vacuum system of the mass spectrometer permits the flow of the sample to the mass spectrometer. The dimensions of the capillary transfer lines in this case are also determined empirically depending on the pumping capacity of the mass spectrometer. Mass Spectrometers. Two 5970 Hewlett-Packard (Palo Alto, CA) quadrupole mass spectrometers were used for the experiments. The AGHIS was interfaced to a mass spectrometer running under a Pascal data acquisition and reduction system (HP 59970B Workstation). The DBIS was installed into a 5890 Series II Hewlett-Packard gas chromatograph that was interfaced to a mass spectrometer running under an MSDOS data acquisition and reduction system (HPG1034C MSChemStation). The data obtained from both systems were analyzed using ASTM and proprietary FORTRAN77 programs. A summary of the experimental parameters is given in Table 1. Perfluorotributylamine (PCR Incorporated, Gainesville, FL) was used to calibrate the mass spectrometers. The HP Autotune program was used to obtain the following peak ratios for perfluorotributylamine: m/z 69, 100%; m/z 219, 55 ( 5%; and m/z 502, 5 ( 2%. The peak width at half-height was tuned to 0.50 ( 0.05 amu. Samples. Most of the work done in our laboratory involves the analysis of naphtha, middle distillate, and lube oil samples. These samples represent different distillation boiling point ranges. Naphtha samples contain compounds boiling lower than 210 °C (411 °F). Middle distillate samples contain

parameters

AGHIS

DBIS

amt of sample introduced (µL) method of sample introduction column head pressure split ratio injection port temperature reservoir volume reservoir/oven temperature (°C) interface temperature (°C) source temperature (°C) source pressure (Torr) mass spectrometer mass range scanned (amu) scans/s run time (min)

1-3 ampule, gallium frit n/a n/a n/a 1L 300 300 220 5.0 × 10-7 HP 5970 25-800 0.55 10-15

0.2 split/splitless injector 3.0 psi of He 100:1 300 °C 2 mL 275 300 220 5.0 × 10-5 HP 5970 33-800 0.50 15

hydrocarbon compounds boiling within the 204-343 °C (400650 °F) boiling point range. Aromatic and saturate lube oil fractions contain compounds typically boiling within the 205538 °C (400-1000 °F) boiling point range. Representative samples, with well-known long-term reproducibility were used to evaluate the performance of the DBIS. Samples were obtained from Imperial Oil and Exxon sources. Liquid Chromatography (LC) and Supercritical Fluid Chromatography (SFC). Mass spectrometric results have been compared to those obtained using LC and SFC analytical methods. Open column chromatography provided total saturate and total aromatic percent values by cyclohexane solvent elution of the saturate components from silica gel followed by elution of the aromatic components using toluene. SFC analysis was used to provide information on the content of saturates and 1-ring, 2-ring, 3-ring, and 4-ring aromatics in middle distillate samples. SFC experiments were performed using a Lee Scientific (Salt Lake, Utah) Series 600 chromatograph with flame ionization detection. A stainless steel 500 × 1 mm column packed with Silica, 60 Å pore size, 10 µm particle size (YMC Inc., Morris Plains, NJ), was used for the analysis. HPLC characterization of lube oil samples was done with a Varian 9012 system equipped with a diode array detector (DAD) and an evaporative mass detector (EMD). Details for the HPLC experimental procedures can be found elsewhere.22

Results and Discussion Principles of Hydrocarbon Compound Type Analysis by Low-Resolution Mass Spectrometry. Low-resolution mass spectrometers compared to highresolution instruments are simpler to maintain and operate. For these reasons, several standard ASTM test methods using low-resolution mass spectrometry have been developed and are widely used in the petroleum industry.32-35 The older magnetic sector mass spectrometers such as the Consolidated Electrodynamics Corp. (CEC) instruments used extensively in the petroleum laboratories are gradually being replaced by (32) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 519-524. (33) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 424-429. (34) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 512-518. (35) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 586-598.

882 Energy & Fuels, Vol. 11, No. 4, 1997

smaller quadrupole mass spectrometers.36 The quadrupole instruments have demonstrated excellent capabilities to reproduce critical hydrocarbon compound type data previously obtained only with magnetic sector instruments. To evaluate the performance of the DBIS compared to the existing AGHIS, low-resolution hydrocarbon compound type experiments were conducted and the mass spectra were analyzed using standard ASTM and Imperial Oil computer programs. The principles of compound type analysis by low-resolution mass spectrometry are briefly presented below. The measured peak intensity in the mass spectrum of a complex mixture is a linear function of the individual mixture components that can generate ions with the same mass to charge (m/z) value. The experimental mass spectral data can then be represented as a set of linear equations:

i1 ) a11p1 + a12p2 + . . . + a1npn i2 ) a21p1 + a22p2 + . . . + a2npn ... im ) am1p1 + am2p2 + . . . + amnpn im is the measured peak intensity at mass m in the mixture spectrum, amn is the peak intensity at mass m due to unit pressure of component n, and pn is the partial pressure of component n in the mixture. In matrix algebra notation the equations become

{I} ) [A]{P} The unknown partial pressures {P} of the n components in the mixture can be calculated using the measured intensities {I}, and the known peak intensities due to unit pressure [A] of the n components:

{P} ) [A]-1{I} The inverse matrix [A]-1 is typically determined from the analysis of mixtures of compounds with known concentrations. In inlet systems operating under molecular flow conditions, the partial pressures of the mixture components are proportional to their concentrations. This approach can be used for any type of quantitative compound mixture analysis. A simplification in the approach is the use of a single characteristic mass or mass series for each component (or compound type). This allows for the treatment of a linear system of n equations with n unknown concentrations. A compound type is represented by the ion intensity summation of characteristic ions produced in the 70 eV EI mass spectra of the compound type. For example, the abundances of m/z 43, 57, 71, 85, etc. (∑43), are summed to obtain a single ion abundance value for all paraffins in the mixture. Similarly, the abundances of m/z 91, 105, 119, 133, etc. (∑91), are summed to produce a single ion abundance for all alkylbenzenes in the mixture. By working in this fashion with a set of samples with known hydrocarbon type concentrations, one can obtain the inverse matrix for a petroleum sample from the known concentrations and the measured ion abun(36) Ashe, T. R.; Colgrove, S. G. Energy Fuels 1991, 5, 356-360.

Roussis and Cameron

Figure 3. Total ion current produced from the introduction of a middle distillate sample via the all-glass heated inlet system. The sample is allowed to flow for approximately 15 min, at which time data acquisition is terminated and the sample is evacuated from the reservoir.

dances. Consequently, using the determined inverse matrix and the measured ion abundances, one deduces the concentrations of the mixture components. The method is more accurate when it is applied to samples very similar in nature to the calibration set. Typically, different inverse matrices are determined depending on the chemical nature, boiling range, and average molecular weight properties of the sample.32-35 Sample Introduction Using the AGHIS. Figure 3 displays the total ion current (TIC) produced upon the introduction of a middle distillate sample via the AGHIS into the mass spectrometer. Data acquisition starts as soon as the sample is introduced through the gallium frit into the glass reservoir. It takes approximately 4 min for the TIC to reach its maximum value. This is the time required for the sample components to expand into the reservoir and via the transfer line reach the ionization source of the mass spectrometer. Once the TIC has reached its maximum value, it remains stable for a long period of time (more than 30 min). For the purposes of our experiments, long sample residence in the reservoir is not required; therefore, after approximately 15 min, data acquisition is terminated and the sample is evacuated from the reservoir. The time profiles of selected ions produced in the analysis of a naphtha sample, a middle distillate sample, a saturate lube oil fraction, and an aromatic lube oil fraction are shown in Figures 4-7, respectively. These profiles are reconstructed from the raw data obtained by the analysis of the samples. After a given time, the ion signal reaches a maximum value and it then decays slowly. Some ions in the heavier samples (e.g., m/z 92 in Figure 7 and m/z 196 in Figure 6) show a pronounced signal increase followed by a decrease at approximately 10 min. This may be due to differences in the origin of these ions (e.g., molecular vs fragment ions) or due to differences in the nature of the compounds generating these ions (e.g., boiling points). It is observed in Figures 4-7 that a longer period is required for the heavier samples to reach the ionization source. This is due to the molecular and boiling property differences of the samples. Introduction of the naphtha sample is very rapid due to its low boiling point (lower than 210 °C). It takes less than 2 min for the naphtha sample to reach its maximum TIC value. The lube oil samples are introduced via the heated ampule (200-550 °C boiling range) and require more than 8 min to reach the maximum TIC values.

Simplified Hydrocarbon Compound Type Analysis

Figure 4. Time profiles of selected ions produced in the analysis of a naphtha sample using an all-glass heated inlet system: (×) m/z 78; (0) m/z 106; (b) m/z 120; (9) m/z 134.

Figure 5. Time profiles of selected ions produced in the analysis of a middle distillate sample using an all-glass heated inlet system: (×) m/z 100; (0) m/z 134; (O) m/z 184; (9) m/z 254.

Figure 6. Time profiles of selected ions produced in the analysis of a saturate lube oil fraction using an all-glass heated inlet system: (2) m/z 140; (0) m/z 196; (b) m/z 308; (O) m/z 364.

Sample Introduction Using the DBIS. The total ion current produced from the introduction of a middle distillate sample using the DBIS is shown in Figure 8. Sample (0.2 µL) was introduced via a split injector

Energy & Fuels, Vol. 11, No. 4, 1997 883

Figure 7. Time profiles of selected ions produced in the analysis of an aromatic lube oil fraction using an all-glass heated inlet system: (2) m/z 92; (×) m/z 178; (O) m/z 358.

(split ratio 100:1). Since the DBIS does not have capabilities for sample evacuation, it is important to introduce small quantities of sample since the entire amount introduced in the expansion volume will reach the ionization source. The TIC profile obtained in Figure 8 shows a rapid increase of the signal in the earlier stages of the analysis (∼4 min) followed by a slower signal decrease in the later parts of the analysis (∼10 min). This is due to the fact that the flow is more restricted in the transfer line connecting the expansion unit to the mass spectrometer, compared to the restriction in the transfer line connecting the injector to the expansion unit. The overall effect is to maintain the sample in the expansion volume for a given period of time. The residence time for a given expansion volume can be changed depending on the transfer line dimensions, the nature of the carrier gas, and the oven temperature. Selected ion profiles generated from the introduction of the same four samples analyzed previously by the AGHIS (i.e., naphtha, middle distillate, saturate lube oil, and aromatic lube oil) are shown in Figures 9-12. The signal starts increasing at approximately the same time (∼4 min) independently of the nature of the sample. However, it takes longer time for the heavier lube oil samples to be transferred to the mass spectrometer under the same experimental conditions. The DBIS experiments shown in Figures 9 and 10 for the naphtha and middle distillate samples were done with the GC oven temperature maintained at 275 °C. For the analysis of the lube oil samples, the GC oven was maintained at 275 °C for 10 min and it was then programmed at 30 °C/min to 350 °C, where it was maintained for 4 min. This was done to shorten the run time for the analysis of the heavier samples. The effect of the oven ramping after 10 min is shown in Figures 11 and 12. Comparison of Figures 4-7 and Figures 9-12 for the samples analyzed by using the AGHIS and DBIS, respectively, shows that the relative ratios of the monitored selected ions in both cases are similar. For example, in both Figures 4 and 9 for the analysis of the same naphtha sample by the AGHIS and DBIS, respectively, the ion abundances of the four monitored ions increase in the same order: m/z 78 > m/z 106 > m/z 120 > m/z 134. The same is also true for the

884 Energy & Fuels, Vol. 11, No. 4, 1997

Roussis and Cameron

Figure 8. Total ion current produced from the introduction of a middle distillate sample using a dynamic batch inlet system.

Figure 9. Time profiles of selected ions produced in the analysis of a naphtha sample using a dynamic batch inlet system: (×) m/z 78; (0) m/z 106; (b) m/z 120; (9) m/z 134.

analysis of the other samples. This indicates a similarity in the flow processes of compounds introduced via the AGHIS and the DBIS. However, similarity in the flow process from the expansion volume to the mass spectrometer is not a necessary requirement for hydrocarbon compound type analysis by low-resolution mass spectrometry. The most important requirement is the introduction of the entire sample without material loss or compositional change. Provided that the entire sample is introduced, and the final mass spectrum used for the calculations is obtained by the summation of all acquired mass spectra across the run, both AGHIS and DBIS are expected to generate similar results. The results obtained with the two introduction systems for several representative samples are discussed below. Comparison of Data Obtained Using the AGHIS and the DBIS. Acquisition of data using both introduction systems was done for approximately 15 min. A final mass spectrum was obtained by the summation of all acquired mass spectra. The mass spectral ion abundances of the summed mass spectrum were normalized on the basis of the most intense ion abundance

Figure 10. Time profiles of selected ions produced in the analysis of a middle distillate sample using a dynamic batch inlet system: (×) m/z 100; (0) m/z 134; (O) m/z 184; (9) m/z 254.

in the spectrum. Table 2 shows an example of data obtained with the two systems by the analysis of a naphtha sample. The DBIS analysis was conducted by sample injection using an HP 7673A autosampler. The use of an autosampler is very convenient for routine analysis of samples. The average and standard deviation values are derived from data acquired over a period longer than 6 months. Both the DBIS and AGHIS display similar margins of long-term repeatability for the analysis of the naphtha sample. The differences for the alkylbenzenes and indans/tetralins are wider thanthe repeatability ranges of both methods. The results corresponding to the paraffins, monocycloparaffins, and dicycloparaffins are within the margins of long-term repeatability of the methods. Tables 3 contains data obtained by using the two systems for a saturate lube oil. The long-term repeatability of the AGHIS appears to be better than that of the DBIS for the D2786 saturate lube oil analysis. With the exemption of the 2-ring cycloparaffins, all other D2787 compound types produce results within the margins of error of the two methods. Table 4 contains

Simplified Hydrocarbon Compound Type Analysis

Energy & Fuels, Vol. 11, No. 4, 1997 885 Table 3. Comparison of Volume Percent Data Obtained for the Analysis (D2786-91) of a Saturate Lube Oil Fraction Using a DBIS and an AGHIS DBISa

AGHISb

types

average

std dev

average

std dev

0-ring paraffins 1-ring cycloparaffins 2-ring cycloparaffins 3-ring cycloparaffins 4-ring cycloparaffins 5-ring cycloparaffins 6-ring cycloparaffins mono-aromatics

29.2 20.8 16.7 15.5 8.3 4.3 1.4 3.9

1.9 1.3 1.0 1.1 0.8 0.7 0.7 0.8

29.1 19.6 18.4 15.7 8.3 3.5 1.1 3.8

0.7 0.8 0.3 0.2 0.3 0.3 0.2 0.5

a DBIS data obtained from 10 replicate measurements conducted over a long-term period (more than 6 months). b AGHIS data obtained from 18 replicate measurements conducted over a long-term period (more than 6 months).

Figure 11. Time profiles of selected ions produced in the analysis of a saturate lube oil fraction using a dynamic batch inlet system: (2) m/z 140; (0) m/z 196; (b) m/z 308; (O) m/z 364.

Table 4. Comparison of Volume Percent Data Obtained for the Analysis (D3239-91) of an Aromatic Lube Oil Fraction Using a DBIS and an AGHIS DBISa types alkylbenzenes naphthene benzenes dinaphthene benzenes naphthalenes acenaphthenes/ dibenzofurans fluorenes phenanthrenes naphthenephenanthrenes pyrenes chrysenes perylenes dibenzanthracenes benzothiophenes dibenzothiophenes naphthobenzothiophenes total unidentified

Figure 12. Time profiles of selected ions produced in the analysis of an aromatic lube oil fraction using a dynamic batch inlet system: (2) m/z 92; (×) m/z 178; (O) m/z 358. Table 2. Comparison of Weight Percent Data Obtained for the Analysis (D2789-90) of a Naphtha Sample Using a DBIS and an AGHISa DBISb

AGHISc

types

average

std dev

average

std dev

paraffins monocycloparaffins alkylbenzenes dicycloparaffins indans and tetralins

37.9 44.7 14.4 1.9 1.1

1.3 0.8 0.9 0.4 0.5

37.6 45.5 16.0 1.5 0.3

1.1 0.6 0.5 0.3 0.1

a An HP 7673A autosampler was used for the DBIS sample introduction. b DBIS data obtained from 10 replicate measurements conducted over a period longer than 6 months. c AGHIS data obtained from 18 replicate measurements conducted over a period longer than 6 months.

results obtained from the analysis of an aromatic lube oil fraction by method D3239. With the exemption of fluorenes and benzothiophenes, the remaining compounds types are within the margins of error of the two methods. The results obtained in Tables 2-4 show that there is a good overall agreement between the AGHIS and DBIS data. The percent average absolute difference between samples analyzed by the AGHIS and the DBIS, respectively, was found to be less than 1.0% (0.8%, 0.6%, and 0.6% for D2789, D2786, and D3239, respectively).

AGHISb

average std dev average std dev 29.0 15.5 13.1 3.2 5.8

2.4 0.6 0.9 0.9 0.3

29.5 15.1 11.8 2.7 5.2

2.1 0.5 0.4 0.4 0.4

5.9 3.1 1.9 1.2 0.8 0.6 0.3 3.6 3.2 0.7 12.3

0.2 0.4 0.6 0.4 0.6 0.2 0.1 1.2 0.3 0.1 1.5

5.0 2.3 2.8 1.4 0.8 0.6 0.3 5.5 3.0 0.6 13.5

0.3 0.4 0.3 0.2 0.1 0.1 0.1 0.6 0.3 0.1 1.8

a DBIS data obtained from 10 replicate measurements conducted over a long-term period (more than 6 months). b AGHIS data obtained from 15 replicate measurements conducted over a long-term period (more than 6 months).

This calculation was based on the comparison of all compound types reported by each method. However, the results in Tables 2-4 show that there are also differences for some compound types. These differences may be due to the different sample introduction, mass spectrometric, or data acquisition systems used for the AGHIS and DBIS experiments, respectively. Since, however, most differences are within the error margins of the two methods, it is difficult to assess the significance of the observed differences. An approach to obtain additional information about the significance of the observed differences between the AGHIS and the DBIS is to compare results obtained by the mass spectrometric methods to those obtained by using other methods of analysis (e.g., LC, SFC, etc.). Overall differences in the results obtained by using the AGHIS and the DBIS should not exceed the differences between the mass spectrometric methods and the other methods of analysis. Comparison of Data Obtained Using Mass Spectrometric and Other Analytical Methods for Hydrocarbon Compound Type Analysis. Several petroleum fractions have been analyzed by AGHIS lowresolution mass spectrometry, SFC, HPLC, and open

886 Energy & Fuels, Vol. 11, No. 4, 1997

Roussis and Cameron

Table 5. Comparison of Total Saturates Weight Percent Data Obtained for the Analysis of Several Petroleum Fraction by MS (Robinson Total Oil), SFC, HPLC, and Open Column Chromatographic (OCC) Methods sample

MS

std dev

SFC

std dev

crude #1 (naphtha cut) diesel oil lube oil #1 lube oil #2 lube oil #3 crude distillate crude #2 (70-343 °C cut) crude #3 (70-343 °C cut)

84.2 78.4 78.6 78.8 44.1 38.0 79.5 80.5

1.7 0.9 0.8 0.9 0.5 0.5 0.9 0.9

84.4 77.0

0.2 0.2

77.8 78.3

Table 6. Comparison of Weight Percent Data Obtained for the Analysis of a Petroleum Fraction (Crude #2, 70-343 °C Cut) by MS (Robinson Total Oil) and SFC Methods MS

SFC

HPLC

std dev

OCC

std dev

77.4 75.5 43.2 40.8

2.2 2.2 1.2 1.2

80.4 82.1

0.4 0.4

0.2 0.2

analytical methods in Tables 5 and 6 is 1.7%. This difference is within the same order of magnitude as the overall difference (less than 1%) obtained between the AGHIS and DBIS data.

compound type

average

std dev

average

std dev

Conclusion

total saturates monoaromatics diaromatics triaromatics

79.5 15.6 3.5 0.4

0.9 0.2 0.1 0.1

77.8 17.0 4.2 1.0

0.2 0.1 0.1 0.1

The use of the DBIS as a sample introduction system offers the opportunity to simplify hydrocarbon compound type analysis by using a smaller, lower cost, easy to maintain, and highly transportable expansion unit that can be installed into any commercial GC/MS system. The DBIS has an excellent potential to be used for field applications requiring rapid quantitative methodologies for the analysis of complex mixtures. In this work, comparisons revealed good overall agreement between results obtained by the DBIS and the conventional AGHIS, respectively. However, differences were also observed for some compound types. Overall differences were found to be on the same order of magnitude as those found between the mass spectrometric and other analytical methods. Future work could address in more detail the margins of acceptable error in hydrocarbon compound type analysis by an interlaboratory study of a large set of samples following a rigorous analytical protocol.

column chromatography (OCC) analytical methods. The data were calculated from a program based on Robinson’s compound type analysis method37 which permits the determination of 25 saturate and aromatic compound types in petroleum fractions. Robinson’s method is particularly useful because it applies to samples with wide boiling point ranges (e.g., 200-1100 °F) without the need for physical separation into saturate and aromatic fractions. The results obtained for the total saturates weight % content are summarized in Table 5. Table 6 contains a summary of the results obtained from the analysis of a sample by MS and SFC. The data given in Tables 5 and 6 represent an average from at least two replicate determinations. Tables 5 and 6 show a very good agreement between the mass spectrometric and the other analytical methods. The percent average absolute difference between the data obtained by the mass spectrometric and the other (37) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434.

Acknowledgment. The authors thank D.G. Lindsay for the LC and SFC data. EF960221J