Hydrocarbon Compound Type Analysis by Mass Spectrometry: On the

Energy & Fuels 2001, 15, 477-486

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Hydrocarbon Compound Type Analysis by Mass Spectrometry: On the Replacement of the All-Glass Heated Inlet System with a Gas Chromatograph Stilianos G. Roussis* and W. Pat Fitzgerald Research Department, Products and Chemicals Division, Imperial Oil, Sarnia, Ontario, Canada, N7T 8C8 Received October 11, 2000. Revised Manuscript Received December 20, 2000

Hydrocarbon compound type analysis by mass spectrometry is a simple and powerful method that provides extensive compositional information about complex petroleum fractions. Sample introduction into the mass spectrometer has been successfully done for over 30 years using the all-glass heated inlet system (AGHIS). However, the limited transportability of the AGHIS has considerably restricted the wider usage of mass spectrometry for hydrocarbon compound type analysis. In this work, a gas chromatograph (GC) has been examined as a system equivalent to the AGHIS for the introduction of petroleum fractions into the mass spectrometer (MS). It is demonstrated that the GC is a versatile system that offers many advantages over the AGHIS, including simplicity of operation, ease of maintenance, automation, and transportability. Most importantly, the use of the GC allows for the detection of individual compounds, a feature not available with the AGHIS. The capabilities of the GC/MS system have been examined by the analysis of several representative petroleum fractions. The differences in the results obtained by the two sample introduction methods are of the same order of magnitude as the repeatability of the two methods. The ability of the GC/MS system to determine individual compounds in lowboiling-range samples has been illustrated by the use of selective-ion monitoring (SIM) experiments. Individual compounds in heavier samples have been determined by using the automated deconvolution program AMDIS to separate overlapping components.

Introduction Refinery process optimization and the ability to meet product specifications greatly depend on the availability of standard methodologies that are able to provide compositional information about feeds and products. Due to its capability of providing detailed and repeatable compositional information, mass spectrometry (MS) has become an important analytical tool for the characterization of petroleum fractions. The first mass spectrometric methods for mixture analysis were introduced over 50 years ago.1,2 Hydrocarbon compound type methods were originally developed for the analysis of gasoline,3 light distillates,4 gas oil, and heavy distillates.5-9 More recently, wider boiling range methods utilizing * Author to whom correspondence should be addressed at Research Department, Products and Chemicals Division, Imperial Oil, 453 Christina St. S., P.O. Box 3022, Sarnia, Ontario, Canada, N7T 8C8. Tel: 519-339-2441. Fax: 519-339-4436. (1) Brown, R. A.; Taylor, R. C.; Melpolder, F. W.; Young, W. S. Anal. Chem. 1948, 20, 5-9. (2) Thomas, B. W.; Seyfried, W. D. Anal. Chem. 1949, 21, 10221026. (3) Brown, R. A. Anal. Chem. 1951, 23, 430-437. (4) Lumpkin, H. E.; Thomas, B. W.; Elliot, A. Anal. Chem. 1952, 24, 1389-1391. (5) O’Neal, M. J., Jr.; Wier, T. P., Jr. Anal. Chem. 1951, 23, 830843. (6) Lumpkin, H. E.; Johnson, B. H. Anal. Chem. 1954, 26, 17191722. (7) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946-1948. (8) Hastings, S. H.; Johnson, B. H.; Lumpkin, H. E. Anal. Chem. 1956, 28, 1243-1247. (9) Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 41, 1548-1554.

both low-10 and high-resolution instruments11-13 have expanded the applicability range and considerably improved the accuracy of the analyses.14,15 Hydrocarbon compound type analysis is based on the use of calibration mixtures with known compositions to obtain the relative instrument response factors for the different compound types.12,16 These response factors are used in combination with the measured mass spectrum to determine the composition of unknown samples. Typically, for higher accuracy, ion abundance summations of a series of characteristic fragments or molecular ions are used instead of the abundance of single ions. The fragmentation patterns of the different compound types are obtained from model compounds. Calibration mixtures must be very similar in nature to the samples of interest. The accuracy of the quantification greatly depends on the use of appropriate calibration mixtures. For that reason, method development can be timeconsuming due to the need for determination of the (10) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434. (11) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833-1838. (12) Teeter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143. (13) Bouquet, M.; Brument, J. Fuel. Sci. Technol. Int. 1990, 8, 961986. (14) Fafet, A.; Bonnard, J.; Prigent, F. Oil Gas Sci. Technol. 1999, 54, 439-452. (15) Fafet, A.; Bonnard, J.; Prigent, F. Oil Gas Sci. Techno. 1999, 54, 453-462. (16) Roussis, S. G.; Cameron, A. S. Energy Fuels 1997, 11, 879886.

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relative response factors for all compound types in the samples of interest. However, once developed, hydrocarbon compound type methods can be very easily applied to the routine analysis of a wide range of petroleum samples producing information that cannot be easily obtained by other methods. A major hurdle in the wider application of the mass spectrometric methods for compound type analysis has been the difficulty associated with sample introduction into the mass spectrometer. Because petroleum fractions extend over a wide boiling range (e.g., gases to vacuum residues), it was originally difficult to develop simple sample introduction systems for all of the different petroleum fractions.17-20 Eventually the all-glass heated inlet system (AGHIS) was introduced as a suitable sample introduction system. 21,22 The AGHIS successfully introduces most boiling petroleum fractions into the mass spectrometer and has been reliably used for more than 30 years in petroleum and other analytical laboratories. However, there are several disadvantages associated with the use of the AGHIS. It generally requires a dedicated mass spectrometer since its typical bulk and fragile nature tend to discourage frequent interchanges between instruments or other sample introduction systems such as the direct insertion probe. Moreover, operation and maintenance require highly trained personnel. These characteristics restrict the transportability of the AGHIS and the general usage of the approach for routine hydrocarbon compound type analysis. To alleviate some of the limitations of the AGHIS, a dynamic batch inlet system (DBIS) was developed to simplify sample introduction into the mass spectrometer.16 The DBIS displayed several desirable characteristics, including simplicity of operation, low cost, ease of maintenance, automation, and transportability. In the current work, the use of a gas chromatograph (GC) has been examined as a sample introduction system, capable of introducing hydrocarbon mixtures into a mass spectrometer, in a fashion equivalent to that of the AGHIS and the DBIS. Recent changes in the research and development needs of the petroleum industry require the use of versatile sample introduction systems that are able to perform a variety of analytical tests with the minimum hardware changes. The GC is a versatile system and offers the additional advantage of using an analytical column that permits the separation of individual compounds. Although the separation of individual compounds in the very heavy fractions is restricted due to the extreme complexity of the fractions, individual compounds can be separated in the lower boiling ranges (e.g., gasoline, naphtha, middle distillate). Previous studies have recognized the capabilities of GC/MS for hydrocarbon compound type analysis. For example, Gehron and Yost23 developed a GC/MS method for the analysis of jet fuels. A GC/MS method with a (17) Genge, C. A. Anal. Chem. 1959, 31, 1747-1748. (18) Lumpkin, H. E.; Taylor, G. R. Anal. Chem. 1961, 33, 476-477. (19) Boyer, E. W.; Hamming, M. C.; Ford, H. T. Anal. Chem. 1963, 35, 1168-1171. (20) Teeter, R. M.; Doty, W. R. Rev. Sci. Instrum. 1966, 37, 792793. (21) Peterson, L. Anal. Chem. 1962, 34, 1850-1851. (22) Stafford, C.; Morgan, T. D.; Brunfeldt, R. J. Int. J. Mass Spectrom. Ion Phys. 1968, 1, 87-92.

Roussis and Fitzgerald

packed column was developed by Castex et al.24 for the quantitative analysis of IBP-C20 hydrocarbon fractions. Fafet and Magne-Drisch25 reported the use of GC/MS for the analysis of middle distillate samples. More recently, a GC/MS method was developed to characterize crude oils and determine their boiling point distributions.26 However, the possibility of transferring existing hydrocarbon compound type methods from the AGHIS to the GC/MS system has not been previously evaluated in a systematic manner. This is achieved in the first part of the current work, by the analysis of a set of representative samples (propylene tetramer, naphtha, middle distillate, and saturate and aromatic lube oil fractions) of well-known compositions, by both the AGHIS and GC/MS systems. The effects of experimental parameters on both systems are considered, and conditions are determined that permit the acquisition of similar composite mass spectra by the GC/MS and the AGHIS. The capability of GC/MS to perform hydrocarbon compound type analysis in a manner analogous to that of the AGHIS is demonstrated by the good agreement between the data obtained by the two systems. In the second part of the paper, the unique capabilities of the GC/MS system for the detection of individual compounds in petroleum fractions are illustrated. Selective-ion monitoring (SIM) is used to detect target compounds such as benzene, toluene, xylenes, etc., in a light petroleum fraction (naphtha). The capability for detection of individual compounds in the heavier fractions is illustrated by the application of the NIST (National Institute of Standards and Technology) program AMDIS (Automated Mass Spectral Deconvolution and Identification System). An example is given where AMDIS is used to detect a trace aromatic sulfur compound in a middle distillate sample. In another example, the distributions of n-alkane compounds in a middle distillate sample are determined by using AMDIS to separate the n-alkane spectra from those of overlapping hydrocarbons. Experimental Section All-Glass Heated Inlet System (AGHIS). The AGHIS (Brunfeldt Co., Bartlesville, OK) used for the experiments has been described previously.16 Liquid samples (∼1-3 µL) are introduced into the heated inlet via a gallium-covered frit. A capillary pipet filled with the liquid sample is immersed into the molten gallium, which acts as a vacuum seal. The differential pressure allows the sample to enter from the capillary pipet into the low pressure (∼10-3 Torr) expansion chamber (1 L). Higher-viscosity samples and solids are introduced via an ampule inlet. Optically flat surfaces are used to mate the ampule inlet with the vacuum lock. In that fashion, no grease is needed to achieve a vacuum-tight seal. The AGHIS heating zones can reach temperatures as high as 400 °C. For the purposes of our experiments, temperatures were maintained at ∼300 °C. (23) Gehron, M. J.; Yost, R. A. In Novel Techniques in Fossil Fuel Mass Spectrometry; ASTM STP 1019, Ashe, T. R., Wood, K. V., Eds.; American Chemical Society for Testing and Materials: Philadelphia, 1989; pp 24-37. (24) Castex, H.; Boulet, R.; Juguin, J.; Lepinasse, A. Rev. Inst. Franc. Petrole 1984, 39, 175-187. (25) Fafet, A.; Magne-Drisch, J. Rev. Inst. Franc. Petrole 1995, 50, 391-404. (26) Roussis, S. G.; Fitzgerald, W. P. Anal. Chem. 2000, 72, 14001409.

Hydrocarbon Compound Type Analysis by MS

Energy & Fuels, Vol. 15, No. 2, 2001 479

Table 1. Summary of Experimental Parameters for Hydrocarbon Compound Type Analysis Experiments Using an All-Glass Heated Inlet System (AGHIS) and a Gas Chromatography/Mass Spectrometry System (GC/MS)

a

parameters

AGHI (Brunfeldt)

GC/MS D2424, D2425, D2789

GC/MS D2786, D3239

sample size (µL) dilution factor injection type AGHIS reservoir temp. (°C) injection port initial temp. (°C) injection port heating rate (°C/s) injection port final temp. (°C) GC oven initial temp. (°C) GC oven heating rate (°C/min) GC oven final temp. (°C) GC oven final time mass spectrometer tune masses (PFTBA) mass range scanned (amu) mass spec. source temp. (°C) interface temperature (°C)

1-3 none whole sample 300 n/a n/a n/a n/a n/a n/a n/a HP 5890 68.9, 218.7, 501.4 25-800 220 300

0.1 none split 50:1 n/a 0 12 350 0 5 300 0 HP 5890 68.9, 218.7, 501.4 25-250 (650)a 220 300

0.5 20:1 split 50:1 n/a 50 12 400 50 10 380 10 HP 5890 68.9, 218.7, 501.4 25-800 220 300

D2425.

Gas Chromatograph. All chromatographic analyses were done using a Hewlett-Packard (HP) 5890 series II gas chromatograph. A 15 m × 0.25 mm i.d. DB-1 HT J&W (J&W Scientific, Folsom, CA) fused-silica capillary column with 0.1 µm film thickness was used for the experiments. For the light samples (propylene tetramers, naphthas, middle distillates) the GC oven temperature was programmed from 0 °C to 300 °C at a rate of 5 °C/min. For the heavier samples (saturate, aromatic lube oil fractions) the GC oven temperature was programmed from 50 °C to 380 °C at a rate of 10 °C/min. The GC oven remained at the maximum temperature for 10 min. Samples were introduced into the gas chromatograph via a Gerstel injection system (Cooled Injection System-CIS 3, Gerstel, Germany). For the light samples, the injection system temperature was programmed from 0 °C to 350 °C at a rate of 12 °C/s. The heavy samples were introduced at 50 °C. The injection system reached 400 °C at a rate of 12 °C/s. It remained at the maximum temperature for 10 min. Samples were introduced into the Gerstel injector with a 7673A HP autosampler. Mass Spectrometers and Data Reduction. Experiments were performed using two 5970 Hewlett-Packard quadrupole mass spectrometers. The instruments were interfaced to the AGHIS and GC systems via heated capillary transfer lines (300 °C). The mass spectrometers were operated in the electron ionization mode (70 eV). Perfluorotributylamine (PCR, Inc., Gainesville, FL) was used as calibration compound. The experimental parameters are summarized in Table 1. Instrument control and data acquisition on both AGHIS and GC/ MS systems were done with Windows-based HP ChemStation software. Data obtained by the two systems were treated by standard ASTM (American Society for Testing and Materials) and Imperial Oil proprietary programs. AMDIS was obtained from NIST (Gaithersburg, MD) as part of the NIST’98 release of the NIST/EPA/NIH mass spectral library. Samples. Most commonly analyzed samples in our laboratory are the following: propylene tetramers (compounds with average carbon number between C9 and C15), naphthas (compounds with boiling points less than ∼210 °C), middle distillates (samples boiling in the 200-343 °C range), and saturate and aromatic lube oil fractions (samples boiling in the 200540 °C range). Representative samples, with well-known compositions and long-term repeatabilities, were used for the evaluation of the performance of the GC/MS system and for the comparisons with the AGHIS. Samples were obtained from Imperial Oil and Exxon sources.

Figure 1. Sample introduction via the AGHIS: (A) total ion current produced from the introduction of a propylene tetramer sample; (B) composite mass spectrum obtained by summation of all spectra acquired in the run.

Results and Discussion Sample Introduction Using the AGHIS. Figure 1A shows the total ion current (TIC) produced by the introduction of a propylene tetramer sample via the AGHIS into the mass spectrometer. Data acquisition starts immediately after the sample is introduced into the mass spectrometer. Due to the volatile nature of the propylene tetramer sample it takes less than 3 min to expand into the heated expansion chamber and reach the mass spectrometer via the transfer line. Once the TIC reaches its maximum value, it remains approximately constant for more than 30 min. Acquisition is allowed to continue for a period of time that depends

480


Figure 2. Sample introduction via the AGHIS: (A) total ion current produced from the introduction of a naphtha sample; (B) composite mass spectrum.

on the volatility of the samples. Typical runs for light samples are ∼10 min and for heavy samples ∼15 min. A composite mass spectrum is obtained by the summation of all spectra acquired over the selected time period. Figure 1B shows the composite mass spectrum obtained by summation of the spectra acquired from the beginning of the run until ∼10 min. The total ion currents obtained from the introduction of a naphtha and a saturate lube oil fraction are shown in Figures 2A and 3A, respectively. The naphtha sample is even more volatile than the propylene tetramer, reaching the mass spectrometer in less than 2 min. The saturate lube oil fraction is considerably less volatile, requiring more than 5 min to reach the mass spectrometer. However, the TICs of both samples reach maximum values before 15 min, thus making the typical run times for samples analyzed by the AGHIS no longer than 15 min. After data acquisition is terminated, the sample is evacuated from the heated glass chamber, and a new sample is loaded for analysis. The composite mass spectra obtained for the naphtha and saturate lube oil fractions are shown in Figures 2B and 3B, respectively. Sample Introduction Using the GC/MS. The total ion currents and composite mass spectra for the propylene tetramer, naphtha, and lube oil saturate fraction samples acquired by using the GC/MS system are shown in Figures 4-6, respectively. Although run times with the GC/MS system can be long (i.e., more than 60 min) in general, depending on the chromatographic requirements for peak separation, peak separation to the baseline is not a requirement for compound type analysis using the GC/MS system. Based on the fact that only a composite mass spectrum is needed for the analysis, run times for type analysis by GC/MS can be within the same order of magnitude as those utilizing the AGHIS


Figure 3. Sample introduction via the AGHIS: (A) total ion current produced from the introduction of a saturate lube oil fraction; (B) composite mass spectrum.

(i.e., less than ∼30-40 min). The most significant criterion for hydrocarbon compound type analysis by GC/MS is to scan the mass spectrometer at fast enough rates to obtain an adequate number of mass spectra for the characterization of chromatographic peaks. The same criterion is not important for the AGHIS experiments because all compounds are thoroughly mixed in the heated expansion chamber and the composition of the mixture does not change as a function of time. This, of course, is not the case with the GC/MS system where compounds are separated in time by the column, and there is a finite period of time for compound elution. Therefore, it is very important in the GC/MS experiments to scan fast enough to obtain an adequate number of scans per chromatographic peak. However, scanning too fast can also introduce several undesirable problems related to the integrity of the mass spectra. At very fast scan rates, isotopic distributions are not always correct and mass spectrometric peaks of weak abundances may not be detected due to the lack of time for the scan. We have found that for the chromatographic conditions used in this work, 2-3 scans/s can adequately sample the eluting components of the mixtures. Longer compound elution times can be obtained by using longer chromatographic columns or slower GC oven heating profiles. Using shorter, wider-bore columns can result in longer compound elution times and shorter runs, but chromatographic separation would be severely compromised. Fast chromatography can also be used to accurately describe very narrow chromatographic peaks by replacing the quadrupole mass spectrometers used in the current study with time-of-flight (TOF) instruments27 which are able to acquire hundreds of spectra per second.28 (27) Cotter, R. J. Anal. Chem. 1999, 71, 445A-451A.


Figure 4. Sample introduction using the GC/MS: (A) total ion current produced from the introduction of a propylene tetramer sample; (B) composite mass spectrum obtained by summation of all spectra between 4 and 12 min.

The total ion current for the propylene tetramer sample in Figure 4A is complex because of the large number of isomeric mono-olefin structures produced in the manufacturing process. Appropriate chromatographic optimization could effectively improve peak separation but it was not necessary for the purposes of the present work. A significant advantage of the GC/ MS over the AGHIS is that in addition to the mass scale, the retention time scale can be used for comparisons of samples. Such comparisons can provide useful information about boiling range differences in addition to the compositional information provided by the hydrocarbon compound type analysis. The naphtha total ion current (Figure 5A) is simpler than those of the other two samples due to its low boiling range (i.e., lower than 210 °C). The sample is a distillation cut, obtained directly from a crude oil, without any processing. Chromatographic separation could be improved but it was not necessary for the current analysis. The total ion current of the saturate lube oil fraction is extremely complex (Figure 6A). This is due to the very large number of possible isomeric structures present in the lube oil boiling range (300540 °C). For such high-boiling samples, it is not possible to optimize the chromatographic conditions for separation of isomers. However, even for such complex samples, the GC/MS system has advantages over the AGHIS. A solvent can be used to dissolve the sample prior to analysis with the GC/MS system. The column will separate the solvent from the sample components. This (28) Gardner, B. D.; Holland, J. F. Proceedings of the 44th ASMS Annual Conference on Mass Spectrometry and Allied Topics; American Society for Mass Spectrometry: Portland, Oregon, 1996; p 1119.


Figure 5. Sample introduction using the GC/MS: (A) total ion current produced from the introduction of a naphtha sample; (B) composite mass spectrum obtained by summation of all spectra in the run.

Figure 6. Sample introduction using the GC/MS: (A) total ion current produced from the introduction of a saturate lube oil fraction; (B) composite mass spectrum obtained by summation of all spectra acquired between 5 and 17 min.

is not possible with the AGHIS, which would report severely distorted results for a solvated sample. Dilution with convenient solvents (e.g., toluene, cyclohexane, carbon disulfide) can be used for the introduction of heavy petroleum fractions. The approach has been used

482



Table 2. Comparison of Weight % Data Obtained for the Analysis (Modified D2424) of a Propylene Tetramer Sample Using an AGHIS and a GC/MS System AGHISa

Table 3. Comparison of Weight % Data Obtained for the Analysis (D2789) of a Naphtha Sample Using an AGHIS and a GC/MS System

GC/MSb

AGHISa

GC/MSb

types

average

std. dev.

average

std. dev.

types

average

std. dev.

average

std. dev.

mono-olefins di-olefins tri-olefins alkylbenzenes

97.3 1.0 0.3 1.4

0.28 0.31 0.16 0.24

97.2 1.6 0.5 1.2

0.22 0.25 0.12 0.26

paraffins monocycloparaffins alkylbenzenes dicycloparaffins indans & tetralins

37.6 44.8 16.0 1.6 0.3

1.90 1.10 1.10 0.35 0.02

38.3 44.1 15.3 2.3 0.3

0.86 0.86 0.61 0.50 0.09

a AGHIS data obtained from 101 replicate measurements conducted over a period longer than 2 years. b GC/MS data obtained from 164 replicate measurements conducted over a period longer than 2 years; an HP 7673A autosampler was used for GC/ MS sample introduction.

successfully before to introduce crude oils into a GC/ MS system.26 The composite mass spectra obtained for the three samples (propylene tetramer, naphtha, saturate lube oil fraction) by the AGHIS and the GC/MS are shown in Figures 1B-3B and Figures 4B-6B, respectively. Comparison of the spectra obtained by the two methods reveals an excellent similarity between them. This demonstrates that experimental parameters can be determined for the GC/MS that produce spectra similar to those obtained by the AGHIS. There are several points of importance to consider in the GC/MS analysis. We have found that the AGHIS is typically free of background air peak contributions. This is because the AGHIS comes in contact with background air only during sample introduction. Little background air is introduced via the ampule and even less via the gallium frit. This results in high vacuum operation (e.g., (1-5) × 10-7 Torr) with significant benefits to long-term stability and high-quality mass spectra (i.e., free of background peaks). The GC/MS system operates at higher pressures (e.g., (1-5) × 10-5 Torr) due to the continuous flow of helium and has a higher amount of background air peaks than the AGHIS. We have found that operating the GC/MS system in the split injection mode has the advantage of lower background air peak contributions; however, they cannot be completely eliminated. An additional problem that can greatly affect the results of the analysis using the GC/MS system is column bleed signal contributions at high GC oven temperatures (e.g., higher than 300 °C). This problem can be attenuated by the use of low-bleed columns especially designed for high-temperature operation. Column bleed peaks can also be removed by subtracting the mass spectra of blank runs.26 For the present experiments, we have found that background peak considerations are only necessary for samples eluting at GC oven temperatures higher than ∼300 °C. For those samples, composite mass spectra of blank runs are subtracted from the spectra of the samples. Comparison of Data Obtained by AGHIS and GC/MS. Table 2 contains the results obtained by the analysis of a propylene tetramer sample using the AGHIS and GC/MS systems, respectively. The same computer program was employed for data treatment of the spectra acquired by the two introduction systems. Peak abundances in each composite mass spectrum were normalized to the intensity of the most abundant peak in the spectrum. A modified ASTM D2424-67 method was used for the analysis. The method is applicable to the analysis of hydrocarbon types in

a AGHIS data obtained from 100 replicate measurements conducted over a period longer than 2 years. b GC/MS data obtained from 32 replicate measurements conducted over a period longer than 2 years.

Table 4. Comparison of Weight % Data Obtained for the Analysis (Modified D2425) of a Middle Distillate Sample Using an AGHIS and a GC/MS System AGHISa

GC/MSb

types

average

std. dev.

average

std. dev.

paraffins cycloparaffins alkylbenzenes indans+tetralins indenes naphthalene C11 + naphthalenes acenaphthenes acenaphthalenes tricyclicaromatics

52.1 18.3 10.2 3.6 1.3 0.0 8.4 3.6 1.5 1.2

0.49 0.28 0.07 0.00 0.07 0.00 0.07 0.07 0.07 0.00

50.5 18.6 10.3 3.8 1.2 0.0 8.7 3.9 2.0 1.2

0.78 0.07 0.28 0.14 0.07 0.00 0.21 0.07 0.07 0.07

a AGHIS data obtained from 2 replicate measurements. b GC/ MS data obtained from 2 replicate measurements. Data reflect magnitude of short-term repeatability.

propylene polymers with average carbon number values between C9 and C15. Table 2 contains the average weight percent values and standard deviations obtained from many replicate analyses conducted over a long period of time (i.e., longer than 2 years). The AGHIS and the GC/MS show equivalent margins of long-term repeatability. The sample consists mainly of mono-olefins (97%) and small amounts of di- and tri-olefins, and alkylbenzenes. Very similar results are obtained by the two methods. The results obtained from the analysis of a naphtha sample by the two methods are given in Table 3 (ASTM method D2789). The method is applicable to samples boiling in the gasoline boiling range (i.e., less than ∼210 °C).29 Paraffins and monocycloparaffins are the most abundant compound types in the sample. Very similar results are produced by the two methods. Table 4 contains the data obtained from the analysis of a middle distillate sample utilizing a modified ASTM D2425 method.30 Compound types boiling in the 200-343 °C boiling range, containing paraffins from C10 to C18, can be analyzed by the method. The modified D2425 method permits the direct analysis of the sample without the need for prior separation into saturate and aromatic fractions. The results in Table 4 show a very good agreement between the two methods. Only two replicate measurements were done to illustrate the magnitude of the short-term repeatability of the two methods. The GC/MS results show a greater variability than the (29) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 519-524. (30) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 424-429.



Table 5. Comparison of Volume % Data Obtained for the Analysis (D2786) of a Saturate Lube Oil Fraction Using an AGHIS and a GC/MS System AGHISa

Table 7. Comparison of Volume % Data Obtained for the Analysis (Robinson Total Oil) of a Petroleum Fraction (343-565 ˚C Cut) Using an AGHIS and a GC/MS System

GC/MSb

AGHISa

GC/MSb

types

average

std. dev.

average

std. dev.

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

31.5 20.0 18.0 15.7 7.5 3.1 0.9 3.3

1.01 0.62 0.61 0.43 0.30 0.28 0.22 0.30

33.0 19.2 18.3 16.2 6.8 4.1 0.6 3.0

1.06 0.98 0.71 0.74 0.74 1.13 0.39 0.52

total saturates mono-aromatics di-aromatics tri-aromatics tetra-aromatics penta-aromatics thiopheno-aromatics unidentified aromatics

80.0 7.9 6.9 0.6 0.7 0.8 1.7 1.4

0.84 0.50 0.53 0.12 0.09 0.13 0.14 0.35

78.9 9.2 7.6 0.9 0.3 0.8 1.5 0.9

2.28 0.55 0.89 0.39 0.15 0.22 0.19 0.42

a AGHIS data obtained from 28 replicate measurements conducted over a long-term period (more than 1 year). b GC/MS data obtained from 6 replicate measurements conducted over a longterm period (more than 6 months).

a AGHIS data obtained from 51 replicate measurements conducted over a long-term period (more than 2 years). b GC/MS data obtained from 12 replicate measurements conducted over a longterm period (more than 6 months).

Table 6. Comparison of Volume % Data Obtained for the Analysis (D3239) of an Aromatic Lube Oil Fraction Using an AGHIS and a GC/MS System AGHISa

GC/MSb

types

average

std. dev.

average

std. dev.

mono-aromatics di-aromatics tri-aromatics tetra-aromatics penta-aromatics thiopheno-aromatics unidentified aromatics

55.5 12.8 5.0 2.4 1.0 8.8 14.5

2.16 0.50 0.32 0.27 0.13 0.37 1.79

56.0 14.7 4.7 2.3 0.8 8.9 12.6

2.33 0.33 0.65 0.17 0.24 0.79 2.42

a AGHIS data obtained from 31 replicate measurements conducted over a long-term period (more than 1 year). b GC/MS data obtained from 6 replicate measurements conducted over a longterm period (more than 6 months).

AGHIS results for the most abundant compound type (paraffins). In the GC/MS analysis, caution is required to avoid inadvertent saturation of the chromatographic peaks. Certain samples contain disproportionate concentrations of compounds that can cause the saturation of the chromatographic column. Chromatographic runs should be inspected for the possibility of peak saturation. If this is the case, chromatographic parameters such as column type, split-ratio, injection volume, dilution factor, etc., should be changed to eliminate this effect. In some cases, the unusual presence of peaks due to additives or contaminants (e.g., phthalates) has been detected in the GC/MS analysis. Detection of these unusual compounds is not possible by the AGHIS due to the lack of chromatographic separation. Erroneous compound type results would be produced by the AGHIS depending on the relative abundance of the contaminants in the sample. Tables 5 and 6 contain the results obtained for a saturate and an aromatic lube oil fractions analyzed by the AGHIS and GC/MS systems, respectively. ASTM methods D278631 and D323932 were used for the analysis. The methods permit the analysis of petroleum fractions boiling in the 200-540 °C boiling range. Mass spectrometry is one of the most powerful tools for the characterization of heavy petroleum fractions. It is (31) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 512-518. (32) Manual on Hydrocarbon Analysis, 5th ed.; Drews, A. W., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 586-598.

Figure 7. Comparison of compound type data (weight percent) obtained from 12 naphtha samples analyzed using the GC/MS and AGHIS sample introduction methods.

difficult to obtain the same degree of compositional information for these samples by other analytical methods in a routine manner. The results in Tables 5 and 6 show that the differences in the amounts of the compound types are of the same order of magnitude as the long-term repeatabilities of the two sample introduction methods. A similar conclusion is drawn from the analysis of a petroleum fraction (343 to 565 °C cut) by Robinson’s total oil method (Table 7).33 The standard deviations of the compound types determined by the GC/ MS method are slightly larger than those obtained by the AGHIS method (Tables 5-7). This can perhaps be attributed to the smaller set of samples analyzed by the GC/MS. Another possible explanation for the wider longterm fluctuations of the data is the dependence of the GC/MS repeatability on the experimental conditions of (1) the mass spectrometer, (2) the injector, and (3) the gas chromatograph. The repeatability of the AGHIS depends almost exclusively on the experimental conditions of the mass spectrometer. Very little changes in time with the AGHIS system. We have found that acceptable levels of long-term repeatability with the GC/ MS system can be achieved by using quality control samples and scheduled maintenance of its various parts. An overall comparison example for samples analyzed by the AGHIS and the GC/MS is shown in Figure 7. The data from 12 naphtha samples were used to create the parity plot. The GC/MS data are plotted against (33) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434.

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Figure 8. Comparison of compound type data (weight percent) obtained from 9 middle distillate samples analyzed using the GC/MS and AGHIS sample introduction methods.

those obtained by the AGHIS. The plot shows a very good agreement between the methods. The average absolute difference between the two methods for all compound types is 0.59%. The R2 value for the naphtha plot is 0.9978 (Figure 7). A second overall comparison of samples analyzed by the GC/MS and the AGHIS is shown in Figure 8. The figure shows the parity plot obtained from the analysis of 9 middle distillate samples. The results obtained by the two methods are in excellent agreement. The average absolute difference obtained for all compound types measured by the middle distillate method is 0.38%. The R2 value of the middle distillate plot is 0.9989 (Figure 8). Accuracy of the Measurements. The current work does not address the accuracy of the measurements. This was done, in a limited fashion, in a previous work.16 In that study, several petroleum fractions were analyzed by the AGHIS (Robinson Total Oil method), SFC, HPLC, and open-column chromatographic methods. The difference between the data obtained by the quadrupole mass spectrometric and the other analytical methods was in the same order of magnitude as the difference between the AGHIS and DBIS mass spectrometric measurements. Caution is, however, required in such comparisons as to the use of appropriate response factors for the mass spectrometric calculations. Experiments can be designed to test and improve the accuracy of the different methods, but this is out of the scope of the current work. Detection of Individual Compounds in Light Fractions by GC/MS. A significant advantage of the GC/MS system over the AGHIS is its capability to detect individual compounds. The detection of individual compounds in light petroleum fractions by GC/MS is illustrated in Figure 9. This is achieved by monitoring the signal of ions characteristic to specific compounds. For example, the amount of benzene in the naphtha sample can be determined by selectively monitoring the current of its molecular ion at m/z 78 (Figure 9B). Similarly, the exact amounts of toluene and the xylenes in the sample can be determined by monitoring the ion at m/z 91 (Figure 9C). Other characteristic ions of these compounds such as their molecular ions can be equally used. Quantification of individual compounds in the light fractions with the GC/MS system can be very accurate because the response factors of individual compounds can be accurately determined, and reliable

Figure 9. Detection of individual compounds by GC/MS: (A) total ion current of a naphtha sample; (B) selected ion profile for benzene (m/z 78); (C) selected ion profile for toluene and xylenes (m/z 91).

calibration curves can be generated by internal or external calibration methods for each compound of interest. Standard GC/MS methods have been recently developed for the detection of benzene, toluene, and total aromatics in finished gasoline.34,35 Determination of Individual Compounds in Heavier Fractions by AMDIS. Accurate quantitative determination of individual compounds in the heavier petroleum fractions is limited due to the inability to resolve chromatographic peaks to the baseline. The total ion current is very complex due to the coelution of many components. To aid in the analysis of individual compounds in the heavier samples one can use the detailed mass spectral information acquired in the GC/MS experiment. Although no improvement is brought to the chromatographic separation by the mass spectrometer, the chromatographic profiles of characteristic fragment ions in the mass spectra of overlapping peaks can be used to deconvolute overlapping components and determine their relative amounts in the samples. The potential of GC/MS for the separation of overlapping chromatographic peaks was recognized early in the development of the technique.36-38 However, only very recently has a general-purpose GC/MS deconvolution method become available for the identification of compounds in complex mixtures.39,40 Here, we report the (34) Method ASTM D 5769-95. Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1998; Vol. 05.03, pp 929-939. (35) Nero, V. P.; Drinkwater, D. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 992-928. (36) Biller, J. E.; Biemann, K. Anal. Lett. 1974, 7, 515-528. (37) Dromey, R. G.; Stefik, M. J.; Rindfleisch, T. C.; Duffield, A. M. Anal. Chem. 1976, 48, 1368-1375. (38) Hargrove, W. F.; Rosenthal, D.; Cooley, P. C. Anal. Chem. 1981, 53, 538-539.


Figure 10. Unknown component determination by AMDIS: (A) part of the total ion current of a middle distillate sample; (B) profiles of the most abundant ions; (C) unprocessed mass spectrum at 17.49 min; (D) extracted mass spectrum at 17.49 min; (E) library spectrum of best match.

application of the Automated Mass Spectral Deconvolution and Identification System (AMDIS) to the determination of individual compounds in petroleum fractions. AMDIS was originally developed by NIST (National Institute of Standards and Technology) to aid compliance to the Chemical Weapons Convention treaty. However, it can be used in a general fashion for the determination of individual compounds in other types of complex mixtures. The principles of AMDIS have been discussed elsewhere.39-41 The method is divided into four main sequential data analysis steps: (1) noise analysis, (2) component perception, (3) spectrum deconvolution, and (4) compound identification.40 Identification of compounds is done by comparison of the measured spectra with those of reference compounds. Best matches are obtained for all deconvoluted mass spectra by comparison with standard libraries of spectra (e.g., NIST, Wiley, etc.). Retention times or retention indices can be optionally used in the comparisons. A significant feature of AMDIS is that being a standard method developed and maintained by NIST, permits a high level (39) Davies, A. N. Spectrosc. Eur. 1998, 10, 22, 24-26. (40) Stein, S. E. J. Am. Soc. Mass Spectrom. 1999, 10, 770-781. (41) Halket, J. M.; Przyborowska, A.; Stein, S. E.; Mallard, W. G.; Down, S.; Chalmers, R. A. Rapid Commun. Mass Spectrom. 1999, 13, 279-284.


Figure 11. Target compound determination by AMDIS: (A) total ion current of a middle distillate sample; (B) profiles of most abundant ions; (C) extracted mass spectrum at 17.54 min; (D) library spectrum of best match.

of inter-laboratory reproducibility for the determination of high-interest molecules in selected matrixes (e.g., toxic compounds in environmental samples, heteroatomic compounds in petroleum fractions, etc.). The use of AMDIS for low-level component identification by spectral deconvolution is exemplified in Figure 10. Figure 10A shows part of the total ion current of a middle distillate sample. Five components were detected by AMDIS within the 17.43 and 17.58 retention time interval. The most abundant ion profiles of one of the components are shown in Figure 10B. Ions with masses at m/z 212 and m/z 105 maximize at 17.49 min. The total ion current of the sample maximizes at a later retention time (17.54 min). The mass spectrum of the unknown component is deconvoluted from the raw mass spectral signal by consideration of the maxima of the individual ion profiles in the time scale. The unprocessed mass spectrum containing features from all coeluting components is shown in Figure 10C (17.49 min). Figure 10D shows the extracted mass spectrum of the component at 17.49 min. The extracted mass spectrum is free of the alkyl fragments originating from the coeluting hydrocarbons. The component was automatically detected by AMDIS. The best match library spectrum (2,8dimethyl(B,D)dibenzothiophene) is shown in Figure 10E. In addition to its ability to detect low concentration unknown compounds, AMDIS can be used to detect

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selected target compounds. This ability is demonstrated by the determination of n-alkane molecules in a middle distillate sample (Figure 11). The determination of n-alkanes in hydrocarbon matrixes is highly desirable because these molecules can have a significant effect on the properties of feeds and products. Figure 11A shows the total ion current of a middle distillate sample. A partial view of the total ion current of the middle distillate sample was shown previously in Figure 10A. Treatment of the middle distillate sample by AMDIS resulted in the detection of 348 components. Userspecified deconvolution parameters are available with AMDIS to control the degree of peak resolution and sensitivity. Depending on the experimental conditions used, it is possible to increase the number of components detected by selecting the high-resolution and highsensitivity options. However, caution is needed to avoid erroneous peak splitting and the detection of noise peaks. We have found that the medium-resolution and medium-sensitivity options generate reliable peak separation and quantification. Experimental and computational optimizations are usually done in the initial stages of method development and remain the same for samples boiling in the same boiling ranges (e.g., naphthas, middle distillates, etc.). From the 348 detected components in the middle distillate sample, 12 were identified as n-alkane compounds. Target compound analysis is much faster than identification of unknown compounds because the deconvoluted spectra are compared only to the reference spectra of the target compounds and not to the entire NIST library. The identified compounds n-C11 to n-C21 are labeled in Figure 11A. As shown in the chart, peak separation to the baseline is achieved by AMDIS. Figure 11B shows the profiles of the most abundant ions for the detected component at 17.54 min. The extracted mass spectrum and the corresponding best match library spectrum (nonadecane) are shown in Figure 11C and 11D, respectively. The repeatability of n-alkane determination by AMDIS is illustrated in Figure 12 which shows the results obtained from the repeat analysis of a middle distillate sample. The utility of AMDIS for the determination of target molecules is evident from the n-alkanes example. Of equal importance, however, is the ability of AMDIS to


Figure 12. The repeatability of individual compound determination by AMDIS: n-alkane results obtained from the repeat analysis of a middle distillate sample.

detect low-level, unknown components, such as the sulfur compound detected in the example of Figure 10. Conventional GC/MS cannot be used for the detection of low-concentration, coeluting components. AMDIS can be successfully employed for the detection of low concentration, usually problematic, heteroatomic compounds in petroleum fractions. Conclusion The GC/MS system is a versatile system that permits the introduction of petroleum fractions into a mass spectrometer for hydrocarbon compound type analysis. The results produced by the GC/MS system are in good agreement with those produced by the conventional AGHIS. In addition to the traditional hydrocarbon compound groups, the GC/MS method also permits the analysis of individual compounds, a capability not available with the AGHIS. Quantification of wellresolved chromatographic components can be done by selected-ion monitoring (SIM). The NIST automated deconvolution program AMDIS can be used for the separation of overlapping components. Acknowledgment. The review of the manuscript and the valuable comments of Dr D. J. Bristow are highly appreciated. EF000225V

Hydrocarbon Compound Type Analysis by Mass Spectrometry: On the

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