Diesel Fuel Analysis by GC-FIMS: Normal Paraffins, Isoparaffins, and

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Energy & Fuels 2001, 15, 996-1002

Diesel Fuel Analysis by GC-FIMS: Normal Paraffins, Isoparaffins, and Cycloparaffins Y. Briker,* Z. Ring, A. Iacchelli, N. McLean, and C. Fairbridge National Centre for Upgrading Technology, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada

R. Malhotra, M. A. Coggiola, and S. E. Young SRI International, Menlo Park, California 94025 Received March 12, 2001. Revised Manuscript Received May 17, 2001

A reliable and convenient characterization method that provides a detailed hydrocarbon composition profile for transportation fuels is an important part of process optimization directed at reducing regulated emissions. In our previous study (Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Rahimi P. M.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15 (1), 23-37) we described the development of a modified gas chromatography, field ionization mass spectrometry method for detailed hydrocarbon type characterization of diesel fuel. The method proved to be an invaluable technique for rapid analysis of diesel fuel. It was less timeconsuming and more informative than the existing mass spectrometry methods, from a characterization point of view, and it was user-friendly and did not require any major modification to an existing commercial instrument, from an instrumentation point of view. This method correlated well with the other methods for total aromatic and saturate groups, and the data for aromatic subgroups were verified against the data obtained from other mass spectrometric and nonmass spectrometric techniques. This paper describes a continuing effort to verify the new GC-FIMS method and produce correlations for the saturate groups. The purpose of this study was to validate GC-FIMS measurements of various saturate types using samples of physically separated fractions, enriched in the individual types by various LC methods described in the literature. In this study, the non-normal paraffinic portion of the saturate fraction of the selected diesel cut was separated into iso- and cycloparaffins. The normal paraffins were quantitatively removed by molecular sieve and additionally determined by high-resolution gas chromatography of the saturate fraction. The contents of iso- and cycloparaffins were calculated gravimetrically in each separated fraction and also analyzed by mass spectrometry methods. The results obtained for the saturate types by different methods were all in good agreement, which demonstrates the applicability of the GC-FIMS method for saturates analysis.

Introduction communication,1

In our resent we described a characterization method for diesel fuels based on gas chromatography-field ionization mass spectrometry (GC-FIMS). We showed that this technique provides rapid diesel fuel analysis because it does not require separation of the sample into saturate and aromatic fractions prior to analysis. The results obtained with this method correlated very well with the results from the traditional liquid chromatography (LC) method, ASTM 2549,2 in terms of class-type separation. Fur* Author to whom correspondence should be addressed at National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta, Canada T9G 1A8. Phone: (780) 987-8700. Fax: (780) 987-5349. E-mail: [email protected]. (1) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Rahimi P. M.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15 (1), 23-37. (2) 2. Annual Book of ASTM Standards: Petroleum Products, Lubricants, and Fossil Fuels. Designation D2007-93 and Designation D2549-91 (Reapproved 1995). American Society for Testing and Materials: West Conshohocken, PA, 1997; 05(01), pp 651-657 and 889-894.

thermore, the results of detailed hydrocarbon composition calculated by the new method were in good agreement with the results obtained by ASTM methods based on electron impact mass spectrometry (EIMS). The accuracy of the method for the aromatic types was successfully verified against other available ASTM methods such as LC/GC-MS, supercritical fluid chromatography (SFC, CAN/CGSB-3.0 No. 15.094), highpressure liquid chromatography (HPLC, IP 391/95), and hot fluorescent indicator absorption (Hot FIA, UOP 50183). We calculated the normal paraffins in several diesel samples by HRGC, but additional calculations for other saturate types were necessary to verify the reliability of the GC-FIMS method. Therefore, we employed a strategy of combining various separation and analytical techniques to get as much information on the composition of saturates as possible. Unlike other GC-MS methods, ASTM D2786, ASTM D3239, and the Robinson method, the GC-FIMS method is able to determine iso- and normal paraffins as separate hydrocarbon groups. The relative contents of

10.1021/ef010057m CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001

Diesel Fuel Analysis by GC-FIMS

iso- and normal paraffins, as well as cycloparaffins, in diesel fuel play important roles in combustion. This is reflected in their effect on the cetane number. Therefore, the ability to distinguish among these three groups of hydrocarbons is important in assessing the impact of diesel fuel quality on engine performance and emissions.3 In this study, we calculated GC-FIMS results for the three saturate hydrocarbon types and compared them with the results obtained by other methods. There are not many ways to determine the contents of iso- or cycloparaffins by methods other than mass spectrometry. One way to quantify these hydrocarbon groups is to separate the saturate fraction into subfractions by liquid chromatography, each representing a separate hydrocarbon type. This is not an easy task. Fractionation of paraffins into normal paraffins, isoparaffins, and cycloparaffins can be achieved by treatment with urea4 or molecular sieve.5 However, complete separation of isoparaffins from cycloparaffins, and further separation of the latter by types and ringnumber, is much more complex. Methods that provide reasonable separation include azeotropic distillation6 as well as liquid displacement partition chromatography with aniline.7 Mair et al.8 investigated the separation of petroleum hydrocarbons by liquid chromatography with Sephadex LH-20, a methylated cross-linked dextran that has been used primarily for gel filtrationsa technique that segregates compounds according to their hydrodynamic volume. They studied a number of model compounds and demonstrated good efficiency of separation between normal paraffins and cycloparaffins, and also between isoparaffins and cycloparaffins. In the latter case, by combining and reprocessing, a final fraction containing 96% isoparaffins was obtained. Their investigation was later extended to wide-range petroleum fractions.9 We applied this approach to separate a non-normal paraffinic portion of the saturate fraction of the diesel cut into isoparaffins and cycloparaffins. The normal paraffins were quantitatively removed by molecular sieve. The various paraffinic fractions obtained as the result of these separations were used to validate GC-FIMS measurements. Experimental Section A hydrotreated oil sands distillate sample was distilled into several narrow cut fractions, and the light diesel fraction boiling between 200 and 250 °C was used in this study. The reason for choosing such a narrow fraction for analysis was the limitation of the LC separation that works best when the molecules in the cut do not differ by more than 4-5 carbon atoms. The narrow cut also allowed the use of PNA (paraffins, naphthenes, and aromatics) analysis (Mode 50) and simplified the HRGC (high-resolution gas chromatography) analysis. (3) Neill, W. S.; Li, X.; Chippior, W. L.; Gu¨lder, O ¨ . L. Canadian Diesel Fuel Composition and Emissions-II; Combustion Research Group, Institute for Chemical Process and Environmental Technology, National Research Council Canada: Ottawa, Ontario, 1999. (4) Mair, B. J.; Mareulaitis, W. J.; Rossini, F. D. Anal. Chem. 1957, 29, 92. (5) Norris, M.; O’Connor, J. G. Anal. Chem. 1959, 31, 275. (6) Mair, B. J. Anal. Chem. 1956, 28, 53. (7) Sauer, R. W.; Washall, T. A.; Melpolder, F. W. Anal. Chem. 1957, 29 (9), 1327-1331. (8) Mair, B. J.; Hwang, P. T. R.; Ruberto, R. G. Anal. Chem. 1967, 39, 838. (9) Talarico, P. C.; Albaugh, E. W.; Snyder, R. E. Anal. Chem. 1968, 40 (14), 2192-2194.

Energy & Fuels, Vol. 15, No. 4, 2001 997

Figure 1. Block diagram of analytical procedure. Figure 1 shows the set of procedures used for analysis. Initially, the light diesel sample was separated into saturate and aromatic fractions using the LC ASTM 2549 method. The fractions were later analyzed by the electron impact GC-MS, and their hydrocarbon composition was calculated by the ASTM2786 and the ASTM3239 methods for saturate and aromatic fractions, respectively. The total sample was analyzed by the GC-FIMS. The saturate fraction was analyzed by HRGC to identify and quantify the normal paraffin portion of the fraction. The normal paraffins were also quantified by the molecular sieve adsorption method.10 After removal of normal paraffins by molecular sieve adsorption, the non-normal fraction of the saturates was analyzed by the GC-MS and by extended PNA using an AC PIONA analyzer, based on the HP GC 5890, and run under mode 50 (paraffins, naphthenes, and aromatics up to 270 °C). This fraction was also separated on a cross-linked dextran (Sephadex LH-20) column9 into iso- and cycloparaffins. The fractions separated by the dextran column were analyzed by the GC-MS. GC-MS Analysis. For GC-MS analysis we used a HewlettPackard GC-MS instrument with HP 5973 MSD, HP 7673 GC/ SFC injector, and HP 6890 GC. The column used in this work was a HRGC Column J122-5532 DB-5MS from Chromatographic Specialties Inc., length, 30 m; ID, 0.25 mm; film, 0.25 µm. An injection of 0.1 µL was made with the help of a cool on-column injector, heated from 60 °C at a rate of 20°/min to 285 °C. The GC oven was held at 35 °C for 3 min and heated at 10°/min to 280 °C. MSD temperature was 285 °C. The ASTM 2786 method11 was used to calculate the saturate hydrocarbon types, and ASTM 3239 method12 was used to calculate the aromatic hydrocarbon types. The software for the calculation of hydrocarbon types was developed by R. Teeter, based on the original methods, and was supplied by PCMSPEC.13 GC-FIMS Analysis. For the GC-FIMS we used a 30 m × 0.25 mm × 0.25 µm HP1-MS nonbonded column. The injection (0.2 µL; 19:1 split) was made with the oven at 45 °C. The oven was heated at 10 °C/min to 300 °C. The instrument was a Hewlett-Packard GC-MS, equipped with an SRI volcano FI ionization sourcesreplacing the HP EI ionization sourcesand equipped with HP 7673 GC/SFC injector and HP 5890 gas (10) Selucky, M. L.; Chu, Y.; Ruo, T.; Strausz, O. P. Fuel 1977, 56, (October), 369-381. (11) Annual Book of ASTM Standards: Petroleum Products, Lubricants, and Fossil Fuels. Designation: D2786-91 (Reapproved 1996). American Society for Testing and Materials: West Conshohocken, PA, 1997; 05(02), pp 145-151. (12) . Annual Book of ASTM Standards: Petroleum Products, Lubricants, and Fossil Fuels. Designation: D3239-91 (Reapproved 1996). American Society for Testing and Materials: West Conshohocken, PA, 1997; 05(02), pp 337-349. (13) Teeter, R. M. Software for calculation of hydrocarbon types; PCMASPEC: Walnut Creek, CA, 1992.

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Table 1: Mass Spectrometric Analysis of Fractions Separated from Sephadex LH-20 Columna,b,c composition in wt % by GC-MS (ASTM2786) fraction number

c

HC types

Z

20

21

22

23

24

25

26

27

28

29

30

31

32

33-36

37-41

0-Ringsd 1-Ring 2-Rings 3-Rings 4-Rings 5-Rings 6-Rings monoaromatics

2 0 -2 -4 -6 -8 -10 -6

93.3 6.2 0.5 0.0 0.0 0.0 0.0 0.0

92.1 7.1 0.5 0.0 0.0 0.0 0.0 0.3

88.4 10.3 1.3 0.0 0.0 0.0 0.0 0.0

80.2 16.5 3.3 0.0 0.0 0.0 0.0 0.0

71.0 23.0 5.9 0.0 0.0 0.0 0.0 0.0

60.4 30.3 9.1 0.2 0.0 0.0 0.0 0.0

47.3 38.6 13.4 0.7 0.0 0.0 0.0 0.0

31.9 47.6 19.3 1.3 0.0 0.0 0.0 0.0

19.4 53.1 25.0 2.5 0.0 0.0 0.0 0.0

11.5 54.0 30.4 4.2 0.0 0.0 0.0 0.0

6.3 51.8 35.7 6.2 0.0 0.0 0.0 0.0

2.7 45.7 43.3 8.3 0.0 0.0 0.0 0.1

1.0 35.6 50.1 13.2 0.0 0.0 0.0 0.1

0.0 2.4 61.6 34.4 0.2 0.0 0.0 1.4

0.0 0.0 21.9 69.6 0.0 0.0 0.0 7.8

a Fractions 1-19 contained only acetone. b “0.0”% means not detected by MS or the number was too small and lost due to rounding. Some columns may not add to 100% due to rounding. d Isoparaffins.

chromatograph. Calculation was done with the help of software developed in collaboration between SRI International and NCUT1. High-Resolution Gas Chromatography. To determine the normal paraffin content, the saturate fraction of the light diesel sample was analyzed by a HRGC technique. The analysis was performed on a gas chromatograph equipped with flame ionization detector (GC-FID). This gas chromatograph was also fitted with a RESTEK RTX-1 PONA column coated with 100% dimethylpolysiloxane (100 m × 250 µm × 0.50 µm). A constant flow rate of 0.5 mL/min was maintained during the analysis. The injection (0.1 µL; 100:1 split ratio) was made with the GC oven at 120 °C. The hold time at this temperature was 20 min, and then oven temperature was raised to 280 °C at a rate of 2 °C/min. At these conditions good separation of normal paraffins was achieved. The normal-paraffin content of the saturate fractions was calculated by using the calibration factors obtained from running normal paraffin calibration standards (C5 to C26). Separation of Saturate Fraction by LC. We first used the ASTM 2549 LC separation method to separate the saturate and aromatic fractions of the diesel sample. Next, the saturate fraction (0.9 g), obtained in the previous step, was dissolved in cyclohexane (50 mL) and mixed with 5 Å molecular sieve preactivated for 5 h at 300 °C. The sample was maintained at 60 °C for 6 h10 under reflux. The normal paraffins were adsorbed on the sieve and later recovered by Soxhlet extraction with hexane. The non-normal portion of the light diesel sample was filtered off. The solvent used in the molecular sieve separation was carefully distilled off and the residual nonnormal fraction was separated into isoparaffins and cycloparaffins by using a Sephadex column.9 The separation scheme was as follows: 200 g of Sephadex LH-20 gel was allowed to stand for 16 h in contact with an excess of acetone; the expanded gel was then transferred to a glass column, 180 cm in length and 2 cm in diameter, with the plug of glass wool added to the top and to the bottom of the column; a flow rate of 0.8 mL/min was maintained throughout the experiment, and the effluent was collected in 6 mL fractions. A charge of 0.4384 g of the non-normal fraction was used for this study. Acetone was used as the solvent to elute the fractions.

Results The initial LC separation of the light diesel sample yielded 28 wt % saturate fraction and 72 wt % aromatic fraction. The normal paraffin portion, after molecular sieve adsorption, was 13.7 wt % of saturates or 3.8 wt % of total sample. The remaining 86.3 wt % of saturates (iso- and cycloparaffins) were separated by the Sephadex column into 41 fractions. Each of those fractions was analyzed by GC-MS and the results are shown in Table 1. The first 19 fractions contained only acetone. Fractions 20-25, representing 47.6 wt % of the total sample,

were enriched with isoparaffins that ranged from 60.4 to 93.3 wt %. Fractions 26-31, representing 41.0 wt % of the total sample, contained the highest concentration of one-ring cycloparaffins ranging from 38.9 to 54.0 wt %. Fractions 32-36, representing 10.5 wt % of the sample, contained the highest concentration of two-ring cycloparaffins ranging from 50.1 to 61.6 wt %. Finally, Fractions 37-41, representing 0.9 wt % of the sample, contained the highest concentration of three-ring cycloparaffins. Where Fraction 25 only contained 0.2 wt % three-ring cycloparaffins, these last five fractions contained 69.6 wt %. The mass compositions of individual fractions are shown in Table 2. Each of these compositions was determined by multiplying the GC-MS result of the fraction in Table 1 by the mass of this fraction determined gravimetrically and appearing in the bottom row of Table 2. For example, the mass of Fraction 20 from LC separation was 0.00420 g (shown in Table 2 as the “total mass” of Fraction 20) and the GC-MS analysis of this fraction resulted in 93.3% of 0-Ring hydrocarbon type compounds (shown in Table 1 as the 0-Rings HC Type for Fraction 20). The mass of 0-Ring compound type in Fraction 20 (and shown in the first row of the first column in Table 2) was calculated by multiplying 0.00420 × 0.933 ) 0.00391 g. The rest of the results were calculated in a similar fashion. The results shown in column “total sample” were calculated by summing the values in each row corresponding to a particular hydrocarbon type. The last three columns of this table contain the results re-normalized, based on the total non-normal paraffin portion of saturates from the diesel sample (100%), the total saturate fraction (86.3%), and finally, the total diesel sample (28.0%). The last column “% of total cut” added up to 24.2% instead of 28.0%, since it did not include 3.8% of normal paraffins removed by molecular sieve adsorption. These results were then compared with the results obtained with other methods shown in Table 3. The results shown in Tables 1 and 2 also demonstrate relatively good separation of cycloparaffins according to the number of rings. In general, the ring number increased with the elution volume (i.e., fraction number), but there was some overlap between the various saturate types. Figure 2 further supports this conclusion. It shows the results of separation of the nonnormal paraffin fraction by the Sephadex column, in terms of the relative concentrations of various saturate types plotted as functions of the fraction number. The relative concentrations were calculated as the ratio of

Energy & Fuels, Vol. 15, No. 4, 2001 999

a

Total sample/0.42479 × 100%. b % of sample × 0.863(percent of nonparafins in the saturates). c % of saturates × 0.28(percent of saturates in the distillation cut).

d

Isoparaffins.

11.3 7.2 4.5 1.1 0.0 0.0 0.0 0.0 24.2 40.29 25.77 16.13 3.97 0.00 0.00 0.00 0.14 86.30 46.68 29.86 18.70 4.60 0.00 0.00 0.00 0.16 100.00 0.19830 0.12686 0.07942 0.01955 0.00000 0.00000 0.00000 0.00067 0.42479 0.00000 0.00000 0.00077 0.00245 0.00000 0.00000 0.00000 0.00028 0.00350 0.00000 0.00060 0.01550 0.00865 0.00000 0.00000 0.00000 0.00035 0.02510 0.00020 0.00701 0.00987 0.00260 0.00000 0.00000 0.00000 0.00002 0.01970 0.00064 0.01080 0.01030 0.00197 0.00000 0.00000 0.00000 0.00002 0.02373 0.00135 0.01101 0.00762 0.00132 0.00000 0.00000 0.00000 0.00000 0.02130 0.00285 0.01340 0.00754 0.00104 0.00000 0.00000 0.00000 0.00000 0.02483 0.00560 0.01510 0.00720 0.00072 0.00000 0.00000 0.00000 0.00000 0.02862 0.01090 0.01630 0.00660 0.00045 0.00000 0.00000 0.00000 0.00000 0.03425 0.01960 0.01600 0.00555 0.00028 0.00000 0.00000 0.00000 0.00000 0.04143 0.02160 0.01067 0.00320 0.00007 0.00000 0.00000 0.00000 0.00000 0.03554 0.03033 0.00983 0.00254 0.00000 0.00000 0.00000 0.00000 0.00000 0.04270 0.04962 0.01023 0.00205 0.00000 0.00000 0.00000 0.00000 0.00000 0.06190 0.04090 0.00480 0.00060 0.00000 0.00000 0.00000 0.00000 0.00000 0.04630 0.00391 0.00027 0.00002 0.00000 0.00000 0.00000 0.00000 0.00000 0.00420 0-Ringsd 1-Ring 2-Rings 3-Rings 4-Rings 5-Rings 6-Rings monoaromatics total mass

0.01080 0.00084 0.00006 0.00000 0.00000 0.00000 0.00000 0.00000 0.01170

37-41 33-36 32 31 30 29 28 27 26 25 24 23 22 21 20 HC Types

fraction number

calculated amount of fractions, g

Table 2: Calculated Composition of Non-Normal Paraffinic Portion of Saturate Fraction

total sample

% of % of % of samplea saturatesb total cutc

Diesel Fuel Analysis by GC-FIMS

the masses of the particular saturate type (shown in Table 2) and of the total non-normal paraffin fraction. For example, data point “2” for the “z ) 2” series was calculated as 0.00391/0.42479 × 100 ) 0.92. Figure 2 also indicates that reasonable separation between branched paraffins and cycloparaffins was accomplished in one pass. In the first two groups of columns, Table 3 compares the analytical results corresponding to the non-normal paraffin portion of the diesel saturates by GC-MS and PNA. In the first group, the GC-MS results for the total non-normal paraffin fraction of saturates are compared with the results obtained by summation of the GC-MS results for the individual LC fractions (Column 4 in Table 3 and the last column of Table 2). Column 1 shows the results of the direct GC-MS analysis of the nonnormal paraffin fraction. These results were then renormalized on the basis of the % of this fraction to the total saturates, and shown in Column 2. For example, 0-Rings iso-paraffins were calculated as 38.1% × 0.863 ) 32.9%. Since the saturate fraction represented 28.0% of the total sample, the results in Column 2 were multiplied by 0.28 and then entered in Column 3. Similarly, in the next group of columns, the results in column “% of fraction” were obtained from the direct PNA analysis of the non-normal fraction and re-normalized based on the total saturate fraction and then the total sample. The next two columns contain the results of analysis of the saturate fraction by HRGC and GC-MS, and are shown as the % of total sample. The GC-MS method calculates the results for the saturate fraction based on the total sample when the % of saturates is entered into the program. In our case, it was 28.0%. The last column shows the results of analysis of the total light diesel sample obtained by GC-FIMS. All the individual methods now can be conveniently compared, with all these results recalculated on the total sample basis. The non-normal paraffin fraction of saturates recovered after removal of normal paraffins by molecular sieve, was analyzed by GC-MS. The calculation was done by ASTM D2786 method. This method does not distinguish between iso- and normal paraffins, and reports them as total paraffins. Since the normal paraffins were removed from this fraction, the amount of hydrocarbon types determined as paraffins by ASTM D2786 represented only the isoparaffins (9.2%). After combining this amount with the amount of normal paraffins separated by the molecular sieve adsorption (3.8%), the total amount of paraffins (iso- and normal paraffins) was calculated as 13.0%. In a similar fashion, when the same fraction was analyzed by PNA, only the isoparaffins and the cycloparaffins were determined. The result for isoparaffins was 9.2% of the total cut, and for total paraffins, 13.0% after the normal paraffins (3.8%) were added. This comparison showed excellent agreement between GC-MS and PNA results. The results calculated by the summation of the GC-MS results of the LC fractions were also very similar. The isoparaffins were calculated as 11.3%, rendering the total amount of iso- and normal paraffins as 15.1%. These small differences between the results, obtained for the total sample and for the sums of the fractions,

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Table 3: Comparison of Saturate Hydrocarbon Types Determined by Different Methods for 200-250 °C Distillation Cut GC-MS

PNA

HRGC

isoparaffis/cycloparaffins fraction of 200-250 °C cut % of % of % of sum of LC fractions% of % of % of fraction saturates total cut % of total cut fraction saturates total cut 0-RingsIsoparaffins 0-RingsNormal Paraffins Total Paraffins 1-Ring 2-Rings 3-Rings 4-Rings 5-Rings 6-Rings Total Cycloparaffins Monoaromatics Total Saturates a

38.1

11.3

38.1

32.9

GC-MS

% of total cut

% of total cut

% of total cut

N/A

10.1

N/A

3.1

13.0

12.7

13.2 11.3 6.6 4.6 0.0 0.0 0.0 22.5 0.0 35.7

32.9

9.2

N/A

N/A

3.8a

N/A

N/A

13.0

15.1

27.2 23.3 9.4 0.0 0.0 0.0 59.9

23.5 20.1 8.1 0.0 0.0 0.0 51.7

6.7 5.6 2.3 0.0 0.0 0.0 14.6

7.2 4.5 1.1 0.0 0.0 0.0 12.8

61.9

53.4

15.0

7.5 5.6 2.0 0.0 0.0 0.0 15.1

2.0 100.0

1.7 86.3

0.4 28.0

0.1 28.0

100.0

86.3

28.0

0.2 28.0

3.8a

GS-FIMS

saturates saturates 200-250 °C cut

9.2 3.8a

3.5

Normal paraffins obtained by molecular sieve adsorption.

Figure 2. Simultaneous separation of branched paraffins from cycloparaffins (where z ) 2 represents 0-Ring cycloparaffins (iso-paraffins), z ) 0 represents 1-Ring cycloparaffins, z ) -2 represents 2-Ring cycloparaffins, and z ) -4 represents 3-Ring cycloparaffins).

can be explained by the discrepancies in calculating the hydrocarbon types by the GC-MS method for a very narrow cut (represented by the LC fraction) as compared to the total light diesel sample. In addition, some error also could have been introduced during gravimetric determination of the fractions that involved solvent removal, fraction transfer, and weighing. For comparison, the normal-paraffin content was also determined by HRGC analysis of the saturate fraction obtained from LC ASTM 2549 separation. The chromatogram of this GC run is presented in Figure 3. It displays the peaks of five normal paraffins, C10 to C14. The normal paraffins calculated by this method amounted to 3.5%. This value compared well with 3.8% of normal paraffins, determined gravimetrically from the molecular sieve adsorption. After the saturate fraction was treated with the molecular sieve and the normal paraffins were recovered by Soxhlet extraction, the concentrated solution of the extract was also analyzed by GC-MS. The chromatogram of this run, presented in Figure 4, displays practically no other peaks except for the five that were confirmed to be C10 to C14 normal paraffins. This clearly demonstrates good quality of separation. The last two columns in Table 3 contain the results of the GC-MS analysis of the saturate fraction and the

Figure 3. Total ion chromatogram of the GC-MS analysis of saturate fraction using high-resolution GC column.

Figure 4. Total ion chromatogram of the GC-MS analysis of 5 Å molecular sieve adduct.

GC-FIMS analysis of the total light diesel cut. As mentioned earlier, the GC-MS method reported only the total amount of paraffins (12.7%). The GC-FIMS method reported that the amount of isoparaffins and normal paraffins were 10.1% and 3.1%, respectively. This sums up to 13.2% total paraffins. These amounts are quite similar to the amounts calculated by other methods discussed above.

Diesel Fuel Analysis by GC-FIMS

Figure 5. Correlation of total cycloparaffin content measured by different methods (the dotted line represents perfect agreement between methods).

The cycloparaffin types determined by the GC-FIMS were slightly higher than those determined by other methods (11.3% for one-ring vs 7.5% determined by GC-MS of saturates, 6.7% by GC-MS of the non-normal fraction, and 7.2% by summation of the LC fractions). This discrepancy could be explained by relatively high concentrations of cycloparaffins compared to normal and isoparaffins. Considering the fact that the sample analyzed by GC-FIMS had a very narrow boiling range, and was not diluted as in the case of GC-MS analysis, the linearity limit for the response factors for these hydrocarbon types (cycloparaffins) may have been exceeded. Normally, the problem does not occur with the samples having wide boiling ranges and a wide variety of compounds, which is usually the case in a real fuel analysis situation. Correlation of Cycloparaffins Measured by Different Methods. In this work, the various components of the saturate fraction were determined by means of adsorption and LC separation, and their contents were compared with the GC-MS calculated values. As shown in Table 3, the GC-FIMS yielded higher values for cycloparaffin types, compared to other methods, due to the reason explained earlier. However, the values for iso- and normal paraffins calculated by GC-FIMS, were in good agreement with the values obtained for these hydrocarbon types by the molecular sieve adsorption, LC separation, GC-MS, and PNA. Further comparison of the results for the cycloparaffin types (total, mono-, and di- + polycycloparaffins) obtained by LC/GC-MS, Robinson14,15 and GC-FIMS methods, in the 24 diesel fuel samples is shown in Figures 5, 6, and 7. It suggests that there were better correlations between GC-FIMS and LC/GC-MS data for total cycloparaffins (R2 ) 0.81) and monocycloparaffins (R2 ) 0.90) than those between the Robinson and LC/GC-MS data (R2 ) 0.73 and 0.47, respectively). In the case of di- + polycycloparaffins, the correlation was the same (R2 ) 0.63 and 0.63, respectively). This suggests that the GC-FIMS method for diesel analysis can be as good as the ASTM LC/GC-MS method; however, in most cases, it is superior to the Robinson method that, similarly to GC-FIMS, does not require separation of the sample prior to analysis. (14) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434. (15) Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 41, 1548-1554.

Energy & Fuels, Vol. 15, No. 4, 2001 1001

Figure 6. Correlation of monocycloparaffin content measured by different methods (the dotted line represents perfect agreement between methods).

Figure 7. Correlation of di- + polycycloparaffin content measured by different methods (the dotted line represents perfect agreement between methods).

Conclusions This paper is a result of the continuation of work on the development of the GC-FIMS method for diesel fuel analysis. This method did not require sample preseparation. It utilized a conventional Hewlett-Packard (HP) instrument with FI source and modified ChemStation software. This method allowed the calculation of total saturate and aromatic hydrocarbons, and the total paraffins were split into normal and branched types. In addition, the calculation of hydrocarbon types included distribution by carbon number. The results of GC-FIMS analysis of diesel samples from various sources compared well with the results obtained by other ASTM methods, particularly for the aromatic types. The main emphasis was placed on more detailed GCFIMS analysis of saturated hydrocarbons. Five different approaches to characterize the saturate fraction were taken, including molecular sieve adsorption of normal paraffins, HRGC, physical separation of the non-normal paraffin fraction into different compound types, PNA analysis, and LC/GC-MS. The results obtained for the saturate types by different methods were all in good agreement, which demonstrated that the new GC-FIMS method is well suited for diesel fuel analysis. While it compared well with the standardized and widely used methods based on electron impact mass spectrometry, GC-FIMS is simpler in execution and offers additional

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information. At this point, for diesel range materials, we can confidently determine the contents of normal paraffins, isoparaffins, cycloparaffins (1- to 3-Ring + poly), and aromatics (1- to 3-Ring + poly), using this GC-FIMS method. Acknowledgment. Partial funding for NCUT has been provided by the Canadian Program for Energy

Briker et al.

R&D (PERD), the Alberta Research Council and the Alberta Energy Research Institute. The majority of the test fuels were blended by Shell Canada Ltd. and additional fuel properties were analyzed by the National Research Council Canada, Syncrude Canada Ltd. and Shell Canada Ltd. EF010057M