Anal. Chem. 1986, 58, 2389-2396
2389
Automated Liquid Chromatographic Compound Class Group-Type Separation of Crude Oils and Bitumens Using Chemically Bonded Aminosilane Patrick L. Grizzle* a n d D o n n a M. Sablotny S u n Exploration and Production Co., P.O.Box 936, Richardson, Texas 75080
A rapld, automated hlgh-performance llquld chromatographlc (HPLC) system has been developed for the semtpreparatlve separations of crude dls, bitumens, and related materiais Into elther compound class grouptype fractions (saturate, aromatic, and polar) or aromatfc rlng-number fractions (saturate, mono-, dl-,tri-, and pdyaromatlc, and polar). Separatkms are performed wlth comnercla#y availaMe HPLC components and chemlcally bonded sika-NH, columns. The reproduclblllty In the quantttative data obtained from the method ls excellent for crude and process dls with an average percent devlatlon of approxlmateiy 2 % for any compound class group-type fractfons. The importance of molecular structure and substituent effects on the separation of aromatic hydrocarbons accordlng to the number of aromatk rlngs was determined by uslng 57 model compounds typlcaUy found In fogsH fuels. The degree of overlap between aromatic rlng-number fractlons is mlnlmal using the method as assessed by gas chromatographlc/mass spectrometric Z series analyses of each rlngnumber fractlon obtained from a highly aromatic crude oil.
Detailed characterization data of crude oils are important in all segments of the petroleum industry: exploration, production, and refining. Due to the complexity of petroleum and related nonconventional liquid fossil fuels such as heavy oil, shale oils, and coal liquids, these materials require preparative separations prior to instrumental analyses. Although the schemes for the separations are quite varied, they typically involve separations of oils according to functionality (Le., acids, bases, and neutrals) followed by chemical group-type (compound-class) fractionation (1-6). High-performance liquid chromatographic (HPLC) chemical group-type separation of the neutral fraction in petroleums and related materials has received considerable attention (7-15). Most of this attention has been focused on detailed studies of molecular structure and substituent effects on the retention characteristics of aromatic hydrocarbons on various sorbents. Of the sorbents @-IO), silica-R(NH,), studied, chemically bonded silica-", (7-10), (2,4-dinitroanilinopropyl)silica(silica-DNAP) (7,111, and alumina (12-15) have been considered superior for aromatic ring-number separations in that molecular structure and substituent effects which perturb class separations are minimized. Although these studies have provided important data, publications on the application of these sorbents and others to the rapid, automated, routine separations of total crude oils have been few. The major limitations of these reported preparative HPLC systems and procedures for routine, automated applications involve the use of noncommercial HPLC hardward components and columns, extensive column reequilibration or reactivation times, and undesirable sample preparation requirements. Reported preparative systems utilizing alumina as the sorbent clearly demonstrate these problems. Early alumina preparative systems employed large, noncommercial
columns and high-volume pumps (16). The size of the columns was dictated by the low sample capacity of alumina. Although excellent aromatic-ring separations were made on these systems, total automation was limited due to poor retention time reproducibility caused by variations in the alumina activity (water content) (17). The use of a moisture control system to alleviate the activation problem has recently been reported (18). Its application to routine, automated separations, however, is limited by the complexity of the system and the necessary sample preparation common to all alumina methods. Although a silica column can be used in conjunction with alumina to afford the separation of the alkanes and cycloalkanes from the aromatic hydrocarbons (16,19), irreversible adsorption of the heteroatom-containing (N, S, and 0) components (polars) prevents the direct separation of total crude oils and related materials. Successful elution of the polar materials in crude oils or related materials from silica-NH2 (8,20) and dual-functional alkylaminealkylnitrile bonded silica columns (21,22) without extensive column reequilibration has been reported. Of these two sorbents, the former is readily commercially available, well characterized with respect to structure and substituent effects (8),and considered as the column of choice for an automated method. This paper describes a modular semipreparative HPLC system for the rapid, routine, automated separation of crude oils, bitumens, and related materials into either compound class group-type fractions (saturate, aromatic, and polar) or aromatic ring-number fractions (saturate, mono-, di-, tri-, and polyaromatic, and polar). The system was developed by using commercially available HPLC components and silica-", columns. The method employs both flow and gradient programming to optimize separation time. To assess the degree of separation of the aromatic ring-number fractions, the columns have been evaluated in detail using 57 model compounds either expected or known to be present in liquid fossil fuels and by gas chromatographic/mass spectrometric (GC/MS) analyses of aromatic-ring fractions obtained from a highly aromatic crude oil. EXPERIMENTAL SECTION Liquid Chromatographic System. Chromatographic separations were performed on a modular system composed of a WISP 710B autoinjector (Waters Associates, Milford, MA), a Model 680 automated gradient controller (Waters) interfaced to two Model 510 pumps (Waters), a V4 variable UV detector used at 254 nm (ISCO, Lincoln, NE), a Model 401 RI detector (Waters), and a Foxy fraction collector (ISCO). Separations were made with two 25 cm X 10 mm 1Ou bonded silica-", columns in series. Columns used were Alltech NH, (Alltech Associates, Deerfield, IL)or RSIL NH2 (Alltech). Back flush of the columns was achieved with a HGA valve controller (Waters)and a Rheodyne Model 7001 valve. Analog signals from both the RI and UV detectors were interfaced to a Hewlett-Packard 3357 LAS/1000 system. The configuration of the system is shown in Figure 1. Gas Chromatographic and Mass Spectrometric Analyses. Gas chromatographic analyses of crude oil group-type fractions were performed on a Siemens Sichromat 2 multidimensional
0003-2700/86/0358-2389$01.50/0 1986 American Chemical Society
Tab=. Mobile Phase Conditions Used in Crude Oil/Bitumen GroupType Separations and Column Studies GRADIENT CONTROLLER
model compound
CONTROL OF: FRACTION COLLECTOR VALVE CONTROLLER
crude oil separation
I I
I_ _
I
e--
AUTOMATION SVSTEM
2u
- SOLVENT FLOW - - MICROPROCESSOR CONTROLS
Figure 1. Modular preparative HPLC system for crude oil and bitumen group-type separation.
chromatograph (E & S Industries, Marlton, NJ) using two 25 m x 0.2 mm (i.d.) 100% methylsilicon (0.33-pm film) bonded fused silica columns (Hewlett-Packard, Avondale, PA) and helium as a carrier gas (23 cm/s, combined linear velocity for both columns). Samples were introduced by use of a 1OO:l split injector at 275 "C. Both of the ovens were operated with a 10-min hold at 100 OC after injection and programmed to 210 "C at 4 "C/min, followed by a second linear program to 325 "C at a rate of 1.5 "C. Analog signals obtained from the flame ionization detector operated at 325 "C were interfaced to the LAS/lOOO system for data processing. Compound identifications were obtained on the group-type separated fractions by gas chromatography mass spectrometric (GC/MS) analyses. The GC/MS system consisted of a Hewlett-Packard 5890 gas chromatograph fitted with a 50 m X 0.2 mm (i.d.) 100% methylsilicon (0.1-pm film) bonded fused silica column (Hewlett-Packard) directly coupled to a Hewlett-Packard 5970 mass selective detector (interface temperature, 280 "C). Chromatographic conditions were the same as those used in the gas chromatographic analyses with the exceptions that the injector split ratio was 101 and the helium flow rate was 1.5 mL/min at 42 psig. The mass spectrometer was operated at 70 eV, and the data were collected from 40 to 450 amu at a rate of 1.97 scans/s. Compound types were identified based on relative retention times, molecular ion profiles, and fragmentation patterns compared with published data. Reagents. Solvents used were Burdick and Jackson Distilled in Glass grade (B&J, Muskegon, MI). Model compounds were obtained from various commercial sources and used as received. Procedure for Crude Oil/Bitumen Group-Type Separations. Asphaltenes were precipitated from a weighed sample of the crude oil (0.5 g) or bitumen (0.5-0.05 g) using hexane. Removal of the asphaltenes from the oil/hexane mixture was made with a weighed Millipore 0.45-pm syringe filter (Millipore, Bedford, MA). The hexane and volatile hydrocarbons were removed from the extract by using a Minivap (Organomation, South Berlin, MA) nitrogen purge system, the recovered oil sample was weighed, and hexane was added to make a 20% (w/w) oil/hexane solution. The prepared sample subsequently was injected (270-450 KL or 30-60 mg of oil) onto the column and eluted using hexane and methylene chloride according to Table I. Elution of the column with forward flow yielded the saturate and aromatic or aromatic ring-number fractions for collection. Elution and collection of the polar fraction were accomplished by back flushing the column after the elution of the four-ring aromatic hydrocarbons. Quantitation of the individual fractions was made gravimetrically upon solvent removal. Quantitation of the asphaltenes was obtained based on the weight difference of the filter before and after asphaltene removal. Procedure for Model Compound Studies. Model compounds were dissolved in n-hexane or n-hexanelbenzene mixtures and stored at 0 "C. Concentrations ranged from 10 mg/mL for monoaromatic compounds to 0.1 mg/mL for four- through six-ring compounds. Each model compound was injected subsequently (5-100 pL) onto the columns and eluted according to the conditions shown in Table I. A standard blend of benzene, naphthalene, phenanthrene, benz[a]anthracene, dibenz[a,c]anthracene,
initial conditions composition (AB)c flow (mL/min) linear gradient segment start time (rnin) final time (min) final composition ( A B ) final flow (mL/min) linear gradient segment start time (rnin) final time (rnin) final composition (AB) final flow (mL/min) step gradient segment (back flush) start time (min) final time (rnin) composition ( A B ) flow (mL/min) reequilibration segment start time (min) final time (min) composition ( A B ) flow (mL/min) reequilibration segment start time (rnin) final time (min) composition ( A B ) flow (mL/min)
studies NHzn NH;
1oo:o
1000
1oo:o
6.0
6.0
6.0
3.0 7.0 85:15 8.0
3.0 32.0
1.0 32.0 70:30
40:60 6.0
6.0
7.0 17.0 70:30 8.0 17.0 25.0 1090 10.0
25.0 30.0 1000 9.0 30.0
32.0
40.0
45.0
1000
32.0 45.0 1oo:o
6.0
6.0
6.0
1oo:o
O25 cm X 10 mm (id.) Alltech NHz and RSIL NHz columns in series. *Two 25 cm X 10 mm (id.) RSIL NH2 columns in series. c A and B correspond to n-hexane and 60:40 (v/v) n-hexane:methylene chloride, respectively. and dibenzo[def,p]chrysene was run after every fourth model compound. The retention times of the model compounds were corrected for slight variations using eq 1where t'is the corrected
retention time, tE is the measured retention time for the model compound, ts is the time of the nearest peak in the closest standard blend analysis, Es is the average time of that same standard compound, to is the retention time of an unretained compound (heptane). RESULTS AND DISCUSSION Preparative Group-Type Separations of C r u d e Oils and Bitumens. HPLC systems and procedures described in the literature for the group-type separations of total crude oils, bitumens, and related liquid fossil fuels generally have not been directly applicable to automated, routine analyses. Although the limitations of these systems for routine applications are many, the use of noncommercial HPLC hardware components and columns, extensive column reequilibration or reactivation times, and extensive sample preparation requirements have been the major limitations. Consequently, the described system was developed to circumvent these deficiencies. The choice of the HPLC hardware was based on two criteria. First, the system of choice should be highly integrated to allow for total automation from injection to fraction collection and applicable to gradient elution semipreparative separations (20-80 mg/injection). Second, the system should feature capabilities such as back flush valves and programmable flow control, which minimize the time required for sample separation and column reequilibration. An integrated modular system best meets these criteria. The configuration
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
of the system developed is shown in Figure 1. Automation of the total system is achieved by the integration of the three microprocessor-based functional controllers. The autosampler controls the injection of the samples and initiates the start of the solvent gradient. In addition to controlling the solvent gradient, the gradient controller starts the fraction collector, activates the back flush valve, initiates data collection on the central computer, and provides flow control capabilities to minimize analysis time. The microprocessor-based fraction collector allows for programmable collection of the various group-type concentrates. The extent of sample preparation prior to group-type separations is principally application dependent. For example, the functional separation of an oil into acidic, basic, and neutral components may be necessary if detailed analysis of the acidic and basic fractions is an end objective (23, 24). Conversely, if detailed analysis of the saturate and aromatic compounds is the ultimate objective, the isolation of the acidic and basic components is time-consuming, and the method developed should not include this isolation step. Obviously, the extent of sample fractionation prior to hydrocarbon group-type separation influences the choice of both sorbent and mobile phases to be used in any method. Our laboratory is specifically dedicated to geochemical petroleum exploration. The detailed characterizations of the saturate and aromatic hydrocarbons in crude oils and source rock bitumens by GC/MS for oil-oil and oil-source rock correlations are our ultimate objectives. Consequently, we have attempted to minimize all unnecessary sample preparations such as acid and basic component isolation and fractionation. A large number of sorbent evaluations for the chemical class separation of the neutral fraction obtained from liquid fossil fuels have been reported (7-14). Most of these evaluations have involved detailed studies of molecular structure and substituent effects on the retention characteristics of aromatic hydrocarbons. Although chemically bonded silica-NH2(&IO), silica-R(NH& (7-10), (2,4-dinitroanilinopropyl)silica(silicaDNAP) (7, I I ) , and alumina (12-15) have been shown to be superior for aromatic ring-number separations, the direct separation of total crude oils or related materials is not possible on these substrates. Preliminary sample preparation is required to prevent irreversible adsorption and/or precipitation of the polar components onto the sorbents. Irreversible adsorption/ precipitation of these materials not only deteriorates column performance with time but also affects the reproducibility of quantitative group-type data. The polar components in crude oils and bitumens historically have been considered to be subdivided into two groups, the resins and the asphaltenes. From a chemical viewpoint, the two groups are virtually identical and differ only by gross molecular weight and, consequently, solubility. Of the four sorbents typically used, the silica-NHz has been shown to not irreversibly adsorb the resin materials commonly found in crude oils ( 8 , 2 0 ) . However, precipitation of the asphaltene material from the oil is not preventable using the solvents typically used for class separations on this substrate. Consequently, a minimum sample preparation step of removing the asphaltenes was incorporated into our separation scheme prior to group-type separation. Refractive index and ultraviolet chromatograms demonstrating the separation of a highly aromatic crude oil (PG-219) using two 25 cm x 10 mm (i.d.) silica-", (Alltech NW2 and RSIL NH,) columns and hexane/methylene chloride as the mobile phases (see Table I) are shown in Figure 2. The chromatograms of the aromatic subfractions (mono-, di-, tri-, and polyarornatics) obtained from the preparative separation are also shown. The use of two columns in series was required to completely resolve the saturate and monoaromatic hy-
2391
n
I 4.0
TIME ( M I N U T E S )
p+l
POLYAROMATICS
TRIAROMATICS
,
I
I 14.0
24.0
IV'2,
J L DIAPOMATICS
MONOAROMATICS
Flgure 2. Refractive index and UV chromatograms of PG-219crude oil and aromatic ring-number separated fractions.
Table 11. Reproducibility and Recovery Data for Group-Type Separations of Crude Oils and Process Oils sample
sample wt, mg
wt %
saturates aromatics polars recovered
crude oils NS-F
CD-E HA-I
39.0 78.0 41.6 83.2 40.0 80.0
76.4 78.5 42.3 44.2 46.5 48.9
18.2 18.9 37.7 38.5 40.0 41.6
2.8 2.8 16.8 17.2 9.3 9.5
97.4 100.2 96.9 99.9 95.8 100.0
32.3 29.0 34.2 31.4 32.1 37.3
86.2 87.6 47.6 49.1 18.6 19.8 1.9
8.7 7.7 43.9 42.4 58.9 59.0 2.0
4.6 4.0 7.8 7.9 22.0 20.8 2.1
99.5 99.3 99.3 99.5 99.5 99.6 0.9
process oils HPO
Golden Sundex av % dev
drocarbons. The cut-points for the various aromatic-ring number fractions were established based on model compound studies and GC/MS analyses of the individual subfractions (see discussion below). Elution of the polar (resin) material is achieved by back flushing the columns using a step gradient and flow control (see Table I) after the forward elution of the four-ring aromatic components. Separation time of the threeand four-ring aromatic hydrocarbons and reequilibration time of the columns are also minimized using flow control. As apparent in the figure, excellent resolution of the aromatic ring-number fractions is achieved by use of the system. Although no attempt has been made, the model compound studies and GC/MS analyses of the subfractions and the resolution of the chromatogram suggest that separation of the aromatic fraction by the number of r electrons may be possible as previously reported (7, 1I , 25-27). The reproducibility of the method and the recovery of compound class group-type fractions (saturate, aromatic, and
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ANALYTICAL CHEMISTRY, VOL. 58,
NO. 12, OCTOBER 1986
~~~
Table 111. Reproducibility and Recovery Data for Aromatic Ring-Number Separations of Crude Oils wt %
aromatics (ring number) sample
saturates
1
S134-W
75.2 78.4 48.8 49.9 19.2 19.1
14.8 10.9 21.0 21.1 25.1 25.2
S134-D TO04
2
3
4+
2.5 2.1 0.4 3.1 1.6 0.4 7.6 9.4 1.6 8.8 8.5 1.2 7.7 11.8 2.2 7.1 11.9 2.7
polars recovered 2.8 2.2 10.7 10.1 36.4 34.7
98.0 96.6 99.1 99.6 102.4 100.7
polar) from the columns are shown by the data in Table 11. Three crude oils and three process oils were chosen for the study. The oils were chosen to represent a wide range of chemical compositions. Crude oils NS-F and HA-1 are highly paraffinic and aromatic, respectively, while crude oil CD-E is biodegraded and contains a high concentration of aromatic and polar materials. Crude oil NS-F was produced from the Tertiary Sheep Pass Formation in Nevada. Crude oil HA-1 was obtained from the Williston Basin in Montana and produced from a Mississippian Madison reservoir. Crude oil CD-E was obtained from Lake Maracaibo, Venezuela, and produced from an Eocene reservoir. The process oil samples similarly represent a large range of oil composition from highly paraffinic (HPO) to highly polar (Sundex). The HPO and Golden samples are distillate process oils. Initial, final, and mean (50% off) boiling points as obtained by simulated gas chromatographic distillation for the HPO and Golden samples are 317,483, and 402 OC and 356,505, and 440 "C, respectively. The Sundex sample is an aromatic extract with a wide molecular weight distribution ranging from hydrocarbons containing 16-38 carbon atoms. The volatile components in the crude oil samples were removed prior to separations to investigate percent recovery. The duplicate separation of each crude oil, with two different sample weights injected onto the columns, was made to assess the reproducibility of the method as a function of column capacity. The asphaltenes in the crude oils were removed prior to group-type separation. The process oils did not contain appreciable asphaltenes and were separated directly. The reproducibility in the weight percent data (Table 11) is excellent for the crude and process oils with an average percent deviation of approximately 2% for any fraction. The high recoveries obtained for both the crude and process oils verify that the polar materials are eluted quantitatively from the silica-", columns under the solvent conditions used. The slight improvement in the recoveries for the process oil samples as compared to the crude oils probably reflects the loss of more volatile hydrocarbons from the crude oils after the separations. The reproducibility of the method for aromatic ring-number separations is shown in Table 111. Three additional crude oils were used in this study. The composition of the oils varied from highly paraffinic (S134-W) to highly asphaltic (T004). Crude oil S134-W was produced from the Devonian Gordon formation in West Virginia, whereas the S134-D oil is from Alabama and produced from the Jurassic Choctaw Formation. The crude oil designated as TO04 was produced in offshore California from the Miocene Monterey Formation. The volatile components and asphaltenes were removed from each oil prior to separations. T o minimize errors introduced from weighing the smaller fractions obtained from the aromatic ring separations, approximately 90 mg of each oil was separated. The data in the table show that excellent aromatic ringnumber quantitation can be made by using the method and that the reproducibility is similar to that obtained in the
compound class fractionation (Table 11). Although a detailed study of column capacity has not been made, these data and subsequent GC/MS analyses of the aromatic ring-number fractions show that the quantitation of group-types is good up to a t least 90 mg/injection. To minimize compound-class overlap (contamination) for samples with a wide range of chemical compositions, sample weights of 30-60 mg are typically used in the analyses. Evaluation of Semipreparative Silica-", Columns for Aromatic Group-Type Separations. Detailed evaluation of two sets of chemically bonded silica-NH, columns (Alltech NH,-RSIL NH, and RSIL NH,-RSIL NH,) was made by using 57 aromatic hydrocarbons and detailed GC/MS analyses of the separated aromatic ring-number fractions. The selection of the model compounds allowed for the assessment of both molecular structure and substituent effects on the aromatic ring-number separations. The evaluation of two sets of different columns was made to assess the effect of variations in the capacity factors between commercially available columns on aromatic ring-number separations. Initial studies of preparative commercially available silica-", columns indicated that the capacity factors between columns are significantly different. Capacity factor variations were observed not only between columns containing different silica-", sorbents but also in columns with the same sorbent. For example, significant differences in relative retention strengths have been observed between different RSIL NH, columns, as well as between RSIL NH2 and Alltech silica-NH, columns. In the present study, the relative retention strength of the RSIL silica-", columns in series is less than that of the Alltech silica-NH,/RSIL NH, column system. The capacity factors for the RSIL column system correspond more closely to that of pBondapak NH, (8), whereas the retention strength of the Alltech NH2-RSIL NH2 system is significantly higher. Although no attempt was made to investigate these differences, they most probably relate to the preparation of the sorbent by the vendor, the surface area of the silica used, or the degree of end-capping of the sorbent to remove the active Si-OH sites. Considering the solvent strength required to elute the aromatic hydrocarbons from the Alltech NH2 and RSIL NH2 column system, the presence of active silica sites is strongly suggested. It should be noted, however, that the number of active sites must be low. Retention index data for this column system are quantitatively similar to those reported for pBondapak sili(8) and silica-R(NH,), (7) and different from those ca-", reported for silica (8) (see discussion below). The use of a silica-", sorbent with a higher relative retention strength has direct pragmatic significance in the group-type separations of petroleum. Separations data re(20) and ported for crude oils on both pBondapak silica-", silica-R(NH,), (7) using isocratic hexane show that the cross-contaminations (overlap) between saturate and monoaromatic fractions and monoaromatic and diaromatic fractions are significant. This overlap problem is directly related to the low relative retention strengths of these sorbents. In the described system, we have used the commercial silica-NH2 columns with the greatest retention strengths. This has significantly reduced the overlap between compound-type fractions and improved our ability to obtain chemically well-defined fractions for detailed characterization. Retention Indexes (I,) were employed as a means to evaluate the two sets of columns and to compare them with other sorbents typically used for aromatic hydrocarbon separations. As previously shown (7), the retention indexes obtained using a gradient solvent system can be directly compared to isocratic index data if the gradient is adjusted so that the elution of retention index standards (aromatic ring-num-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
Table IV. Retention Indexes for Selected Aromatic Hydrocarbons on Silica-",,
I compound benzene toluene ethylbenzene propylbenzene isopropylbenzene butylbenzene hexylbenzene cyclohexylbenzene pen tadecylbenzene nonadecylbenzene o-xylene 1,3,5-trimethylbenzene 1,2,4,5-tetramethylbenzene pentamethylbenzene hexamethylbenzene 1,3,5-triisopropylbenzene indane 1,2,3,4-tetrahydronaphthalene octahydroanthracene dodecahydrotriphenylene
NHZb,' NH,b.d DNAP'
lop I" R(NH,),
1.00 0.88 0.98 0.90 0.95 0.88 0.81 1.06 0.70 0.61 1.26 0.87 1.18 1.31 1.4j 0.55 1.09 1.15 1.17 1.69
1.00 0.92 0.84 0.75 0.76 0.72 0.67 0.91 0.49 0.42 1.27 0.75 0.93 0.92 0.98 0 35 1.00 1.01 0.99 121
1.00 1.04 1.oo 0.97 0.96
1.00
1.04 0.97 0.96 1.02 1.04 1.11 1.25 1.47 0.81 1.12 1.13 1.15 1.53
1.27 0.65 0.84 1.15 1.09 1.23 1.35 1.35 1.09 1.27 1.28 1.32 1.48
1.02 26.13
0.85 26.74
1.08 lci.5i
1.13 20.31
2.00 2.42 1.68 2.06 1.92 1.84 1.79 1.54 2.11 1.87 1,2,3,4-tetrahydrophenanthrene 2.24 2.27 1,2,3,4-tetrahydroanthracene 9,lO-dihydrophenanthrene 3.14 9,lO-dihydroanthracene 2.29 1.2,3.6,7,8-hexahydropyrene
2.00 2.21 1.52 1.82 1.73 1.55 1.55 1.54 1.72 1.50 1.92 1.92
2.00 2.01 1.53 2.04 1.97 1.25 1.81 1.61 1.42 1.24
2.00 2.11 1.71 1.97 1.91 1.65 1.83 1.82 1.68 1.62
2.17
2.29
2.51 1.85
2.08 18.21
1.81 15.59
1.73 18.4'7
1.87 10.67
av % std dev
naphthalene biphenyl indene 2-methylnaphthalene 2-ethylnaphthalene 2-isopropylnaphthalene 2-butylnaphthalene 2-isobutylnaphthalene 2-methylbiphenyl 2-isopropylbiphenyl
av
70std dec
1.19 1.13 1.m 1.09
2393
Silica-DNAP, and Silica-R(NH,), 1 NHzbvC NHZbsd DNAP'
compound phenanthrene anthracene fluorene acenaphthalene 2,6-dimethylanthracene 1-phenylnaphthalene 5,12-dihydrotetracene
log I" R(NHz)z
3.00 3.12 2.88 2.25 3.09 2.86 4.01
3.00 2.93 2.43 2.02 2.67 2.63 3.51
3.00 2.98 2.43 2.54 3.20 2.19 3.24
3.00 2.94 2.45 2.59
3.03 15.94
2.74 15.89
2.80 13.53
2.85 13.18
4.00 3.39 4.06 3.58 3.68 4.07 3.43 3.16 3.90 3.89 2.88
4.00 3.29 4.05 3.42 3.37 4.11 3.19 2.84 3.74 3.64 3.20
4.00 3.70 4.07 3.63 3.49 4.20
4.00 3.39 4.08 3.49 3.44 4.12
3.52 3.82
3.12 3.80
3.64 10.28
3.53 11.06
3.69 10.69
3.68 9.42
dibenz[a,c]anthracene benzo[e]pyrene
5.00 4.39
5.00 4.43
5.00
5.00
dibenzo[def,p]chrysene benzo[ghi]perylene
6.00 4.67
6.00 4.73
6.00 6.07
6.00 4.80
av % std dev
benz[a]anthracene pyrene chrysene fluoranthene benzo[b]fluorene triphenylene 1-methylpyrene 1-butylpyrene 12-methylbenz[a]anthracene 7,12-dimethylbenz[a]anthracene 9-phenylanthracene av 70 std dev
2.79
ORetention indexes obtained by using hexane as the mobile phase as previously reported (7). bRetention indexes obtained by using the mobile phase conditions given in Table I. '25 cm X 10 mm (id.) Alltech NHZ and RSIL NH, columns in series. d T ~ 25 o cm X 10 mm (i.d,) RSIL NH, columns in series. eRetention indexes for silica-DNAP as previously reported (7).
ber) is linear with time. Figure 3 shows the corrected elution times (t') for the retention index standards on the two preparative silica-NH2column systems as a function of aromatic ring-number. For comparison, data for silica-DNAP are also shown in the figure (7). The difference in capacity factors for the two sets of silica-NH2columns is obvious in Figure 3, considering the mobile phase conditions used for each column system (see Table I). For the gradient elution system, the retention indexes were obtained by using eq 2, where t!, is the corrected retention
iF SIIIca-DNAP
LC
-
10 --
time of each model compound, t is the corrected time of each standard with the same number of aromatic rings as the model compound, and th+lis the corrected retention time of the standard with the next higher number of aromatic rings. The retention index standards used and their value of I N in eq 2 are as follows: benzene, 1;naphthalene, 2; phenanthrene, 3; benz[a]anthracene, 4; benz[a,c]anthracene, 5 ; and dibenzo[def,pJchrysene, 6. The retention index data for the two silica-NH2 column systems are given in Table IV. For comparison, index data for silica-DNAP and silica-R(NHJ2 are also presented (7).For
0 2
3
4
5
6
NUMBER A R O M A T I C RlNQS
Figure 3. Dependence of retention time on the number of aromatic rings for Alltech silica-NHP/RSIL silica-"2, RSIL silica-NHP/RSIL silica-"2, and silica-DNAP.
the silica-R(NH2), isocratic data, the retention indexes are expressed as the log I,. For brevity, the retention index data for the column system employed in the routine method (Alltech NH2-RSILNH2)are used in the following discussion.
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ANALYTICAL CHEMISTRY, VOL. 58, N 6 . 12, OCTOBER 1986
-16
-16
r
1
-16
-12s
-14
li
-
I
-14
I
I
1
1251
'11
-12s
n
-12s
L
Figure 4. Gas chromatogram of triaromatic I subfraction with compound identification based on Zseries. -16, -12S, and -14 correspond to alkylfluorenes, alkyldlbenzothiophenes, and alkyltetrahydrophenanthrenes, respectively.
The data for the two RSIL NH2 columns demonstrate, howcolumns are not equivalent and that ever, that all silica-", detailed characterization of the columns used in a routine, automated group-type separation method is necessary. Detailed model compound studies on alumina, silica-R(NH,),, and silica-DNAP yielded the following conclusions concerning the ring-number separations of aromatic compounds (7). First, pericondensed compounds in general elute faster than catacondensed compounds on each substrate. This as shown in Table same trend is also noted for the silica-", IV. Quantitatively, the magnitude of the effect for the silica-", is comparable to silica-R(NH2), and less than that observed for silica-DNAP and alumina. Naphthenic and alkyl substitution greatly perturbs the elution characteristic of aromatic hydrocarbons. The effect is similar to that observed of alkyl substitution on silica-", for ~ilica-R(NH,)~ and silica-DNAP. As evident in the case of monoaromatics, the elution is accelerated with increasing alkyl chain length and retarded with the increasing degree of alkyl substitution. Quantitatively the perturbations are generally larger for naphthenic substituents than the length of alkyl group. Based on the magnitude of the percent standard deviations, the perturbations observed on the silica-", are larger than silica-R(NH,), or silica-DNAP. It is interesting to note that although qualitatively the substituent effects are similar on both sets of silica-", columns, quantitatively the perturbations are quite different. Steric effects arising from alkyl substitution are noted on each sorbent, although the effects are noticeably different. For example, data for naphthalene, 2-ethyl-, 2-isopropyl-, and 2-butylnaphthalenes on s i l i ~ a - R ( N Hand ~ ) ~silica-DNAP show that branching of an alkyl group on an a-carbon atom produces a decrease in the adsorption energy when the alkylated molecule is planar ( 7 ) . This a-effect is not observed in the case of silica-NH2. This effect has also been observed for alumina (12,15). Alternatively, steric effects resulting from
alkylation of basic ring structures which yield nonplanar aromatic hydrocarbons are observed for each sorbent. This effect is readily evident for alkylated biphenyls. Substitution of an alkyl group in the 2 position generally decreases the retention indexes for all three column systems, although the silica-", system does appear to be less sensitive to this steric effect. In general, the retention index data show that the silica-NH2 system is similar to other sorbents characterized in detail and suffers from the same problems for aromatic ring-number separations. Steric effects appear to be slightly smaller quantitatively compared to silica-DNAP and silica-R(NH,),, whereas the variation in elution characteristics due to alkyl and naphthenic substituents is larger. To further evaluate the degree of aromatic ring-number overlap (contamination between individual aromatic ringnumber fractions) incurred in the separations and to confirm the results of the model compound studies, the highly aromatic oil (PG-219) shown in Figure 3 was further subfractionated into seven fractions, and the diaromatic, triaromatic, and polyaromatic subfractions were analyzed by GC/MS. The major compound types present in each subfraction were identified from molcular ion profiles according to -2 series obtained from the full-scan total ion chromatograms. Quantification of the compound types in the individual subfractions was made by flame ionization gas chromatographic analyses assuming equal response factors for each compound type (-2 series). In this context, the term -2 series refers to homologous series of aromatic hydrocarbons with the empirical formula C,H2n-z. As an example, Figure 4 shows the gas chromatogram of the triaromatic I subfraction and the 2 series identification of the individual components. In the case of the triaromatic I subfraction, the dominant 2 series of -16, -12S, and -14 correspond to alkylfluorenes, alkyldibenzothiophenes, and alkyltetrahydrophenanthrenes,respectively. Table V gives the cut-points of the designated subfractions
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
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Table V. Principal Aromatic Hydrocarbons in Subfractions of PG-219Crude Oil Based on -Z Series fraction designation diaromatic I
retention time, min
probable compound type alkylnaphthalenes alkylhexahydropyrenes
75.7 11.1 6.0
12 14 16
86.6 6.2 2.7
14 16 12
59.4 25.9 2.2
16 12 (S) 14
76.7 17.9 1.2
18 20 16
37.4 31.1 5.6 5.0
22 24 22 (S) 26
10.05-11.40
alkyltetrahydrophenanthrenesand/or alkylacenaphthenes
alkylhexahydropyrenes alkylnaphthalenes triaromatic I
-Z
7.55-10.05
alkyltetrahydrophenanthrenes
diaromatic I1
re1 area 70a
11.40-12.20
alkylfluorenes alkyldibenzothiophenes alkyltetrahydrophenanthrenes
triaromatic I1 12.20-13.55 alkylphenanthrenes alkyldihydropyrenesand/or alkyltetrahydrochrysenes and/or alkyltetrahydrotetracenes alkyldihydrophenanthrenes
polyaromatic
>17.0
alkypyrenes alkylchrysenes and/or alkyltetracenes alkylbenzonaphthothiophenes
alkylnaphthenopyrenes Obtained by flame ionization gas chromatography assuming unity response factors for all compound types. and the identities and relative area percents for the principal compound types in each subfraction according to -2 series. As noted in Figure 2, the times for the subfraction cut-points correspond to valleys observed in the UV chromatogram for the sample. Several conclusions can be made based on the data in Table V. First, this crude oil is relatively simple in its composition. The bulk of each subfraction can be characterized by only three or four -2 series. For example, 92.8% of the diaromatic I fraction is composed of compound types in the -12, -14, and -16 2 series. Based on the starting mass of each molecular ion series observed and fragmentation patterns, these series most probably correspond to alkylnaphthalenes (-E),alkyltetrahydrophenanthrenes (-14), and alkylhexahydropyrenes (-16). Detailed examination of the GC/MS data shows that the elution of two-, three-, and four-ring aromatic hydrocarbons is consistent with model compound studies and that overlap between ring-numbers is minimal. In fact, the amount of overlap between subfractions is even quite small. The diaromatic I and I1 subfractions are dominated by alkylnaphthalenes (-12) and alkyltetrahydrophenanthrenes (-14), respectively. The overlap of these compounds between the two subfractions reflects the effect of alkyl substituents. For example, the alkyltetrahydrophenanthrenesin the diaromatic I fraction have alkyl groups containing three or more carbons, whereas this compound type in the diaromatic I1 fraction is dominated by aromatic structures with a small degree of alkyl substitution (1-4 carbons). This variation of elution characteristics with degree of alkylation is not observed in the case of the alkylhexahydropyrenes. Based on retention times and fragmentation patterns, the distribution of these compounds is the same in both subfractions. This observation would suggest that the effect of alkyl substitution of polyaromatic hydrocarbons may be less than for two- and three-ring aromatic compounds. This conclusion cannot be made from model compound studies due to the scarcity of alkylated polyaromatic hydrocarbons. Based on model compounds, alkylacenaphthenes (-14) and alkylbiphenyls (alkyldiphenylalkanes (-14) should elute in the diaromatic subfractions. The presence of the alkylacenaphthenes in the diaromatic I1 subfraction and their absence in diaromatic I is consistent with model compound studies. No evidence for the presence of alkyldiphenylalkanes or alkylbiphenyls was
observed in either fraction based on fragmentation patterns, although the concentrations may have been too low to detect. The triaromatic I and I1 fractions are dominated by the presence of alkylfluorenes (-16) and alkylphenanthrenes (-18), respectively. No overlap of these compound types was observed. The compounds in the -16 series in triaromatic I1 are alkyldihydrophenanthrenes and not alkylfluorenes, based on retention times and fragmentation patterns. The occurrence of dihydrophenanthrenes in this fraction is consistent with the model compound studies. The isolation of the alkylfluorenes in the triaromatic I subfraction further supports the use of silica-NH2for the separation of aromatic hydrocarbons by the number of T electrons (7,11,24-26). Interestingly, this effect is also observed for aromatic hydrocarbons containing sulfur. The triaromatic I fraction contains a high concentration of alkyldibenzothiophenes (-lZS), which are structurally analogous to the alkylfluorenes. Similarly to the fluorenes, no dibenzothiophenes were observed in the triaromatic I1 fraction. The only significant overlap of compound types observed in these fractions is the presence of a small amount of alkyltetrahydrophenanthrenes (-14) in the triaromatic I subfraction. Although no attempt was made to subfractionate the polyaromatic fraction, the results obtained are qualitatively consistent with the model compound studies. Alkyldihydropyrenes and alkyltetrahydrochrysenes (tetracenes) (-20) are observed in the triaromatic I1 subfractions, whereas the aromatic homologues are dominant in the polyaromatic fractions. Other compound types observed in the polyaromatic fraction include alkylnaphthenopyrenes (-26) and alkylbenzonaphthothiophenes (-22s).
CONCLUSIONS In summary, we have developed a rapid, automated HPLC system for the compound class group-type fractionation of crude oils, bitumens, and related materials. The HPLC system is modular and consists of commercially available components and chemically bonded silica-", columns. The silica-NH2 columns chosen for the separations have a higher relative retention strength than other silica-", sorbents studied. The use of these columns has reduced the overlap between compound class fractions and, consequently, improved the chemical uniqueness of the fractions for detailed characterization studies. The reproducibility of the method for the quanti-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
tation of compound classes is excellent, as is the recovery of the polar constituents from the columns. The system has been routinely used in our laboratory for one and one-half years, and approximately 500 crude oils and bitumens have been separated with no apparent change in chromatographic resolution or column efficiency. Evaluation of several commercially available chemically bonded silica-NH2 columns has shown that the relative retention strength between columns is quite variable and that detailed studies of model compounds to assess the effects of molecular structures and substituents on aromatic ring-number separations are necessary for the selection of columns for optimum separations. The effects of molecular structure and substituents on the ring-number separation of aromatic hy)~, drocarbons are similar for silica-NH2, s i l i ~ a - R ( N H ~and silica-DNAP sorbents. As compared to the other sorbents typically used, silica-NH2 quantitatively is less affected by steric effects, whereas the variations in aromatic hydrocarbon elution characteristics due to alkyl and naphthenic substituents are larger for silica-NH2. GC/MS characterization of the aromatic ring-number fractions isolated from a highly aromatic crude oil according to -2 series showed that chemically well-defined aromatic fractions and subfractions are obtained from the described separation method. Overlap between aromatic subfractions is minimal, with the overlaps observed consistent with the model compound studies. This result has direct pragmatic significance in the detailed characterization of liquid fossil fuels. Simplification of these complex materials enhances our ability to characterize them chemically and, consequently, improves our ability in discovering and utilizing these natural resources.
ACKNOWLEDGMENT The authors thank Jane S. Thomson and John B. Green for supplying standard hydrocarbons used in this study and for useful discussions.
LITERATURE CITED (1) Green, J. B.; Grizzle, P. L.; Thomson, J. S.;Hoff. R. J.; Green, J. A. Fuel 1985, 6 4 , 1581-1590. (2) Green, J. B.; Hoff, R. J.; Woodward, P. W.; Stevens, L. L. Fuel 1984, 63, 1290-1301. (3) Grizzle, P. L.; Green, J. B.; Sanchez, V.; Murgla, E.; Lubkowltz, J. The Future of Heavy Crude Oil and Tar Sands 2nd International Conference; Meyer, R. F., Wynn, J. C., Olson, J. C., Eds.; McGraw-Hill: New York, 1984; Chapter 42. (4) Grizzle, P. L.; Green, J. B.; Sanchez, V.; Murgla, E.; Lubkowitz, J. Prepr.-Am. Chem. SOC.,Div. Pet. Chem. 1981, 2 6 , 839-850. (5) Green, J. B.; Hoff, R. J. J. Chromatogr. 1981, 2 0 9 , 231-250. (8) Mckay, J. F.; Amend, P. J.; Harnsberger, P. M.; Cogswell, T. E.; Latham, D. R. Fuel 1981, 6 0 , 14-16. (7) Grizzle, P. L.; Thomson, J. S. Anal. Chem. 1982, 5 4 , 1071-1078. (8) Wise, S. A.; Chesler, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 4 9 , 2306-2310. (9) Popl, M.; Dolansky, V.; Mostecky, J. J. Chromatogr. 1978, 117, 117-1 27. (IO) Chmielowiec, J.; George, A. E. Anal. Chern. 1980, 5 2 , 1154-1157. (11) Thomson, J. S.;Reynolds, J. W. Anal. Chem. 1984, 5 6 , 2434-2441. (12) Popl, M.; Mostecky, J. J. Chromatogr. 1974, 9 1 , 649-658. (13) Popl, M.; Mostecky, J.; Dolansky, V.; Kuras, M. Anal. Chem. 1971, 43, 518-521. (14) Snyder, L. R. Principles of Adsorption Chromatography;Marcel Dekker: New York, 1968; Chapters 10, 11. (15) Snyder, L. R. J. Chromatogr. 1986, 2 5 , 274-293. (16) Vogh, J. W.; Thomson, J. S. Anal. Chem. 1981, 53, 1345-1350. (17) Matsunga, A.; Yagi, M. Anal. Chem. 1978, 5 0 , 753-756. (18) Radke, M.; Wlllsch, H.; Welte, D. H. Anal. Chem. 1984, 56, 2538-2546. (19) Sawatzky, H.; George, A. E.; Smiley, G. T.. Montgomery, D. S. Fuel 1976, 55, 16-20. (20) Dark, W. A. J. Liq. Chromatogr. 1982, 5 , 1645-1652. (21) Bollet, C.; Escaller, J. C.; Souteyrand, C.; Caude, M.; Rossett, J. J. Chromatogr. 1981, 206, 289-300. (22) Miller, R. Anal. Chem. 1982, 5 4 , 1742-1746. (23) Green, J. B.; Hoff, R. J.; Woodward. P. W.; Stevens, L. L. Fuel 1984, 63, 1290-1301. (24) Green, J. B.; Grizzle, P. L. Trace Ana/ysis; Lawrence, J. F., Ed.; Academic Press: New York, 1982; pp 223-265. (25) Boduszynskl, M. M.; Hurtubise, R. J.; Allen, T. W. Anal. Chem. 1983, 55. 225-231. -- , -- - - . (28) Chmielowlec, J. J. Chromatogr. Sci. 1981, 19, 296-307. (27) Wise, S.A.; Bonnett, W. J.; May, W. E. Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects; Bjorseth, A,; Dennis, A,, Eds.; Battelle Press: Columbus, OH, 1981; pp 791-806.
RECEIVED for review April 4, 1986. Accepted June 3, 1986.
