Anal, Chem. 1088, 60,200-204
200
High-speed and Super-speed Size-Exclusion Chromatography of Polymers for Process Analysis Curtiss N. Renn and Robert E . Synovec* Center for Process Analytical Chemistry, Department of Chemistry, BG-IO, University of Washington, Seattle, Washington 98195
w-
superspeed liclukl chromatography ( H p w is becomlng a popular analytical tool. AppUcaUon of superspeed HPLC in process analysls may provide near real-time feedback control. Nonaqueous slze exciuslon chromatography (SEC) wHh sllca-based stationary phase columns was studled In the htgh-speed and super-speed reglmes. The dependency of solute resoiution and chromatographic analysis time on experimental parameters, such as column back pressure, was investlgated. A column selection procedure Is proposed and applied by extrapolating information from the hlgh-epeed regbne to the super-speed regime. Resolution and detection of a 500 000 g mol-' polystyrene and toluene mlxture in less than 3 s analysls time was obtained for nonaqueous SEC. The experimental results compared favorably to the results suggested by the column selection procedure. Reproduciblilty of the system was examined.
High-performance liquid chromatography (HPLC) is a popular tool for routine problem solving in analytical chemistry. HPLC is characterized by many dependable advantages, providing both qualitative and quantitative chemical information with fast analysis time (I).The capability of extremely short analyses has carried with it some new definitions, such as high-speed HPLC when the eluent dead time is about 10 s, and super-speed HPLC when the eluent dead-time is 1 s or less (2). The requirements for performing super-speed HPLC are quite rigorous (3,4)since solute band broadening becomes more sensitive to instrumental characteristics. Careful attention to instrumental details such as reducing extracolumn band broadening due to the injector, detector, connecting tubings, and fittings has allowed the application of accepted chromatographic theory for describing band broadening due to the separation mechanism in high-speed HPLC (1-7). Chromatographic theory has led to the understanding that short analyses are optimized in terms of solute resolution by using small HPLC columns, which seems obvious, while packing the columns with materials of 3 - ~ m diameter (1,2,4,5,8,9). Further, data acquisition requirements are quite stringent, makiig important the choice of the detedor, i.e., response time and time constant, and subsequent interface devices, such as a computer (10). Process analysis is an area of research which has become quite active (11).The application of HPLC in process analysis has been limited, generally due to the relatively slow analysis times obtained for many separations. The development of high- and super-speed HPLC theory has clear ramifications for process analysis ( 2 ) . Routine, continuous process monitoring via HPLC requires a reliable system that provides chemical information at speeds that allow confident feedback control (12). Unfortunately, continuous operation puts a tremendous burden on any HPLC system and can only be alleviated if precautions are taken. Specifically,for high-speed HPLC, use of a small packing material diameter, ca. 3 wm, quickly produces a column back pressure that often is too high 0003-2700/88/0360-0200$0 1.50/0
for continuous use in process analysis (9).Thus, the pressure drop, or back pressure, for a given HPLC system must be considered in detail for process analysis, since the minimum analysis time is available only when operating at or near the maximum available pump pressure (1,5). A trade-off exists between analysis time and solute resolution, within the context of available, and reasonable, pump pressure capacity. Polymer characterization is an important area of chemical analysis, particularly at the process level. Size-exclusion chromatography (SEC) is very useful for characterizing polymer/monomer mixtures. Recent developments in SEC column technology have produced some interesting results for aqueous SEC with analysis times reduced to about 6-10 min (13,14).Nonaqueous SEC with silica-based packing materials should perform well for high- and super-speed HPLC since silica materials can withstand higher back pressures than semirigid or soft SEC gels. The limits of optimization should be outlined with readily available SEC theory (15). Physical separation via SEC of polymer/monomer mixtures is particularly important for characterization since the chemical similarities make spectral deconvolution without separation, via chemometrics, somewhat limited. Much of the previous work in high-speed HPLC has focused on reversed-phase HPLC (RP-HPLC), with many useful applications (3,8,9,16), yet RP-HPLC is not as well suited as SEC for polymer characterization. Therefore,the optimization and application of high- and super-speed nonaqueous SEC, using silica-based packing materials specifically for process analysis conditions, is the focus of this work. A method is presented that allows one to optimize analysis time and solute resolution under specific instrumental constraints, Le., primarily a pressure drop constraint, for packed nonaqueous SEC columns. Setting a pressure drop constraint is critical to maintaining system performance for a longer period of time between service. Briefly, the method involves two steps. First, a column is chosen that provides more than enough solute resolution for species of interest, such as in the analysis of a polymer/monomer mixture containing as few as two major components. Second, the data from this first column can be readily applied to choose a second column that optimizes the analysis time, i.e., reduces, while maintaining a desired level of solute resolution, all within the constraints set on instrumental levels, Le., column back pressure. General chromatographic theory is assumed and supported by the experimental results.
THEORY Given a more than adequate separation from a typical HPLC column, the goal is to devise a method of choosing a HPLC column with appropriate smaller dimensions. Basic chromatographic relationships must be employed for this purpose. First, the back pressure for the HPLC system must be considered. For a packed HPLC column the pressure drop, AP,is given by ( 1 )
AP = 47uL/dp2
(1)
where 4 is a dimensional structural constant, 7 is the mobile 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
phase viscosity, u is the mobile phase linear velocity, L is the column length, and d, is the average particle diameter. For two columns similarly packed, i.e., same material and method, yet of different physical dimensions, one obtains
with subscripts 1 and 2 denoting the two HPLC columns. Since u = L/to,where tois the dead time, related to the void or dead volume, Vo,and volumetric flow rate, F, by to = Vo/F, one can write
:)( ?)(2 )
201
with H = L / N ( N is the number of theoretical plates), and CY = kb’/k,’ (subscript b denotes the later eluting peak), the analysis time t is related to R, by (17)
It was noted (17) that eq 8 suggests that simultaneously optimizing R, and analysis time, t , requires an understanding of H / v behavior. This is apparent when one considers eq 9 for two columns where thermodynamicproperties are the same
(3)
fll= f 1 2 ( Note that the dead volume is proportional to ar2L,where r is the column inside radius. For two similarly packed HPLC columns this provides (4)
Initially, let us assume one obtains a chromatogram for a column with a pressure drop of AP2. What is of concern is how much resolution may be sacrificed while still separating components of interest. This must be achieved vis-a-vis a short analysis time, determined by the requirements of a given analytical control system. Simultaneously, one must be concerned with the hardships imposed upon the instrumental apparatus. Foremost, the performance of the pump and other components in light of the pressure drop must be kept within acceptable limits. Thus, a trade-off between resolution and analysis time is complicated in a practical sense by the pressure drop increase generally encountered when one attempts to decrease analysis time by increasing flow rate, etc. It is practical to suggest that the second column (subscript 1) of smaller physical dimensions should be operated at the same back pressure as the initial column (subscript 2). This constraint is mathematicallywritten as AP, = APz. Combining this constraint with eq 3 and 4 provides
if column 2 is the initial column, t2, and ( H / u ) , are all known. The problem reduces to trying to achieve an acceptable R , , (aslarge as possible) and t , (as small as possible). Thus, eq 9 suggests that the quantity ( H / u ) , must be minimized as suggested (17). The problem is complicated since the dependence of H vs u is difficult to predict a priori. Alternatively, one may simplify eq 9 to
assuming Hl data (from column 1)can be reasonably predicted from H 2 data. This may be realized when one considers the contributionsof H in HPLC. Specifically, for polymer analysis via SEC, H can be described by (15)
H=A+Cu+P
(11)
where A is the multipath contribution, Cu is the resistance to mass transfer term, and P is the contribution due to solute polydispersity. Note that longitudinal diffusion has been neglected in eq 11. From eq 10 the ratio H2/H1may be written as
H2 _ H,
- A’+ C V ~
(12)
A’+ Cv,
for a given solute eluted from two columns similarly packed but of different physical dimensions, where A‘ = A P. If one defines X = A’/Cv2, then
+
which relates the volumetric flow rates, F1 and F2,of the two HPLC columns. The relationship in terms of analysis time can now be obtained. The capacity factor is defined in the usual way, Le., k’ = ( t - to)/to.Since the two columns should function the same in terms of thermodynamic properties, i.e., k,’ = K i for a given solute, one realizes to,l/to,2 = tl/t2. Thus, given the constraint that AP, = AP2, substitution of eq 4, using t l / t z into eq 5 gives instead of to,*=
-)
tl = t2( Lld,,2
2
which provides an estimate of the minimum value of H 2 / H 1 for all positive X,thus safely overestimating the loss of resolution upon going from column 2 to column 1. The result of eq 13 is even more accurate for monodisperse solutes (P = 0). As column packing technology improves, this will result in smaller A terms, further improving the applicability of eq 13. Since we have
L2d,,l which compares the analysis times for the two columns constrained by the pressure drop equivalence. Now, chromatographic resolution, R,, must be considered. For two adjacent bands or peaks, subscripts a and b, the standard deviation of the peaks calculated from the peak width can be considered approximately equal, Le., ua = ub If this is not the case the larger of the two should be used, which is assumed to be b b in this case. Resolution is given by tb
- ta
R, = 4ab
(7)
then eq 10, 13, and 14 may be combined to provide
for a reasonable estimate of R,,,. The result of eq 15 can be related in chromatographic terms to eq 9. Basically, eq 15 suggests that for two similarly packed SEC columns of different inside diameter and length, the H / u vs u plots are superimposable, or at least to a good approximation. Thus,
202
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
in eq 9 the ratio (H/u)~/(H/u)~ is equal to 1. The pressure dependence may be constrained such that AP1 = AP2, at a practical level. The result, eq 6, may be substituted into eq 15 to yield
The ratio L1/L2can be readily calculated via eq 6 by considering eq 15. Further, previous results (9) indicate that the optimum particle diameter, d,, is as small as possible, within a practical limit. Now, HPLC columns with readily available inside radii may be chosen according to eq 5, since column lengths have been constrained according to eq 6,15, and 16. By use of acceptable Fi and ri values according to eq 5, the constraint that AP1 = AP2 can be met, while the relationship between analysis time and resolution will be described by eq 15. Application of this theory is explored, and possibilities for relieving the pressure drop constraint will be discussed in light of experimental results. EXPERIMENTAL SECTION The experimental system was state-of-the-art for conventional and microbore super-speed HPLC and data acquisition. The HPLC system consisted of a syringe pump ((ISCO, LC-2600, Lincoln, NE) connected through an in-line filter to a 1.5-wL injection loop (Rheodyne,Model 7125, Cotati, CA) which was at the head of an analytical column. Two different columns were used 250 X 4.6 mm Macrosphere,300-A pore, 7 pm, C8 (Alltech, Deerfield, IL), and 30 X 2.0 mm Macrosphere, 300-A pore, 5 Km, C8 (Alltech,Deerfield, IL). The analytical column was connected directly to the input tubing of a UV-vis absorbance detector (ISCO, Model V4,Lincoln, NE) with a 3.5-yL flow cell (ISCO, Series 0080-057, Lincoln, NE) operating at 260 nm for the detection of the polystyrene standards (Polymer Laboratories, Amhurst, MA) and HPLC grade toluene (Baker, Phillipsburg, NJ). The mobile phase for each column was composed of Spectrograde tetrahydrofuran (THF) (Burdick and Jackson, Muskegon, MI). Reversed-phase conditions were studied by adding distilled water to the THF, with use of the larger C8 column. It was important to consider instrumental band broadening, as well as extracolumn band broadening, for the super-speed analyses (1-7). The flow cell volume of 3.5 pL was just adequate for the analyses with the smaller C8 column. This was later confirmed by duplicating the study with a 0.5-pL flow cell (ISCO, Series 0080-072, Lincoln, NE). Some loss of resolution with the 3.5-pL flow cell did occur but did not impede the analysis or discussion of the method. A variety of time constants were available on the detector making this detector choice quite appropriate. A 0.05-s time constant was applied with analyses using the small C8 column. Further, the Nyquist criteria for sampling was satisfied by collecting 40 data points per second with this short time constant. The data were collected via a laboratory interface (MetraByte,DASH-16, Taunton, MA) that facilitated the analog to digital (A/D) conversion for subsequent storage and analysis with a personal computer (IBM-XT, Armonk, NY). All software used in this work was written in-house. Each sample was injected at least three times for a given set of experimental conditions. RESULTS AND DISCUSSION SEC of polymers is often performed with columns packed with semirigid gels such as polystyrene-divinylbenzene (PS-DVB). PS-DVJ3 packed SEC columns perform reasonably well a t moderate flow rates and linear flow velocities. Unfortunately, semirigid packing materials compress readily under high back pressure making it difficult to decrease analysis time by operating at high linear flow velocities without damaging the column. SEC based upon rigid packing materials, such as silica, solves many of the problems associated with use of semirigid gels (13-15). For this work, 300 8, pore silica, derivatized with
I "
I
'.'.
300 R P C 8 250 X 4 . 6 M M
L n
1.5
I 2.0
I
I
2.5
3.0
I I
3.5
Figure 1. Calibration curve, In (molecular weight) vs solute retention volume, for the silica-based 250 X 4.6 mm, 300-A pore, 7 ym, C8 column with THF mobile phase.
300 R P C 8
v)
m
a -I
W
a 40
50
60
70
80
TIME, sec Figure 2. SEC of a mixture containing toluene and the following polystyrenes: 1 130 000,500000,170 000,68 000,34 500,9000,and 3250 g mol-'. The silica-based column is described In Figure 1, operating at 2.50 mL/min and 1000 psi. Absorbance detection at 260 nm was used. Injected quantities of each solute ranged from 8.5 to 12.5 kg.
octyl functional groups, was chosen. A reasonably small particle diameter, d,, was used to ensure adequate column efficiency (9) while not so small as to cause excessive back pressure. This column provided an excellent calibration curve, In (molecular weight) vs solute retention volume, shown in Figure 1, which substantiates the SEC behavior (15). Retention volumes of each solute were constant to within 2% over the flow rate range tested, 0.35-2.5 mL/min. The low dead volume is the key to reduced analysis time. Favorable analysis time and resolution are shown in Figure 2, at a relatively moderate back pressure of 1000 psi. Evidently, THF has an effectively low viscosity, which allows relatively high linear flow velocities without raising the back pressure beyond reasonable limits for continuous process analysis conditions. If RP-HPLC character in the separation is required to improve resolution, water may be added to the THF. The data in Figure 3 demonstate how this dramatically increases the back pressure due to increased eluent viscosity. More importantly, it points toward the need to reduce the effective mobile phase viscosity. The goal was to design a simple method for choosing a column of correct physical dimensions, given data from an initial column, to provide an optimized analysis time and solute resolution. Six solutes of the mixture in Figure 2 were studied to measure column efficiency. H vs u data were calculated and plotted in Figure 4 for four of the standards. Note that the largest polystyrene studied, 1130000 g mol-', was totally excluded from the 300-A pores, which is consistent with a Stokes radius calculated at 301 A in this solvent system. Toluene, the totally permeating solute, produced an H of
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
2
.-fn
c
2 I
0
/
4 I
6 i
203
8
0
/x
'
P Y
.
-
I
I 3
01 I
2
\
THF: H20 X 75:25 0 90:10 0100: 0 1 I 1 5 6
4
.
I
I 0
501
OL
v , mm sec-1
I
10
0
Flguro 3. Pressure drop, A P , measured as a function of the linear flow velocity, v , for three THF/H,O combinatlons, for the sllica-based column described for Figure 1.
I
I
30
20
40
v , mm s e d Flgure 5. H l v (s)vs v (mm s-') for 500 000 g mol-' polystyrene with the 250 X 4.6 mm silica-based SEC column (upper v scale, 0 )and the 30 X 2.0 rnm sillca-based SEC column (lower v scale, 0). 300 R P C 8
1
0.2
cn m a
E E 0 .l-,x
I
+-+-+-+-
4 W
e PS i33oooo I /+ 0 P S 170000 X PS 34500 + PS 3250
0.0
I
0
I 1
I 2
I 3
v
I 4
I 5
I 6
a 1
Flguro 4. H(mm) vs v(mm s-') for the foliowlng polystyrenes using the silica-based SEC column described for Figure 1: 1 330 000, 170 000, 34 500, and 3250 g mol-'.
4
0
, mm sec-1
~ I M (Es e~i ) Flgure 6. SEC of 500 000 g mol-' polystyrene (246 ng injected) and toluene (114 ng Injected) wlth the silica-based 30 X 20 mm, 300-A pore, 5 pm, C8 column, wlth THF mobile phase at 3.33 mL/rnin and 3100 psi.
nearly constant value, 0.015 mm, independent of u relative to the polystyrene behavior. Typical SEC behavior was observed (15). Given these H vs u data, it was possible to estimate the trade-off between resolution and analysis time upon going from the initial column to a smaller column, packed with the same material, as outlined in the derivation of eq 15. Note that it is essential to work at linear flow velocities well beyond the minimum H in HPLC to keep analysis times acceptable (1-6).
Instead of plotting H vs u,H / v vs u was plotted for 5OOOOO g mol-' polystyrene and shown in Figure 5. From the consideration of eq 4 it is clear that
for two similarly packed columns of different dimensions. This relationship is useful when comparing H / v vs u plots for the high-speed and super-speed SEC regimes. From eq 17 it was found that u1 is about 5 times up assuming the same volumetric flow rate F. This facilitates the plotting of the data for columns 1 and 2 with a more easily visualized x axis, Le., u. Plots similar to Figure 5 are representative of all species studied in this work, i.e., Figure 4. It has been suggested that minimizing analysis time requires minimizing H / v (17). See eq 8 and 9. It is clear from Figure 5 that the H / v vs u curves approach the same asymptote. Indeed, this behavior is described mathematically in the derivation starting with eq ll and ending with eq 15. If one requires short analysis times, operation in the asymptote region is no doubt a necessity. Thus the derivation of eq 13 to calculate H 2 / H l in terms of u2/u1 is justified for this asymptote regime. Since the idea is to infer the behavior of column 1 from column 2, using data
on column 2, one can only assume superimposable asymptote behavior. Data plotted in Figure 5 for the two columns support this idea. For this work it was of interest to simulate the separation of a monomer peak, i.e., reactant, from the polymer peak, i.e., product, as in the analysis of a polymer synthesis process. It was important to achieve an adequate resolution, i.e., Rss1= 1, while keeping the analysis time at a minimum for optimum feedback control. This should support predictions about the behavior of the SEC system in the limit of the super-speed regime. For the initial column the chromatogram in Figure 2 provides R,, = 7 for the pair of analytes, polystyrene 500000 g mol-' and toluene 91 g mol-'. The analysis time for the initial column was about 80 s (Figure 2). Equation 15 predicts that a second, smaller column, similarly packed, should provide R,' = 1 with an analysis time of 1.6 s. If the particle diameter of the packing material is equal, eq 6 predicts that the smaller column should have a length, L,, of 35 mm to ensure a similar pressure drop as for the initial column. Finally, if the volumetric flow rate F is kept constant, eq 5 predicts that the smaller column should have an inside radius, r l , of 0.9 mm (1.8 mm i.d.). If more resolution is required for quantitative analysis, Le., more solutes separated, start with eq 15, setting R,,l requirements, and similarly proceed with the calculations of the smaller column dimensions. From this calculation we chose a 30 X 2.0 mm (length X inside diameter) column packed with the same material as the initial column (250 X 4.6 mm). A slightly different particle size was used. For this smaller column, the chromatogram of the polymer/monomer simulated sample is shown in Figure 6. It is important to minimize extracolumn band broadening contributions to H, i.e., injector, detector, and connecting
Anal. Chem. 1988. 60,204-209
204
tubings. The chromatogram in Figure 6 compares favorably with the calculation based upon eq 5,6, and 15 and serves to visually document the trade-off between analysis time and resolution vis-a-vis Figure 2. Further, the asymptote behavior for the H / u vs u plot of 500000 g mol-' polystyrene at large u is nearly coincidental for both columns, as shown in Figure 5. The asymptote predicts a value for C, eq 11, of about 0.020 s for both the super-speed and high-speed SEC systems for this solute. Evidently, contributions to band broadening due to nonequilibrium effects are minimal. This makes sense since the SEC mechanism is physical instead of chemical in nature. Further, the solutes most likely affected are of large molecular weight, Le., small diffusion coefficient, which elute first, and interact with the column the least in SEC. Another potential problem associated with super-speed SEC of polymers is the possibility of shear degradation (18). This problem requires further study. A slightly higher back pressure, AP,was observed compared to the predicted value for the experimental conditions. Since the two columns were packed with slightly different particle size diameters, d,, this is reasonable. Intermediate molecular weight polystyrene standards had elution volumes between the two peaks observed in Figure 6. Also, the super-speed separation could be readily automated with excellent reproducibility. Three replicate injections for the super-speed SEC system provided reproducible retention times and peak heights to better than 2.5% relative standard deviation. This capability should be ideal for feedback control applications in process analysis (I1,12). Future work toward combining super-speedSEC and chemometric techniques may
prove useful for extending the molecular weight resolution capacity of SEC techniques. Registry No. Polystyrene, 9003-53-6. LITERATURE CITED Katz. E.; Scott, R. P. W. J. Chromatogr. 1982, 253, 159-178. Erni, F. J. Chromatogr. 1983, 262, 371-383. Gfeller, J. C.; Haas, R.; Troendle, J. M.; Erni, F. J. Chromatogr. 1984, 294, 247-259. Jinno, K. Anal. Lett. 1984, 77(A10), 933-943. Katz, E.; Ogan, K. L.; Scott, R. P. W. J. Chromafogr. 1984, 289, 65-83. Dezaro, D. A.; Dvorn, D.; Horn, C.; Hartwick, R. A. Chromafographia 1985, 20, 87-96. Rocca, J. L.; Higglns, J. W.; Brownlee, R. G. J. Chromafogr. Sci. 1985, 23, 106-113. Rice, L. G. J. Chromatogr. 1984, 317, 523-526. Takeuchi, T.; Ishll, D.; Nakanishi, A. J. Chromafogr. 1984, 285, 97-101. Durden, D. A.; Balky, B. A. J. Chromafogr. 1988. 368, 49-58. Callls, J. B.; Illman, D. L.; Kowalski, B. R. Anal. Chem. 1987, 5 9 , 624A-637A. Ruslnov, L. A.; Kurkina, V. V. J. Chromafogr. 1988, 365, 367-374. Hlrayama, C.; Ihara. H.; Hamada, K.; Kinoshita, S.; Yonemura, S.; Motozato, Y. J. Chromatogr. 1988, 368, 391-394. Dawkins, J. V.; Gabbott, N. P.; Montenegro, A. M. C.; Lloyd, L. L.; Warner, F. P. J. Chrometogr. 1988, 371, 283-291. Dawkins, J. V.; Yeadon, G. Polymer Sclence and Technolcgy; Cooper, A. R., Ed.; Plenum: New York, 1982; Vol. 16, pp 27-33. Dong, M. W.; DlCesare, J. L. J. Chromatogr. Sci. 1982, 2 0 , 517-522. Karger, B. L.; Snyder, L. R.; Horvath, C. An Introduction to Separation Sclence; Wiley: New York, 1973; pp 153-155. Barth, H. G.; Carlin. F. J., Jr. J. Liq. Chromafogr. 1984, 7, 1717-1738.
RECEIVEDfor review July 28,1987. Accepted October 6,1987.
On-Line Fractionation and Identification of Diesel Fuel Polycyclic Aromatic Compounds by Two-Dimensional Microbore High-Performance Liquid Chromatography/Capillary Gas Chromatography Ilona L. Davies* and Keith D. Bartle
Department of Physical Chemistry, The University of Leeds, Leeds, LS2.9JT, United Kingdom
Paul T. Williams a n d Gordon E.Andrews Department of Fuel and Energy, The University of Leeds, Leeds, LS2.9JT, United Kingdom
Automated hlgh-performance amlnosllane sllka packed mlcrobore IlquM chromatographic (HPLC) fractlonatlon Is coupled on-llne to caplliary gas chromatography (GC). Under nornabphase HPLC condltlons, aHphatlc compounds are unretalned whlle one- to four-rlng polycyclic aromatlc hydrocarbons elute accordlng to rlng slze and are detected by ultraviolet absorbance. HPLC fractlons of 100 HL are transferred dlrectly Into the GC through a 10-port valve Interface and a 25-m retentlon gap. Indlvldual components are resolved by temperature-programmed capillary GC wlth flamelonlzatlon detectlon. A two-dlmenslonal model of HPLC/GC retentlon Indexes Is used to Identlty two-to four-rhrg polycyclfc aromatic compounds in diesel fuel.
Petroleum-derived diesel fuels are complex mixtures of hydrocarbons that boil within the distillation range of ap-
proximately 180-360 OC. These fuek are e k n t i d y olefin-free with typical chemical class compositions of 1530% aromatics and 7 0 4 5 % saturated aliphatica (1). The polycyclic aromatic fraction of grade A-2 diesel fuel contains two- and three-ring aromatics and their alkylated isomers (2). These compounds are thought to contribute toward diesel exhaust polycyclic aromatic compounds (PAC), both as unburned fuel and through a pyrosynthetic route (3). Many PAC are listed as Environmental Protection Agency (EPA) priority pollutants because they are known or suspected mutagens and/or carcinogens (4). Recent surveys indicate that the percentage of diesel-powered vehicles on the roads is expected to increase significantly in the near future owing to their superior fuel economy over gasoline-powered vehicles (5). As diesel exhaust emissions are increasingly being viewed as an important contributor to environmental pollution, accurate analysis of PAC in both diesel fuels and exhaust particulates is necessary if we are to understand and control this problem.
0003-2700/88/0360-0204$01.50/00 1988 American Chemical Society
