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On the Limitations of UV-Fluorescence Spectroscopy in the Detection of High-Mass Hydrocarbon Molecules Trevor J. Morgan, Marcos Millan, Mahtab Behrouzi, Alan A. Herod, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, South Kensington Campus, Imperial College London, London SW7 2AZ, United Kingdom Received May 4, 2004. Revised Manuscript Received September 14, 2004
This work compares UV-fluorescence (UV-F) and UV-absorption (UV-A) as detection methods in the analysis of coal and petroleum-derived materials, using size exclusion chromatography (SEC). A UV-F spectrometer that was equipped with a flow cell was connected in series to an SEC chromatograph with a conventional UV-A detector. Samples were examined via SEC, using both UV-F and UV-A detectors that were operating in tandem. They included asphaltenes from heavy petroleum residues and three fractions of a coal tar pitch obtained by solvent solubility separation. The chromatogram of the lightest fraction of the coal tar pitch (the acetone solubles) showed a single peak, with close agreement between both detection systems. The rest of the samples showed an early-eluting peak that corresponded to material excluded from the column porosity, in addition to a retained peak. UV-F showed little sensitivity to material eluting under the excluded peak in any of the samples and also was less effective than UV-A in detecting the material eluting at shorter times under the retained peak, only responding to the smallest molecules. Number and weight averages of the molecular mass distributions calculated for the retained material from UV-A were significantly higher than those calculated from UV-F data. UV-F fails to detect the entire range of compounds present in these complex samples, and it is particularly insensitive to the heavier ends. It seems that detection by UV-F is more dependent on structural features than UV-A.
Introduction Some years ago, we used a UV-fluorescence (UV-F) spectrometer that was equipped with a flow cell as a detector for size exclusion chromatography (SEC).1 The spectrometer was connected in series with a conventional UV-absorption (UV-A) detector. This was done as part of a search for novel detection methods that might improve our understanding of large-molecularmass materials in coal-derived liquids. The results were disappointing. These measurements were made as part of a coal liquefaction experiment, where the samples of the product stream exiting from a semicontinuous reactor were sequentially examined. The two spectrometric methods showed broadly similar signals for the fractions extracted at lower temperatures (the early-exiting fractions from the reactor). However, as the extraction deepened and the extents of coal conversion increased, we observed a UV-A signal that appeared at progressively shorter elution times, whereas the signal from UV-F remained essentially unchanged. In SEC, shorter elution times normally signify the presence of largersized molecules. For the heaviest coal extract fraction isolated in these experiments, UV-A showed that the sample was eluting from 10 min onward. In contrast, * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Li, C.-Z.; Wu, F.; Xu, B.; Kandiyoti, R. Fuel 1995, 74, 37-45.
no fluorescence signal was detected before ∼14 min. In terms of the polystyrene calibration of the particular column used in the study, this corresponded to a value of ∼log(3.5), which is slightly less than 3200 u. This was the upper mass limit of sample that could be detected by the UV-F spectrometer. We concluded and reported at the time1 that we were unable to observe some of the apparently higher-mass material (easily observed via UV-A) with detection by UV-F spectroscopy. These observations were made during SEC experiments where tetrahydrofuran (THF) had been used as an eluent. In subsequent work, we replaced THF with the more-polar 1-methyl-2-pyrrolidinone (NMP). The latter solvent was first used as an eluent in SEC by Lafleur and Nakagawa.2 Comparing chromatograms obtained with THF and NMP, it became clear that an additional early eluting peak that was observed with (the stronger solvent) NMP was not observed when THF was used as the eluent. Examination of the guard column used during SEC with THF revealed deposited material that could be washed clean with NMP.3 THF had to be abandoned as a valid eluent for the samples on hand, because precipitation from solution had indicated a partial loss of sample. (2) Lafleur, A. L.; Nakagawa, Y. Fuel 1989, 68, 741. (3) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Carter, D. M.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1995, 9, 1446-1451.
10.1021/ef049885g CCC: $30.25 © 2005 American Chemical Society Published on Web 11/20/2004
UV Detection of High-Mass Hydrocarbon Molecules
Following the adoption of NMP as the preferred eluent for SEC, a new attempt was made to use the UV-F spectrometer as a detector.4 These measurements were conducted on the pentane-insoluble fraction of a coal extract and hydrocracking products of this fraction, which were treated catalytically at 320-460 °C. The earlier data1 had been acquired at a single UV-F excitation (at 254 nm) and a single emission (at 420 nm) wavelength. This second study using NMP as the eluent was somewhat more detailed. A wider selection of excitation-emission frequency combinations was used: 300/325, 325/350, 350/400, 375/425, 400/450, 425/475, and 450/500 (values given in nanometers). However, using the same SEC column (Mixed-E; Polymer Labs., U.K.), but with NMP as the eluent, no signal could be observed from the SEC column at elution times earlier than 14 min. In contrast, when using UV-A detection, the higher solvent power of NMP allowed the detection of large-sized material eluting at the exclusion limit of the chromatographic column, which was observable from 6.5 min onward.4 There are good reasons for expecting the UV-F intensity to decrease with increasing molecular size and with increasing sizes of polycyclic aromatic (PCA) units embedded within large molecules. The effects are explained in terms of intramolecular energy transfer within large molecules and diminishing quantum yields for larger PCA units.5,6 Thus, while attempting to use a UV-F spectrometer as the detector, we seem to have stumbled on a method for estimating the upper mass limit of coal-derived samples that could be detected by this technique. There the matter rested, until recent measurements that reported that UV-F polarization failed to detect material much larger than ∼1000 u in samples of petroleum asphaltenes.7 However, there was a difference between the two lines of work. The work outlined previously, which showed the shortcomings of UV-F spectroscopy, was performed using coal-derived liquids. These samples are usually considerably more aromatic and polar than petroleum-derived samples. Therefore, the shortcomings of UV-F spectrometry observed in our work could not be extended a priori to evaluate work on petroleum asphaltenes. There are significant differences between the coal tar pitch fractions that we have used and the petroleum asphaltene fractions that we have examined. First, the relative proportions of high-mass material in the petroleum-derived samples were considerably smaller. The pyridine-insoluble fraction of the coal tar pitch that was used as a laboratory standard is ∼30%, whereas the heptane (a less-aggressive solvent)-insoluble fractions of the petroleum residues that we considered were of the order of, at most, several percent of the total sample. Second, the data showed narrower mass distributions for the heaviest petroleum asphaltene fractions, compared to heavy coal-derived fractions. However, in two successive studies on petroleum asphaltenes,8,9 we reported evidence for the presence of (4) Herod, A. A.; Zhang, S.-F.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1996, 10, 743-750. (5) Li, C.-Z.; Wu, F.; Cai, H.-Y.; Kandiyoti, R. Energy Fuels 1994, 8, 1039-1048. (6) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York and London, 1983. (7) Groenzin, H.; Mullins O. C. Energy Fuels 2000, 14, 677-684.
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material in samples of petroleum asphaltenes of considerably greater mass than those reported in ref 7. These studies were performed using size exclusion chromatography and matrix-assisted laser desorption/ ionization (MALDI) mass spectrometry: up to ∼5000 u for a Forties field (North Sea) vacuum residue and a second (un-named) vacuum residue. Furthermore, the heptane-insoluble fractions of two residues showed traces of material up to m/z ∼15 000 with peak intensities (Mp) between m/z 500 and m/z 700. These latter data reinforce our view that fractionation is a necessary step to examine the properties of lessabundant high-mass materials, in the presence of the more-abundant low-mass material normally found within complex samples. More generally, the resolution of analytical techniques is usually improved by narrowing the mass range of samples prior to examination. The array of samples studied in our laboratory has been drawn from relatively light crudes. The slow depletion of convenient crudes in many basins and the availability of large reserves of little-used “heavy crudes”, notably in the North Slope (Alaska) fields, lends urgency to the characterization of heavy petroleum fractions. Within this framework, it seems necessary to reexamine the capabilities of UV-F spectrometry, first as a technique that is possibly useful in examining structural features and, second, possibly as a method for estimating molecular mass distributions of petroleumderived heavy fractions. Experimental Section Solvent Fractionation. Solvent solubility separation of the coal tar pitch involved acetone and pyridine and followed the method given elsewhere, using acetone rather than acetonitrile.10 Petroleum residue samples were stirred with excess heptane overnight and filtered to separate the asphaltene fraction. Portions of the Forties sample and sample 2 (2.1% and 1.6%, respectively, by weight) were isolated as the heptane insolubles. Of the vacuum bottoms samples A, B, and C, 0.4%, 1.6%, and 1.8%, respectively, were determined to be heptaneinsoluble. For the Maya heptane-insoluble asphaltene sample,11 a further 1.29% was observed to be soluble in heptane; the heptane-insoluble fraction was ∼11.3% of the crude,11 and, correspondingly, for the Petrox [Concepcion, Chile] vacuum residue, 0.10% was heptane-insoluble. The fractionations of petroleum materials follow the method described in ref 11 and follow the ASTM procedures.12 Size Exclusion Chromatography. Procedures for SEC have recently been reviewed elsewhere,13,14 where polystyrene was determined to predict the molecular mass of pitch fractions up to at least 3000 u. In experiments reported below, SEC with NMP as the eluent was performed using a Mixed-A polystyrene/polydivinylbenzene column (particle size of 20 µm; Polymer Laboratories Ltd, U.K.) that was operated at a rate (8) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 429-437. (9) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1-14. (10) Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795801. (11) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquin, G.; Garcia, J. A.; Tenorio, E.; Torres, A. Energy Fuels 2002, 16, 1121-1127. (12) Standard Test Method for n-Heptane Insolubles, ASTM Method D-3279-97, 2001 ASTM Annual Book of Standards, American Society for Testing and Materials, West Conshohocken, PA, 2001. (13) Herod, A. A.; Zhuo, Y.; Kandiyoti, R. J. Biochem. Biophys. Methods 2003, 56, 335-361. (14) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004 18, 778-788.
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of 0.5 mL/min at room temperature. Polystyrene molecular mass standardssfrom log (630 u) ) 2.8 to log (1 995 262 u) ) 6.3swere resolved by the column and elute with a linear relationship14 between log(molecular mass) and elution volume (or elution time). Three-dimensional standards with a diameter of 1-40 nm were eluted within the exclusion region of the column, independent of sample density. Detection by UV-Absorbance (UV-A). The detectors were an Applied Biosciences Diode Array detector (supplied by Perkin-Elmer, Beaconsfield, U.K.) with a Perkin-Elmer LC290 variable wavelength detector in series linked to a computerbased data acquisition system. Samples have been examined by UV-A detection at wavelengths of 280, 300, 350, 370 and 450 nm. Detection by UV-Fluorescence (UV-F). The procedure has been described in detail elsewhere.1,4 A Perkin-Elmer LS50 luminescence spectrometer was set to run in “time drive” mode. As in ref 4, qualitatively similar results were obtained over a range of excitation and emission wavelengths. The combination used for results reported below were as follows: excitation wavelength, 350 nm; excitation slit width, 15.0 nm; emission wavelength, 450 nm; emission slit width, 10.0 nm; duration of scan, 1800 s; data interval, 0.10 s. A quartz, continuous-flow cell that was 25 mm long was used. The spectrometer featured automatic correction for changes in source intensity, as a function of wavelength. The SEC column was connected to the Diode Array UV-A detector (280, 300, 350, and 370 nm), the Perkin-Elmer LC290 UV-A detector (450 nm) and the UV-F flow cell in series. The flow system was calibrated to compensate for time lags between the detectors, by running 9-methyl anthracene through the system. This compound gives a single sharp peak in both UV-A and UV-F detection. The lag between the responses from the two detectors was determined to be 2 min ((5 s). In the figures shown, this delay has been taken into consideration and compensated correspondingly, so that both detectors start at zero time. Clearly, neither the UV-A detector nor the UV-F detector is sensitive to purely aliphatic material. Although petroleum asphaltenes are known to contain material with a morearomatic character than lighter fractions, the chromatograms and results shown in this paper contain no information on aliphatic species. Work performed in this laboratory, using an evaporative light scattering (ELS) detector, with heptane as the eluent, to characterize some of the lighter petroleumderived material is being prepared for publication. Refractive index detectors are much less sensitive than the ELS detector, by a factor up to 10; the UV-F detector is more sensitive than the UV-A detector, which has a sensitivity that is approximately similar to that of the ELS detector. When using a refractive index detector,15 the sample loading sufficient to give a signal overloaded the exclusion region of the column, which appears partly as a nonzero intensity region between excluded and retained peaks. Such an overload would seriously overload the fluorescence detector and lead to self-absorbance of the emitted fluorescence wavelengths.
Results and Discussion To establish a base case, the coal tar pitch fractions were examined first, by tandem UV-A-UV-F detection. Figure 1 presents SEC chromatograms to compare detector responses. Figure 1a, which represents data for the lightest (acetone soluble) of the pitch fractions, shows similar responses from UV-A and UV-F detection. The fluorescence and absorption peaks could reasonably be presumed to detect the presence of the same mate(15) Pipatmanomai, S.; Islas, C. A.; Suelves, I.; Herod, A. A.; Kandiyoti, R. J. Anal. Appl. Pyrolysis 2001, 58, 299-313.
Morgan et al.
Figure 1. Size exclusion chromatography (SEC) chromatograms with simultaneous detection by UV-A at 300 nm and UV-F of (a) acetone-solubles of pitch, (b) pyridine-solubles of pitch, and (c) pyridine-insolubles of pitch.
rial. The forward edge of both peaks were observed at ∼19 min; this corresponds to ∼4000 u, according to this column’s polystyrene-poly(methyl methacrylate) calibration;14 the UV-A signal in this diagram shows a narrower distribution than the UV-F chromatogram, probably because the smallest aromatics have the strongest fluorescence. The absorbance (UV-A) chromatograms of the three fractions in Figure 1 show a progressive movement toward shorter elution times (i.e., larger apparent molecular masses), from acetone-solubles (Figure 1a) to acetone-insolubles and pyridine-solubles (Figure 1b) to pyridine-insolubles (Figure 1c). In Figure 1b, the resolved part (smaller-sized material) of the chromatogram of the pyridine-solubles fraction showed an absorbance signal starting from 18 min and a fluorescence signal from ∼19 min. However, the UV-F detector seems to have entirely missed the material shown by the excluded peak (centered near 14 min) of the UV-A chromatogram.
UV Detection of High-Mass Hydrocarbon Molecules
Greater differences were observed between the UV-A and UV-F chromatograms for the pyridine-insoluble fraction of the pitch (Figure 1c). This fraction has already been identified as being the most aromatic (by 13C-NMR and Fourier transform infrared (FT-IR) spectroscopy), the most polar (in terms of chromatographic behavior), and the largest-molecular-mass fraction (by MALDI-mass spectroscopy (MALDI-MS) and SEC, using UV-A detection) of the pitch.16,17 Much of the larger-molecular-mass material in the resolved part of the UV-A chromatogram (the peak centered on ∼19.5 min) was not detected by UV-F. Although the pitch fraction seemed to dissolve completely, the possibility that some of the larger molecules may have been dispersed in the solvent cannot be discounted. However, the operation of the column is such that molecules up to at least 40 nm in diameter would elute in the exclusion region of the column, even if present in a colloidal state.14 Furthermore, the forward edge of the UV-F signal in panels b and c of Figure 1 was identical, at 19 min. Once again, the early eluting material shown under the excluded peak (between ∼12 and 15 min) of the UV-A chromatogram seems to have been missed completely by the UV-F detector. Synchronous UV-F spectra of pitch fractions taken from thin-layer chromatography (TLC)17 show a steady shift to longer wavelengths with increasing size as indicated by SEC, as well as decreasing fluorescence intensity, indicating the presence of larger aromatic systems in the larger molecules where the energy from absorbing a photon may decay by pathways other than fluorescence. Taken together, discrepancies observed between UV-A and UV-F detection in Figure 1 correspond quite closely to the results described previously.1,4 Compared to the previous study, the maximum molecular mass detected via UV-F was observed to move from a value of ∼3200 to a value of ∼4000 u. Size Exclusion Chromatography of PetroleumDerived Samples. The samples fractionated for this work included a vacuum residue from the Petrox refinery (Concepcion, Chile), a Forties field sample, samples 1 and 2 (from BP), samples A, B and C (from Shell), and a Mayan (heptane) asphaltene from Mexico. In each case, the resolved peak of the heptane-insoluble fractions eluted earlier than the resolved material of the heptane-solubles fractions (not shown). Only a limited amount of data will be presented for the purposes of this paper. The preparation of a heptane-soluble fraction from the Maya asphaltene by extraction using heptane alone resembles the fractionation of a similar Maya asphaltene18 and showed that the SEC chromatogram shifted to longer elution times (smaller molecules) in the extracted material, comparable to the data of Figures 5b and 6b of ref 9. Figure 2a compares SEC chromatograms of Maya asphaltene, which were acquired by tandem detection with UV-A and UV-F. Because the UV-F detector is more sensitive, the sample concentrations that were injected were, at times, rather low, giving noisy UV-A (16) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1813-1823. (17) Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 2004, 1024, 227-243. (18) Douda, J.; Llanos, Ma. E.; Alvarez, R.; Navarette Bolan˜os, J. Energy Fuels 2004, 18, 736-742.
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Figure 2. SEC chromatograms with simultaneous detection by UV-A at 300 nm and UV-F of heptane insolubles of (a) Maya crude, (b) Forties vacuum residue, and (c) vacuum bottoms B.
chromatograms. However, the nature of the data was comparable with those of the pitch pyridine-insoluble fraction (see Figure 1c). The asphaltene from the Forties vacuum residue showed a similar trend (see Figure 2b). Although the heptane-insolubles of the vacuum bottoms samples A, B, and C also showed similar trends, a lowintensity signal that was centered around 13-13.5 min could also be discerned for this set of samples. Figure 2c shows one of these three chromatograms. Some variability between responses of the UV-F spectrometer to different petroleum-derived asphaltenes seems to exist. It is likely that the nonzero (albeit very low)intensity excluded peak observed at short elution times in the latter chromatogram was due to large-molecularmass material with relatively small, embedded PCA units. The large masses would cause early elution, whereas the small size of the PCA units would lead to measurable fluorescence. Fluorescence spectrometers are far more sensitive than UV-A detectors and operate
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Table 1. Molecular Mass Averages Calculated from the Retained Peaks of SEC Chromatograms with Detection by UV-A and UV-F Mn Values samplea
Mw Values
Mp Values
UV-A 350
UV-F
UV-A 350
UV-F
pitch acetone sol-1 pitch acetone sol-2
166 179
156 177
179 214
177 230
pitch pyridine sol-1 pitch pyridine sol-2
216 378
174 234
378 1249
pitch pyridine insol-1 pitch pyridine insol-2
322 853
174 242
Maya asphaltene-1b Maya asphaltene-2b
350 737
Maya heptane sol-1 Maya heptane sol-2
UV-A 350
Polydispersity
UV-F
UVA
UVF
61
58
1.07 1.20
1.13 1.30
234 498
84
85
1.75 3.30
1.34 2.13
853 2599
242 802
744
78
2.65 3.05
1.39 3.31
233 556
737 1587
556 2050
261
233
2.11 2.15
2.39 3.69
304 603
230 505
603 1402
505 1780
256
225
1.98 2.33
2.20 3.52
Maya heptane insol-1 Maya heptane insol-2
404 1069
259 700
1069 2715
700 2503
332
255
2.65 2.54
2.70 3.58
vacuum bottoms B heptane insol-1 vacuum bottoms B heptane insol-2
716 1608
348 938
1608 3016
938 2676
1470
580
2.25 1.88
2.70 2.85
Forties vac residue heptane insol-1 Forties vac residue heptane insol-2
461 802
296 687
802 1301
687 1913
777
302
1.74 1.62
2.32 2.78
a The number “1” indicates that the signal was proportional to mass, and the number “2” indicates that the signal was independent of mass. b Literature value for Mw (5190) was determined via vapor pressure osmometry (VPO) (from ref 11).
at low sample concentrations, to avoid self-absorption of the emitted fluorescence. It may be noted that, when using UV-F detection, coal tar pitch-derived samples (panels b and c in Figure 1) showed similar upper limits at ∼19 min, whereas, for petroleum-derived samples, the upper limit came earlier, at 18 min. Once again, this seems to result from the smaller size of the PCA units embedded within the asphaltene molecules. Synchronous UV-F spectra of fractions of the Petrox residue collected from TLC17 indicated a shift to longer wavelengths (larger aromatic systems) with increasing molecular size; however, the extent of the shift was smaller than that for the pitch, showing that the aromatic systems were smaller in the petroleum residue than in the pitch. These data are consistent with earlier results obtained using a column with a smaller range of porosity. However, the upper mass limit of detection, according to UV-F spectrometry, was determined to be greater than earlier measurements. Direct comparison between coal and petroleum-derived samples in the present study (i.e., comparing Figures 1 and 2) shows a higher mass limit of detection for petroleum-derived samples. Average molecular masses have been calculated from the polystyrene calibration and the second peak within the range of porosity of the column using the formulas
∑i
signal intensity (Hi) is assumed to be proportional to niMi, then
Mn )
∑i ni
∑i Hi/Mi
and
Mw )
∑i HiMi ∑i Hi
This latter pair of equations represent method 1. If the signal intensity Hi is independent of mass (proportional to ni only), then
Mn )
∑i HiMi ∑i Hi
and
n iM i
Mn )
∑i Hi
Mw )
∑i HM2ii ∑i HiMi
and
Mw )
∑i
niM2i
∑i niMi
where ni is the number of molecules of mass Mi. If the
The latter pair of equations represent method 2. Calculated values for Mn and Mw, using both formulas, together with peak intensity values (Mp) corresponding to the maximum SEC intensity are shown in Table 1. From the calculated values and the comparison of chromatograms by UV-A and UV-F, it is clear that the UV-F detection misses some of the material of the sample eluting in the relatively small molecule peak in
UV Detection of High-Mass Hydrocarbon Molecules
addition to missing the material of the early peak. The Mn values from UV-A all exceed values calculated from UV-F; the Mw values from UV-A all exceed values from UV-F when using method 1 but not always when using the second method, with the signal intensity being independent of mass. These Mw values, which are larger for UV-F than for UV-A, may result from the very low intensities of the UV-A signal, with a loss of sensitivity at the leading (high mass) edge of the chromatograms, which has a disproportionate influence on the Mw values. The value determined for the Maya asphaltene (5190)11 is obviously much larger than the values calculated here but includes the material excluded from the SEC column and excluded from the present calculations (inclusion would increase the Mw value). The excluded peaks observed in the present work contain materials of undefined structure and mass; however, the masses may be considered to be greater than those of the materials of the latter peak. These mass values are in reasonable agreement with data derived from small-angle X-ray scattering of centifugation fractions of asphaltenes,19 where the material with the largest radius of gyration (2.5 nm) in dilute solution in toluene had a weight average mass of 1.56 × 106 u; in our work,14 the elution time of such large molecules would be expected to be 12.5-14 min. The aggregation of small molecules in an NMP solvent has been discounted20 and is not likely at the very low concentrations used here, to avoid overloading the UV-F detector. Conclusions and Summary Size exclusion chromatograms of two heavier coalderived fractions and three petroleum asphaltenes have shown bimodal molecular size distributions, when detection was made by UV-absorption (UV-A). A UVfluorescence (UV-F) spectrometer, when used as a tandem detector, has shown no sensitivity to material excluded from column porosity and some of the early (19) Fenistein, D.; Barre, L. Fuel 2001, 80, 283-287. (20) Thiyagarajan, P.; Cody, G. D. Am. Chem. Soc., Div. Fuel Chem. 1997, 42 (1), 253-257.
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eluting material in the resolved peak. In the case of the lightest fraction (the acetone-solubles of the coal tar pitch), there was agreement between the chromatograms (single peak) from the two detectors. One of the petroleum-derived samples showed a trace intensity of fluorescence for material eluting at the exclusion limit of the column. These data show, first, that UV-F could not detect some of the heavier material within the samples. Not all the undetected material appeared under the excluded peak. It may also be noted that, for all three pitch solubility fractions, and the first two petroleum asphaltenes presented (panels a-c in Figure 1 and panels a and b in Figure 2), the peak traced by the UV-F detector showed elution at almost similar times; the detector showed very little sensitivity to rather important changes in the sample properties. A low-intensity signal at the exclusion limit of the column was observed for sample B in Figure 2c, as well as a leading edge at ∼18 min. The latter elution time corresponds to ∼15 000 u, according to the polystyrenepoly(methyl methacrylate) calibration of the column. We infer, from these findings, that detection by the UV-F detector is dependent on structure to a greater extent than detection by UV-A. It seems inescapable to conclude that UV-F spectroscopy is not capable of detecting the full breadth of the molecular mass distribution in these complex samples. The estimation of molecular mass distributions by fluorescence depolarization, with values well below 1000 u in petroleum asphaltene samples, seems to be an inevitable result of these shortcomings. Acknowledgment. The authors would like to thank Prof. Ancheyta (Mexican Petroleum Institute) for the gift of a sample of Mayan crude asphaltene and Dr. Munsterman (Shell Research and Technology Centre, Amsterdam) for three vacuum residue samples. Funding of this work by the British Coal Utilization Research Association and by the UK Department of Trade and Industry (under BCURA Contract No. 53) is gratefully acknowledged. EF049885G
