ANALYTICAL CHEMISTRY, VOL. 50, NO 6, MAY 1978
753
not only an increase in equilibrium of the mass transfer, but conversely a decrease in equilibrium of the mass transfer by operating the column a t elevated temperature. Any factor which disturbs the equilibrium of sorption/ desorption in the column, will effect a decrease in mass transfer which can occur in either the stationary or mobile phase. Generally, the stationary phase mass transfer effect is relatively small except for heavily loaded liquid-liquid supports and reverse phase packings ( 4 ) . Since the observed resolution loss occurred on two packings (RP-8 and RP-18, both derivatized to 70% ( 5 ) (steric hindrance prevents further silanization of the silica-gel), the effect of stationary phase mass transfer cannot be dismissed. For this to occur, the diffusivity of the stationary phase must become significantly reduced a t 60 "C.
beginning t h e migrations apparently remain constant, and relatively independent of flow rate, for separations with the same gradient profile. This factor can account for the observed reduction (by half) of elution volumes a t the slower flow rate. T h e observed loss in resolution a t elevated temperatures must be taken into consideration when using E Merck 5-ym R P - 8 or RP-18 packings.
CONCLUSION If heating the column packing to 60 "C results in an increase in channeling (and eddy diffusion), thus accounting for the observed loss in resolution a t 60 "C, then no increase in resolution would h a l e been observed a t the lower flow rate at 60 "C, since the contribution of multiple pathways to peak broadening is independent of flow velocity. In addition, if increased molecular (longitudinal) diffusion were primarily responsible for decreasing resolution at 60 "C(relative to room temperature), then resolution should have been degraded still further by decreasing the flow rate a t 60 "C to 0.5mL/min, since longitudinal diffusion is directly proportional to residence time on the column (inversely proportional to flow rate). Reduction in the flow rate will cause a reduction in band broadening from both stationary and mobile phase mass transfer contributions since velocity (flow rate) is directly proportional to the magnitude of non-equilibrium of solute sorption/desorption. T h e increase in resolution at 60 "C obtained by reducing the flow rate (relative to the effect on resolution of reduced flow rate a t room temperature) implies
LITERATURE CITED J. R. Jadamec, W. A. Saner, and Y. Talmi, Anal. Chem., 49, 1316 (1977). W. A. Saner, G. E. Fitzgerald, and J. P. Welsh, Anal Chem., 48, 1747 (1976). M. Telepchak, ES Industries, Marlton. N.J., personal communication, Sept. 1977. "Introduction to Modern Liquid Chromatography", L. R. Snyder and J. J. Kirkland, Ed., Wiley Interscience, New York, N.Y., 1974, pp 31, 171. T. Abstender, EM Laboratories, Elmsford, N.Y., personal communication, Sept. 1977.
RECEIVED for review October 27, 1977. Accepted February 13, 1978.
Separation of Aromatic Compounds in Lubricant Base Oils by High Performance Liquid Chromatography Atsushi Matsunaga" and Masataro Yagi Technical Research Center, Nippon Mining Co., Ltd., 3- 17-35, Niizo-Minami, Toda, Saitama, Japan
A high performance liquid chromatographic method is described for the rapid class separation of aromatic compounds present in the lubricating base stocks. Gradient elution chromatography on activated alumina (rather than silica or polystyrene gel) produces a successful separation of heavier petroleum fractions. The chromatograms reflect the difference in compositional characteristics. The same pattern of separation was also obtained in preparative scale, and the ultraviolet and mass spectrometric analyses of fractions verified the separation with aromatic ring numbers. Quantitative studies were also attempted to determine the content of monoaromatics.
septum and loop (2-mL volume) injectors were used for sample injection; the former for analytical and the latter for preparative work. Stainless steel columns of 50-cm length were used. The internal diameter of the columns was 2.3 mm for analytical and 8 mm for preparative separations. Alumina (LVoelm, Type N18)and silica Waters Associates, Porasil A) were dry-packed. Porous polystyrene gel (Hitachi Gel 5010) was packed by the slurry method. Hexane of liquid chromatographic grade (Wako Chemicals. 'Tokyo) and methylene chloride of reagent grade were used without any purification. Separations were performed at ambient temperature without thermostating the column or the detector. Ultraviolet spectra (UV)were measured with a Hitachi 323 Spectrophotometer. Mass spectra (MS)were obtained using a Hitachi RMU-6L Mass Spectrometer.
There is a need for rapid analysis of aromatic compounds present in the heavy petroleum fractions for evaluating feedstocks and products. T h e existing methods are timeconsuming and expensive to run on a routine basis. Recently, high performance liquid chromatographic (HPLC) separation of lighter petroleum fractions was developed ( I ) . Authors investigated the possibilities of HPLC as a method for analysis of lubricating distillates and products.
Figure 1 presents some results obtained by adsorption chromatography on silica and porous polystyrene gel. In both cases, the good group-separation was achieved for lighter petroleum fractions (e.g., gasolines and kerosenes), but the resolution decreased for heavier distillates. For the separation of lubricating oil fractions, only gradient elution chromatography (GEC) on activated alumina gave successful results. Figure 2 presents the typical chromatograms. As saturated hydrocarbons are transparent in the ultraviolet region, the chromatograms show the group-separation of aromatics including some hetero compounds.
RESULTS AND DISCUSSION
EXPERIMENTAL A Varian Aerograph model 8520 Chromatograph, attached to a spectrophotometer as a detector. waq used for this work. Both 0003-2700/78/0350-0753$01 . O O / O
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Table I. Relative Retention for Standards Relative retention I. Monoaromatics iso-Propylbenzene Toluene 3-Methylthiophene n-Hexylbenzene n-Octylbenzene Te tr a1in Indane n-Dodecylbenzene 1,2,3,4,5,6,7,8-0ctahydrophenanthrene 11. Diaromatics Biphenyl Indene Naphthalene Benzothiophene Acenaphthene
0.25-0.40 0.28 0.30 0.32 0.33 0.33 0.34 0.38 0.38 0.38
Relative retention 111. Polyaromatics Acenaphth ylene Fluorene Phenanthrene Dibenzothiophene Anthracene Fyrene Chrysene 1,2-Benzopyrene IV. Sulfides n-Butyl sulfide Dodecyl sulfide Benzyl sulfide Phenyl sulfide Thioanisole Benzyl disulfide V. Polar compounds Quinoline, Acridine, Skatole, Carbazole, Phenyl sulfoxide, Dibenzothiophene dioxide
0.9-1.1 0.98 0.97 1.00 1.00 1.10
1.2--2.6
1.20 1.28 1.28 1.37 1.84 1.64 2.53 2.45 0.4-2.6 1.02 0.45 1.63 1.00 1.30 1.42 2.74 2.74
1,
D
N co
U
1
10
0
nn
20
:min
1
Figure 2. Separations of lube stocks by GEC on activated alumina. (1) Solvent refined 150 neutral. (2) Hydrofinished 150 neutral. Column: 50 X 0.23 cm, Woelm AI,O, N18 (activated). Eluent: hexane (100) to methylene chloride (100) at 2%/min. 1 mL/min. Detection: 270
0
nm
m
N
U 4
1
3
- --
4
i
0
5
(min i
10
0
5
(min 1
1
0
0
5
1
t
imin I
Figure 1. Separation of light to heavy distillate fractions by adsorption chromatography on silica or polystyrene gel. (1) Kerosene. Column: 50 X 0.23 cm, Porasil A . Eluent: hexane, 1 mL/min. Detection: 280 nm. (2) 40 Neutral lube stock. Conditions as in 1 . (3) 150 Neutral lube stock. Conditions as in 1. (4) Bright-stock. Conditions as in 1. (5)Gasoline. Column: 50 X 0.23 cm, Hitachi Gel 3010. Other conditions as in 1. (6) 60 Neutral lube stock. Conditions as in 5. (7) 150 Neutral lube stock. Conditions as in 5
It is considered t h a t the activity level of alumina for the successful separation must be more than Brockmann's grade I (2). It was impossible to regenerate the adsorbent by means of flushing the column with hexane after a run of GEC. Therefore, the column was activated a t 250 "C for 1.5 h while purging with dry nitrogen before each separation run. Thermally activated alumina column always showed good
0
IO
20
30
/Tin)
Figure 3. Separation of model aromatic compounds. (1) Tetralin, (2) Biphenyl, (3) Naphthalene, (4) Acenaphthene, (5)Acenaphthylene, (6) Phenanthrene,(7) Anthracene, (8) 1,2-Benzopyrene,(9) Chrysene, (10) Impurity. Conditions as in Figure 2 resolution. although some fluctuation in retention times was observed. This activation procedure is considered to be more easy to operate than the use of partially deactnated adsorbents and eluents ( 3 ) .
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, M A Y 1978
755
Table 11. UV and MS Analyses of the Fractions Obtained by Preparative LC -
Fraction No. in Fig. 2
wt %
SR-lb
UV analysis hmax
8.8
E
MS analysis
at Amax
271
515
437
SR-2
59.1
269.5
SR-3
20.9
233 266 233 264
4582
243 259
... ...
270.5 268.5 260
456 539 3800
SR-4
9.2
SR-5
1.9
HF-1 HF-2 HF-3
43.8 50.2 4.4
HF-4
1.6
Molecular ion
Estimated typea
...
Fragment ions
Dinaph thenobenzenes Mononaphthenobenzenes Trinaphthenobenzenes Benzenes Benzenes Mononaphthenobenzenes Dinaphthenobenzenes Trinaph thenobenzenes Benzothiophenes
1 5 7 , 171, 1 8 5 145, 1 5 9 239, 253 1 1 9 , 1 3 3 , 147 119, 1 3 3 , 1 4 7 145, 1 5 9 157, 1 7 1 , 185 239, 2 5 3 1 6 1 , 175, 189
Benzothiophenes CnHZn-6 Naphthalenes Cn H2 n- i z C,H,,-,, Biphenyls or Mononaphthenonaphthalenes Thiaindanes C,H,,-,, Dibenzothiophenes C,H,,-,, Fluorenes CnH2,-,, Phenanthrenes C,H,,-,, Hexahydrodibenzothiophenes CnH2n-6 Same as SR-1 Same as SR-2 Biphenyls C,H,,-,, Thiaindanes(?) C,H,,-,, Benzenes and fluorenes are also detected. Dibenzothiophenes C,H,,-,, Fluorenes CnH2,-,,
161, 175, 1 8 9 155,169, 183 181, 1 9 5 , 209, 223
5125
...
240 260
24220
1 4 9 , 163, 1 7 7 225, 239, 253 207, 221 1 9 1 , 205 217, 231 181, 195, 209, 2 2 3
135, 149, 1 6 3 225, 239, 253 207, 221
shoulder a
The uDDer is more abundant.
Solvent-refined oil.
Hvdrogen-finished oil.
0
-0
40
80
120
20
(mln.1
Figure 5. GEC of oxo-desulfurized lube stocks. Original samples and conditions: same as in Figure 2
i_
Cl-1-2-L31,4
10
(mtn
1
Figure 4. Preparative GEC separations of lube stocks Samples: same as in Figure 2. Flow rate: 2 mL/min
T h e elution behavior of model aromatic compounds and some hetero compounds in GEC on alumina was studied. Results are summarized in Table I. Typical separations are presented in Figure 3. As shown in Table I, the difference in retention values between monoaromatics and diaromatics is sufficiently large, whereas the difference within each compound class is insignificant. Most polyaromatics eluted after diaromatics, but the elution order of each compound did not always correspond to aromatic ring numbers. T h e retention characteristics of sulfides depended largely upon the alkyl or aryl substituents. Polar compounds including nitrogen compounds were adsorbed most strongly. They were eluted as a single peak. Impurities in solvents or adsorbents often gave a spurious peak in the same position. These results suggest that the order of elution in this procedure is: (1) monoaromatics, (2) diaromatics, ( 3 ) polyaromatics, and (4)
polar compounds including impurities. The separations of lubricating oil fractions shown in Figure 2 for the above compound groups were investigated by UV and MS analyses of the fractions collected from preparative separations. Figure 4 shows the chromatograms and the results from spectrometric analyses are summarized in Table 11. It was confirmed from these results that mono-, di- and triaromatics were concentrated in the separated fractions. I t is clear, therefore, that the analytical separations shown in Figure 2 were also performed successfully, as the patterns of the separations are the same. Such separations could not be obtained using silica or polystyrene gel as adsorbent. I t is considered that the selectivity of activated alumina for one compound class is practically independent of substituent, whereas that of silica and polystyrene gel is dependent. T h e chromatograms obtained by GEC on alumina permit us to compare the compositional characteristics of lubricating fractions. For example, we can understand the compositional difference between solvent-refined and hydrogenated oil from Figure 2. T h e procedure has also been applied successfully for the characterization of widely different lubricant feeds and stocks including vacuum distillates and bright-stocks from
756
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
700) in hexane averaged 525 a t 270 nm, with a relative standard deviation of f 1 5 7 ~ .This value is in good agreement with that reported by Snyder on gas oil fractions ( 5 ) . Then, the determination of monoaromatics with UV detection a t 270 nm is possible, without regard to sample types, if the molecular weight can be estimated. Isocratic elution with hexane as eluent is convenient for the determination of monoaromatics, because only monoaromatics are eluted. leaving polyaromatics retained strongly on the adsorbent (activated alumina). In this way, monoaromatics in a series of hydrogenated oils of a lighter lubricating distillate were determined. Calibration was performed from known blends of monoaromatics with saturates from one sample. As the changes i n molecular weights are very small, this calibration could be applied to all samples. Results of determination are shown in Table 111. Little quantitative change was observed with monoaromatics, in spite of the decrease in total aromatics content with the increase of desulfurization.
Table 111. Determination of Monoaromatics in Hydrogenated Oils Desulfurization, %
0 36.2 64.6 72.4 82.9 91.1 a
Total aromatics, wt %a 44.4 42.0 39.8 39.0 38.1
37.2
Monoaromatics, wt % 30.9 30.5 31.3 28.9 30.5 30.3
Results obtained bv silica-gel chromatoaraDhv.
various sources. Sulfur compounds exist in a considerable amount in heavy petroleum fractions. Therefore, GEC was carried out on the oxo-desulfurized ( 4 ) oils. T h e examples are shown in Figure 5 . By comparing Figure 5 with Figure 2, we observe that most sulfur compounds behave as di- or polyaromatics and that the content in polyaromatics is large. Quantitative analysis of aromatics with an ultraviolet detector is rather difficult because each compound has its oum absorptivity. T h e use of a universal detector such as a flame ionization detector would be desirable for quantitation. As for the monoaromatics, however, quantitation with the UY detector is not unreasonable. Preparative separations were carried out to isolate the monoaromatics from widely different samples (vacuum distillates, bright-stocks, solvent-refined, hydrogenated, etc.). The molar absorptivities of the recovered monoaromatics (average molecular weight varied from 300 to
LITERATURE CITED (1) J. C. Suatoni, H. R. Garber, and B. E Davis, J . Chromatogr. Sci.. 13, 367 (1975). (2) H. Engelhardt and H. Wiedemann. Anal. Chem.. 4 5 , 1641 (1973). (3) L. R . Snyder and J. J. Kirkland, "Introduction to Modern Liquid Chromatography'. Wiley-Interscience, New York, N.Y.. 1974. (4) H. V Drushel and A . L. Sommers, Anal. Chem., 39, 1819 (1967). ( 5 ) L. R. Snyder, Ana/. Chem., 3 6 , 774 (1964).
RECEREL) for review August 30, 1971. Accepted February 13, 1978.
Dispersion versus Absorption: Spectral Line Shape Analysis for Radiofrequency and Microwave Spectrometry Alan G. Marshall* and D. Christopher Roe Department of Chemistry, University of British Columbia. Vancouver, B.C. V6T 1 W5. Canada
detection of a coherent absorption or emission process (e.g., NMR, ICR, KQR. ESR: and pure rotational spectrometry, as well as dielectric and ultrasonic relaxation), virtually all existing analyses of spectral response have been based upon either ii) the time-domain transient signal following one or more pulses of coherent oscillatory excitation (usually electromagnetic radiation) ( I 4. or (ii) the frequency-domain a bsor p t i o n-m ode or "magnitude ("absolute - Val lie" steady-state response to a continuous oscillatory excitation (6-9). Provided that the system is linear (namely! that the incident power level is below that at which saturation effects are significant), the time-domain and frequency-domain responses are related by a Fourier transformation (10). T h e reason for choosing these particular spectral displays is historically clear: each tqpe of signal is obtainable as the direct output of a suitable electronic detector. However, with the advent of spectrometers with on-line computers providing for digitization, storage, and manipulation of spectra, t h e spectrometrist need no longer feel restricted to these particular data displays. It is the purpose of this paper to introduce a new form of data reduction, consisting of a plot of dispersion vs. absorption (see Theory). This new data display produces a semicircle reference curve for a simple Lorentzian line shape. A related display has long been used to detect and characterize multiple relaxation in dielectric measurements ( I I ) . In the simpler dielectric or ultrasonic case. the spectral components of the
I n radiofrequency (nuclear magnetic resonance, ion cyclotron resonance, nuclear quadrupole resonance) and microwave (electron spin resonance, pure rotational) spectrometry, it is possible to obtain both absorption and dispersion spectra. For a simple Lorentzian line shape, a plot of dispersion vs. absorption gives a semicircle. I n this paper, dispersion:absorption plots are constructed for the first time for a number of linebroadening mechanisms. I t is shown that such plots are diagnostic for, and can often be used to distinguish between, line-broadening resulting from: unresolved superposition of two Lorentzians of different resonant frequency or different line width; Lorentzian line shapes which have been weighted by either a Gaussian distribution in resonant frequency, or a log-Gaussian distribution in relaxation time or correlation time; and line shapes resulting from "chemical exchange" between two sites of different resonant frequency or different relaxation time. It is proposed that the dispersi0n:absorption plot can serve as a useful means for establishing, from a single spectrum, the mechanism for line-broadening for isolated lines in radiofrequency and microwave spectra. Experimental examples of many of these situations are provided in a companion paper (see following article) for the particular case of nuclear magnetic resonance spectrometry.
"
In those forms of spectrometry featuring phase-sensitive 0003-2700/78/0350-0756$01 O O / O
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