aman Spectra ROLAND C. HAWES, KENYON P. GEORGE, DAVID
C. NELSON,
and RICHARD BECKWITH
Cary Instruments Corp., Monrovia, Calif.
Laser excitation opens new possibilities in Raman spectrometry. Good spectra may be obtained quickly from milligram samples. Fluorescence difficulties are reduced, but are never removed entirely, and sometimes are aggravated. Axial excitation in fused silica capillary sample tubes is preferred over excitation enhancement by multiple passing because OF freedom from losses due to color in samples, as well as smaller sample volumes. Polarization measurements are as useful with this geometry OS with the Toronto arc and capillary cells. Efficiency by He-Ne at 6 3 2 8 A. compares favorably with 4 3 5 8 A. mercury radiation. Application of this optical geometry to a variety of sannpies, including solids, is illustrated. Modifications to the Cary Model 81 spectrophotometer are described.
CHSRAGTERISTICS in a Raman source are suitable wavelength, monochromaticity, both in narro\.yness of exciting line and freedom from radiation of the other wavelengths, high and continuous output power, low power requirements, and no unusual need t o heat, cool, or ventilate. The gas laser combines all these except high output, but it can be made to yield very high beam radiant intensity because of the small beam divergence. A laser beam can be concentrated to a smaller size or kept inside a given cross section area over a greater length than the beam from any other source, by many orders of magnitude. The experiments to be described are the result of an attempt to take full advantage of this property for production of high quality spectra with a concomitant reduction of sample size. The helium-neon laser line at 6328 A. is favorably located in the spectrum for Raman excitation. While the Raman excitation efficiency decreases with the fourth poxer of the exciting wavelength, fluorescence excitation efficiency usually decreases still faster. Also, frequently strongly colored or photosensitive samples can be examined by exciting in the red (4, 6, 11). Only fragmentary information is available about the alleviation of the fluorescence problem for typical samples. Data on this question are being accumulated and a few examples are reported here, Improvement in signal-to-backESIRABLE
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RAMAN RADIATION
Figure 1 .
~
Coaxial excitation optical arrangement
ground ratios may exceed 10-fold, Improvement is quite variable, however, and some samples give poorer results. The most fluorescence-free material we have found for cells and other optics common to the exciting and Raman beam is Suprasil fused silica (Engelhard Industries, Inc., Hillside, PI'. J.), It is difficult to fabricate into capillary cells with fused-on windows, but techniques have been developed which permit the construction of usable capillary cells with low background. Excitation Optics and Efficiency. One of the most valuable advantages of laser excitation is the smaller sample size t h a t is made possible by the very high focussed beam irradiance which mag be obtained when the laser oscillation is confined to a single mode type and the beam diameter is diffraction-limited, The possibilities are enhanced by the action of the Model 81 image slicer optics (d), which utilize rays from almost the entire end of a capillary cell, with equal horizontal and vertical angles, instead of from the usual slit-shaped area which wastes most of the radiation emerging from the cell window. The most efficient geometry for a single pass appears to be when both laser and Raman beams are coaxial. The optical arrangement used experimentally is shown in Figure 1. The Raman radiation is retained within the capillary cell by total internal reflection until it reaches the window. If the cell contains water, the extreme (diagonal) ray may have an angle of incidence on the water-to-glass interface of the end window of 48.9'. At this maximum slope, the coniposite slit image presented by the slicer optics would be a square 0.55 mm. on a side. The beam from this tiny area could completely fill two
slits 10 em. long and 0.5 mm. wide, with a solid angle great enough to illuminate the 10 X 10 cm. aperture of the monochromator a t f/10. In experiments, however, a circular image area about 0.8 nim. in diameter was used, to avoid extreme ray angles. Cells 5 cm. long and 1 mm. 0.d. were used in most experiments. The small 45' prism used to turn the laser beam has 1-mm. square faces, and the lens is 8 mni. in diameter. The loss of Raman intensity because of the occlusion by the prism is only about 3 per cent. For most non-aqueous solutions, open capillaries were used with a drop of glycerol a t each end to limit evaporation. Trials were made with the end against the lens fused-over, but even with a small refractive index difference between sample and silica, the laser focal position is significantly and unpredictably shifted because of the small radius of curvature of the interface. Consequently, cells for aqueous solutions were made up with thin, flat, fused-on silica windows. The first question about this arrangement concerns the Raman indicatrix, or angular distribution of scattering. The energy of a depolarized line is distributed almost isotropically, the vector ranging from 1 to 6/7, with the highest efficiency a t 0 and 180°, and the coaxial direction is also a t peak efficiency for a polarized line, as Damen, Leite, and Porto showed experimentally ( 3 ) . For the coaxial geometry, neglecting the small correction for the change of indicatrix with angle, the emission from the Raman cell within the light grasp of the coupling optics is proportional to the irradiance (the flux per unit area) of the exciting beam, times the solid angle over which the Raman radiation is collected, integrated over the sample volume com-
mon to both beams. If two geometriea are compared for which the solid angle of collection is unchanged, this reduces to integration of the average irradiance across the Raman beam, over the pathlength common t o the exciting and the Raman beam. I t can then be seen that the output is independent of the cross section area of the laser beam so long as it lies within the Raman beam. The Raman beam in a total reflection cell is defined by the outside diameter. T'ne amount of radiation collected is almost independent of diameter because the changing area from which the radiation is collected is compensated by a reciprocal change in solid angle of collection. This assertion has been tested repeatedly, in comparing large with capillary cells illuminated by the Toronto arc, for which the exciting irradiance is substantially constant. Nearly uniform Raman energy results. IThile the Raman output efficiency is unchanged, the average laser irradiance increases in inverse proportion to cell cross section area. Overall efficiency should improve as the cell diameter and sample volume decrease, the greatest intensity being obtained for the least cell diameter allowed by total internal reflection of the Raman radiation. The laser beam can be focussed to the smallest possible area if it operates with only a single mode type. Such lasers with high output are commercially available (Spectra-Physics Corp., Mountain View, Calif.), using the prismatic dominance suppressor described by Bloom ( I ) . They make it possible to leave the outside diameter of the cell unchanged a t the minimum total reflection value, and to further reduce the amount of sample by narrowing the capillary bore to match the smallest laser beam size. The Raman radiation is collected from the cell wall as well as from the sample. Alignment of cell and beam becomes critical, but attention has been given to making this convenient in the laser accessory for the Cary Model 81 Raman spectrophotometer, described below. The minimum diameter for a diffraction limited beam is, by the Lommel formula for the diameter of the Airy circle, equal to 1.22 nxl@-lwhere n is refractive index of the medium, is the vacuum wavelength, and p is the extreme ray angle. If it is desired to contain such a beam in a maximum length cylindrical tube of diameter d, the minimum beam diameter should be 0.5 d, and located midway in --the tuhe. The dianieter is then 2 4 1 . 2 2 nhll,where I is the tube length. Table I gives lengths and corresponding limiting diameters for water, n = 1.34, and sample volumes. Quality of Spectra. The quality of spectra obtained with the best mercury lamp excitation arrangement sets a standard t h a t should be met by laser excitation if it is t o become
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Figure 2. Sample handling conditions vs. background for naphthalene 10% in CC14. All curves run with 25 mw. axialmode laser (6328 A.) Upper curve: 1 mm. 0.d. borosilicate capillary cell. Sample volume 60 pl. Middle curve 2 mm. 0.d. borosilicate capillary cell. Sample volume 160 pl. Lower curve1 1 mm. 0.d. fused silica capillary cell. Sample volume 30 pl.
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widely useful. The comparison calculation that follows omits fluorescence, absorption and refractive index effects, which may be considered separ ately . The most useful parameter determining the quality of spectra is the signal-tonoise ratio ( S I N ) . It is approximately proportional to the square root of the photon flux at the phototube cathode, multiplied by the cathode quantum efficiency. The energy per photon is proportional to the frequency. Thus, based on equal numbers of photons, the “fourth power law” becomes a third power law, and S / N falls with wavelength to the 3//z power. If a nionochromator with the same spacing of grating rulings is used, a further favorable factor in energy proportional to v - ~ applies, because the monochroniator dispersion is almost constant in wavelength units and therefore increases with the square of the wavelength when expressed in frequency units. This assumes that neither grating, mirror, nor lens efficiency changes with wavelength. Gratings of suitable blaze and high efficiency and mirror coatings with high efficiency throughout the entire visible ranges are available. Taking all these considerations into account, the final factor in S I N is proportional to ~ 1 1 2 ,or only 0.83 due to the wavelength shift for equal power. The increasing dispersion a t long wavelength requires wider slits for the same spectral slit width. In fact, the maximum, 0.5-nim., slit width that the image slicer optics handle with full efficiency corresponds only to about 4 cm.-l a t ~ A V= 1000 em.-’ for the red excitation in the Wodel81. Unfortunately, photocathode quantum efficiency varies rapidly over the wavelength region of interest. The best phototubes for red use employ the S-20 tri-alkali cathode, which has about 7% quantum efficiency a t 6328 A. us. 25% at 4358 A., and falls in efficiency 4-fold for every 1000 cm.-l of Stokes shift. The greatest axial mode output power a t 6328 A. available from the SpectraPhysics Model 116 He-Ke laser used in the efficiency comparison experiment is about 30 mw. On the other hand, the 3.5-kw. Toronto arc supplied for the Model 81 puts about 1.4 watts at 4358 A. through the filter into an absorbing sample in the largest (19-mm, diameter) cell. This corresponds to an irradiance of 23 mw./cm.z at the cell surface for a transparent sample. The Toronto arc output was measured by actinometry, following Parker (6, 8) and confirmed approximately by a calorimetric method using 0.017, picric acid solution as the absorber. The laser ouput was measured by a Spectra-Physics Model 401 power meter. Factors which affect S/lV in comparison with 4358 A. excitation are: excita1844
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ANALYTICAL CHEMISTRY
tion power, effective average area of the excitation beam, cell length, the fourth power excitation efficiency law, the number of photons per watt, the grating dispersion ratio and the phototube relative quantum efficiency at 1000 cm.-l shift. The operation point was taken as 1000 cm.-’ from the exciting line, and the capillary cell dimensions assumed were 1.0 mni. diameter and 5 cm. length. The resulting equation for signal-tonoise ratio for laser excitation compared with mercury light excitation is: Laser -= Hg 120 5 1 x -20x - 4.4 x 1400 X 2 m 9 1.45 27 1 X = 1.75 -X 1 1 12
-1
The calculation shows that a slight improvement in S / N ratio should be expected for the same spectral slit width. Further improvement would be expected with the 65-mw. laser. Yet sample volume is reduced from the previous capillary cell volume of about 600 pl. to about 10 111. An experimental test of the predicted relation, using the strong 992-cm.-l line of benzene, confirmed the calculation within about 20%, which is good agreement since phototube efficiency factors were not confirmed and the trials were separated by several weeks. A more significant set of experiments was done in which the contributions to background by the sample cell were also observed. The results are in Table 11. The comparison was somewhat less than fair to the lasers. The Toronto arc tests were done with a new instrument in excellent condition, while the laser experiments were performed using the original, 10 year-old Model 81 prototype. It was, however, equipped with 4 X 5 inch gratings blazed for 7500 A. Another difference was that one Raman phototube was used in the laser experiments, while the standard instrument had two working in push-pull to avoid loss of light from the chopper, giving it a theoretical advantage of &in S I N .
fable 1. Diameters and Volumes of Cylinders Which Enclose a Focussed, Diffraction-Limited Beam, of “Diameter” Determined by Airy Circle
Length, cm. 1.25 5.0 20.
Diameter, cm.
Vol., ml.
0.023 0.045
0.00051
0.091
0.0081 0.13
A few points regarding the data of Table I1 deserve comment. The background height a t 5461 A. may be due to borosilicate glass fluorescence. The high background with the 25-mw. Model 116 laser is not typical, a n d was probably caused by beam misalignment in the rather crude original experimental set-up. The best combination, the 65-mw. Model 125 laser with Suprasil capillary cell, shows tt signal-to-r.m.s. background noise of about 4000. EXPERIMENIAL SPECTRA
Fluorescence. The effect of various sample conditions on background is shown graphically in Figure 2. The sample is 10% naphthalene in CC&. All curves were run with the 25-mw. laser. The upper curve shows the large fluorescence background caused by the borosilicate glass 1-mm. capillary cell. The middle shows much improved signal to background with a larger diameter borosilicate glass capilIary. Not only is there less glass relative to the volume of sample, but the laser beam is more easily aligned to avoid the capillary wall. The lower curve shows the excellent signal to background achieved with only 3 mg. of naphthalene when the Suprasil capillary is used. In efforts t o find an objective way to characterize the improvement in fluorescence background, the Raman spectra of several substances known to fluoresce intensely under 4358-A. excitation were recorded. Quinine is too insoluble in acidified aqueous solution to give a Raman spectrum, and does not fluoresce strongly when dissolved in alcohol.
Table II. Toronto Arc (4358 A. and 5461 A.) vs. Laser (6328 A.)5 6328 A. 6328 A. 4358 A . 5461 A. (25 mw.) (65 mw.) Peak height 79 98 78 74 Peak noise (p./p.) 1.5 1.9 2.7 1.6 49 40 27 50 s/4v (P./P.) 5.0 0.7 Background height 0.7 17 0.1 Background noise (p./p.) 0.3 0.2 0.6 0.03 0.03 Sample volume, cc. 5 0.6 0.6 Sample cell diameter, mm. 7 Fused Fused Sample cell material BoroBoro-
;
silica silicate silicate silica a Sample: CC14, 459 em.-’ peak; Instrument condition: 5-cm.-’ slits, 1-second period, 10-cm. slit height, double slit mode.
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Figure 3. Comparison of spectra of 9,l O-dibromoanthracene, which is highly fluorescent and photosensitive. Concentration 1% in CS2 Upper curve: 4 3 5 8 A. excitation; 2 mm. 0.d. borosilicate cell. Short section near 2400 cm.-' run 3 0 minutes after main curve Lower curve 6 3 2 8 A. excitation; 1 mm. a.d. fused silica cell
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But 9,lO-dibromoanthracene in C52 gave an interesting result (see Figure 3). The Raman lines, which show plainly with laser excitation, are obscured by fluorescence noise in the 4358 A. case. The solution rapidly turned dark under the Toronto arc, as is indicated by the decreased fluorescence shown by the short record near 2400 Acm.-l scanned 30 minutes after the initial run. A similar spectrum of indene is shown in Figure 4. The frequency scale change between the two curves in each case was necessary because the prototype Model 81 does not have the same scales available as the production model. Natural Products. To illustrate the value of laser excitation for examining
II
b Figure 4. Improvement in fluorescence with red excitation for indene Upper curve: 4 3 5 8 A. excitation in 2 mm 0.d. borosilicate cell (0.6 ml. volume). Dashed line marks change o f zero suppression. Lower curve: 6 3 2 8 A. excitation b y 65 mw. laser; 30 p1. in 1 mm. 0.d. fused silica capillary
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I Figure 5. peaks)
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i CROSSED PBLARIZER
Laser spectrum of steroid hormone (SP = solvent Figure 6.
natural products of physiologic interest, Figure 5 shows the spectrum of androstan-3,17-dionea Five of the most intense peaks are due to the chloroform solvent, b u t a t least 15 additional lines can be seen. These were not detected because of the fluorescence background when the sample was excited with the Toronto arc a t 4358 A. Polarization. Polarization ratios do not reach theoretical values in the capillary cells, principally because the Raman radiation is depolarized on repeated reflection from the cell walls. Polarization ratios are amply high t o be useful, however, and the Rank (IO) technique affords correction factors by which the inteiisity ratios may be accurately determined when needed. Polarization was measured by using Polaroid film HK 38 in the Raman beam in parallel with the easy transmission direction of the monochromator and rotating the plane of polarization of the (well polarized) exciting beam from parallel to perpendicular to the Polaroid, by turning a half-wave plate in that
beam through 45' about the beam axis. A test of the polarization effects seen with the smallest diameter, 0.5 mm. i.d., cells, for which depolarization is most pronounced, is shown in Figure 6. The line a t 459 cm. is almost fully polarized (7), the others depolarized. The depolarized lines retain their relative intenwity, while the 459 cm.-l line shows a depolarization of 0.40 instead of ,038. Figure 7 shows the spectrum of a crystal of potassium biphthalate which replaced the capillary cell and was held directly against the flat face of the hemispheric lens. The crystal faces were not polished, and the crystal showed numerous flaws, so that it glowed brilliantly throughout with scattered excitin,o. energy. In adapting the laser accessory to the Cary Model 81 Raman Spectrophotometer, several changes are advantageously made in the basic instrument. The scan range is extended from 13,450 t u 11,100 em.-', to increase the laser shift range. In addition, more efficient coatings for the red are used on both mirrors and transmission optics, but ones which do
Polarization spectra for CC14
not preclude work a t shorter wavelengths, since future developments in ion lasers or other sources may make this attractive. And it is of great importance to employ a phototube compartment modified to accommodate the end window phototubes with 5-20 response. Their higher quantum efiiciency in the red is almost indispensable. To further improve red response, the multiple cathode reflection technique of Rambo (9) is employed, giving a factor of two in sensitivity a t 6328 A. and more a t longer wavelengths. Commercial apparatus to exploit these principles is available both as an attachment for installation on existing instruments and as new equipment. The orientation of its optical parts is shoivn in Figure 8, but not even approximaiely to scale. Particular pains were taken in the design of the accessory to make the alignment of the laser beam with the sample cell easy and reproducible. The prism which turns the beam downward can be turned to center the beam on the entrance face of the prism cemented to the FILTER 1532883
a
4 WAVE PLAT
Si\MPLE CELL
REF BEAM I
Figure 7. phthalate
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ANALYTICAL CHEMISTRY
Figure 8. Optical schematic sf complete laser excitatic^ accessory. Dashed line shows a large change in scale
lens. The cell can be centered on the beam emerging from the hemispherical lens, and also pivoted about that point to align the cell axis with the beam. The carriage for the upper prism is on horizontal ways, so that it can be displaced in the plane of the drawing to carry the beam to the left of the hemispherical lens, for 90" sample illumination. Provision has been made for more than one laser, with convenient and rapid beam interchange b y inserting another p r i m just above the half-wave plate. The second laser is mounted on an attachable frame extending in the opposite direction from the central case. It is possible that for some applications, the high power at 4880 A., presently a t
least 1 wat,t,and other advantages of the argon ion laser, will make it sufficiently attractive to justify its high cost and short life. ACKNOWLEDGMENT
We are indebted to R. N. Jones for the sample of androstan-3J7-dione. LITERATURE CITED
(1) Bloom, A. L., A p p l . Phys. Letters 2, 101 (1963). (2) Caw, H. H. (to Applied Physics
Corp.) U. S.Patent 2,940,355 (June 14, 1960). (3) Damen, T. C., Leite, R. C. C., Porto, S. P. S., Phys. Rev. Letters 14, 9 (1965). (4) Ham, N. S., Walsh, A,, Spectrochim. Acta 12, 88 (1958). ( 5 ) Hatchard, C. G., Parker, C. A.,
Proc. Roy. Soc. (London) Ser. A235,
518 (1956).
(6) King, F. T., Lippincott, E. R., J. Opt. SOC.Am. 46. 661 (1956). ( 7 ) Leite, R. C. C.,'Moore, R. S., Porto, S. P. S., Ripper, J. E., Phys. Rev. Letters
14, 7. iisfi.51 ,-___ ( 8 ) Parker, C.' A., Proc. Roy. SOC.(Lond o n ) Ser. A Z O , 104 (1953). (9) Rambo, B. E., Tech. Document Report No. AL TDR 64-19, April 1964, O.T.S.. DeDt. of Commerce. Washine-
ton 25; D.
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e.
(10) Rank, D. H., Kagarise, R. E., J . Opt. SOC.-4m. 40, 89 (1950). (11) Stammreich, H., Spectrochim. Acta
8, 41 (1966). RECEIVED for review November 15, 1965. Accepted September 19, 1966. This paper was presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy on March 4, 1965.
Quantitative UItravi o let Determina tio n of C,,-C,, Naphthalenes in Hydrocarbon Oils PATRICIA A. ESTEP, EDWARD E. CHILDERS, JOHN J. KOVACH, and CLARENCE KARR, Jr. Morgantown Coal Research Center, Bureau of Mines, lb A method is described for the quantitative ultraviolet determination of alkylnaphthalenesaccording to degree of substitution in neutral oils derived from low-temperature coal tars. The method utilizes elution chromatography on alumina with subsequent analysis of the fractions by ultraviolet spectrometry. The method is based on the emergence of naphthalenes from the column relatively free from other classes of compounds, the separation of naphthalenes as classes according to their degree of substitution, and a wavelength/structure correlation between the most intense band of naphthalenes and the degree of ring substitution. Using this method, amounts for the individual classes of Clo-Cls alkylnaphthalenes can be obtained, although none higher than c16 were found in coal tar neutral oils.
C
Of neutral Oils from low-temperature coal tars required the development of a quantitative determination of alkylnaphthalenes according to their degree of substitution. The near ultraviolet spectrum of naphthalene consists of three principle regions of absorption which, according to Clar's classification ( I S ) , are called the 0-.para-, and &and systems and are related to the three band systems of benzene. The principal bands of these systems are located at 220.7, 275.5, and 311.5 mp (log E 5.08, 3.77, and 2.40, respectively). For the quantitative analysis of C1o-Cll naphthaHARACTERIZBTIOS
U. S.
Department of the Interior, Morgantown, W. Vu.
lenes, the use of the a-bands has been reported by Neimark ( S I ) , Adams (@, and Coggeshall (15). An ASTM method (4) for total naphthalenes in jet fuels using the para bands has been adopted and the extent of interference of other aromatics pointed out. Snyder (34) has said that the hST3I procedure using the para bands for naphthalene determination is inapplicable in gasoline samples because of serious interference from monoaromatics. Mixtures of coal tar neutral oils analyzed in this laboratory (%?) also prohibit the use of the para bands because of possible interference from other aromatics. Therefore, the present paper describes a method developed for the analysis of ClO-Cl8 alkylnaphthalenes using the most intense p-band of naphthalenes, thereby improving sensitivity and increasing selectivity by reducing interference from other classes. EXPERIMENTAL
As part of a recently developed lowtemperature coal tar assay (LS),total naphthalenes were isolated from a highquality neutral oil using liquid chromatography on a gas chromatography analog. The detailed procedure for this chromatographic technique has been described (21, 12). For the separation, a weighed quantity close to 1 gram of the high-quality neutral oil was introduced to a 25-foot length of 3/8-in~htubing packed with 80- to 100-mesh F-20 alumina containing 4 weight-per cent water and prewetted with spectral grade cyclohexane. The charge was eluted with cyclohexane
under 75 p.s.i.g. nitrogen. Fractions of 14 ml. each were collected, and elution of naphthalenes was followed by the autoniatic recording of a chromatogram with an ultraviolet absorption monitoring device. Ultraviolet spectra were obtained on the cyclohexane solutions from tubes corresponding to the chromatogram peaks, using a PerkinElmer 350 ultraviolet spectrophotometer and matched quartz absorption cells. For the absorptivity data, pure Samples of naphthalene and all methyl and ethylnaphthalenes ClO-Cl2 were commercially available. Zone refined Samples from James Hinton, 358 Chicago Ave., Valparaiso, Fla., were used when possible and were found to be the highest purity available. Of the 14 possible isomers of trimethylnaphthalenes, the only commercially available samples were the 1,3,7-, the 2,3,5-, and the 2,3,6 - isomers. All samples were weighed on a microbalance for the absorptivity data. A study of the @-band for naphthalene and monomethylnaphthalenes showed that these classes behaved in accordance with Beer's la157 and it was assumed that higher homologs also obeyed this law in the concentrations used. KO solvent corrections were applied to the literature data, reported both in alcohol and hydrocarbon solvents, because the ultraviolet spectra of hydrocarbons are relatively unaffected by solvent POlarity. RESULTS A N D DISCUSSION
Elution Chromatographic Separation. Naphthalene and its alkyl derivatives emerged from the column VOL. 38, NQ. 13, DECEMBER 1966
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