The chopper used, however, is a two-bladed quartz wheel which is mounted behind the entrance slit of the spectrometer and at an angle of approximately 40' to the normal of the light passing through the slit. When one of the quartz chopper blades intercepts the light, it displaces, by diffraction, all of the spectral lines to one side of their respective exit slits. During this time spectral background is being received by the photomultiplier tubes. As the chopper rotates one quarter revolution further the quartz blade passes by and the light from the entrance slit resumes its normal course with the spectral lines again passing through their respective exit slits. This system has the advantages that conventional exit slits may be used and the total time for integrating the spectral line signal is reduced only by a factor of two instead of by a factor of four as in the case with the offset slit system, This reduction of integrating time, however, is generally not of significance in analysis by dc arc because the intensity of the spectrum is usually substantially greater than is necessary for good phototube signal-to-noise ratio. The disadvantage of the quartz-chopper system is that the background for all the spectral lines of interest must be measured on the same side of the line and at about the same distance (depending, of course, somewhat on wavelength) from the line. This system is, therefore, applicable only to programs with comparatively simple spectra having no interfering spectral lines on the same side and lying within 200 p (for the instrument, quartz-chopper thickness and angle used) from the analytical lines of interest in the program. Offset Exit Slit Using Two Photomultiplier Tubes. Frequently an analytical program contains only one or two elements which are difficult to detect and require background correction, In this case the offset slit is used to advantage
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Figure 5. Background monitoring using offset exit slit and post-mounted mirror without the entrance slit chopper and switching system but utilizing two photomultiplier tubes in conjunction with a post-mounted mirror as shown in Figure 5 . The method used to calibrate the line and background photomultiplier tubes is identical to that described under the system using the chopper. The results observed are essentially the same as for the chopper system.
RECEIVEDfor review August 28, 1968. Accepted October 29, 1968.
Small Sample Handling in Laser Raman Spectrometry Stanley K. Freeman International Flavors & Fragrances, R & D Center, Union Beach, N . J. Donald 0. Landon Spex Industries, Znc., Metuchen, N . J.
SEVERAL REPORTS have appeared recently (1-3) pertaining to the generation of Raman spectra by laser excitation of small samples. Bailey, Kint, and Scherer (3) adopted the axial excitation/viewing geometry (Figure 1, left) of Porto ( 4 ) , and reported spectra from as little as 0.04 p1 of CCl4 in specially fabricated capillary cells. However, this system gives poor depolarization ratios for the 218, 314, and 459 A cm-1 bands. Dr. J. R. Scherer recently advised us that the system has been improved and it now yields good results on 0.04-pl sample quantities. Pez (5) obtained Raman spectra of liquids in 1.0-mm bore melting point capillaries by transverse irradiation/viewing (Figure 1, right). Unlike the tube with a single focused hemispherical lens of the type
(1) R. C . Hawes, K. P. George, D. C . Nelson, and R. Beckwith, ANAL.CHEM., 38, 1842 (1966). (2) A. Lau and J. H. Hertz, Spectrochem. Acta, 22, 1935 (1966). (3) G. F. Bailey, S. Kint, and J. R. Scherer, ANAL.CHEM., 39, 1040 (1967). (4) T. C. Daman, R. C . C. Leite, and S. P. S. Porto, Phys. Reu. Lett., 14, (I), 9 (1965). (5) G. Pez, McMaster University, private communication, 1968.
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employed by Bailey et al., the transverse viewing method allows multipassing of the laser beam. In the former method, a lens at each end of the capillary would be necessary to permit multipassing. To obtain spectra of smaller samples, we have extended Pez's technique to liquids in 0.1-mm bore tubing, as well as to the spectral examination of solids trapped from a gas chromatograph in 0.5-mm bore capillaries. A spectrum of 8 nl of CC14 appears in Figure 2. With this system, the measured depolarization ratios are excellent (pJls and 314: 0.75 + 0.01, p t s s a 0.006 ==I 0.002) Under the same conditions, 8 nl of linalool yielded a usable spectrum (Figure 3). (Note that only a ten-fold signal amplification is required to yield a curve similar to the one obtained on 800 nl of linalool in a 1-nim bore capillary. The intensity difference is due to a difference factor of ten between scattering paths of the two capillaries.) The different spectral backgrounds are attributed to greater fluorescence of the 0. I-mm capillary. As a result of the intense scattering of laser radiation from the walls and windows of capillary microcells, it is essential that a double monochromator be employed to fully realize the capabilities of the micro techniques.
Figure 1. Axial excitation (left);
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Figure 3.
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Upper curve: Cell volume, 800 nl in a 1.0-mm bore capillary; laser power, 75 mW; phototube voltage, 1750; phototube dc amplification, A full scale; spectral slit width, 5 cm-I; time constant, 3 sec; scan rate, 50 cm-l/min Lower curve: Cell volume, 8 nl in a 0.1-mm bore capillary. Instrumental conditions identical to above, but signal ampliA full scale) fication is increased one decade
This approach can be used equally well with either liquids or solids and is particularly suited to investigations of GLC fractions. A curve of about 0.1 mg of GLC-trapped benzoic acid is presented in Figure 4. Subsequent to isolation in an 0.5-mm capillary cell, the acid was concentrated to ca. 2-mm length by appropriate heating and cooling.
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Spectra of CCI,
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VOL. 41, NO. 2, FEBRUARY 1969
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Figure 4. Spectrum of 0.1-mg GLC trapped benzoic acid in a 0.5-mm bore capillary
(Instrumental conditions same as Figure 3)
EXPERIMENTAL
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A Spex laser-Raman system (Spex Industries, Inc., Metuchen, N. J.) was used for this ivvestigation consisting of a Spectra-Physics He-Ne 6328A laser, a 3/4-meter Czerny-Turnerodouble monochromator (1200 gr/mm gratings blazed at SOOOA), an ITT F W 130 photomultiplier tube, and a Spex ER-1 (picoammeter) electronic readout assembly. The combination of a multipassing arrangement and backscattering mirror housed in the sample compartment gave a n approximately fourfold increase in radiation entering the monochromator. Borosilicate capillary tubing (0.1- and 0.5-mm i.d.; 1.1mm 0.d.) was obtained from Wilmad Glass Co., Buena, N. J. Approximately 3-cm lengths were partially filled by capillary action and placed in a holder (Figure 5) which fitted into the spectrometer's sample compartment. Capillaries containing liquids boiling above cn. 100 "C did not require sealing. Prior to recording the spectrum of a sample in an 0.1-mm i.d. capillary, the laser beam was aligned by placing an 0.5-mm capillary containing linalool in the cell holder. The 1648cm-' band intensity was maximized and the capillary replaced by the 0.1-mm tubing. Because the bore generally is not centered in the capillary, the tube must be manually rotated while in the holder until it is visually in line with the beam. RECEIVED for review September 25, 1968. Accepted November 22,1968.
Figure 5. Capillary tube and holder for transverse excitation/viewing 400
ANALYTICAL CHEMISTRY
