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Chemical Instrumentation Edited by GALEN W. W I N G , Seton Hall University, So. Orange, N. J. 07079
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These articles are intended to serve the readers O ~ T H I JOURNAL S by calling attention to new developmenla in the theory, d&, or availability of chemical ladoratory instrumentation, or by presenting useful insights and explanations of topics that are of practical importftnce to those who use, or teach the use of, modem instrumenlation and instrumental techniques. The editor invites correspondence from prospective contributors.
XLVIII.
Raman Spectroscopy-Part
Two
Bernard J. Bulkin, Hunter College of the City University of New York, New York, N . Y. 1 0 0 2 7
...a feature
one has moved into the red region of the spectrum, e.g., excitation by He-Ne or KT, it is difficult to find a sensitive photomult,iplier tube. The overwhelming choice of Raman users has been for an end-on tube with extended S-20 response (trialkali photocathode). This response is shown in Figure 9, from which it can he seen that the sensitivity is best in the blue region, but is still better than other available tuhes well into the red reeion of the
majority of commercial instruments are the EM1 955RB and the I T T FW 130. The former was used in earlier instruments, the latter is the one commonlv ~died .. s u.. with most current Raman mectrometers.
DETECTION AND AMPLIFICATION The problems of detection and amplification of Raman signals, inherently very weak signals, are considerable. Most older Ramaii work was done with photographic detection, with exposures as long as 200 hours being common. Several advances have now obsoleted the photographic detection systems, and the inevitable increasing use of computer averaging techniques in Raman spectroscopy should do more in this direction. Almost all Raman spectrometers now use photoelectric detection with one of three forms of amplification: (1) simple direct current using a picoammeter; (2) lock-in amplifier using a reference signal developed by chopping of the laser beam a t some point; (3) photon counting with discrimination circuitry. To some extent, the choice of which
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detector system is built in to the total Raman system. Probably the SpectraPhysics Raman spectrometer will also have a built-in amplification system, hut this new instrument is still in a state of change and it, is not yet clear just what system that will be. In the CODERG, JarrellAsh, and Spex Raman spectrometers there is really no requirement of a particular amplification system although the brochures describing the CODERG Raman spectrometers usually discuss only DC amplification. The choice of phot,otube will depend on which amplification system and which laser lines will be used. Many uhoto-
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Figure 9. Sensitivity of certified I T ' FW-130 photomullipliers cornpored lo S.20 and S-1 i n d a r d responses. The dashed horizontal lines represent the range of 3 5 0 0 cm'"I Roman shift from tommon loser exciting lines. l c o w ~ e s ySpex Ind~tries.)
pearing on the market may be quite suitable for Raman work. Among these are the Bendix Channeltron tube, an RCA tube with a gallium phosphide dynode, and tuhes made by the Hamamatsu Company in Japan. The need for cooling of the photomultiplier tube t o reduce the dark current is still a topic of some discussion among Raman apectrocopists. With the I T T FW 130 photomultiplier tube, the very small (3-mm diameter) cathode results in a rather low dark current even a t room temperature. In the author's laboratory, for example, a tube of this type gives 63 electronslsec a t room temperature, which is a fairly low background. For most
weak bands are to be measured, e.g., dilute aqueous solutions, etc., it is desirable to further reduce the dark current to as low a value as possible. Th'rs need results from the fact that the dark current fluctuates statistically rather than being a simple, constant background. Tube cooling is fortunately not nearly so difficult a job as it once was. For the t,uhes in question, dark current can be reduced to a very low value (less than 5 counts/sec in n i r experience) by cooling t o -25'C. This can now be accomplished with thermoelectric coolers (Pettier devices), and some very elegant photomultiplier housings using the thermoelectric effect have been made by Products for Research Corp. Figure 10 shows the effect of cooling on dark current for an I T T FW 130 photomultiplier tube. It seems that these coolers, which are efficient and require little or no maintenance, are ideal for Raman work. Although a dry ice or liquid nitrogen cooler is slightly less expensive, it requires frequent attention and nrobablv involves large changes in tube temperature every day. The photomultiplier tubes described (Continued on page A8601
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Figure 10. Use of a thermoelectric refrigerator for l i n g 1 photomultiplier tube from room temperature to -25°C The dark-noise level (counts/sec) is plotted as a function of time ofter the thermoelectric coder has been turned on. (Courtesy Jorrell-Ash Div. of Fisher Scientific C0.l
here, such as the I T T FW 130 generally run under quite high voltages, in the 2000 roll range, although the actual operating voltage must be determined for each tube individually to give optimum signal-tonoim ratios. Considerations in this choice of voltage have been developed in some detail in a paper by Nakamura and Schwartz (4), which is recommended reading for any newcomersto this area. The optimum voltage to he used may change in the early days of tube use, as the characteristics seem t o change slightly during the first period of exposure to light. The photomultiplier tubes should he kept in the dark and under full voltage when not being used. Returning to the discussion of amplification systems, detailed comparison of the lock-in amplifier and photon counting ivstems has been made by Nakamura and ~ c h w a r t z(4). I t seems that from "theoretical considerations" photon counting is the better system for signals in the range of Raman lines. The main advantage of the lock-in system a t the moment appears to be the continuous monitoring of the laser intensity, and the consequent removal of any fluctuations in this intensity from the resulting spectrum. Both dc and photon counting do not provide for any such monitoring, although it could presumably he built into the system, and in the Jarrell-Ash system provision has been made for this. Although it has been argued in the literature that nhoton countins' should provide u p to a factor of three advantage over dc with respect t o signal-to-noise ratio for weak signals, it has been found experimentally by many workers that this is not true imtil one approaches very weak signals indeed. For most Raman hands comparable SIN is found by dc and photon counting. Further, photon counting is considerably more expensive than dc amplification, adding approximately $3100 to the cost of a Raman system.
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There are certain advantages, nevertheless, to a photon counting amplification system. These stem from the essentially digital nature of the photon counting system, as opposed to the analog dc current. First it is possible in photon counting to use discrimination circuitry to remove pulses of higher and lower energy than the Raman pulses. Low energy pulses result from electrons originating down the dynode train rather than a t the cathode. These produce smaller anode pulses which can be eliminated by discrimination in the amplifier stage of the photon counting system. High energy pulses generally provide little interference in Raman spectroscopy. The greatest advantage of photon counting comes in digital applications of the data. If the signal is sent from the pulse height analyzer to a digital ratemeter or counter, or in factdirectly to a computer which can accept pulses, automation of data collection is a simple matter. I n other amplification systems some type of analog to digital conversion system would he needed. Even if the spectrometer is not automated, data collection using a digital ratemeter can rapidly determine depolarization ratios by accumulating all pulses in a spectral region in both positions of the analyzer. Counting statistics then give information about the uncertainties in the values. The digital nature of the photon counting system, especially asmore and more computers arc available in laboratories, is certainly its biggest positive feature. For continuous recording of spectra most Raman spectrometers use an analog ratemeter which records counts/sec and sends this information to a recorder Princeton Applied Research Corp. seems to be the leading sourceof lock-in amplifier systems which are being used for Raman spectroscopy, aside from those amplifiers built in to the Cary and Perkin-Elmer spectrometers. D C picoammeters are available from several sources, although most inusenow come from either Vietoreen or Keithley. Photon counting equipment is again available from a very large number of manufacturers, principally those concerned with nuclear equipment and X-Ray detection and amplification. Many of the automation systems based on photon counting for X-Ray diffractometers appear to be applicable to Raman with little modification.
spectro~copy,because of their low scattering. I t appears that it is only practical to study gases with a He-Ne laser if the sample is placed inside the laser cavity or if photographic detection is used. With an Ar laser it should be possible to obtain good spectra of gases outside the laser cavity using multiple passing through the Raman cell. In general one would have to say that Raman is not a routine technique for study of gases a t the moment. Further developments may change this situation, however. The pioneering effort in sample illumination of liquids was made by the PerkinElmer Corp. with a multi-pass cell. This cell is still used by Perkin-Elmer and by Cary Instruments for illumination of liquid samples, although both have other types of liquid cells available as well. In the P-E cell the laser beam enters at one end of the cell, which is silvered and wedge shaped, and is passed down to the other end in a series of reflections. The Raman scattered light is observed a t 90' to the incident radiation. Subsequent to the development of the Perkin-Elmer cell, several workers showed that it is more advantageous to focus the laser beam to a diffraction limited point in a small sample, rather than try for multiple as sing through a larger cell. If the liquid is clear, the focused beam will pass t,hrough the liquid and can be reflected back again for another pass. Since the exit of a laser is itself a mirror, further reflection to the sample can be achieved. Thus a considerable gain in Raman intensity can be achieved beyond a single pass. This is now the preferred scheme for examination i f liouid samles. with some variations as
SAMPLE ILLUMINATION AND HANDLING As with any spectroscopic method, each user seems to develop his own battery of sampling techniques, depending on his own experience and the types of samples he encounters most frequently. In the Raman effect, it seems that the method of illumination of the sample with the laser beam and collection of scattered radiation may be as important as any of the other instrumental factors described thus far. I t is convenient to separate the sampling problems by phase, although in many laboratories the same or similar systems are now used for powdered solids and liquids. A further classification or suhheading under solids is that of single crystals. Gases are difficult to study by Raman
Figure 11. Transverse excitation-transverse viewing optical system for examining Ramm spectra of small quantities of sample. Light from the laser is focused on to the sample by 11, n d the scattered light from the sample is focused om the mono~hrom~torentrance slit, Sa, by lens I;. Mi collects the back scattered light. My allows for multiple passing, which in practice yields a gain of nearly 2 for clear liquid samples. (Courtesy Spex Industries.)
(Continued on page A862)
Chemical Instrumentation liter or even nanoliter range for liquids. Indeed, since the volume of the focused He-Ne laser beam is only about. S nl at the diffraction limited point,, increasing sample volume does not help. Capillary cells (even inexpensive melting point capillaries are fine) can be used to contain the samples. If the capillary is illuminated along its full length, the rounded bottom of the capillary serves to further focus the radiation. Alternatively, as suggested by Landon and Freeman (5)a transillumination syst,em (Figure 11) is possible. The Cary 81 spectrometer uses primarily coaxial viewing of the Haman radiation, that is, the Raman lines are observed a t 180' to the incident exciting radiat-ion. For small samples a capillary cell is fixed t o a hemispherical lens which collects the Raman light. The light is then focused on the entrance slit of t,he monochromator. If 90' viewing of the Raman light is desired in the Gary 81, the Perkin-Elmer multi-pass cell is used. This is necessary if accurate depolarization ratios are i o be measured, because the internal reflections of the Raman radiation from the walls of the capillary in the 180" viewing mode results in substantial depolarizat.ion. For powdered solids, it is best to use a small amount- of sample and focus the laser beam directly cm to the sample. Although good spectra can he achieved with either 180' or 90' viewing, to observe low-frequency lines it is necessary t o maximize the ratio of Raman to Rayleigh mattering, and this is best achieved by 90' viewing. Solid sample handling is considerably simpler for Raman spectroscopy than it is for infrared work, as a small amount of solid sample can he handled with no matrix such as KBr or Nujol. For both powdered solids and liquids it is worthwhile to have a mirror which takes the back scat,tered light into the monochromator. For solid single crystals it is a simple matter to install a standard goniometer head in the sample compartment. Considerable information about forces in crystals can be obtained from examination of the liaman spectrum of oriented single crystals, using the high degree of polarization of the laser exciting line. A considerable number of studies have appeared on ionic crystals, and it seems likely that an increasing number of spectra of oriented organic crystals will also be forthcoming. There are several other important features in a good Haman sample illuminator. First, for single crystal studies and for all depolarization ratio measurements it should be possible to easily turn the axis of polarization of the incident beam and to have an analyzing polarizer in the path of the liaman scattered light. The use of a half wave plate serves the first pnrpo.se, and a standard piece of Polaroid such as used in photography is suitable as an analyzer. Second, provision should he made for insertion of fillers, both spike filters to isolate the laser line from nonlasing emission lines, and neutral density filters to attenuate the laser beam if necessary. For some samples the high-energy beam will cause decomposition. Third,
if accurate depolarization ratios are to be obtained, a quartz wedge or other device for scrambling the polarizations must be placed between the analyzing polarizer and the gratings. This is because gratings respond differently to light polarized in different directions, and the relative efficiencies to the different polarizations is a function of wavelength. Perhaps the most important requirement for a sample illuminator which is to take full advantage of the microsampling capabilities of Raman spectroscopy is provision for positioning of the laser beam on the small sample and subsequent focusing of the image from this sample into the entrance slit of the monochromator Different manufacturers have handled this problem in different ways. In the Gary and Perkin-Elmer spectrometers, the sample is always held in the same position, exactly, and no provision is made for moving either sample or laser beam to improve imaging. Further, since the distance from the slit remains unchanged, no adjustment is made in this direction eit,heer In the Spex and Jarrell-Ash spectrometers this is not the case. The Spex sample illuminator (Fig. 12) keeps the distance between the sample and the entrance slit fixed, and a lens which focuses the Raman scattered light is usually not adjusted in the course of a measurement. To align laser beam and sample, the Spex illumina-
F u r 12. Schematic d i a g r a m of the Spex idustries No. 1430 Sample Illuminator. [ C o w s s y Spex Industries.)
tor has a floating microscope lens mounted just below the sample. This lens focuses the laser beam on the sample and adjustments of the lens in the x and g directions (laser beam proceeding along the z direction) allow the laser beam to be moved several millimeters in each direction. These adjustments can he made with the sample illuminator cover closed. The Jarrell-Ash illuminator is somewhat more complicated, but also is potentially more flexible. The sample is mounted on an optical bench, and a s two such benches are provided, one below and one inverted above the entrance slit, illumination can take place from either above or below the sample. Also, the distance between the sample and slit can be changed continuously, though it need not he, of course. For positioning of the sample with respect to the laser beam, Jarrell-Ash cells are mounted in spring clips on a goniometer (Continued on page A8641
Chemical Instrumentution head, and the goniometer is adjusted for optimum ilhimination of the sample. The condensing lens between sample and slit ismounted in a sort of floating arrangement., and i t is adjusted in all three directions for peak intensity. Presumably, for similar samples if no change in the position of the goniometer head is made, then little adjustment of the lens is needed. To perform all thcos adjustments, however, the sample compart,ment doors must he open. This holds a certain disadvantage, as with wide slits it will generally be necessary to darken the room to avoid the high background from room lights. The CODEliG spectrometers offer a wide variety of unique sampling cells for liquids and solids, including the only commercially available variable temperat,ure accessories for Raman work.
FUTURE TRENDS There are two leading: areas in which considerable effort is being made at the moment. These are t,he development of small Raman spectromet,ers for routine analytical laboratory use and t,he automation of new and exisi.ing instruments. Several companies are already developing small spectrometers. The new instnment of the Spectra-Physics Corp. has been discussed in this art-ide, and the Perkiii-Elmer spectromet,er has always been of the small type. I n order for such
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an instrument to be successful, however, it must be able to produce good spectra from a wide variety of samples, including what might be considered difficult samples, such as polymers, powders, colored solutions, etc. As each component of the larger Raman instruments has been optimized SO that spectra may be obtained from difficult samples, it. is not yet clear that it TI\\ he possible to produce an instrument a t substantially lower cost which still has the high performance needed. The problems of Raman instrumentation in this regard are more complex than those of infrared spectroscopy, for example. I t is not generally possible to trade off resolution for intensity by opening the slits, because the problems of high backgrounds may make it impossible to obtain spectra a t the wider slit widths. There are several design features of a smaller iristi'ument which would be of convenience to many workers, however. These include preprinted charts (using either a strip chart or X-Y recorder), easy interchange of samples, and compatibility with infrared charts. The latter feature has been incorporated in the PerkinElmer LIi-3, which features charts the same size as those from Perkin-Elmer 21 series infrared instruments. The automation of ltaman instrument is an important area in which the first advances are just now being made. I t is, in principle, rather easy to automate a Raman instmmeni, especially if photon
(Continued on page A868)
counting amplification is being used. This is a digital output which can be punched almost directly for data collection. To digitally record frequency information a stepping motor can be iised as the scanning motor, and the steps either counted from a predetermined reference point or absolutely defined by shaft encoder. The Spex Model 1401 monochromator and the Spectra-Physics instrument already have stepping motors built in as the scanning motors of the instrument. A SloSyn motor made by the Superior Electric Co. and a liesponsyn motor made by the United Shoe Co. are suitable for this application. The problems of collection of frequency and intensity information are thus not difficult ones for Raman. There remains the automation of the scanning of the spectrometer for such things as multiple scanning, solvent spectrum subtraction, radiometric corrections, depolarization ratios, and other signal averaging methods. At least part of this can be done with only slight modification of any one of a number of systems designed for automation of X-ray diffraction equipment. I n this application one wants to step through a small angle of rotation and punch position and counts. This is virtually the same requirement as that of Raman. Automation systems of this sort are currently being made by such companies as Canberra Industries, Digital Automation Co,, and Ortec, Inc. We have generally refrained from giving detailed prices of instruments in this article because they have been changing very rapidly (both up and down, strangely enough) and because there are often many component selections to be made by an individual user. Prices may fluctuate greatly even for the same model instrument. Many useful references on Raman spectroscopy can be found in the biennial review in Analytical Chemistry, and these generally cover theory, applications, and instrumentation. There also exists a Raman Newsletter, which is sent free to those who contribute to it reeularly. Further information can be obtained from Miss P. Wakeling, Raman Newsletter, 1613 Nineteenth Street, N.W., Washington, D. C. 20009.
REFERENCES (4) NAKAMURA, J . K.,
A N D SCHWARTZ, S. E., Appl. Optics, 7,1073 (1968). (5) LANDON, 11. 0. A N D FREEMAN, S. K., Anal. Chum., 41,398 (1969).
(Coming next month: "Continuous Flow Measurement of Beta Radiation Using Suspended Scintillators" by Dr. E. T . McGuinnes.~and Dr. Sf. C. Cullm of Seton Hall University.)
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