Manfred J. D. Low Rutgers-The State University N e w Brunswick, New Jersey
Multiple-Scan Infrared Interference Spectroscopy
Chemists have long recognized the value of infrared spectroscopy for the characterization of materials, and have readily found uses for infrared techniques in practically all phases of basic and applied work. Yet, despite the increasing availability of excellent commercial spectrometers of high resolution and sensitivity, there are a variety of potentially valuable applications for which the otherwise spectacularly successful infrared techniques have not been particularly fruitful. These involve situations in which a conventional spectrometer is required to operate under energy-limited conditions as, for example, in measurements of the infrared spectral emission of solids at low temperatures or of spectral absorption in the far infrared. Conventional spectrometers have inherent features which limit their utility for operating under energystarved conditions. Detectors are not available which will discriminate between the individual components within polychromatic radiation, so that the radiation must he dispersed. This is accomplished by a monochromator using prisms, gratings, or their combination. The intensity of each individual radiation component is then measured. This constitutes what is essentially a radiometric measurement for each individual radiation component, and means that the time spent on each individual measurement is small. Also, in order to permit the dispersion of polychromatic radiation into individual components, the radiation beam entering the monochromator must be defined by narrow slits. This limits the amount of radiation entering the instrument and falling on the detector. Some of the limitations imposed on conventional dispersion spectrometers can be overcome by using a scanning interferometer. The advantages afforded by this are that (a) the narrow entrance slits required by dispersion instruments are not needed, so that the light-gathering capability of the interferometer spectrometer is much greater than that of a dispersion spectrometer of comparable size and resolution; and (b) all of the radiation components are incident on the detector simultaneously and are observed throughout the entire operation cycle. Each factor is beneficial, with the result that the signal-to-noise ratio of the interferometer is greater than that of dispersion instruments of comparable speed and resolution. The interference spectrometer is thus useful especially for energy-limited measurements. The principles of interferometry for the measurement of infrared spectra were recently considered in THIS JOURNAL by Hurley ( I ) , who outlined the history and the general theory of operation of the interference or Fourier spectrometer. The discussion of instrumenta-
tion and of chemical application was, however, restricted to measurements in the far infrared. This paper will consider measurements in the more familiar mid or "fingerprint" region of the infrared spectrum. As the purpose of the present paper is to complement the earlier article, the reader is referred to Hurley's paper and the references therein for an outline of the general theory. However, it is necessary to describe the instrumentation in some detail, because there are significant differences in construction, operation, and data reduction between mid and far infrared spectrometers from the practical point of view, although they are identical in principle. Instrumentation
A block diagram of a mnltiple-scan Michelson interference spectrometer is shown in Figure 1. The diagram and the description of the instrumentation are given in general terms, because it is possible to obtain a variety of components or complete instruments that cover different frequency ranges, although components linked by solid lines are packaged and available as the Block Engineering Model 200 spectrometer (8. !
SWEEP
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i SPECTRUM Figure 1.
Diagram of a multiple-man interference spectrometer.
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Components that may he used for data storage and data reduction are linked with broken lines. Although the complete multiple scan interference spectrometer will yield conventional infrared spectra, i.e., plots of intensity versus wavenumber, there is a fundamental difference in the mode of operation of interference and dispersion spectrometers. The detector of the dispersion spectrometer produces a simple electrical signal which is proportional to the intensity of the monochromatic radiation striking it (after the polychromatic radiation has been dispersed by the monochromator), whereas the output of the detector of the interference spectrometer is a complicated function of the intensity of polychromatic radiation and of time. A digital or analog data reduction must be performed in order to convert the complicated interferometer signal into a conventional spectrum. I n view of this, the operation of the interference spectrometer will be considered in a qualitative manner. Mathematical treatments are given elsewhere (see ref. (1) and references therein). Optical System
The optical system of the interferometer is not a monochromator, hut a device used to perform an optical transformation. Consider the passage of monochromatic radiation of wavelength X through the optical system. The radiation enters the system through a window W and is split int,o two components of equal intensity at the beam-splitter plate B. Plate C, made of the same material, is used to equalize the refractive loss of the reflected and transmitt,ed rays. The reflected ray strikes the stationary mirror M I , passes through plates C, B, and a focusing lens L, and impinges on the detector D. The transmitted ray is reflected a t the mirror M2, reflected at plate B, and passes through the lens L to the detector. If the lengths of the two light paths are identical or differ precisely by an even multiple of X/2 they will he in phase; but there will be destructive interference if the path difference is greater or smaller than X/2. The geometry of the system can he changed by moving mirror M 1by applying a voltage to the trausdncer T. The sweep and drive electronics are adjusted so that the mirror MS undergoes a precisely linear displacement of d mm/sec so that the light path of the transmitted ray is shortened at a constant rate. Under these conditions t,he two rays will be precisely in or out of phase at the detector every time the mirror M z has moved a distance h/4, and the detector will produce a sinusoidal signal of frequency d/(X/4) cps. If the magnitude of d is of the order of 0.1 mm secrl then, for incident infrared radiation, the detector output falls in the low audio frequency range. Note also that at constant mirror velocity d the detector frequency is a linear inverse function of the wavelength of the incident radiation. This means that the optical system causes a precise transformation of a signal from about 10" cps (the frequency of the entering infrared radiation) to the audio range (the detector output is in the range 1&3000 cps). If polychromatic radiation enters t,he optical system, then each radiation component undergoes such a transformation. The detector output corresponds to the sum of all interferences, and is a complex signal of 638
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frequencies in the audio range. Such a complex signal, termed an interferogram, results from one sweep of the mirror M2. The low-frequency interferogram must then be analyzed to extract the desired spectral information by obtaining the Fourier transform of the interferogram. Data Reduction
The computations involved in reducing the interferogram are so complex, lengthy, and tedious that manual computation is out of the question. However, digital computer reduction or analog analysis of the interferogram is possible. The interferogram can be recorded in a suitable fashion by means of magnetic tape, punched cards, or paper tape, and the Fourier transform he performed by a digital computer. This has the advantage that spectra can be corrected for background radiation, detector emission, and the like, and very precise results can be obtained. Digital computer techniques, however, involve the disadvantage of fairly great expense as well as the even greater disadvantage of the time delay between the experiment and the reception of the results. These can he avoided by using an analog data reduction technique. The interferogram can be analyzed by means of a slow scanning wave analyzer. The latter is, essentially, an audio-frequency spertrometer. The interferogram is rapidly and repetitively presented to t~hewave analyzer by means of a tape loop or, preferentially, by the Coadder (this component is described later). The wave analyzer then produces a signal which can be ronveniently recorded by an X-Y plotter or strip-chart recorder. A plot of intensity versus frequency results. As there is a linear relation between the frequencies of the infrared radiation and of the audio signal produced, that plot is an intensity-wavenumber presentation, i.e., a conventional spectrum. This analog procedure is precise enough for almost all applications, is inex~ensive,and has the advantage that data reduction and readout is rapid (using the wave analyzer built into the model 200 system, a spectrum is drawn out by an X-P plotter in about 40 see). Filtering
The detector output is an audio-frequency signal, and consequently can be amplified easily with conventional audio-frequency equipment. An electronic band-pass filter can then be used to eliminate undesirable frequencies prior to data reduction, in a manner entirely analogous to the use of an optical filter. This is described in detail elsewhere (S,4). Multiple Scanning
The transducer is normally fed a saw-tooth signal having a selectable frequency of 4,2, or 1 cps. As each sweep of the mirror Ms brings about one interferogram, a continuous sequence of interferograms is produced. The interferograms may he recorded on magnetic tape (any stereo tape recorder of reasonable quality is suitable for this), or be coherently added in the core memory of a time-averaging computer (Coadder) built into the Rlodel 200 system. As shown in Figure 1, the tape recorded interferograms may be processed for
digital computing or be transferred to the Coadder. r o t e that the tape recorded interferograms retain their identity, so that an individual interferogram or a sequence of interferograms can he transferred to the Coadder. A very flexible data handling system is thus available. If the source which is examined is relatively strong, it is possible to use single interferograms to produce infrared spectra. The interferogram can be recorded on magnetic tape and then be transferred to the Coadder memory, or be stored directly in the latter. This makes it possible to obtain an infrared spectrum over a range of 4-40 in 1 sec, for example. Also, as a sequence of interferograms can be recorded on tape, it is possible to examine a fluctuating source or signal in 1-see intervals, e.g., an infrared analysis of concentration changes in the effluent stream of a reactor could be monitored in 1-sec intervals. Some examples of this are given later. If the source which is examined is stable but very weak, the signal-to-noise ratio of a spectrum produced from a single interferogram is frequently so low that the spectrum is useless. Multiple scanning is then used to increase the signal-to-noise ratio. Successive interferograms are fed to the Coadder computer. An interferogram is digitized and stored in the 1024 channels of the computer core memory, successive interferograms being added coherently. A composite interferogram is thus built up in the core memory. However, noise has random characteristics, i.e., positive and negat,ive fluctuations at random. When noise is scanned, its amplitude is a function of the square of the number of scans. The information signal, conversely, is nonrandom and either positive or negative, and is scanned linearly as a direct function of the number of scans. The signal-to-noise ratio thus increases as a function of the square root of the number of scans. This increase further augments the previously-mentioned beneficial effects due to the absence of slits and the relatively long measuring time, so that a very sensitive spectrometer results. This makes it possible to make spectral measurements of sources that are so weak that such measurements using dispersion spectrometers are impossible or, at best, are extremely difficult. The effects of multiple scanning are shown in Figure 2, which also serves to demonstrate the sensitivity of the instrumentation. Each spectrum shown is the relative spectral emission of a sample of the mineral apatite at 20°C (6, 6). The number next to each spectrum indicates the number of interferograms that were added and then reduced to yield the spectrum. The total intensity as well as the signal-to-noise ratio is seen to increase with increasing number of scans, the emission bands (pointing up) becoming not only larger but better defined.
Figure 2. The effects of multiple scanning. The numbers next to e m h ~ ~ e c t r u indicate m the number of interferogroms added to produce the IpeClrYm.
operation, e.g., absorption lines superimposed on a slanting background. The Coadder, however, can function in a "subtract" as well as an "add" mode. This permits the correction of spectra, so that a doublebeam mode of operation is simulated. This important effect is illustrated by Figure 3.
Coadder
The prime function of the Coadder is to act as a coherent addition and data-storage device, ae described above. In addition to this important function, which helps to bring seemingly "impossible" measurements within the realm of feasibility, the Coadder memory can be used for correctional procedures. The interference spectrometer functions, essentially, as a single-beam spectrometer, and consequently sufiers from all of t,he disadvantages inherent in single-beam
Figure 3. Simulated double-beam operation. A: 3 0 0 scan. of radiation through cell containing the vaporized sample, using 0 ceromic heater source. 8: 3 0 0 scans of background, i.e., radiation passing through the empty cell, were subhacted from spectrum A.
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A sample of 2 p1 of methyl ethyl ketone eluted from a gas chromatograph column was trapped within a small, tubular cell. A total of 300 scans were taken, using a ceramic heater as source. Spectrum A shows the results obtained. The absorption bands of the gaseous methyl ethyl ketone are superimposed on the emission envelope of the source. A total of 300 interferograms of "background," i.e., radiation from the same source passing through the empty cell, were then subtracted from the previously added and stored cumulative "sample" interferogram. The resulting spectrum B is now similar to one obtained by double beam operation. The background was deleted so that the apparent distortion of relative band intensities was avoided, and the absorption spectrum shows clean and well-defined structure. Instrument Characferistics
to trade off resolution for signal-to-noise ratio by opening the slits. This means that, for normal usage, the gap between the resolutions of interferometers and dispersion instruments is narrower than it appears a t first glance. Costs
A multiplescanning interferometer is rather complicated, and its cost relatively high. A complete iMode1 200 system or its equivalent in any spectral range is priced at approximately $27,500. Auxilliary equipment such as tape recorders, filters, and oscilloscope used to monitor the electronics, must also be considered. However, if a digital computer is available, then only the interferometer proper (approx. $10,000) is required, initially a t least, and the considerable cost of the Coadder computer is bypassed. This, however, has the disadvantage that the rapid readout made possible by the complete instrument package is not available.
It is useful to consider some instrument characterInfrared Spectral Emission istics, although a complete cataloguing is neither desirable nor feasible. The optical system and the p r e Infrared emission spectroscopy has been a neglected area of study and is generally not mentioned in text amplifier are mounted in a box (6 X 8 X 2 in., approx. books or monographs dealing with infrared spectral 4 lb), connection to the electronics being made by means of a flexible 10-ft cable, so that the optics can methods and techniques. Little work has been rebe readily incorporated into a variety of experimental ported in the literature, the emphasis being placed setups. Radiation enters the interferometer through predominantly on absorption-transmission methods. a 30-mm aperture covered with an infrared-transThe reason for the neglect seems to rest in the fact that mitting window, although the 15 mm diameter mirrors spectral emission measurements are quite difficult to define the true aperture. carry out with conventional instruments because the The spectral range that can be covered by any one emission of materials near room temperature is very interferometer is defined by the absorption of the low. It has been shown that samples at temperatures optics through which radiation passes, the efficiency of of 100" or so could be examined (7),but the utility of the emission techniques is limited by the sensitivity of the beam splitter, and the detector. The composition of the optics, beam splitter coating, and the nature of commercial dispersion instruments. This limitation the detect,or determine the frequency range covered. can be overcome to a great extent by interference specRanges from 1-3,l-6,3-15, and 4-40 p can be covered. trometers, so that an alternate approach to the measurement of the infrared spectral characteristics of materials Scan times of '/&, '/%, and 1 sec are available. The is available. Some examples are given in Figures 4-6. number of scans that can be added is indefinite: enough The characteristic infrared spectral emission of scans are made until a signal of acceptable signal-tonoise ratio results. Several thousand scans may be inorganic or organic materials can be obtained relatively easily (3-9). Emission spectra of some minerals and necessary when examining the spectral infrared emisrocks at 20° are given in Figure 4, each spectrum, sion of a trace impurity on a surface, while only a hundred or ten scans may be needed to obtain an absorpcorrected for background, resulting from 300 scans (6). tion spectrum of several micrograms of a substance Examples of emission spectra of organic substances using transmission techniques. are shown in Figure 5 (9). Other results are given The spectral resolution obtainable at present varies elsewhere (3-16). from approx. 8&5 em-', depending on the snectral ranee covered. With a Mode1 200 system covering the range 4-40 p, it is 15-18 em-'. This is, of course, poor in comparison to the resolution obtainable by conventional double-beam dispersion spectrophotometers, zg and constitutes a serious disadvantage because g some work requiring closely-spaced bands to be resolved cannot be carried out. However, , the relatively poor resolution is more of a S psychological hazard than a practical deter0 . rent. Although modern double-scan grating w instruments are capable of 0.2-0.3 em-' reso- 9 lution, their full capabilities are rarely used. I n most cases the stress placed on resolution 2! is, in reality, generated by the more prevalent and pressing requirement of greater sensitivk5c itv: a dis~ersioninstrument of high resolvine power is desirable because it is then possible Figure A. Emission spectra of minera!, rockr.
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The emision spectra were Figure 5. Infrared emission spectra at 30'. measured with o Block Model 2 0 0 interference spectrometer covering the range 3-14 p, of o resolution of 3 0 cm~'(emirrionbands point up). The top tracer are the absorption spectra recorded with o Perkin-Elmer Model 5 2 1 rpestrophotometer (obrorption bonds point down). A: butyl rtewate, 1 6 0 sconr. B: ahopine, 3 0 0 rconr. 191
Emission spectra of a large variety of pure materials and of mixtures could be obtained easily, comparison of emission and absorption spectra showing good correspondence between emission and absorption hands. The sample preparation for emission work was minimal, in distinct contrast to that for absorption-transmission methods, particularly with solid samples. As numerous sampling difficulties such as mulling, solvent effects, changes in bands with the state of aggregation, and the like, can be avoided by the emission method, the latter should prove to be a valuable tool for the infrared examination of a wide variety of materials. Also, the techniques should prove to be valuable for microspectrophotometry. Exploratory results have shown the feasibility of obtaining emission spectra of microgram quantitiesof pesticides (10,11). The emission techniques should also prove to be of great value for the study of surface reactions (12, 13). An example is given in Figure 6. When a thin layer of tricresyl phosphate was applied to a steel surface, the emission bands of the compound were superimposed on the emission of the metal, so that an emission spectrum was obtained. The absence of new bands snggests that there was no significant reaction between the tricresyl phosphate and the steel surface at 40'. Some changes were observed after the coated plate had been heated to 400°, however. The P-0-C bands near 1190 and 1242 cm-I hand disappeared, the spectrum showing remnants of the 1300 cm-' band and of the 1500-1600 cm-' bands due to aromatic rings. This suggested that cleavage of the P-0-C linkage occurred in the tricresyl phosphate that remained on the surface and did not evaporate, some products containing P-0 groups and aromatic systems remaining bound to the steel surface after the 400' heating (13). The feasibility of measuring low-temperature emission spectra of surfaces was shown by these and other experiments during which the reaction of oleic acid with metal surfaces was recorded, as well as by other studies (12-1 4). The techniques should he particularly valuable for the study of the surfaces of bulk, opaque materials, e.g., boundary lubrication, adhesion, adsorption, corrosion, oxidation, and heterogeneous catalysis.
The emission spectra rorultod Figure 6. Emission of TCP on steel a t 40'. from 1 2 0 sconr, 0.5 rec each, obtained a t 3 0 em-' resolution using a Block Model 2 0 0 spectrometer. A: emirrion spectra of polished skol wrfoce. 8: emirrion spectrum of tricreryl phosphate-cooled steel. C: obtorption rpectro of tricreryl phosphate recorded with a Perkin-Elmer Model 5 2 1 ipectrophotometer.
Infrared Absorption Spectra
The multiple-scan interference spectrometers are also useful for measurements involving absorption-transmission techniques, especially for cases where speed is required or low relative or total signal intensities are encountered. One application involving both speed and low relative signal is the characterization of the effluent of a vapor phase chromatograph (16, 17). For example, a tubular infrared cell was connected to the end of a column so that the effluent passed through the cell. A 1-p1 sample of acetone was eluted and, as the sample swept through the cell, an interferogram was recorded on magnetic tape every second. Some of the resulting spectra are given in Figure 7. Figure 8 is a summary of such results (16). It was found possible to monitor the effluent stream continuously, recording interfemgrams on tape, or to trap samples within the infrared cell and perform multiple-scan addition. Samples as small as 0.00.5 p1 could be examined by multiple-scanning. Single-scan, 1-see spectra could he obtained with samples larger than 0.2 pl. Such experiments showed the feasibility of analyzing effluent gas streams of all kinds (17). The method should he readily adaptable for any case where a gas or gas stream can be analyzed by infrared, and be particularly valuable in a dynamic situation. Possible applications, the study of some of which is proceeding at preseut, include the analysis of gaseous decomposition products from differential thermal or gravimetric analysis; the analysis of the product stream of a hetemgeneously catalyzed reaction; the analysis of trace Volume 43, Number 12, December 1966
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impurities in gas streams; reaction rate studies; and monitoring the exhaust gases from internal combustion engines and impurities in air pollution studies. A second area of application involves the measurement of the spectral transmittance of optically dense materials. Unlike the first example, which dealt with the measurement of a very small signal (the absorption produced by very small amounts of gas) in the presence of a very large signal (the source radiation), the examination of a sample of very low transparency requires the detection of and discrimination between two very small signals. The transmission of paper is considered as an example. Spectrum A of Figure 9 is the transmission spectrum of one sheet of paper, and shows structure typical of a
CHROMATOGRAM 0
:b
ONE-SECOND SPECTRA
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0 C
I
u, W
I
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Figure 8.
Variation of ocetone concentration. The d i d line giver the gencrol shape of the eluted I PIacetone sample ar shown by the recorder of the gas chromatograph. The broken line rhowr the change in sonsentration of ocetone within the infrared cell. The plot war constructed from such as those of Fig. 7, although not all of the numerous data were used 1161.
Figure 7.
One-second absorption spectra. Spectra were obtained with Block Model 200 ~pestrometer01 o resolution of 1 8 cm.? Interfero&oms obtained a t a rate of one per second were recorded on mognetic tope. The number next to each spectrum gives the time in seconds after the eluted of 1 PI of acetone had begun to enter the infrared cell. The spectra are corrected for bockgmund. The changes in band intenritier d e c t the in concentration occurring within the cell (161.
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cellulose spectrum. Spectrum B of the transmission of two sheets of paper is similar to spectrum A above 1400 em-', but shows a higher transmission below 1400 cm-'. However, the paradox of a thicker and consequently more absorbing sample having a higher transmission than a thinner sample is easily resolved. The thick sample absorbs the incident radiation, is heated, and consequently emits more radiation than the thin sample. The recorded spectrum is therefore distorted, the region of apparently high transmission being an artifact. If the self-emission is suhstracted, spectrum C results. The latter records only the spectral trausmission and shows the thick sample to have a low but measurable transmittance of the order of 0.1% below about 1400 cm-'. Similar measurements of physically thick, highlyscatterina. -, or thin hut hiehlv - " absorbina,-, samdes have been made, e.g., adsorbents and catalyst supports such as finely-divided silicas and aluminas (14). The spectra must be corrected for self-emission. Such measurements and the emission corrections are very difficult to carry out and are generally not attempted with conventional dispersion instruments if the sample transmission is 1 4 . 5 % or lower. Yet, such measurements can be made relatively easily with the multiple-scan interference spectrometer. Interferometry Versus Conventional Spectroscopy
The advantages as well as some of the difficulties of the interferometric method have been briefly outlined
ticated servo loop, the multiple scanning and data reduction requirements of the interference spectrometer are more complex-and more expensive. Also, in addition to technical matters, there is a completely human failing: the interference spectrometers are thoroughly unconventional and, to the chemists weaned on dispersion instruments, may give the impression of being strange and formidable. However, their disadvantages are more than outweighed by their performance. Interferometry offers a valuable approach to d i c u l t infrared spectral measurements. This is shown by the various applications in the fingerprint region and in the far infrared (see the literature cited in Hurley's article). As such areas of application are of great interest to chemists, it seems that multiple-scan interference spectroscopy should become a popular tool. Acknowledgment
Partial support through grants AF'00211-01A3 from the Division of Air Pollution and 5 501 ES00088-02 from the Division of Toxicology, Bureau of State Services, Public Health Service, is gratefully acknowledged.
Figure.9. Tranrmisrion spectra of opaque samples. Absorption-trmr mission spectra of paper. A: 1 sheet, 300 scans. 8: 2 sheets, uncorrected, 400 scans. C: the self-emission of the poper heated by the globar source war subtracted from spectrum 8. The ordinates ore orbihory and displaced.
above and elsewhere (1). The following detractions must also be noted. The optical system of an interferometer is simple, but it must always be correctly and precisely adjusted. Unlike the dispersion spectrometer, which will yield perhaps poor but still usable spectra if the optics are defective or the instrument is improperly used or mistreated, the mix-aligned interference spectrometer yields nothing. The performance of the interference spectrometer is also somewhat more dependent on the quality and performance of the electronic components than the dispersion instrument. Although the modern optical-null spectrophotometer incorporates a quite sophis-
Literature Cited (1) HURLEY, W. J., THIS JOURNAL, 43,236(1966). (2) Block Engineering Co., 19 Blackstone St., Cambridge, Mass. (3) Low, M. J. D., AND COLEMAN, I., Spect~ochimieaA d a , 22, 22, 369(1966). (4) Low, M. J. D., AND COLEMAN, I., Advances in Applied Spectroscopy, Vol. 5, in press. I., Appl.Oplics, 5,14530966). (5) Low, M. J. D., ANDCOLEMAN, I., PTOC. Symp. Electro(6) Low, M. J. D., AND COLEMAN, magnetic Sensing of the Earth from Satellites, in press. (7) Low, M. J. D., AND INOUE,H., Anal. Chem., 36, 2397 (1964); Can. J . Chem., 43,2047(1965). ( 8 ) Low, M. J. D., Nature, 208, 1089(1965). I., Chem. (9) Low, M. J. D., ARUMS, L., AND COLEMAN, Communiealim, 1965, 389. (10) COLEMAN, I., AND LOW,M. J. D., Speclrochim. A d a , 22, 1293(1966). (11) COLEMAN, I., AND Low, M. J. D., Advanees i n Applied Spectroscopy, in press. Low, M. J. D., Nalu~wissmehaften,52,257(1965). Low, M . J. D., J . Catalysis, 4, 719(1965). Low, M. J. D., unpublished results. Low, M. J. D., Ezperienlia, 22,262(1966). Low, M . J. D., Chem. Commtmicalions, 371(1966). S. K., Anal. Chem, in press. Low, M . J. D., a m FREEMAN,
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