Instrumentation
ATOMIC SPECTROCHEMICAL MEASUREMENTS
WITH A FOURIER TRANSFORM SPECTROMETER Gary Horlick and W. K. Yuen Department of Chemistry University of Alberta Edmonton, Alta., Canada T6G 2E1
The development of effective simultaneous multielement analyses based on atomic spectrochemical methods is one of the major goals of a number of academic, government, and industrial laboratories. A key aspect of this development is the design of spectrochemical measurement systems capable of simultaneously measuring spectral information over a wide range of wavelengths. To date, the dominant technique for the measurement of spectra is the dispersive system based on the diffraction grating. The detection of the spectral information in a dispersive system has been accomplished in a number of ways. The two main ways are with a photographic plate or with a combination of an exit slit and a photomultiplier tube. The photographic plate, although capable of recording thousands of lines in a single exposure as a permanent record, has a nonlinear response, limited dynamic range, and tedious readout. Thus, even though the exit slit-PM tube combination is limited to the measurement of one spectral line at a time, its wide linear dynamic range, sensitivity, and the fact that it transduces light intensity directly to an electronic signal have made it the detection system of choice for the ma-
jority of spectrochemical measurements. However, it is difficult to design truly effective and versatile dispersive P M tube-based systems for simultaneous spectrochemical measurements. The classic direct reader, in which an exit slit-PM tube is mounted at each point in the exit focal plane of a dispersive instrument at which measurements are to be made, is a reasonably powerful multichannel measurement system. But even at best, only a very small fraction of the spectral information available in the spectrum can be measured. This limitation is now being overcome by the utilization of electronic image sensors as detectors (1 ) which over a moderate wavelength range provide a continuous multichannel measurement system. Such devices, which include silicon vidicons (2-5), secondary electron conduction (SEC) vidicons (6), and silicon photodiode arrays (7), have recently been applied to a variety of multichannel spectrochemical measurements. In addition to these multichannel systems, rapid scanning sequential systems have also proved useful for multielement analysis. Both a mechanical slew scan system based on a programmable monochromator (8)
and what amounts to an electronic slew scan system based on an image dissector photomultiplier tube (9, 10) have been developed. Another potential approach to the overall problem is to dispense with the dispersive system completely and use a multiplex technique. In a multiplex technique the spectral information is encoded so that a single detector can be used to simultaneously measure a wide wavelength range. The most common examples of multiplex techniques are Fourier transform spectroscopy and Hadamard transform spectroscopy (11). In Hadamard transform spectroscopy, the spectral information is encoded in a binary code based on Hadamard matrices. In Fourier transform spectroscopy, the spectral information is encoded in sine and cosine oscillations. This encoding is accomplished in the optical region of the electromagnetic spectrum by a Michelson interferometer. The encoded signal which is measured with a single detector is called an interferogram, and it is necessary to take the Fourier transform of the interferogram to decode it and obtain the desired spectrum. Fourier transform spectroscopy has been extensively used for spectral
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975 · 775 A
measurements in the infrared regions, but so far it has found little application for atomic spectrochemical measurements in the UV-VIS spectral regions. One potential drawback to this application is that spectral measurements in the UV-VIS regions are normally signal noise limited, and in this type of situation the so-called multiplex or Fellgett's advantage of Fourier transform spectroscopy may not be realized. However, it is clear from the infrared applications of Fourier transform spectroscopy that additional important practical advantages result from the utilization of Fourier transform techniques and instrumentation. Among these are: spectra can be measured with a very accurate and precise wave number axis which is predetermined (no calibration necessary), high resolution can be achieved in a relatively compact system, the resolution function is easily controlled and manipulated as an inherent step in data reduction by use of apodization techniques, and computerization of the spectrometer is facilitated. In addition, with proper utilization of aliasing, a Fourier transform spectrometer can be very versatile in simultaneously covering a wide range of wavelengths. This is one aspect that can be a major limitation of array-based multichannel measurement systems, such as vidicons and photodiode arrays, as a consequence of the finite length and detector element density limitations of these devices. In this article some considerations for using Fourier transform spectroscopy in the UV-VIS regions will be presented along with examples of atomic spectrochemical measurements made with a Fourier transform spectrometer. Multiplex Advantage? Much of the impetus for the development of infrared Fourier transform spectroscopy came from the promise of achieving the multiplex (Fellgett's) advantage. In contrast to a scanning spectrometer, a Fourier transform spectrometer using a Michelson interferometer may achieve, in the same measurement time, a signal-to-noise ratio that is superior by a factor of (M/2) 1/2 , where M is the number of resolution elements being observed (11). The advantage is realized only in measurement situations that are detector noise limited, i.e., the increased light level falling on the detector must not increase the overall limiting noise of the system. Most UV-VIS spectral measurements utilize the photomultiplier tube detector and thus are signal noise limited, with the noise level increasing as the square root of the signal level, thereby eliminating any potential multiplex advantage. However,
the situation is not quite this clear cut. Both the nature of the spectrum and the distribution of the noise in the spectrum can affect the existence of a multiplex advantage in signal (photon) noise-limited measurement situations. As early as 1959, Kahn (12) discussed the signal-to-noise ratio of Fourier transform spectroscopy in the photon (shot) noise limit. He concluded that "the interferometer is better only in those parts of the spectrum where the intensity of the radiation is more than twice the average intensity in the range of frequencies admitted to the instrument." This result essentially means that a multiplex advantage will only be realized for simple spectra consisting of only a few lines (such as atomic spectra) and also only if little or no broadband background radiation is present. Essentially the same conclusion has recently been reached for Hadamard transform spectroscopy in a photon noise-limited situation (13). Filler (14) has also discussed photon-noise-limited Fourier spectroscopy. Again, it was concluded that a multiplex advantage will be present only for line emission spectra. Another factor that must be considered in this discussion is the possibility of a so-called multiplex disadvantage. This arises primarily because the distribution of photon noise in a spectrum measured with a scanning instrument and one measured with a Fourier transform spectrometer is different. In the scanning case, the rms noise level is greatest where the signal is greatest (i.e., at the top of the spectral peaks). In the Fourier case, the noise tends to be spread out more or less uniformly throughout the entire spectrum. Thus, the signal-to-noise ratio of strong peaks should improve, but weak spectral lines may be obscured by the noise from strong lines that ends up distributed along the baseline of the spectrum. This problem is also present in Hadamard transform spectroscopy (13), and some experimental verification of this multiplex disadvantage has already been reported (15). Thus, at this time no definitive conclusions can be reached about the existence and importance of multiplex advantages and/or disadvantages in photon noise-limited multiplex spectrochemical measurements. More experimental work is necessary to clear up the situation. What the above discussion does mean, however, is that the promise of any substantial multiplex advantage cannot be a driving force in extending Fourier transform techniques into the UV-VIS region; in fact, some disadvantages may exist with respect to signal-to-noise ratio in certain measurement situations.
776 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Additional Considerations Signal-to-noise ratio is not always the only and overriding consideration when carrying out a spectrochemical measurement. As mentioned in the introduction, it is clear from the impressive success and capabilities of the Fourier transform technique in the infrared that some important advantages result from the nature of the instrumentation used to implement Fourier transform spectroscopy. These advantages are not dependent on the existence or realization of the multiplex advantage. The digitization of the interferogram in a Fourier transform spectrometer is normally controlled with a clock signal derived from a He-Ne reference laser. This provides a final spectrum which has a very accurate wave number (frequency) axis. In addition, the values along the wave number axis can be easily calculated from the wavelength of the laser line; thus, no external calibration of the spectrometer is necessary. This inherent accuracy of the wave number axis greatly facilitates intercomparison of digitized spectra for small spectral shifts and peak shape perturbations. In fact, a strong case can be made for the statement that a Fourier transform spectrometer is the best system for the measurement of digitized spectra. Perhaps the main instrumental contenders to this statement are dispersive-based systems utilizing electronic image sensors as the detection system, particularly échelle grating spectrometers coupled to area array electronic image sensors (6). A Fourier transform spectrometer is capable of very high resolution while maintaining relatively high throughput and with a relatively compact system. A commercial interferometer (EOCOM Corp., 19722 Jamboree Blvd., Irvine, Calif. 92664) which can easily sit on a tabletop has an optical retardation of 16 cm (mirror movement of 8 cm) which provides a nominal resolution of 0.0625 c m - 1 . This translates to 0.00225 nm at 600.0 nm and 0.00056 nm at 300.0 nm. Also, the form of the resolution function is easily manipulated as an inherent step in data reduction (16). Considered in the context of capability, both the cost and basic simplicity of Fourier transform spectroscopy can be considered advantages. Certainly, the present cost of commercial Fourier transform spectrometers does not back up this statement, but without question, effective systems can be made considerably less expensively. A Fourier transform spectrometer constructed in our laboratory had a component cost (optical, electronic, and mechanical but not including machin-
ing costs and computer) of about $2500. With respect to simplicity, the in terferometer has only one moving part, the interferometer mirror. A sim ple air bearing, which costs about $30, provides an excellent moving mirror support, and most commercial mirror drive systems are based on this device. In another context the simplicity ad vantage was stated by Fellgett several years ago (17): It appears probable, moreover, that an outstanding advantage will eventually prove to be that pro posed by Mertz, namely simplicity. / / this assertion appears at first sight surprising, let us imagine that we have been doing interferometric multiplex spectrometry all our lives, and someone were to come along and propose the grating spec trometer for the first time. We should find that there were slits to cut off most of the light, only one element of the spectrum could be observed at one time, thousands of lines would have to be laid down on the grating with sub-wavelength accuracy, the resolution function would depend on the figure and phase shifts of the optical surfaces. Then there would be questions of overlapping orders, grating aberra tions, blaze, etc. Would it really be surprising if such a proposal were rejected as too complicated? Finally, with proper utilization of aliasing, a Fourier transform spec trometer can be very versatile in si multaneously covering a wide range of wavelengths while maintaining rela tively high resolution. This aliasing advantage will be considered in some detail in the next section. As a result of considering these ad ditional points, we have utilized a re petitive-scanning laser-referenced Fourier transform spectrometer devel oped in our laboratory to carry out some preliminary atomic spectrochemical measurements in the visible region. A photograph of the interfer ometer is shown in Figure 1. The in terferometer has a He—Ne reference laser to control digitization, a white light source to control time averaging, and an air bearing—speaker coil mirror drive system. The moving mirror ve locity for these studies was about 2 mm/sec which results in a modulation frequency for the 632.8-nm He-Ne laser line of about 6000 Hz. A paper is in preparation that will describe the complete interferometer system. The computer system used to acquire the interferogram signals and transform them is a PDP 8/e with an OS/8 oper ating system (18).
Figure 1. Photograph of Fourier transform spectrometer
Aliasing Question
One of the first major points to con sider in utilizing Fourier transform spectroscopy in the UV-VIS region is the sampling rate (19). With the stan dard He-Ne reference laser, the basic sampling interval for the interferome ter system is 0.6328 μπι (one cycle of the laser line modulation). This means that the shortest wavelength of light that can be properly sampled without aliasing is 1.266 Mm (7901 cm" 1 ). Clearly then, to work in the UV-VIS region, either the sampling rate must be increased or aliasing must be toler ated. The frequency of the reference laser modulation can be increased by use of optical techniques such as dou ble passage of the laser through the in terferometer or by electronic frequen cy multiplication using phase-locked loops (20). However, the high sam pling rate necessary to sample the
UV-VIS modulation frequencies with out aliasing quickly results in a prohi bitively large number of data points that must be digitized in the interfero gram to achieve reasonable resolution. Thus, the alternative of aliasing the spectral information in the interfero gram must be considered. Aliasing (19, 20) refers to the undersampling of modulation frequencies in the interferogram. Normally, in Fouri er transform spectroscopy, aliasing is avoided as the undersampled modula tion frequencies show up as spurious spectral information (foldover). How ever, with line spectra, as commonly measured in atomic spectrometry, it is possible to use aliasing to advantage. As mentioned above, the basic sam pling interval of the interferometer system is 0.6328 μηι. The first four specific spectral regions (7901 c m - 1 bandwidth) that can be covered with
Table I. Spectral Regions Covered with 0.6328-μηη Sampling Interval (Direct Sampling Rate) A Region cm ' •
1
0-7,901 15,802-7,901 15,802-23,703 31,604-23,703
--12,656 6,328-12,656 6,328-4,219 3,164-4,219
•
Table I I . Spectral Regions Covered with 1.266-μηι Sampling Interval {+-2 Sampling Rate) Regi on cm ' 1 2 3 4 5 6 7 8
0 - -3,950 7 , 9 0 1 - -3,950 7 , 9 0 1 --11,851 15,802--11,851 15,802- -19,752 2 3 , 7 0 3 --19,752 2 3 , 7 0 3 --27,653 31,605--27,653
o o - -2.5 μ 1.2- -2.5 μ 12,656--8,438 6,328- -8,438 6,328--5,062 4,219- -5,062 4,219--3,616 3,164- -3,616
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 8, JULY
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Table I I I . Spectral Regions Covered with 2.532-μπι Sampling Interval (*4 Sampling Rate) Region cm - 1 A
1
0-1,975
—5 μ
6 7 8 9
11,851-9,876 11,851-13,826 15,802-13,826 15,802-17,777
8,438-10,125 8,438-7,233 6,328-7,233 6,328-5,625
15 16
27,653-29,628 31,605-29,628
3,616-3,375 3,164-3,375
Table IV. Major Emission Lines of Cs, Rb, K, and Li in Near IR Region (Table III) A en
Cs Rb Κ Li
6 6 7 7 7 7 8
this sampling interval are listed in Table I. Regions 2-4 are aliased re gions. Spectral information in all of these regions can be measured simul taneously as long as aliased spectral
8943 8521 7947 7800 7698 7664 6707
11,181 11,735 12,583 12,820 12,990 13,048 14,909
lines do not exactly overlap. With a sampling interval of 1.266 μηι (-ί-2 sampling rate), these four regions be come eight regions, each covering a bandwidth of 3950 c m - 1 (see Table
II). With a 2.532-μπι sampling interval (-j-4 sampling rate), 16 regions each with a bandwidth of 1975 c m - 1 can be simultaneously covered. A partial list of the regions is presented in Table III. Thus, given a fixed number of data points that can be acquired and transformed, spectral lines of widely different wavelength can be simulta neously measured with significantly better resolution than could be achieved if aliasing were avoided. Flame emission spectra of Cs, Rb, K, and Li were measured with our Fouri er transform spectrometer to illustrate this point. The emission lines of these ele ments in the near-IR region are listed in Table IV along with the region from Table III that they fall into. Solutions containing only one element each were aspirated into an air-C2H2 flame, and the emission was measured with the interferometer by use of a silicon pho tocell as the detector. The analog interferograms for the emission signal from each element are shown in Fig ure 2. The beat patterns in the K, Rb, and Cs interferograms clearly indicate that the spectra all consist of two lines, and the frequency of beating can be related directly back to the doublet separation, the potassium doublet having the least separation. These in terferograms were digitized with the
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Figure 2. Analog interferograms of Li, K, Rb, and Cs
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Figure 3. Digitized interferograms of Li, K, Rb, and Cs
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2.532-μπι sampling interval, and a total of 512 points was acquired. Therefore, the nominal resolution was about 8 cm" 1 . The digitized interfero grams are shown in Figure 3. Note the manner in which aliasing has affected the interferogram beat patterns. The spectra obtained by transforming these interferograms are shown in Fig ure 4. If aliasing were completely avoided, a bandwidth of 15,802 c m - 1 would be necessary for these measure ments, which would mean a sampling
interval of 0.3164 μηι. With the same number of points (512), resolution would be only about 80 cm - 1 , or 4096 points would have to be digitized and transformed to achieve the same reso lution. It is interesting to note that similar aliasing capability has recently been incorporated into a commercial Fourier analyzer to increase its fre quency resolution without increasing the number of data points that must be processed (21). None of the aliased lines in Figure 4
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Figure 4. Flame emission spectra of Li, K, Rb, and Cs as measured with Fourier transform spectrometer
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Figure 5. Analog interferogram for simultaneous flame emission determination of Li, K, Rb, and Cs (A); expanded scale (X5) oscilloscope trace of initial portion of interferogram (B)
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 8, JULY
1975
ChelexlOO vs. heavy metals.
Figure 6. Flame emission spectrum of Li, Κ, Rb, and Cs as measured with Fourier transform spectrometer
exactly overlaps; t h u s , t h e s e e l e m e n t s can b e d e t e r m i n e d s i m u l t a n e o u s l y . T h e analog interferogram measured w h e n a solution c o n t a i n i n g all four el e m e n t s w a s a s p i r a t e d i n t o t h e flame is s h o w n in F i g u r e 5 (A). A n e x p a n d e d (X5) oscilloscope t r a c e of t h e first por tion of t h e analog i n t e r f e r o g r a m is s h o w n in F i g u r e 5 (B), a n d t h e c o m plex b e a t p a t t e r n r e s u l t i n g from t h e m o d u l a t i o n frequencies of t h e seven lines c a n easily b e seen. T e n r e p e t i t i v e i n t e r f e r o g r a m s were digitized a n d time averaged with t h e same sampling i n t e r v a l (2.532 μτα) a s before. T h e r e s u l t i n g s p e c t r u m is s h o w n in F i g u r e 6. Although the measurement band w i d t h is only 1975 c m - 1 , s p e c t r a l in f o r m a t i o n over a r a n g e of 5926 c m - 1 (regions 6 - 8 of T a b l e III) h a s b e e n m e a s u r e d while still m a i n t a i n i n g t h e 8 c m - 1 resolution. I n t h i s region of t h e s p e c t r u m , a 5926 c m - 1 b a n d w i d t h r e p r e s e n t s 3797 Â. Also, t h e t o t a l m e a s u r e m e n t t i m e for t h i s s p e c t r u m w a s 3.3 sec (10 scans, 0.33-sec m e a s u r e m e n t t i m e p e r s c a n ) . T h i s s p e c t r u m is n o t of high q u a l i t y a n d only r e p r e sents preliminary results obtained prim a r i l y t o i l l u s t r a t e t h e u s e of aliasing. H o w e v e r , it was m e a s u r e d in a relatively s h o r t p e r i o d of t i m e c o n s i d e r i n g t h e effective w a v e l e n g t h r a n g e covered. Conclusions I n t h i s article we h a v e t r i e d t o p r e s e n t s o m e p e r s p e c t i v e on t h e p o t e n t i a l a p p l i c a t i o n of F o u r i e r t r a n s f o r m s p e c troscopy to atomic spectrochemical m e a s u r e m e n t s . I n p a r t i c u l a r , t h e aliasing a s p e c t of F o u r i e r t r a n s f o r m spectrochemical measurements can p r o v i d e u n i q u e c a p a b i l i t y in t h e m e a s u r e m e n t of s p e c t r a l d a t a . C e r t a i n l y
more work a n d results are necessary t o d e m o n s t r a t e t h e t r u e overall capabilit y of F o u r i e r t r a n s f o r m t e c h n i q u e s for m e a s u r e m e n t s in t h e U V - V I S region. References (1) R. M. Barnes, Anal. Chem., 46,150R (1974). (2) K. W. Jackson, Κ. Μ. Aldous, and D. G. Mitchell, Appl. Spectrosc, 28, 569 (1974). (3) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 2074 (1974). (4) D. O. Knapp, N. Omenetto, L. P. Hart, F. W. Plankey, and J. D. Winefordner, Anal. Chim. Acta, 69, 455 (1974). (5) M. J. Milano and H. L. Pardue, Anal. Chem., 47, 25 (1975). (6) D. L. Wood, A. B. Dargis, and D. L. Nash, Paper No. 310, Pittsburgh Confer ence, Cleveland, Ohio, March 3-7, 1975. (7) G. Horlick and E. G. Codding, Appl. Spectrosc, 29,167 (1975). (8) E. Cordos and H. V. Malmstadt, Anal. Chem., 45, 425 (1973). (9) A. Danielson, Paper No. 311, Pitts burgh Conference, Cleveland, Ohio, March 3-7, 1975. (10) A. Danielson, P. Lindblom, and E. Soderman, Chem. Scripta, 6, 5 (1974). (11) A. G. Marshall and Μ. Β. Comisarow, Anal. Chem., 47, 491A (1975). (12) F. D. Kahn, Astrophys. J., 129, 518 (1959). (13) N. M. Larson, R. Crosmun, and Y. Talmi, Appl. Opt., 13, 2662 (1974). (14) A. S. Filler, J. Opt. Soc. Am., 63, 589 (1973). (15) F. W. Plankey, T. H. Glenn, L. P. Hart, and J. D. Winefordner, Anal. Chem., 46, 1000 (1974). (16) E. G. Codding and G. Horlick, Appl. Spectrosc, 27, 85 (1973). (17) P. Fellgett, J. Phys. (Paris), 28, C2165 (1967). (18) G. Horlick, E. G. Codding, and S. T. Leung, Appl. Spectrosc, 29, 48 (1975). (19) G. Horlick and H. V. Malmstadt, Anal. Chem., 42,1361 (1970). (20) H. V. Malmstadt, C. G. Enke, S. R. Crouch, and G. Horlick, "Optimization of Electronic Measurements," Benjamin, Menlo Park, Calif., 1974. (21) H. W. McKinney, Hewlett-Packard J., 26 (April), 20 (1975).
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