Multiplex Methods in Atomic Spectroscopy - American Chemical Society

Instrumentation

Kenneth W. Busch Larry D. Benton Department of Chemistry Baylor University Waco, Tex. 76798

Multiplex Methods in Atomic Spectroscopy

Figure 1 . M i c h e l s o n i n t e r f e r o m e t e r (S) Source; (M1) stationary mirror; (B) beam splitter; (M2) movable mirror; (D) detector

Multielement analysis is an area of research aimed at increasing the efficiency of analytical atomic spectroscopy (1, 2). Ever since the development of spectrography, workers have consistently sought electronic alternatives to photographic detection. This article is concerned with electronic multiwavelength detection methods, collectively referred to as multiplex methods. Multiplexing The term multiplexing refers to the result produced by encoding a number of independent messages so that they can be sent or received over a common transmitting medium. Because a number of messages are transmitted over a common transmission medium, a key feature of multiplexing is to encode each message in some readily distinguishable manner prior to transmission, so that after transmission the multiplexed message can be decoded 0003-2700/83/0351-445AS01.50/0 © 1983 American Chemical Society

to retrieve each individual message. The chief benefit of multiplexing is the increased efficiency in transmitting information as compared with nonmultiplexed forms of transmission. Analytical instruments have many similarities with communication systems. Both encode the desired information into electrical signals by means of a transducer, and both transmit the final decoded information to an observer. Characteristics of Analytical Atomic Spectroscopy In analytical atomic spectroscopy, the desired information consists of the intensities of a selected number of wavelengths indicative of certain elements. The amount of information contained in a complete spectrum from, say, 200 nm to 800 nm, is enormous (3, 4). Fortunately, not all of the information contained in a complete

spectrum is required. Since the line widths associated with atomic transitions are fortunately quite narrow, and since most analyses do not require information about all the elements in the periodic table, information is required only over specific narrow wavelength intervals characteristic of each element of interest. For each element of interest, information about the atomic line intensity and its adjacent background is required. The situation in analytical atomic spectroscopy consists of appropriately selecting and monitoring the intensities of a selected number of wavelength intervals in which the desired spectral information on the elements of interest is contained. Unfortunately, the desired information is not evenly distributed throughout the spectrum. Even though large regions of the atomic spectrum may not contain any desired information, high res·-

ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983 · 445 A

Table I. Classification of Multiplex Approaches in Atomic Spectroscopy Single-detector systems a) Time-division multiplexing • Rotating filters • Linear scanning systems (rapid linear scanning) • Programmed monochromators (slew scanning) • Image dissector systems b) Frequency-division multiplexing • Tuned amplifier systems • Interferometric systems c) Transform methods • Fourier transform • Hadamard transform Multiple-detector systems a) Wavelength division multiplexing by direct-reading spectrometers b) Wavelength division multiplexing with integrated solid-state array detectors

olution is often required to resolve the randomly situated atomic lines of interest from the unwanted spectral background. Thus, to be efficient, an ideal instrument for analytical atomic spectroscopy should have high resolution in the regions of interest and low resolution in spectral regions that do not contain desired information. These considerations have important consequences when comparing various multiplex methods.

Classification of Multiplex Approaches The objective of all multiplex approaches in atomic spectroscopy is to transmit the intensities corresponding to some given number of resolution elements over a common transmission channel so that the individual intensities in each resolution element can be recovered and monitored. The common transmission channel may be either a common optical pathway or a common electronic channel. In either case, some form of encoding is required to distinguish between the various resolution elements. Broadly speaking, multiplex methods in atomic spectroscopy may be classified into two main groups: 1) those that employ a single detector, and 2) those that employ multiple detectors (Table I).

Single-Detector Approaches Time-Division Multiplexing. Frequencies in the optical region of the electromagnetic spectrum are much too high (1014 Hz) for detection systems to respond directly (5). Consequently, if a single detector is to distinguish between a number of different wavelengths of radiation, this information must be encoded in some manner prior to the radiation striking the detector. This can be accomplished in a variety of ways, as shown in Table I. When different optical

wavelengths are sequentially arranged to strike the detector, this form of multiplexing is known as time-division multiplexing. In time-division multiplexing, a particular wavelength is always transmitted at a given time after the initiation of a spectral sweep. Naturally, any spectral sweep instability will lead to decoding errors that result in wavelength registration problems. In addition, since each wavelength is monitored for only a fraction of the total scan time, shot noise and source fluctuation noise will lead to uncertainties in the monitored intensities. The presence of these noise sources imposes a limitation on the speed with which time-multiplexed spectrometers can gather data. To gather spectral information at high speed implies an adequate electronic bandwidth to accurately keep up with the rate of data transmission. But the wider the electronic bandwidth, the more noise transmitted by the system. Thus a compromise exists.between immunity from source-generated noise (the limiting noise), on the one hand, and the speed of data acquisition, on the other. The signal-to-noise (S/N) ratio obtained with time-multiplexed systems may be improved in two ways. A smaller frequency response bandwidth can be used (which implies a slower scan rate) or signal averaging can be used to average the accumulation of an appropriate number of spectral sweeps. In either case, a longer time is required to obtain the desired information. The ultimate rate of data transmission is therefore source-limited and cannot be exceeded by some clever instrumental multiplexing arrangement. There are two ways time-division multiplexing can be accomplished. The entire spectrum may be sequentially scanned, one resolution element at a time. This is the approach used in

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rapid-scanning spectrometers (6-14). Alternatively, selected wavelengths of interest may be sequentially interrogated, entirely skipping broad regions of the spectrum that do not contain information of interest. This is the approach used in programmed (slewscan) monochromators (15-17) as well as nondispersive systems using rotating filters (18,19). It can be shown that this approach is more efficient for atomic spectroscopy because only certain specified wavelength regions are of interest, not the entire spectrum. An important aspect of the performance of time-multiplexed systems is the total time required to collect the information on all the desired elements in the sample. With rapid-scanning systems that monitor all the resolution elements in the spectrum, signal averaging (or very slow scanning) will be required to reduce the effect of statistical fluctuations inherent in the source. Since the entire spectrum is monitored, the minimum observation time will be set by the time required for the weakest signal of interest (in the presence of statistical fluctuations in the source intensity) to achieve an adequate S/N ratio. Slew scan systems are expected to be more efficient in terms of the total observation time required to collect information on all the desired elements in the sample because the fraction of the total scan time spent observing signals of interest is higher. The fraction of a given scan time spent observing a given element decreases as the number of elements under consideration increases. Therefore, the total observation time required with these systems increases with the number of elements under consideration. In contrast to linear scan systems, the length of time spent observing each element need not be constant with slew scan systems. Thus, the weakest signal does not set the required observation time for the stronger signals, as in the case of linear scan systems. Instead, the observation time for each element may be independently adjusted. With most of these dispersive systems (linear scanning or slew scanning), wavelength variation is accomplished by moving a particular optical component in the wavelength dispersion system, i.e., a mirror or the dispersing element. Wavelength registration becomes a central consideration when these systems are used in analytical atomic spectroscopy. The line width of an atomic line under ordinary spectroscopic conditions is on the order of 0.005 nm. To rapidly and reproducibly jump a monochromator from one wavelength to another under high-resolution conditions so that selected spectral lines are reproducibly

imaged on the detector calls for very high tolerances in the mechanical movement of the given optical element. The near physical impossibility of attaining the required tolerances to permit such jumping is the primary reason why no commercial instrument with these characteristics is available. Much greater wavelength registration can be achieved if the components of the optical dispersion system are kept stationary. One way in which this can be accomplished is by using an electronically scanned detector such as the image dissector (20-25). The image dissector is a scanning photomultiplier tube. Light focused on the detector produces a 1:1 photoelectron image on an aperture plate within the tube. Only that portion of the photoelectron image that passes through the aperture reaches the dynode chain and is amplified. The particular portion of the image that is interrogated can be varied electronically by deflecting the photoelectron image. Since wavelength scanning is accomplished electronically rather than mechanically, scanning can be accomplished at high speed with a minimum of wavelength registration problems. Since the image dissector is not able to monitor more than one picture element, i.e. resolution element, at a time, it falls in the time-multiplexed category. Another novel device for time-multiplexing spectral information that does not require moving parts has recently been described (26). With this device the time required for different wavelengths to travel down an optical fiber is used as the basis for distinguishing different optical frequencies. Unfortunately, the device does not provide sufficient resolution to be useful for atomic spectroscopy. Recently we have been evaluating a time-multiplexed instrument with a stationary dispersive system in our laboratory. The system is based on multiple-entrance-slit technology (27, 28) and uses an optical multiplexer (29). The optical multiplexer consists of a microprocessor-controlled mirror system that scans the undispersed radiation from the spectral source past an array of fiber optic light guides arranged in a circle. Each individual optical fiber conveys light to a selected entrance slit, appropriately situated in the entrance focal plane so that light of a desired wavelength will be passed by the exit slit to the photomultiplier. Because the entire dispersive system is stationary, wavelength registration problems are avoided. The undispersed beam from the spectral source can be made to rotate past each fiber optic light guide in a continuous manner or to rotate to a particular light guide and stop for a given time period

before moving on to the next one. Only wavelengths of interest are monitored. Also, since scanning is accomplished by jumping the undispersed beam from one entrance slit to another—i.e., one optical fiber to another—scan time between different wavelengths of interest is independent of wavelength separation. Background correction adjacent to a spectral line can be accomplished by wavelength modulation with a refractor plate at the exit slit in the conventional manner (30), although this may produce some wavelength registration problems as a result of refractor plate beam shifting. Because each entrance slit is independently adjustable, a wide range of intensities can be monitored with this instrument. , Frequency-Division Multiplexing. Frequency-division multiplexing (31 ) is another way optical wavelengths can be encoded. With frequency-division multiplexing, more than one wavelength at a time is allowed to strike the detector. By modulating each wavelength of interest at a different frequency, wavelength discrimination can be accomplished electronically after detection by some means—for example, by using separately tuned amplifiers to respond to individual optical wavelengths. Modulating frequencies must be sufficiently different to avoid crosstalk between wavelength channels. This tends to restrict the number of channels because each channel requires adequate bandwidth, analogous to the situation in radio broadcasting. Another factor limits the number of channels available with this approach when a photomultiplier is used as a detector. A photomultiplier responds to the rate of arrival of photons. This rate is not entirely constant but varies according to Poisson statistics, so that the fluctuation in the rate is proportional to the square root of the rate. The resulting variation in electrical signal, or shot noise, is therefore proportional to the square root of the total intensity striking the detector. Therefore, as the number of channels increases, the total intensity striking the detector increases, and the noise associated with the total signal increases as well. Thus, for a photomultiplier, the presence of other signals received simultaneously with the one of interest does affect the ability to detect the one of interest. Transform Procedures. Transform procedures represent still another potential approach to multiplexing spectral information. Such approaches have been successfully applied in other analytical areas, such as infrared (IR) spectrometry and nuclear magnetic resonance (NMR) spectrometry (32).


Before discussing the application of transform techniques to UV/VIS spectroscopy, it is instructive to consider some of the potential advantages that have been achieved in IR and NMR spectrometry. One characteristic of transform methods is that intensities from more than one resolution element strike the detector simultaneously. This is a distinct advantage in the IR region of the spectrum, where thermal detectors rather than photomultipliers are used and where detector noise becomes the limiting noise. It is also an advantage to have intensities from more than one resolution element strike the detector if the detector has a large threshold sensitivity. In addition, transform procedures generally have a higher radiational throughput than conventional dispersion systems, which is an advantage under conditions of low radiant flux. The increased S/N ratio compared with conventional dispersive systems obtained by the transform method as a result of monitoring intensities in more than one resolution element at a time is known as the multiplex advantage or Fellgett's advantage (33, 34). The performance gain obtained because of the larger throughput (sometimes referred to as étendue, from the French meaning extent) available with transform techniques is referred to as the throughput advantage or Jacquinot's advantage (35). Both of these advantages are readily realized in the IR region of the spectrum. In spite of the success of transform procedures in other areas of spectroscopy, their success in the UV/VIS region of the spectrum using photomultiplier detection has been only marginal (36-50). The reasons why these techniques have not lived up to their anticipated potential will be discussed in the following sections. The two transform techniques that have been most widely applied in spectroscopy are Fourier and Hadamard transform procedures. Fourier Transform Techniques. Most Fourier transform (FT) techniques (41-44) employ some form of a Michelson interferometer (Figure 1) to gather spectral information over a given frequency range (the free spectral range) simultaneously. The Michelson interferometer is actually a device that performs frequency-division multiplexing, in that each wavelength in the spectrum is modulated at a characteristic frequency determined by the speed of the mirror. The complex pattern that results when all of the modulation waveforms are simultaneously received by the detector is called the interferogram. To decode or transform the interferogram (a plot of signal amplitude vs. time) into a spectrum (a plot of signal amplitude

Figure 2. Hadamard spectrometer (a) Source; (b) entrance slit; (c) dispersing element (typically a grating); (d) slotted mask in focal plane; (e) post-dispersion optics, which collect the radiation passed by the mask and relay it to the detector (f). The mask is stepped across the focal plane so different combinations of resolution elements strike the detector

vs. frequency), use is made of the mathematical procedure known as Fourier transformation (32). A key feature of encoding spectral information through the use of a Michelson interferometer involves the stability of the spectral source as well as the stability of the signal generated by the detector. The accuracy of the encoding process depends to a large extent on the stability of both the source and the signal generated by the detector. Ideally, as the mirror moves during a scan, each tiny change in the signal produced should reflect only variations in the modulation pattern of the interferogram. Thus, any other extraneous variations in total signal as a result of either source fluctuations during a scan or shot noise on the signal (from random arrival of photons at the detector) will be misinterpreted during the Fourier transform. These extraneous signals result in noise. As in the previous discussion of the effect of shot noise in frequency-division multiplexing where a single detector was employed with several tuned amplifiers, in FT spectroscopy in the UV/ VIS, the noise is also spread uniformly throughout the spectrum (36-41). In both cases, however, the effect is the same; that is, weak lines suffer in S/N ratio when in the presence of strong lines. The effect is particularly bad if a complete high-background spectrum is monitored, such as might be encountered using a flame as a spectral source. Thus, with a photomultiplier detector the best results are obtained by restricting the number of wavelengths simultaneously incident on the detector and avoiding intense lines in the presence of weak lines. The performance of FT systems can be improved with multiple-entranceslit technology (27, 28). Analytical atomic spectroscopists are not really interested in the entire spectrum from one wavelength limit to another. Instead they are interested only in spe-

cific wavelengths of interest. By using a multiple-entrance-slit spectrometer prior to the Michelson interferometer, it should be possible to restrict the wavelengths incident on the detector to the narrow regions around the various spectral lines, thereby avoiding a large amount of the source background, which is not of any interest. The price for this reduction in unwanted background comes in a loss of throughput. Even the problem of the intense line in the presence of the weaker lines can be avoided using the multiple-entrance-slit spectrometer as a bandwidth selector prior to the Michelson interferometer. Since each entrance slit behaves independently, it is a simple matter to attenuate the intense line to some convenient level either by using a neutral density filter in front of that optical fiber or by simply employing a smaller entrance slit width for that particular slit. Hadamard Transform Techniques. Hadamard transform techniques (32, 45-50) are based on the idea that instead of measuring each resolution element separately, combinations of resolution elements can be measured simultaneously. If the combinations are chosen correctly, and if enough combinations are measured, there should be enough information to mathematically determine the individual intensities corresponding to the various resolution elements that make up the spectrum. If there are M resolution elements in a spectrum, M independent combinations of intensities from various resolution elements in the spectrum are required to solve the system of equations. In comparison to FT techniques, the mathematics associated with Hadamard transform techniques is simpler. As with FT methods, the potential advantages of Hadamard transform techniques lie in the potential for a multiplex advantage and a throughput advantage. The through-


put advantage arises because various resolution elements are recombined after dispersion to simultaneously strike the detector. This is accomplished with a special mask consisting of a series of appropriately placed slots or openings (Figure 2). As the mask is stepped across the dispersed beam, various combinations of appropriately selected resolution elements are made to simultaneously impinge on the detector. The total intensity monitored by the detector at any given moment is a linear combination resulting from several resolution elements. This approach is most useful for systems where detector noise is the predominant noise. The most accurate encoding of the spectral information in Hadamard transform spectroscopy will occur in the absence of source fluctuation noise and shot noise. As the mask is stepped across the spectrum, it is assumed that the fluctuations in the signal are due to the various combinations of resolution elements being simultaneously imaged on the detector, not by the random fluctuations caused by source flicker and shot noise. Any uncertainty in the measured total signal due to shot noise or source fluctuation noise when the various combinations of resolution elements are monitored must lead to uncertainty in the individual resolution elements when the message is decoded, i.e., when the system of equations is solved. Because of the distributive effect of the multiplex technique, the noise is uniformly distributed throughout the spectrum (48-50). Therefore, a high intensity at a particular wavelength will adversely affect any weaker lines that may be present. Plankey, et al. (49) observed for a simple spectrum containing two spectral lines of different intensity that a signal 3% of the most intense signal is lost in the baseline noise. Multiple-Detector Approaches It should be clear from the previous discussion that for a shot noise and/or source fluctuation noise limited situation, the highest multiplex efficiency (that is, the most accurate encoding of information) can be obtained with discrete integrating detectors, each simultaneously monitoring a resolution element of interest throughout the entire observation period. Use of multiple detectors to monitor different resolution elements in a spectrum requires some form of wavelength selection. This may be as simple as a series of detector/filter combinations, but this generally does not provide adequate resolution for analytical atomic spectroscopy. More sophisticated instruments rely on optical dispersion produced with either a diffraction grating, a prism, or a com-

bination of both. This form of multiplexing can be described as spatial optical multiplexing or wavelength-division multiplexing. In spatial optical multiplexing, the spectral information is encoded in terms of position in the exit focal plane of the instrument and is carried in separate optical channels that are monitored simultaneously. Unfortunately, the spectral information of interest is not uniformly distributed throughout the spectrum. This means that the dispersion required to separate a given spectral line from other unwanted spectral features is not the same for all lines of interest. When monitoring a particular combination of elements, the minimum resolution allowable is set by the spectral line that is most difficult to separate from the interfering spectral background. For systems with a constant dispersion, this means that large portions of unwanted spectral background must be monitored at the resolution set by the combination of elements being simultaneously monitored. Since the purpose of multiplexing is to increase the efficiency of the transmission process, anything that can improve the information density will make the encoding process more efficient. Spatial optical multiplexing can be divided into two broad categories: 1) singleentrance-slit/multiple-detector systems and 2) multiple-entrance-slit/ multiple-detector systems. Single-Entrance-Slit/MultipleDetector Systems. One of the most familiar systems of this type is the direct-reading spectrometer (51). Direct-reading spectrometers have been used successfully for quite some time for analytical atomic spectroscopy. This instrument consists of a dispersion system with an array of exit slits arranged at appropriate locations. Behind each exit slit is a photodetector, usually a photomultiplier. There are several advantages to the direct reader for analytical atomic spectroscopy. First, each exit slit-detector combination is independently adjustable, allowing the simultaneous monitoring of both intense and weak spectral lines. Second, the system is highly efficient in terms of the amount of useful information collected. In other words, no unnecessary wavelengths are monitored from the spectrum. Finally, each resolution element of interest is simultaneously monitored for the entire observation period. This permits continuous signal integration of each resolution element for the entire observation period. This feature of signal integration is extremely important; it not only reduces the effect of shot noise, but it also permits the instrument to be used with spectral sources, such as arcs and

Figure 3. Principle of échelle dispersion (EG) échelle grating; (P) prism; (FP) focal plane; ( 1 , 2, 3, 4) different diffraction orders. The échelle grating disperses radiation in the horizontal direction. This radiation consists of many overlapping orders. The prism disperses the overlapping orders vertically, producing a two-dimensional spectrum. Collimating and focusing elements have been omitted to simplify the diagram, and some of the angles have been exaggerated

sparks, which are subject to a high degree of source fluctuation noise. Direct readers are not, however, without certain disadvantages. High dispersion and complicated mirror systems are necessary to permit detector placement of the bulky photomultiplier tubes at the appropriate positions in the exit focal plane. This is especially difficult because of the random positioning of the wavelengths of interest. As a result of the necessity for high dispersion, direct readers are usually large instruments. The long focal length means that the optical flux or throughput of the system will be low. Because of the high dispersion generally used, exit slit positioning becomes critical, and temperature variations and other parameters can cause misalignment of the exit slits. Finally, correction for adjacent background, while possible, requires further optical complications. The need for high dispersion and complicated mirror systems, which are often necessary to permit detector placement at the appropriate positions in the exit focal plane of direct readers when bulky photomultiplier tubes are used, can be mitigated by the use of integrated detector arrays. The smaller the sensing elements, the closer together they can be placed in the focal plane. This means that the minimum allowable dispersion can be determined by the spectrum itself and not by detector placement considerations. Therefore, the use of integrated array detectors may reduce the minimum allowable dispersion required. This will reduce the focal length of the dispersion system, which


determines the overall size of the instrument. Integrated Solid-State Array Detectors. A complete discussion of integrated solid-state array detectors is beyond the scope of this paper. The interested reader should consult the excellent reviews of this subject (52, 53). Assuming that future developments in array detector technology lead to a generation of detectors with performance characteristics equal to or superior to photomultipliers, efficient utilization of these devices will be required if they are to be useful for analytical atomic spectroscopy. For example, if an array detector is mounted at the focal plane of a conventional grating spectrometer, there is a compromise between the resolution obtained and the wavelength range monitored (54, 55). To access any combination of spectral lines in the region from 200 to 800 nm, so that the desired combination is monitored simultaneously, would require a detector array wider than is currently available. For example, to photograph the complete spectrum from 200 to 800 nm with a conventional grating instrument having a reciprocal linear dispersion of 1.6 nm/mm would require a photographic plate 375 mm wide. While it is conceivable that it might eventually be feasible to fabricate an unintensified photodiode array this wide (or use a number of arrays), this approach is certainly not the most economical and seems remote when considering other types of array detectors. One way to monitor widely separated spectral lines at the required

resolution is to jump from one spectral window to another (56). Since there may be several elements that can be monitored in each window, this approach is more efficient than time multiplexing with a system employing a photomultiplier, where each element must be monitored sequentially for some time period. Compared with systems that can monitor all the wavelengths of interest simultaneously, a longer total observation time will be required with the system that jumps from one window to another. In addition, such a system is not suitable for use with time-varying sources. With time-varying sources such as arcs, equivalent integration at various stages within a burn will not necessarily produce equivalent signals, because of selective volatilization. A number of different approaches have been applied to alter the format with which spectral information is imaged on the detector so that the spectral information may be more efficiently collected by the array detector. This has been accomplished in two basic ways: 1) by overlapping spectral information on the detector so that more resolution elements of interest will simultaneously be monitored by the detector than would be possible with a conventional dispersion system; and 2), by dividing the spectrum up into segments and stacking the segments on top of one another like the printed lines in a book. Instruments in the single-entranceslit category have used the second approach. This has been accomplished using either the well-known échelle dispersion system originally developed in the late 1940s (Figure 3) or a modified conventional dispersion system. The échelle dispersion system (21-25; 57-59) is capable of very high resolution. It makes use of a specially ruled grating that is used in very high orders to produce the desired high resolution. The échelle grating is used to disperse the spectrum in the horizontal direction. A cross-dispersion element, which can be either a prism or another grating, is used to disperse the spectrum into a two-dimensional format. The function of the cross-dispersion element is to disperse the radiation from the échelle grating in the vertical direction, thereby separating the large number of overlapping orders produced by the échelle grating. In most commercial instruments, the cross-dispersion element is a prism. Since the dispersion of a prism is not constant with wavelength, the various rows of spectral information produced are not evenly spaced. Nevertheless, the two-dimensional spectral format produced by an échelle spectrometer can be used to advantage when an array detector is used in

Figure 4. Modified dispersion system for use with image detectors (a) Source; (b) collimating mirror; (c) diffraction grating; (d) stacked array of plane mirrors; (e) focusing mirror; (f) image detector; (g) entrance slit. Ray tracing indicates position of focus for a single wavelength (Reference 60)

conjunction with these systems, since it allows the spectral information to be more efficiently imaged on the array detector compared with a conventional dispersion system. The échelle system is not without disadvantages, however. The nonuniform spectral format that results when a prism is used as a cross-dispersion element makes programming the scanning pattern of the image detector more complex. As a result of the crossed dispersion system, the spectral information is imaged on the detector as points of light rather than spectral lines. This causes intense spectral lines to be focused over a smaller number of pixels (picture elements), which can cause charge-spreading problems (blooming) when photodiode arrays are employed. Although this is less of a problem with some of the newer charge-coupled devices, it can limit the overall dynamic range of the instrument when photodiode arrays are used. Another approach to increase the efficiency of the dispersion system with regard to image detectors is to modify a conventional dispersion instrument. This has been accomplished in two ways: 1) by modifying the collimating/focusing system on single-entrance-slit instruments; and 2), by using multiple entrance slits with a conventional dispersion system. In the first case (60) (Figure 4), the beam of radiation from the entrance slit is collimated, and the collimated beam is subsequently divided into a series of horizontal strips, each of which is made to encounter the diffraction grating at a different angle. Since each collimated horizontal strip strikes the grating with a different angle of incidence, each strip will subsequently focus a different spectral region on the target of the image detec-


tor. In this manner, more spectral information may be imaged on the detector simultaneously than with a conventional system. In terms of the efficiency of imaging only information of interest on the detector, the modified dispersion approach is more efficient than the échelle system. With the échelle system, the entire spectrum is simultaneously imaged. With the modified dispersion system, spectral regions of no interest need not necessarily be imaged on the detector. This feature can be useful in avoiding blooming caused by spectral regions with high source background and no analytical information of interest. One drawback with this approach is that it may be difficult to adjust the instrument to monitor different spectral regions. Another problem involves the significant drop in luminosity for each spectral region, which can lead to reduced S/N ratios. All single-entrance-slit systems require that the dynamic range of the detector be matched to the range of intensities in the spectrum. If the range of intensities in the spectrum exceeds the dynamic range of the detector, it will not be possible to measure the intensity of those lines that exceed the upper limit of the dynamic range. The problem is particularly important for those image detectors employing photodiode sensors. The dynamic range of these devices is determined by the amount of charge that can be stored on a given photodiode. When an intense beam of light is focused on an image detector, the electron-hole pairs that form tend to repel one another if generated in sufficient quantity. This causes a depletion of charge in adjacent pixels. This phenomenon is known as blooming and causes a loss of resolution in the immediate area of

the focused beam. To avoid blooming when monitoring a source with both intense and weak spectral regions, the slit width and/or the observation time must be decreased to reduce the effect of the intense light. In either case, this reduction in light to accommodate the intense radiation makes the weak radiation even more difficult to detect. Again, we seem to encounter the same problem discussed in connection with single-detector systems—i.e., the problem of being able to simultaneously sense weak lines in the presence of intense lines. This problem can be reduced significantly with multiple-entrance slit systems. Multiple-Entrance-Slit/MultipleDetector Systems. There are actually two problems that must be overcome to effectively use image detectors for analytical atomic spectroscopy. The first is to devise an efficient imaging system to permit a wide spectral window to be simultaneously imaged on the detector with adequate resolution. This problem is overcome with the single-entrance-slit systems, which provide a two-dimensional spectral format. The second problem is to match the dynamic range of the spectrum to the linear dynamic range of the detector. To be effective in multielement atomic spectroscopy, an image detector must be able to monitor weak lines in the presence of intense lines (or background). With many solid-state detectors, the presence of intense lines results in blooming that can spread in such a way as to obscure spectral information adjacent to it. The problem is to avoid this detector saturation while at the same

Figure 5. Multiple-entrance-slit spectrometer showing switchboard plate for fiber optic light guides (Reference 27)

time not attenuating the weak lines of interest. This problem has been overcome with single-entrance-slit systems by employing a random access scanning pattern of the tube target (61, 62). In this manner, portions of the target subject to intense radiation are refreshed with additional charge more frequently than those regions not subject to intense radiation. This allows more integration time for the weaker portions of the spectrum to accumulate on the target before being erased, thereby extending the effective dynamic range of the detector.

This procedure requires software control to be specifically written for each analytical situation. Another way this problem can be solved is by using several entrance slits simultaneously. When multiple entrance slits are used simultaneously, each entrance slit independently produces its own particular spectral region on the target of the image detector because each entrance slit subtends a different angle of incidence with the diffraction grating (27, 28). There are two ways the multiple entrance slits may be ar- · ranged (28). For detectors with a sin-

Sr Sr 460.7

Li 670.8

Li

Cr 427.5 Cr 429.0 I / C r Mn 403.2

425 4

-

\

B a 553.6

*^W*K

^^ffi^w^ ^ J 4 J *

(c)

(d)

(e)

Figure 6. Spectra obtained with one-dimensional multiple-entrance-slit spectrometer (a) Mn, 3 ppm, entrance slit #3 (402.4-442.4 nm); (b) Cr, 3 ppm, entrance slit #3 (402.4-442.4 nm); (c) Sr, 1 ppm, entrance slit #6 (440.8-480.8 nm); (d) Ba, 3 ppm, entrance slit #14 (543.2-583.2 nm); (e) Li, 1 ppm, entrance slit #23 (658.4-698.4 nm); (f) composite spectrum obtained simultaneously using entrance slits 3, 6, 14, and 23. Multielement mixture contained 5 ppm Cr, Mn, and Ba, and 1 ppm Li and Sr (Reference 28)


gle raster scan pattern or a one-dimensional array, the multiple entrance slits are arranged in a single row in the entrance focal plane. Individual placement of particular entrance slits in the entrance focal plane depends on the resolution elements of interest in the spectrum. The aim is to place the various entrance slits in the focal plane in such a way that the resolution elements of interest are simultaneously imaged on the detector without being interfered with by other sample constituents. Since source background is easily removed by subtraction, the only concern in terms of spectral interference is spectral lines (or background) produced by other sample constituents. This procedure is possible with simple spectra because, as mentioned earlier, the spectral information is not uniformly distributed throughout the spectrum, and there are often broad areas with no spectral information at all. This approach therefore increases the efficiency with which the image detector monitors spectral information of interest without simultaneously degrading the resolution. By employing a regularly spaced row of entrance slits and fiber optic light guides, a switchboard system has been developed (Figure 5) that makes it convenient to modify the instrument so as to monitor different composite spectra (27, 28) (Figure 6). The system offers other benefits in addition to the increased efficiency in detector usage. Careful placement of the entrance slits can be used to avoid areas of intense background (which would ordinarily saturate the detector) while still monitoring the spectral elements of interest. Intense lines can be attentuated with filters or by using a smaller entrance slit width without simultaneously attenuating weaker

Figure 7. Multiple-entrance-slit spectrometer with two-dimensional array of entrance slits

Figure 8. Spectrum obtained with multiple-entrance-slit spectrometer having twodimensional array of entrance slits Spectrum obtained using three entrance slits, each entrance slit having a different column-row coordinate in the array

lines of interest. This is accomplished by using several entrance slits to image a region with both intense and weak lines. In this way, both can be transmitted to the detector in an optimum manner. Since all the slits are active simultaneously, a composite spectrum is produced that is simultaneously integrated, allowing the system to be used under source fluctuation noise or shot-noise-limited situations or even with time-varying sources. For detectors that permit a twodimensional scan pattern, the multiple entrance slits can be arranged in a two-dimensional array (Figure 7) in the entrance focal plane (28). If a stigmatic dispersion system is employed, each row in the array of entrance slits will image a particular spectral segment on the detector with one row arranged above the other, the separation between rows being determined by the vertical separation between the rows in the entrance slit array (Figure 8). The particular spectral segment sampled will be determined by the horizontal position of the slit in the given row. This configuration provides the maximum amount of flexibility in arranging the spectral information on the image detector. Spectral regions of high background can be avoided. Within a given row, several entrance slits can be employed simultaneously to achieve a desired result, as in the one-dimensional case. Since there is less overlapping of spectral information, there is less chance for spectral interference. As before, since each entrance slit is used simultaneously, the system will simultaneously integrate spectral information in all the spectral segments observed. This reduces the effect of source fluctuation noise and shot noise. The intensities in different spectral segments may be independently varied because each segment is


independent of the others. This permits the attenuation of intense spectral lines without simultaneously attenuating weak lines. Finally, since more than one entrance slit is employed simultaneously, a throughput advantage is possible compared with single-entrance-slit systems. Conclusions

Although not all multiplex approaches are equally efficient, the ultimate performance achievable in the UV/VIS region by any proposed multiplex approach is limited by the statistical fluctuations inherent in the source. Any multiplex approach proposed for the UV/VIS region must be able to operate not only in the presence of these statistical fluctuations, but must also be capable of monitoring intensities of widely different magnitude if that approach is to be effective for analytical atomic spectroscopy. Efforts to increase the efficiency of spectroscopic systems by overlapping spectral information on photomultiplier detectors, as in frequency-division and Hadamard transform approaches, have not been entirely successful because of the shot noise behavior of photomultipliers. It is difficult to see how this dilemma can be avoided. As a result, the best hope for increasing the efficiency of spectroscopic detection systems lies in multiple detector systems, such as the various array detectors. Although the current state-of-the-art image detectors may not be ideal as spectroscopic detectors, there is reason to believe that improved second-generation devices will be developed. Because array detectors permit the simultaneous monitoring of different resolution elements, variations in source intensity are less serious than with systems that sample a given resolution element for a brief

t i m e interval during t h e observation period. Also, since t h e solid-state array detector is basically an integrat ing device, shot noise a n d source fluc t u a t i o n noise t e n d to be averaged o u t d u r i n g an observation period. T h i s al lows spectral information to be over lapped to produce composite s p e c t r a without t h e deleterious shot noise ef fects observed with p h o t o m u l t i p l i e r s . Signal averaging also can be used to further increase t h e S/N ratio. Two potential p r o b l e m s m u s t be solved to effectively use image d e t e c tors for analytical a t o m i c spectrosco py. First, m e a n s m u s t be found t o in crease t h e efficiency with which spec tral information is imaged on t h e d e tector. Second, m e a n s m u s t be devised to p e r m i t intense lines t o be s i m u l t a neously m o n i t o r e d along with weak lines w i t h o u t detector s a t u r a t i o n a n d / or blooming. Work on t h e s e p r o b l e m s is c u r r e n t l y u n d e r w a y / a n d several so lutions have already been proposed.

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Kenneth W. Busch (left) is associate professor of analytical science in the chemistry department at Baylor Uni versity. He received his BS in chemis try in 1966 from Florida Atlantic University and his PhD in analytical spectroscopy from Florida State Uni versity for his work on low-pressure microwave-induced plasmas. His re search director was T. J. Vickers. After graduation, he spent two years in the laboratory of G. H. Morrison at Cornell where he became interested in image devices as detectors for spec troscopy. Lately his research has fo cused on various multiplex approaches

that might increase analytical atomic


the efficiency spectroscopy.

of

Larry D. Benton (right) is current ly in his third year of graduate stud ies for the PhD degree in analytical chemistry at Baylor University. His research interests include atomic spectroscopy, applications of micro computers and microprocessors in the control of instrumentation, and ap plications of information theory to spectroscopy. He completed his BS degree in chemistry at Howard Payne University in Brownwood, Tex., in 1980.

Multiplex Methods in Atomic Spectroscopy - American Chemical Society

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