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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6 , MAY 1979
Multiple Entrance Slit Vidicon Spectrometer for Simultaneous M~1Itielement Ana Iy sis K. W. Busch” and
B. Malloy
Department of Chemistty, Bay/or University, Waco, Texas 76703
Y. Talml Princeton Applied Research Corporation P.O. Box 2565, Princeton, New Jersey 08540
A vldicon spectrometer employing multiple entrance slits to slmultaneously focus several 40-nm windows on the SIT detector Is described. The system may be visualized as the reverse of a dlrect reader, since multiple entrance sllts and a single muitlchannei detector are used Instead of a single entrance slit and multiple detectors. Detection limits obtained with the system for Mn, Cr, Sr, Ba, Li, and K by atomlc emlsslon with a nitrous oxlde-acetylene flame are reported. The advantages of this system for simultaneous muitleiement analysls are discussed.
I n recent years, great interest has been shown in developing spectroscopic systems for simultaneous multielement analysis (SMA) (1-3). Busch and Malloy ( 4 ) have reviewed the various systems which have been proposed to date, and have discussed the role of image devices in SMA. The development of image detector spectrometers for SMA has proceeded along two basic lines of development based on the mode of dispersion. In one-dimensional configurations, the spectral information is dispersed across t h e tube target with a conventional spectrographic system as a single horizontal band of information. One-dimensional systems are based on a window concept (5), resulting in a compromise between resolution and wavelength coverage. Under certain favorable circumstances (6-8), widely spaced spectral lines may sometimes be monitored without unduly increasing t h s size of the wavelength window (and thereby suffering a loss in resolution) by monitoring spectral lines in overlapping orders. This approach, however, is not successful in every case, and the limited wavelength window obtained by mounting an image detector a t the focal plane of a conventional spectrograph remains the most serious problem associated with this optical arrangement (4). To avoid the limited wavelength coverage associated with one-dimensional systems, several groups (9-1 3) have assembled and demonstrated the feasibility of echelle systems. Although these systems provide wide wavelength coverage under high resolution conditions, the optical system is more complicated and expensive, and computer control of the scanning circuitry is required for accurate wavelength registration. This paper describes a multiple entrance slit vidicon spectrometer employing a one-dimensional dispersive system which overcomes many of the limitations associated with previous one-dimensional systems.
EXPERIMENTAL Table I lists the experimental facilities used in this study. Multichannel Detection System. The multichannel detection system employed in this study consists of an SIT vidicon detector and an optical multichannel analyzer. The optical multichannel analyzer accumulates spectral information in 500 electronic channels, and has two separate memories that permit 0003-2700/79/0351-0670$01 .OO/O
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Table I. Experimental Facilities burner Varian Techtron 5-cm slot burner for nitrous oxide-acetylene monochromator Spex, Model 1870, 0.5-m CzernyTurner spectrograph, fl6.9, with 600 grooves/mm grating blazed for 500 nm. Reciprocal linear dispersion 3.2 nm/mm in the first order detector Model 1205D silicon intensified target detector head, Princeton Applied Research Corp. optical multichannel Model 1205A, Princeton Applied analyzer Research Corp., with Option 3 serial converter. readout Tektronix oscilloscope, 604 Monitor Sargent-Welch Recorder, Model SRG flow meters Brooks Full-view Rotameters calibrated for nitrous oxide and acetylene, Brooks Instrument Division fiber optics Approximately 200 (2.5-mil diameter) glass fibers noncoherently bundled in flexible poly(vinylch1oride) jacket; approximate transmission 65% from 400-2000 nm for 30-cm length. Edmund Scientific, Barrington, N.J. input lenses Precision-molded acrylic terminal lenses. Edmund Scientific, Barrington, N.J. the storage of a data spectrum and a blank spectrum. An arithmetic unit permits channel-by-channel subtraction of memory B from memory A. Optical System. F,gure 1 shows the optical arrangement employed. A Spex 0.5-m Czerny-Turner spectrograph with a camera mirror which is larger than the collimating mirror was modified in the following way. The entrance slit was removed and the vidicon detector was mounted in the focal plane of the entrance port. A multiple entrance slit assembly was fabricated and mounted in the focal plane of the exit port of the spectrograph. The multiple entrance slit assembly consisted of an aluminum plate with a horizontal row of 1.588-mm diameter holes spaced on 4-mm centers. This arrangement produced a horizontal row of 29 holes across the 12-cm exit port opening. Each hole was masked from behind the plate with two pieces of black tape to form an entrance “slit” 1.6 mm high. On the basis of spectral resolution measurements using the chromium triplet (425.4,427.5, 429.0 nm), the slit widths obtained by this procedure are on the order of 300 pm. The vertical position of the row of holes was adjusted to that height which produced a maximum signal from the vidicon. Because of the Qpticaldifficulties presented when illuminating multiple slits simultaneously with a single source, fiber optic light guides were chosen to convey the light from the light source to the individual slits. The diameter of the fiber optic light guides was selected so that each fiber optic strand could be plugged into 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
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Focal plane o f entrance
slits Figure 1. Optical arrangement. M,, 45O mirror; M I , focusing mirror; M,, collimating mirror; G, grating
Figure 3. Multiple entrance slit vidicon spectrometer. (1) nitrous oxide-acetylene flame, (2) SIT vidicon detector, (3) fiber optic input lenses, (4) fiber optic entrance slit system, (5) 0.5-m Czerny-Turner spectrograph, (6) optical multichannel analyzer, (7)oscilloscope display
RESULTS A N D D I S C U S S I O N
Figure 2. Fiber-optic system showing input lenses and switchboard entrance slit system
the hole corresponding to any one of the 29 possible entrance slit positions. Those holes corresponding to entrance slit positions not in use were blocked by means of opaque plugs. To improve the light gathering ability of each fiber optic light guide, the input end of each strand was fitted with an input lens. The lenses were mounted on a vertical slotted plate to permit each of seven lenses to be individually adjusted vertically from 3 to 40 mm above the burner head. Figure 2 illustrates the complete entrance slit assembly, showing the vertically adjustable input lenses, the fiber optic light guides, and the entrance slit assembly plate mounted in the focal plane of the instrument. Figure 3 shows the entire instrumental setup used. Note that the entire system is no larger than a conventional flame emission spectrometer. The slot burner has been turned so that the slot is parallel to the lens mounting plate to provide a wide source to adequately illuminate the lenses. Experimental Conditions. The spectra shown in Figure 4 were determined under the following conditions: nitrous oxide flow, 9.5 L/min; acetylene flow, 6.0 L/min. Vertical positions of the fiber optic light guides were optimized for each element. All spectra in Figure 4 are the result of 100 accumulation cycles. The accelerating voltage on the intensifier stage of the SIT was set for 7 kV. The detection limit data shown in Table I1 were obtained using the same oxidant and fuel flow rates given above, as well as the same accelerating voltage. Data spectra were accumulated in memory A by aspirating single element solutions containing lo00 ppm Cs as an ionization buffer into the nitrous oxide-acetylene flame for a period of 100 accumulation cycles. Blank spectra were accumulated in memory B by aspirating blank solutions containing lo00 ppm Cs into the flame for the same number of accumulation cycles. The background-corrected spectra were monitored using the A minus B mode of the multichannel analyzer. The detection limit was taken as that concentration giving a signal-tenoise ratio (S/N) of 2. This value was obtained by extrapolation from S / N measurements made at concentrations slightly above the detection limit. The noise was calculated as the standard deviation of 10 adjacent background channels on either side of the analytical line.
The vidicon spectrometer described in this paper employs multiple entrance slits arranged so that radiation from various spectral regions can be simultaneously imaged on the target of the image detector. The system may be visualized as the reverse of a direct-reading spectrometer; thus, instead of using a single entrance slit and multiple exit slits, as in t h e case of the direct reader, the present system employs multiple entrance slits and a single detector. In contrast to a direct reader which typically requires a long focal length dispersive system to achieve sufficient dispersion in the exit plane to permit multidetector placement, the vidicon spectrometer system described here can employ a short focal length dispersive system. In the direct-reader system, spectral resolution is determined by how closely the multiple exit slits can be placed in the exit focal plane. In contrast to the direct reader, the spectral resolution of this vidicon spectrometer is not limited by how closely the multiple entrance slits can be placed in the entrance focal plane. Entrance slit position is not a t all related to resolution, but instead simply controls the spectral region imaged on the vidicon detector. Resolution depends, rather, on the entrance slit width and t h e reciprocal linear dispersion of the optical system. T h e single multichannel detector employed reads the overlapping spectral information in 500 electronic channels, spaced on 25-km centers, and consequently requires only a short focal length system to give adequate resolution. With a grating producing a reciprocal linear dispersion of 3.2 n m / m m , each separate entrance slit images a 40-nm spectral window onto the 13-mm SIT detector faceplate. Since the entrance slits are spaced over a 12-cm segment of the focal plane, any wavelength within a range of 384 nm can be imaged on t h e detector. By employing several entrance slits simultaneously, widely separated spectral lines of interest may be simultaneously imaged on the detector with adequate resolution. Figure 4 shows the spectra of Mn, Cr, Sr, Ha, and Li obtained with this system using a nitrous oxide-acetylene flame. T o simultaneously monitor this combination of elements with a conventional one-dimensional dispersive system would require a 267-nm window. The spectra illustrated in (a)-(e) represent the spectrum obtained for each element when a single fiber optic light guide is plugged into the appropriate entrance slit (#1-29). T h e spectrum shown in (f) represents the composite spectrum obtained when all five fiber optic light guides are simultaneously plugged into their respective entrance slit holes. T h e background subtraction capability of t h e vidicon/ optical multichannel analyzer system is essential to the success
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 d n
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(a) (b) (C) Figure 5. Impdance of background subtraction. (a) ComposRe flame background from inputs 3, 6, 14, and 23; 100 accumulation cycles; N,O, 5.7 L/min; C,H,, 4.5 L/min. (b) Composite spectrum without background subtraction. Mn, Cr, Ba, 5 ppm; Li and Sr, 1 ppm. Scale expanded slightly over that shown in (a)to show positions of analytical
lines. (c) Background-corrected spectrum
Flgure 4. Spectra obtained with system. (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 by simultaneously using entrance slits 3, 6, 14, and 23. Multielement mixture contained 5 ppm Mn, Cr, and Ba, and 1 ppm Li and Sr
Table 11. Detection Limit Data wavelength, nm 403.0
eninput trance lens slit position, nummma berb 28 3
6,
OD In
detection limit, dmLC
element Mn 0.46 Cr 425.4 28 3 0.56 Sr 460.7 28 6 0.041 0.50 4 14 Ba 553.6 Li 670.8 0.01 3 28 23 0.84 28 29 K 7 66.5 a Vertical distance above burner head. Entrance slit holes on 4-mm centers, numbered from left to right. Accelerator voltage, 7 kV; concentration giving S / N = 2; all solutions buffered with 1000 ppm Cs. of this optical arrangement. When various windows are overlapped as they are in this instrument, the flame background alone can become very large as shown in Figure 5a, where the radiation from a nitrous oxide-acetylene flame is simultaneously monitored from five inputs. Figure 5b shows t h e spectrum of the five elements without background correction. Figure 5c shows the same spectrum after background correction. Table I1 gives detection limit data for Mn, Cr, Sr, Ba, Li, a n d K obtained with the multiple entrance slit vidicon spectrometer. These detection limits were obtained individually under the same set of flame conditions. The vertical positions of the individual input lenses above the burner head are also indicated. These detection limits are about a factor
0
z
(a) Figure 6. Spectrum of tap water. N,O, 9.5 L/min; C,H,, 6.0 L/min; 500 accumulation cycles. (a) Entrance slit #15, (b) entrance slit #IS,
(c) entrance slit #17 of 400 worse than the best values reported in the literature ( 1 4 ) for an SIT detector with an ordinary one-dimensional dispersive system. I t should be remembered that these data represent preliminary results obtained with a prototype system, and it can be anticipated that better slits, higher quality fiber optics and input lenses, and an improved flame geometry will result in lower detection limits. Even with this relatively crude system, the detection limits are all less than 1 ppm, which is adequate for many samples. T h e fiber optic “switchboard” system offers several important advantages for SMA which should be emphasized. Since a line of any given wavelength may be imaged on the detector from any of 3 entrance slit positions, spectral lines may be moved across the target in such a way as to avoid
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
potential spectral interferences as well as regions of intense background. Figure 6 illustrates a typical example of this for sodium in tap water. Note the very intense CaOH band a t 554 nm, which could easily saturate the detector in certain samples, may be easily moved off the target, thereby avoiding problems with detector saturation and blooming. In addition, analytical lines may be selected on the basis of spectrochemical considerations, since any line within a 384-nm region may be accessed. Thus, in contrast to earlier one-dimensional systems, primary resonance lines normally used in analytical work may be employed rather than less satisfactory emission lines which happen to fall in a certain restricted wavelength window. Another important advantage of the switchboard system is that the problems normally associated with the simultaneous determination of major, minor, and trace elements in a single sample can be ameliorated. In single-entrance slit systems, for example, the presence of intense emission from major analyte species may often result in target blooming. In most cases, this situation may easily be avoided with the multiple entrance slit approach. With the switchboard optical system, each element (or, a t most 2 to 3 elements) is monitored by an individual fiber optic light guide, permitting essentially individual adjustment. Intense emission radiation striking the detector target may be attenuated or avoided in several ways: (1)variable neutral density filters may be inserted in front of the particular fiber optic strand(s) without simultaneously attenuating the intensities for all elements; (2) a less sensitive analytical line may be selected in place of a very intense primary resonance line if the emission is due to a major or minor analyte; or (3) a smaller entrance slit may be utilized a t t h a t position. Thus the switchboard system offers many of the individual adjustment features of the direct-reading spectrometer with the added advantages of compactness, flexibility to monitor different combinations of analytical lines, and the ability to monitor background adjacent to a spectral line. Finally, individual observation height adjustment of each fiber optic guide is possible, thereby reducing the severity of
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the compromise conditions required for SMA (15) by eliminating observation height as a factor. In conclusion, it should be noted that the multiple entrance slit approach is not limited to detectors with one-dimensional scan patterns, and may easily be extended to systems with two-dimensional target interrogation capabilities. Also, the multiple entrance slit approach is not limited to vidicon detectors, and may be used with any type of multichannel detector which permits background subtraction. The ultimate criterion for the evaluation of multielement analytical systems is their utility in analyzing real samples. Experiments are currently being carried out in this laboratory to improve the instrumental design of this system based on analytical experience with different multielement samples.
LITERATURE CITED (1) P. W. J. M. Boumans, Fresenius' 2. Anal. Chem., 278, 1 (1976). (2) J. D. Winefordner, J. J. Fitzgerald, and N. Omenetto. Appl. Spectrosc., 29, 369 (1975). (3) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 712A (1973). (4) K. W. Busch and B. Malioy, in "Multichannel Image Devices in Chemistry", Y. Talmi, Ed., ACS Symposium Series, in press. (5) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem.. 46, 575 (1974). (6) K.W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 1231 (1974). (7) N. G. Howell, J. D. Ganjei, and G. H. Morrison, Anal. Chem., 46, 319 (1976). (8)J. D. Ganjei, N. G. Howell, J. R. Roth, and G. H. Morrison, Anal. Chem., 48, 505 (1976). (9) A. Danieisson and P. Lindblorri, Phys. Scripta, 5 , 227 (1972). (10) A. Danieisson and P. Lindblom, Appl. Spectrosc., 3 0 , 151 (1976). (1 1) A. Danielsson, P. Lindbbm, and E. S&rman, Chem. Scripta,6. 5 (1974). (12) D. L. Wood, A. B. Dargis, and D. L. Nash, Appl. Spectrosc., 28, 310 (1975). (13) H. L. Felkel and H. L. Pardue, Anal. Chem., 49, 11 12 (1977). (14) N. G. Howell and G. H. Morrison, Anal. Chem., 49, 106 (1977). (15) D. F. Brost, B. Mailoy, and K. W. Busch, Anal. Chem., 49, 2280 (1977).
RECEIVED for review November 17,1978. Accepted January 25, 1979. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Acknowledgment is also made to the Robert A. Welch Foundation for partial support of this research.
Trace Element Analysis of Bulk Metals with a Hollow Cathode Discharge and a Quadrupole Mass Filter C. G. Bruhn,' B. L. Bentz, and W. W. Harrison* Department of Chemistty, University of Virginia, Charlottesville, Virginia 2290 1
Use of a hollow cathode ionization source with a quadrupole mass filter forms a solids mass spectrometer well suited for trace element analysis of bulk metals. Cathode geometry, particularly bore diameter and nozzle-orifice length, and discharge current effects were studied to yield optimum ion sensitivity. Reproducibility for minor to trace constituents in steel and brass standards was 2-6 YO RSD. Sensitivities were in the low ppma to sub-ppma range.
Trace element analysis by mass spectrometry has generally been carried out using one of two techniques: (a) spark source 'Present address: D e p a r t m e n t o de A n d i s i s I n s t r u m e n t a l , Esc. d e Q u i m i c a y Farmtlcia, U n i v e r s i d a d d e Concepcibn, Casilla 237, Concepci6n, Chile.
mass spectrometry (SSMS) or (bl secondary ion mass spectrometry (SIMS), depending upon the analysis requirements. SSMS ( 1 , 2 ) has the advantages of excellent sensitivity and broad application, but the demonstrated limitations in accuracy and precision, as well as high cost and complexity of operation, have restricted the extent t o which this otherwise powerful technique is employed. SIMS ( 3 , 4 ) , including microanalysis capabilities, is of particular value for surface studies. However, quantitative analysis can involve many difficulties and is often not routine. As an alternative to the spark source mass spectrometer used in our laboratories, we became interested in a gas discharge as a sputter atomization-ionization source for trace element analysis. Instead of relying on the direct secondary ion production as with SIMS, we wished to take advantage of the large sputtered neutral atom population, a portion of
0003-2700/79/0351-0673$01.00/0C 1979 American Chemical Society
