Anal. Chem. 1085, 57, 2049-2055
Values obtained by the two methods were not significantly different (p = 0.788, paired t test for equality of means).
Table I. Control Plasma Data assay day 10
2 3
between-dayb
at 75 ng/mL
at 700 ng/mL
71.30 f 12.03% 76.18 i 12.22% 75.72 i 12.97% 74.40 i 3.62%
691.22 f 3.17% 703.62 5.47% 690.92 i 3.48% 695.25 i 1.04%
*
Mean f relative standard deviation of three replicates at each concentration. Mean f relative standard deviation of daily means. Table 11. Comparison of GC/MS and Microbiological Assays for Quantitation of Plasma Levels in a Cancer Patient Receiving 18 (mg/m2)/day as a 96-h Constant Rate Intravenous Infusion time. h
acivicin concn, (ng/mL) GC/MS microbiological During Infusion
12 48 60 72 96
250 340 320 390 310
150 320 350 360 380
Postinfusion 0.5 1 4 6 12 18
2049
300 290 230 210 150 90
ACKNOWLEDGMENT We acknowledge the assistance of H. Harpootlian and R. C. Thomas, Drug Metabolism Research, The Upjohn Co., in preliminary GC/MS studies. Registry No. Acivicin, 42228-92-2. LITERATURE CITED Poster, D. S.; Bruno, S.; Penta, J.; Nell, G. L.; McGovren, J. P. Cancer Ciin. Trials 1981, 4, 327-330. O'Dwyer, P. J.; Alonso. M. T.; Leyland-Jones, 0. J. CHn. Oncoi. 1984,
2, 1064-1071. Houchens, D. P.; Ovejera, A. A.; Sherldan, M. A.; Johnson, R. K.; Bogden, A. E.; Neil, G. L. Cancer Treat. Rep. 1979, 63, 473-476. Neil, G.L.; Berger, A. E.; Bhuyan, B. K.; Blowers, C. L.; Kuentzel, S. 1. In "Advances In Enzyme Regulatlon"; Weber, G., Ed.; Pergamon Press New York, 1979, Vol. 17, pp 375-398. Denton, J. E.; Lul, M. S.; Croki, T.; Sebolt, J.; Weber, G. Life Sci. 1982, 30, 1073-1080. Maroun, J.; Makysmiuk, A.; Eisenhauer, E.; Stewart, D.; Young, V.; Pater, J. Proc. Am. SOC. Clin. Oncol. 1984, 3, 218. Weiss, G. R.: McGovren, J. P.; Schade, D.; Kufe, D. W. Cancer Res.
1982, 42, 3892-3895. Sridhar, K. S.; Ohnuma, T.; Chablnian, A. P.; Holland, J. F. Cancer Treat. Rep. 1983, 67, 701-703. Murphy, W. K.; Burgess, M. A.; Valdivieso, M.; Bodey, G. P. Proc. Am. SOC. Ciin. Oncol. 1982, 1, 23. Earhart, R. H.; Koeller, J. M.; Davis, T. E.; Borden, E. C.; McGovren, J. P.; Davls, H. L.; Tormey, D. C. Cancer Treat. Rep. 1983, 67,
683-692.
320 310 250 200 130
70
Within-day precision ranged from 12.03% to 12.97% (relative standard deviation) at 75 ng/mL and from 3.17% to 5.47% at 700 ng/mL. Recoveries from the control standards ranged from 95.1% to 101.6%. Table I1 shows a comparison of the GC/MS and microbiological assays for measurement of during and post infusion acivicin plasma levels in a cancer patient treated with 18 (mg/m2)/day for 4 days by constant rate intravenous infusion.
McGovren, J. P.; Nell, G. L.; Sem, P. C. C.; Stewart, J. C. J. Pharmacol. Exp. Ther. 1981, 276, 433-440. McGovren, J. P.; Stewart, J. C.; Elfring, G. L.; Smith, R . B.; Soares, N.; Wood, J. H.; Poplack, D. G.; VonHoff, D. D. Cancer Treat. Rep. 1982, 66. 1333-1341. Rosenblum, M. G., M.D. Anderson Hospital and Tumor Institute, Houston, TX, personal communlcation. Ames, M. M., Mayo Clinic, Rochester, MN, personal communlcation. Yamamoto, S.; Kiyama, S.; Watanabe, Y.; Makita, M. J. Chromafogr. 1982, 233, 39-50. Hanka, L. J.; Gerpheide, S. A.; Spieles, P. R.; Martln, D. G.; Belter, P. A.; Coleman, T. A.; Meyer, H. F. Anfimicrob. Agents Chemofher. lQ76. ., 7 ,. 807-810 -- - .
Kelly, R. C.; Schletter, I.; Stein, S. J.; Wlerenga, W. J. Am. Chem. SOC. 1979, 101, 1054-1056. "Technical Bulletln"; Aldrich Chemical Co.; Mllwaukee, WI.
RECEIVED for review February 22, 1985. Accepted May 20, 1985.
Optical Imaging Spectrometers John W. Olesik' and Gary M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Two optical systems for acquisition of two-dimensional images are described. The systems, respectively, an electronic slltless spectrograph and a monochromatic imaging spectrometer, both use a Crerny-Turner mon6chromator for spectral discrimination. A silicon intensified target vidicon detector provides quantitative images In time-integrated or time-resolved modes. By use of simple single-lens imagetransfer optics with a monochromator, spatial resolution on the order of 0.3 mm is attainable. Appiicatlans of the imaging spectrometers for spatial mapping of inductively coupled plasmas and flames are illustrated.
'Present address: Department of Chemistry, Venable and Kenan Laboratories 045A, University of North Carolina, Chapel Hill, NC 27514.
Spatial resolution in two or three dimensions in needed for many optical spectroscopic measurements. Sources for atomic spectrometry including flames, arc and spark discharges, and inductively coupled plasmas (ICP) all exhibit spatial heterogeneity (1-6). In order to understand and ultimately control the processes involved in conversion of sample material to emitting or absorbing atoms, spatial maps of ground-state and excited-state species are required. Similarly, studies of combustion systems such as internal combustion engines (7,8), gas turbine engines, or laboratory flames (9,10)often employ spatially resolved optical probes (11) of atomic and molecular species. Many analytical samples also exhibit chemical or physical spatial heterogeneity (12, 13). Therefore, analysis of the sample must include a description of the analytes that are
0003-2700/85/0357-2049$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
present, the concentration or amount, and their spatial location within the sample. Optical techniques based on fluorescence, absorption, scattering or other processes could be used to obtain the desired information as a chemical image (14). A number of approaches have been used previously to obtain spatial resolution with optical probes. Point-by-point measurements can be made in order to construct a complete map. The required optical system and its calibration are relatively simple. Also, a low-cost, sensitive detector such as a photomultiplier can be used. However, the measurement process is slow if a large number of spatial positions must be viewed sequentially. For example acquisition of a 500 X 50 array of points with a 1-s per point integration time would require 6.9 h. If the chemical spatially resolved measurements are based on atomic or molecular emission or absorption, tomography (15)or Abel inversion (16,177 of the laterally resolved, lineof-sight data is required to obtain three-dimensional or radially resolved data. Both techniques require a large number of line-of-sight intensity values to be obtained. The Abel inversion technique also has a number of experimental requirements such as cylindrical or spherical symmetry, positional stability, and a lack of self-absorption (18). Probes based on fluorescence (IP-22) or saturated absorption (23-25) can obtain three-dimensional spatial resolution directly, in a point-by-point manner. These methods use a combination of imaging on the entrance slit of a monochromator and the incident laser beam size or the overlap of two crossed beams, respectively, to provide spatial resolution. While these methods do not require Abel inversion for three-dimensionalresolution, they can be time-consuming due to the point-by-point mapping. Long acquisition times are often required in order to obtain good signal-to-noise ratio because of the low duty cycle of the pulsed laser sources. Linear photodiode arrays (LDA) (26,277 and intensified LDAs (28)have been used to view a one-dimensional slice of light from atomic or combustion sources. The LDA-based systems can result in a reduction in the acquisition time of up to a factor equal to N , the number of detector elements. However, because the LDA has a lower gain than a photomultiplier tube and requires a longer acquisition time for a good signal-to-noiseratio, the fador of N may not be achieved. If the entire two- or three-dimensional image could be detected simultaneously, there would be a greater reduction in acquisition time. Also, nonreproducible or transient systems such as turbulent combustion or high-energy plasmas could be studied. Experimental systems which simultaneously acquire data in two dimensions have been developed. If an interference filter provides sufficient spectral discrimination, a two-dimensional imaging detector, such as a vidicon, can be used. Radially resolved emission data can be obtained using Abel inversion, if the necessary conditions (18) are met. Similarly, three-dimensional spatial resolution can be obtained directly by using a thin sheet of exciting radiation or a multipass cell and monitoring the resulting fluorescence (2S32)or Raman (33,34)signal. Two spatial dimensions are then viewed simultaneously by an imaging detector while the resolution in the third dimension is provided by the width of the narrow sheet of exciting radiation. However, the interference filter possesses a number of limitations as a spectral discriminator. A different filter is required for each spectral line or band. Further, the spectral band-pass (10 nm, typically) may be too large for many applications such as monitoring atomic emission lines in electrical discharges. In this paper we describe two optical systems using monochromator-based spectral discrimination which provide
SOURCE
DEMAGNIFIED IMAGE OF SOURCE
IMAGING OPTICS
n
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,
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DEMAGN'F'ED IMAGE OF SOURCE
Figure 1. Electronic slitless spectrograph: A, aperture; L, lens; M i ,
spectrograph collimating mirror; G, grating; M2, spectrograph focusing mirror.
DISPLAY
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Figure 2. Monochromatic imaging spectrometer: L1, colllmating lens; focal length of lens L1; Si,entrance slit; M1, M2, concave mirrors;
fL,,
G, gratlng; S2, exit silt; L2, focusing lens; f,,
focal length of lens L2.
detection of two-dimensional images. A "slitless Spectrograph" can be used to obtain simultaneouslyboth spectral resolution and two-dimensional imaging. In this arrangement, the usual entrance slit is removed from the spectrograph. A demagnified image of the source is formed a t the object plane of the spectrograph and effectively serves as the entrance slit. A photographic plate has been used as the detector previously (35). However, the acquisition of quantitative data from the photographic plate is complicated by its nonlinear response and time required for point-by-point densitometry. In the present article, an electronic slitless spectrograph (ESS) is described (shown in Figure 1) that provides quantitative two-dimensional data directly by using a silicon intensified target (SIT) vidicon detector. Further, the inherent temporally resolved readout or time-gating capability of the SIT vidicon can be used to provide time resolution from seconds to microseconds (36-38). Unfortunately, the electronic slitless spectrograph suffers from a trade-off between spectral and spatial resolution. In order to obtain high spectral resolution (a small band-pass), narrow slits should be used, requiring great demagnification of the source. However, because the obtainable spatial resolution should be limited by the smallest detector element, the limiting resolvable distance in the source will be increased as the source image size is decreased. Conversely, as the source image size is increased to improve spatial resolution, the experimental band-pass is increased, degrading the spectral resolution. If the image of the source is totally out of focus at the entrance and exit slits of a monochromator, and reconstructed after passing through the exit slit, the spectral and spatial resolution can be controlled independently. The monochromatic imaging spectrometer (MIS) described here (shown in Figure 2) is based on this idea using a CzernyTurner monochromator. Light from the source is collimated before being fed into the monochromator. When the distance from M1 (normally the collimating mirror) to the grating is equal to the focal length of M1, an image of the source is formed on the grating. The light is recollimated by M2 (normally the focusing mirror) before passing through the exit slit. Secondary optics re-form the image of the source outside of the monochromator, where the imaging detector is placed. The experimental band-pass is controlled by the entrance and exit slit widths, the grating angles, and the focal length of the
-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985 WAVELENGTH (nm)
monochromator. The spatial resolution is determined by the optics outside of the monochromator, the detector, and the fidelity of the entire optical system. EXPERIMENTAL SECTION Electronic Slitless Spectrograph Optics (Figure 1). A Jobin-Yvon (Model HR-1000) 1-m Czerny-Turnermonochromator was used with a 2400 lines/mm holographic grating. The source (described below) was imaged onto the normal position of the entrance plane with a magnification of 0.2 by a plano-convex quartz lens (7.5 cm focal length, Oriel Corp.). The lens aperture was adjusted (1.5 mm, typical) to provide a detectable signal without vidicon saturation. The entrance slit was fully opened (2 mm) to allow a 10 mm wide portion of the plasma to be viewed. A silicon intensified target vidicon (Princeton Applied Research Corp., Model 1205D) was mounted onto the camera port of the monochromator. Lens L was repositioned each time the wavelength was changed in order to compensate for chromatic aberration. The correct focal position was determined by backlighting a precision screen target and positioning lens L for best image fidelity. The vidicon-reportedspatial position was calibrated by backlightingwith diffuse light a target having millimeter divisions. A reference target which was 10 mm above the ICP load coils was backlit to determine the vertical position of the detected source image. Monochromatic Imaging Spectrometer Optics (Figure 2). A Heath (Model EU-700) 0.35-m Czerny-Turnermonochromator was used with a 1200 lines/mm grating. Light from the source (flame or ICP) was collimated by either of two plano-convex fused silica lenses: 35-cm focal length (Oriel) or 25.4-cm focal length (Esco Products, Model A120100, S1-UV grade). Light passing through the monochromator was reimaged by a 15-cmfocal length, plano-convex, fused silica lens (Oriel, Model 4177, UV grade research quality) onto the face of the vidicon. This lens (L2) was placed on a stage which could be translated along the optical axis without opening the enclosure between the monochromator and the detector housing. Lens L1 was mounted on an optical rider (39,40)which was easily and precisely translated along the optical axis. The slit widths were set depending on the signal intensity and desired spectral resolution. The lens apertures were set to provide detectable signals without saturation of the detector (typcially 1 cm or less). Lens L1 was positioned at a distance from the source calculated to be its focal length at the particular wavelength passed through the exit slit. Lens L2 was then positioned for best resolution of an image formed by backlighting a 250- or 325-pm screen with diffuse light. The vidicon-reported distances were calibrated as described above. Optical Multichannel Analyzer (OMA). A Model 1205 OMA system (Princeton Applied Research Corp.) was used including Model 1205A console with optional two-dimensionalscan card and Model 1205D silicon intensified target vidicon. For time-gated experiments,a high-voltage pulser (PARC Model 1211) was used to gate the vidicon intensifier. The SIT vidicon was cooled to approximately -40 "C with dry ice (PARC Model 1212 cooled housing), for some experiments. Specific experimental conditions used for each detected image will be listed with the experimental results. Quantitative intensity data are provided by the vidicon. However, the sensitivity is a function of the particular resolution element, called a pixel, or spatial position on the detector face. The relative standard deviation in response is specified to be &lo%. The response could be measured experimentally (36,37, 41) and experimental data corrected for pixel-to-pixel variation by ratioing. Data Acquisition System . The data acquisition and control system consisted of a DEC MINC-11 laboratory minicomputer, Z-80 microprocessor (to generate A/D conversion trigger signals for each of the detector elements, or pixels, whose signals were to be digitized and stored) and a circuit to enable/disable the electron-beam readout in the vidicon and triggering of the high-voltage pulser for time-gated detectisn. The data acquisition and control system is described in detail in ref 42. Inductively Coupled Plasma System. The ICP system consisted of a 2.5-kW, 40.68-MHz crystal-controlledpower supply (Plasma-Therm, Inc., Model HLF-2000D), an automatic impe-
2051
589.0
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8.9
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LATERAL POSITION
(mm)
Flgure 3. Laterally and vertically resolved emission due to the sodium D lines emitted by an lnductlvely coupled plasma as reported by the
electronic slltless spectrograph. Experimental conditions were as follows: ICP, low flow (5.9 L/mln coolant), low power (300 W); detection, 15 frames summed in memory, 48 tracks over the 10 mm detector height, collected tracks 10-48 (observing heights in the ICP from 8 mm below the top of the load coil to 32 mm above the load coil), channels 200-299.
dance-matching unit (Plasma-Therm, Inc., Model PT/AMN/ RCM), a conventional-sized, water-cooled three-turn load coil, and a low-flow, low-power torch design (43). Power and argon flow rates are listed with each example described in the text. Air-Acetylene Flame. The flame was supported on a modified (44) Meker burner. Air and acetylene flow rates were approximately 16 L/min and 2.4 L/min, respectively.
RESULTS AND DISCUSSION Electronic Slitless Spectrograph. The horizontal axis of images detected through the electronic slitless spectrograph is a function of both wavelength and lateral position within the light source; the vertical axis is simply height within the source for a stigmatic spectrograph. Images of the emitted sodium D lines from a nebulized solution of sodium chloride introduced into an inductively coupled plasma are shown in Figure 3 and illustrate this behavior. When radiation consists of wide bands or continuum radiation, the wavelength and lateral spatial information overlaps. For example, the background continuum emission from the ICP cannot be laterally resolved. Molecular bands could also not be laterally resolved if the distance along the focal plane is of the same order of magnitude as or larger than the spatial lateral distance within the light source. Allemand (45) has described a subtractive double monochromator arrangement which overcomes this problem. The trade-off between spectral and spatial resolution is governed by the size of the demagnified image which acts as the entrance slit. For Figure 3, the effective entrance slit width was approximately 1.2 mm, as defined by the width of the Na emission plume in the ICP and the demagnification. At 589 nm the reciprocal linear dispersion of the spectrometer is approximately 0.4 nm/mm, resulting in a band-pass of 0.5 nm. The observed spatial resolution will be a function of the detector limiting resolution, optical aberrations, and the size of the final image. Each pixel of the vidicon is approximately 25 pm in diameter. However, blooming (36,37)results in a smearing of the detected image; the resulting vidicon-limited resolution is approximately 10-15 line pairs/mm for valleys with 50% the intensity of the peak signal.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
B
Flgure 4. Images detected by (A) electronic slitless spectrograph and (B) monochromatic imaging spectrometer due to backlighted test target. The target line spacing in line pairs per millimeter is (1) 2.24, (2) 2.00, (3) 1.78, (4) 1.59, (5) 1.41, (6) 1.26, (7) 2.00, (8) 2.24, (9) 2.52, (10) 2.83, (11) 3.17, (12) 3.56, and (13) 1.00.
Optical aberrations include chromatic aberration, coma, astigmatism, and spherical aberration (46,47). Loss of image fidelity due to chromatic aberration is eliminated by repositioning the lens for each significant change in wavelength. The source must also be repositioned if the magnification is to be held constant since the effective focal length of the lens is a function of wavelength. Coma introduced by the Czerny-Turner monochromator is small due to the arrangement of mirrors. The lens will introduce little coma since it is used on axis. However, the coma will increase for portions of the object which are removed from the optical axis. The most significant aberration expected to be introduced by the ESS optical system is astigmatism. Astigmatism is minimized by use of small off-axis angles for the concave monochromator mirrors. It is also possible that the grating acting as a nonideal plane mirror will result in a loss of image fidelity. As can be seen from Figure 4A, a resolution (horizontal) of approximately 2 line pairs/mm in the source was observed in the present system with valley intensities 50% of peak intensities reported by the ESS. The most significant limitation on spatial resolution is the detector. If a perfect image were produced with a magnification of 0.2, the detector-limited resolution would be 2 to 3 line pairs/mm. The experimental measure of resolution indicates that the detector is the most significant limitation on the detected image resolution. The vertical resolution was further limited by the detector, in particular, by the number of tracks used. Because our applications demanded a higher spatial resolution in the horizontal than in the vertical dimension, 32 to 96 tracks (vertical resolution elements) were used compared to 500 horizontal channels. As a result, the vertical resolution obtained is 5 to 15 times poorer than the horizontal resolution. Of course, the detector-limited spatial resolution could be improved by using auxiliary optics to magnify the images at the spectrometer focal plane onto the faceplate of the vidicon. Allemand's subtractive double monochromator based electronic slitless spectrograph (45) uses a 20-fold demagnification of the source and subsequent 20-fold magnification, apparently with good fidelity. The sensitivity of the electronic slitless spectrograph is high. For example, a 1.5-mm aperture was used for the images detected in Figure 3. Moreover, the sensitivity of the SIT vidicon was reduced by more than 2 orders of magnitude so that it was similar to that provided by an unintensified vidicon. Although a large sodium concentration was used (350 ppm), detection of the emission image with a small light collection angle (effective f / # is 50) and no intensification demonstrates
the high sensitivity of the system. With an increase of the intensifier voltage to its typical value, a similar signal would be obtained for a solution of less than 4 ppm. For more dilute solutions, the lens aperture could be increased. The ability to use a small-aperture light-collection system provides two advantages. Aberrations, such as spherical aberration, are minimized and the depth of field is large. This characteristic is advantageous when observing sources with a large depth along the optical axis and when Abel inversion is to be used to transform the laterally resolved intensity data into radially resolved information. M o n o c h r o m a t i c Imaging S p e c t r o m e t e r . The spatial resolution provided by the monochromatic imaging spectrometer (Figure 2) is determined by the image magnification, aberrations, and the detector resolution but is independent of the spectral resolution. In turn, the image magnification is controlled by the collimating and reimaging optics. For example, if the light is collimated entering and exiting the monochromator,the magnification is equal to the ratio of the focal lengths of the collimating to reimaging lenses. In fact, in our system the distance from mirror M1 (normally collimating mirror of the monochromator) to the grating is approximately 34 cm while the distances from the slit to the mirror and the mirror focal length are 35 cm. Therefore, an intermediate source image is formed 1cm past the grating and approximately 34 cm in front of mirror M2. As a result, a virtual image is formed approximately 1240 cm in front of mirror M2. The virtual image then acts as an object for the reimaging lens, L2. Because the object distance is large compared to the focal length of L2, the distance from L2 to the image it forms is similar to that if the light incident on L2 were collimated (15.2 cm rather than 15.0 cm). Caution must be exercised when the collimating and reimaging optics are configured with a particular monochromator. Field-limiting and vignetting effects are especially important and often subtle. For example, the size of the intermediate image must not be larger than the grating or the grating will act as a field stop and part of the image will be lost. The magnification of this intermediate image is controlled by the ratio of the focal length of monochromator mirror M1 to that of lens L1. Also, as the grating angle is changed, the effective width of the grating perpendicular to the optical path is altered. Vignetting effects might occur also at the slits of the monochromator if the light is not collimated at that point. The effect can be very subtle and will result in changes in the light-gathering efficiency with position within the optical source. Vignetting by the slits is minimized by placing the external lenses as close to the slits as is practical. One might be tempted to pay little attention to the distance of lens L1 from mirror M1 since the light is collimated by L1. However, a field-limiting effect can occur if the aperture of lens L1 is imaged near the intermediate or final source image. For example, if lens L1 were placed 2 m from mirror M1, the lens aperture would be focused 42 mm past mirror M1, or 7 cm past the grating. The demagnified (0.21X) image of the lens aperture would then act as a field stop for the intermediate image of the source formed near the grating. Again, this effect is minimized by placing L1 as close to the entrance slit as possible. A number of aberrations must be considered when assessing the spatial resolution provided by the MIS. In particular, one must not overlook the aberrations introduced by the monochromator and the external optics. Loss of image fidelity due to chromatic aberration is minimized by repositioning the two lenses for each wavelength. Astigmatism caused by the monochromator will be significant due to the relatively large off-axis mirror angles used in the Heath 0.35-m monochro-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
mator compared to the 1.0-m JY monochromator used in the ESS. Coma introduced by the Czerny-Turnermonochromator will be small. A measure of the observed lateral spatial resolution obtained with our system is indicated by detecting the image of a backlighted test target as shown in Figure 4B. The image magnification was approximately 0.42. The observed resolution is better than 3.5 line pairs/mm with valley intensities 50% of peak intensities reported by the MIS. If perfect image fidelity were obtained, a vidicon-limited resolution of 4 to 6 line pairs/mm would be expected. While the spatial resolution could be improved by increasing the magnification of the image, a smaller portion of the source would then be detected by the 10 x 12 mm vidicon active area. The usable size of the vidicon is further limited by distortion effects which are most severe near the edges (36, 37). The vertical spatial resolution is limited by the number of tracks (lateral slices) which are used to read out the vidicon signal. When the maximum number of tracks is used (256 in our system), the vertical resolution is approximately 2 times poorer than the lateral resolution. Typically, 32 to 96 tracks were used and provided a vertical spatial resolution of 0.3 to 0.1 mm at the vidicon or 0.6 to 0.2 mm in the source when a magnification of approximately 0.5 was used. The optical fidelity could be improved in a number of ways. The most significant aberration is probably astigmatism. A monochromator with smaller off-axis mirror angles (such as the 1.0-m monochromator used for the ESS) would produce less astigmatism (46,47). Also, it is possible to compensate for astigmatism by using collimating and reimaging mirrors in an over-under arrangement with the appropriate off-axis angle in place of the lenses (46,47).The all-reflective optical system would also suffer no chromatic aberration. The importance of the characteristics of the monochromator used to process two-dimensionalimages was illustrated when a concave grating monochromator with large off-axis angles (a Jobin-Yvon H-20) was used. With lenses placed close to the entrance and exit slits, an image of the source was produced on the grating. While the image fidelity appeared adequate when zero-order light was passed through the exit slit, the spatial resolution degraded as the wavelength and the grating angle were increased. The sensitivity of this system is less than the electronic slitless spectrograph since smaller slit widths are employed. However, the sensitivity was sufficient to require introduction of a 1.6 optical density neutral density fiiter when Ar 415.9-nm light was viewed from a 300-W ICP. Further, light from the Ar 415-nm line was detected during time-gated operation using a gate width of 50 ps each 5 ms (a duty cycle of 0.01). Signals from a 500 ppm solution of sodium introduced through a pneumatic nebulizer required introduction of a 1.0 optical density neutral density filter to prevent detector saturation. Therefore, similar signals would be obtained from a 50 ppm solution of sodium without the neutral density filter. The monochromatic imaging spectrometer is somewhat similar in principle to the molecular optics laser examiner (MOLE) system (48,49).However, the MOLE system employs a double monochromator with concave gratings for spectral resolution and imaging of the entrance slit on the focal plane. In contrast to the MIS, the internal optics of the MOLE monochromator are not used to collimate light from the source or reimage collimated light from the monochromator. Therefore, the lenses used to image the source on the grating and transfer that image outside of the monochromator must be placed as close as possible to the entrance and exit slits in order to minimize vignetting. Since light fed to the entrance slit of the MIS is collimated and light exiting the monochromator is nearly collimated, it is not imperative that
50 ppm Ca
2053
50 ppm Ca
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Figure 5. Laterally and vertically resolved Ca I (422.7 nm) emission from a low-flow, low-power ICP. ICP and detector parameters are listed in Figure 3.
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Figure 6. Spatially resolved, time-integrated Na emission from a nitrogen-sheathed, air-acetylene flame. Sodium was introduced as 60 pm diameter droplets of 50 ppm NaCl at a rate of 496 Hz. Detection conditions were as follows: 20 frames summed in memory (40 s total); 64 tracks total; collected signal from tracks 6-35 and channels 1362 9 6 vidicon cooled with dry ice. The 0 to 7 and 0 to 15 scales indicate the relative intensity of tracks 11 and 21.
the external optics be placed close to the slits. (However, the external optics must then be large enough to prevent fieldlimiting effects as discussed above.) As a result, simple spherical mirrors could be used as the external optics for the MIS to provide an all-reflective optical system or compensate for astigmatism. Such an arrangement could not be used with the MOLE. Applications of Imaging Spectrometers to the Spatial Mapping of Atomic Sources. The imaging spectrometers described above have been used for fundamental investigations of inductively coupled plasma and flame sources. A continuous detection mode was used to study the effect of the addition of easily ionizable elements on calcium atom and ion emission. Figure 5 shows a portion of the detected images for calcium atom emission. The vaporization and atomization of sample material in flames have been studied by introducing monodisperse droplets along a reproducible path into an air-acetylene flame. This scheme (44,50-53) allows a separation of the vaporization and atomization processes in time and space, unlike the situation when an aerosol with a range of droplet sizes is sprayed into the source. Figure 6 shows the time-integrated detection of sodium emission from vaporizing particles of sodium
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
trometer. Characterizations under way include ray tracing and third-order aberrations analysis and more complete experimental measurement of the two-dimensional spatial resolution and aberrations including astigmatism, possible field distortion or curvature, and wavelength (grating angle) dependent effects. Reflective optical systems to compensate for aberrations such as astigmatism and eliminate chromatic aberrations are being designed and tested.
VERTlCAL POSITfON I
2.5
5
3
I
I
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LATERAL POSITION
Figure 7. Spatially resolved, time-gated Na emission. Source and sample Introduction are the same as those llsted In Flgure 6. Detection conditions were as follows: 10 frames summed, 32 tracks total; collected signal from tracks 4-23, channels 159-408, vidicon cooled with dry ice, 10 ps gate triggered 0.50 ms after the beginning of atomization (indicated by the appearance of emission),stroboscopic measurement with a total of approximately 5100 gates (indivkluai atom clouds). chloride. In order to view directly atom clouds in the flame, a time-gated detection mode was used. Figure 7 shows atom clouds detected stroboscopicallywith a 50-ps time gate. Data detected in this manner allow the direct measurement of the flame-gas velocity, particle vaporization rates, and diffusion rates of the resulting atoms (50-53). CONCLUSIONS The imaging spectrometers described here can provide a quantitative two-dimensional image of optical sources. The simultaneous detection in two dimensions dramatically reduces the time required for many experiments. Some experiments which could not reasonably be done with a point-by-point detection scheme because of the necessary acquisition time or source irreproducibility can be performed with an imaging spectrometer. Radial spatial information could be obtained by Abel inversion of the laterally resolved data. The systems described here could also be used with absorption, scattering, or fluorescence probes with similar spatial resolution and with spectral resolution far better than that available from interference filter based systems. Improvements to the system described here could be accomplished with better detectors. The blooming, lag, and narrow dynamic range of the SIT vidicon (particularly of this first generation OMA system) limit the performance of the system. Use of second-generation vidicons or newly developed charge-coupled device or charge-injection device detectors would result in improved capabilities. Changes in the data acquisition system would significantly improve the overall performance of the imaging monochromators. The vidicon-based system used here was not originally designed for two-dimensional detection. As a result, the frame readout time is long (32 ms times the number of tracks) when a large number of tracks are detected. The computer data collection system could only store images of 5000 or fewer detection elements. Video data collection systems using frame grabbing buffer systems would allow real time signal averaging and background subtraction. Also, newer versions of the optical multichannel analyzer are more suited to two-dimensional detection than the system used for this work. If one compares the electronic slitless spectrograph (ESS) and monochromatic imaging spectrometer (MIS), the ESS is most useful when spectral and spatial resolution can be traded for higher sensitivity and when nearby wavelength images do not overlap. Also, the ESS is a less complicated optical system. In contrast, the MIS can provide high spectral and spatial resolution simultaneously if sufficient light is available. Although the performance of the ESS and MIS described here are adequate for many applications, work is continuing (J.W.O.) to further improve and characterize the optical fidelity and aberrations of the monochromatic imaging spec-
ACKNOWLEDGMENT The work of Ray Sporleder who designed the Z-80 microprocessor and wrote software for it and Robert Deutsch who aided in development of the MINC software is acknowledged. Registry No. Na, 7440-23-5; Ca, 7440-70-2. LITERATURE CITED Rann, C. S.;Hambly, A. N. Anal. Chem. 1965, 37,879. Walters, J. P.; Goldstein, S.A. Spectrochlm. Acta, Part B 1984, 398, 693. Blades, M. W.; Horllck, G. Spectrochim. Acta, Part B 1981, 368,861. Koirtyohann, S.R.; Jones, J. S.;Jestor, C. P.; Yates, D. A. Spectrochlm. Acta, Part B 1981, 368, 49. Blades, M. W.; Horllck, 0.Spectrochim. Acta, Part B 1981, 368,881. Furuta, N.; Horllck, G. Spectrochim. Acta, Part B 1982, 378, 53. Abata, D. L; Myers, P. S.;Uyehara, 0. A. SOC.Automot. Eng. 1979, paper 780970. Klick, D.; Marko, K. A.; Rimai, L. Appl. Opt. 1981, 20, 1178. Crosley, D. R., Ed. "Laser Probes for Combustion Chemistry"; Amerlcan Chemical Society: Washlngton, DC, 1960; ACS Symp. Ser. No. 134. Kychakoff, G.; Klmball-Linne, M. A.; Hanson, R. K. Appl. Opt. 1983, 22, 1426. Eckbreth, A. C.; Bonczyk, P. A.; Verdieck, J. F. Appl. Spectrosc. Rev. 1977, 73 (I), 15. Drummer, D. M.; Morrison, G. H. Anal. Chem. 1980, 52, 2147. Vandeginste, 9. G. M.; Kowalski, B. R. Anal. Chem. 1983, 55, 557. Callls, J. 8.; Bruckner, A. P. I n "Multichannel Image Detectors"; Talml, Y.. Ed.; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. 236, Vol. 2, p 234. Bennett, K.; Byer, R. L. Opt. Left. 1984, 9 , 270. Cremers, C. J.; Blrkeback, R. C. Appi. Opt. 1966, 5 , 1057. Choi, B. S.; Klm, H. Appl. Spectrosc. 1982, 36, 71. Scheeline, A.; Walters, J. P. Anal. Chem. 1978, 48, 1519. Piepmeier, E. H. Spectrochlm. Acta, Part B 1972, 431. Ornenetto, N.; Hart, L. P.; Benettl, P.; Winefordner, J. D. Spectrochim. Acta, Part B 1973, 288, 301. Daily, J. W.; Chan, C. Combust. Flame 1978, 33,47. Omenetto, N.; Winefordner, J. D. Prog. Anal. At. Spectrosc. 1979, 2 , 1. Walters. P. E.; Long, G. L.; Winefordner, J. D. Spectrochim. Acta , Part B 1984, 39,69. Walters, P. E.; Lanauze, J.; Winefordner, J. D. Spectrochim. Acta, Part B 1984, 398, 125. Kychakoff, G.;Howe, R. D.; Hanson, R. K. Appl. Opt. 1984, 23, 1303. Horlick, G.;Codding, E. I n "Contemporary Topics in Analytical and Clinical Chemistry"; Hercules, D. M., Hieftje, G. M., Synder, L. R., Evenson, M. A., Eds.; Plenum Press: New York, 1978; Vol. 1, p 195. Edmonds, T. E.; Horlick, G. Appl. Spectrosc. 1977, 31, 536. Alden, M.; Edner, H.; Holmstedt, G.; Svanberg, S . ; Hogberg, T. Appl. Opt. 1982, 21, 1236. Dyer, M. J.; Crosley, D. R. Opt. Left. 1982, 7 ,382. Kychakoff. 0.;Howe, R. D.; Hanson, R. K. Appl. Opt. 1984. 23, 704. Daugal, R. A.; Willlarns, P. F.; Pease, D. C. Rev. Sci. Instrum. 1963, 5 4 , 572. Dyer, M. J.; Crosley, D. R. O p t . Len. 1964, 9 ,217. Hartley, D. L. I n "Laser Raman Gas Dlagnositics"; Lapp, M., Penney, C. M., Eds.; Plenum Press: New York, 1974; p 311. Long, M. B.; Fourguette. D. C.; Escoda, M. C. Opt. Left. 1983, 8 , 244. Horlick, G.; Furuta, N. Spectrochim. Acta, Part B 1982, 37, 999. Oleslk, J. W.; Walters, J. P. I n "Multichannel Image Detectors"; Talmi, Y ., Ed.; Amerlcan Chemical Soclety: Washlngton, DC, 1983; ACS Symp. Ser. No. 236, Vol. 2, Oleslk, J. W.; Walters, J. P. Appl. Spectrosc. 1984, 38, 578. Talmi, Y. "Optoelectronic Image Detectors in Chemistry, an Overvlew", I n "Multichannel Image Detectors"; Talrni, Y., Ed.; Amerlcan Chemical Society: Washington, DC, 1979; ACS Symp. Ser. No. 102, Vol 1. Walters, J. P. I n "Contemporary Topics in Analytlcal and Clinical Chemistry"; Hercules, D. M., Hieftje, G. M., Synder, L. R., Evenson, M. A., Eds.; Plenum Press: New York; 1978; Vol. 3, p 91. Coleman, D. M.; Walters, J. P. Spectrochlm.Acta, Part B 1976, 318, 547. Colgate, S.A,; Moore, E. P.; Colburn, J. Appl. Opt. 1975, 1 4 , 1429. Oleslk, J. W.; Deutsch, R. D.; Sporleder, R.; Hieftje, G. M. submitted for publication In Anal. Chim. Acta. Rezaaiyann, R.; Hieftje, G. M.; Anderson, H.; Kaiser, H.; Meddings, B. Appl. Spectrosc. 1982, 36, 627. Clampltt, N. C.; Hiefile, G. M. Anal. Chem. 1979, 4 4 , 1211. Allemand, C. Appl. Opt. 1983, 22, 16.
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RECEIVED for review November 29, 1984. Resubmitted May
2055
13, 1985. Accepted May 13, 1985. This work was supported by the National Science Foundation through Grants CHE 82-14121 and CHE 83-20053 and bv the Office of Naval Research. Portions of this work were presented at the Tenth Annual Meeting of the Federation of Analytical Chemistry and SpectroscopySocieties (Sept 1983, Philadelphia, PA) and the Thirty-fifth Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy (March 1984, Atlantic City, NJ).
Controlled Potential Electrolysis Coupled with a Direct Sample Insertion Device for Multielement Determination of Heavy Metals by Inductively Coupled Plasma Atomic Emission Spectrometry Magdi M. Habib and Eric D. Salin*
Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada
The appilcatlon of controlled potential electrolysis with both graphite electrodes and a hanglng mercury drop electrode as a separatlon and preconcentratlon technique for Inductively coupled plasma (ICP) atomlc emission spectrometry uslng the direct sample lnsertlon device (DSID) is described. Heavy metal Ions in aqueous solution are determined. With a deposltlon time of 5 min the detection llmlts under compromise condHions are 2.4, 680, 2.0, 175, 25, and 259 ng/mL for Cu, Pb, Zn, Cd, Ni, and Co, respectively. A determination of Cu at the 63 ng/mL level In artlflclai seawater (3.5% salinity) was made with a 4% error.
The application of inductively coupled plasma atomic emission spectrometry (ICP-AES) to the simultaneous determination of major and minor and trace level elements in various matrices has been well documented (1-5). Pneumatic nebulization appears to be the most popular method of sample introduction although the sensitivity attainable is not sufficient for the ICP analysis of many elements which are present in the nanogram per gram range (6). A number of studies have concentrated on developing methods for isolating trace elements from complex matrices including coprecipitation (7), chelation (8, 9), chromatography (IO),and conversion into hydrides (11). In all of these isolation methods large volumes of additional chemicals are brought into contact with the samples and thus may introduce contaminating or interfering species. In addition, some of these techniques are time-consuming and tedious. Ultrasonic nebulizers have demonstrated improvements in working range by factors of 1.1-12 (12-14) in various matrices; however, considerablequestion still exists about the general reliability and freedom from interferences of these devices (15). Separation of heavy metal ions from various matrices by controlled potential electrolysis is often a very useful method in trace analysis (16-19). By the use of mercury electrodes, a number of heavy metals can be deposited even from acidic aqueous solution because of the broad cathodic potential range 0003-2700/85/0357-2055$01.50/0
(20). This separation technique has found wide application in the field of atomic spectrometry. Higher sensitivity than conventional techniques was obtained because of the sample preconcentration during electrolysis and the potential power of the technique to separate trace elements from complex interfering matrices. Electrolysis has been performed on metal wires (21-25), carbon rods (26, 2 3 , hanging mercury drop electrodes (28, 29), and tubular pyrolytic graphite-coated electrodes (30)for spectrochemical applications. The technique has also been applied to flame AA, using a thin film of mercury deposited on a wax-impregnated graphite rod (31), to a direct current arc using a hanging mercury drop electrode (HMDE) (32) and to a helium microwave induced plasma (He-MIP) (33). A wall-jet electrochemical cell for preconcentration of trace metals from flowing streams prior to their determination by ICP with conventional pneumatic nebulization was also described (34). We previously reported preliminary results on controlled potential electrolysis coupled with ICP-AES for the determination of copper in aqueous solutions (35). The method involved the electrodeposition of copper from an aqueous solution of copper nitrate onto a piece of spectrographic graphite electrode previously coated with mercury. After the completion of the electrolysis the electrode was demounted from its holder and mounted on the top of the quartz rod of the direct sample insertion device (DSID) (36). The electrode was then inductively dried for 1 min at a forward power of 30 W prior to ICP analysis. This study is an evaluation of this technique for the simultaneous determination of heavy metals (Cu, Pb, Cd, Zn, Ni, and Co) in aqueous solution as well as an evaluation of the performance with a difficult sample matrix.
EXPERIMENTAL SECTION Table I lists the principal components of the instrumentation. Figure 1 is an illustration of the electrochemical cell. The cell body (a) is made from Teflon and has a volume capacity of 40 mL. The cell lid (b) contains three holes to fit the reference electrode (c),the working electrode (d), and the auxiliary electrode (e). The lid also contains two small holes (f) for nitrogen input 0 1985 American Chemical Society
