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Optical System for Gathering Simultaneous Time and Spatial Data from Transient Events S. G. Salmon and J. A. Holcombe" DepaHment of Chemistry, The University of Texas at Austin, Austin, Texas 78712
A rotating disk containing multiple dits, which replaces a dngle field stop in a spectroscopic optical system, is used to provide simultaneous time and spatially resolved spectral information. The rotating disk serves as a spatial Isolation wheel ( S I W ) and is used to gather data for three-dimensional profiles of absorbance vs. time vs. distance above a carbon filament atomizer. Profiles are obtained for the flameless absorption of Pb with pure Ar sheath gas and with Ar sheath gas containing 0.15 % 02. Changes in the appearance temperature of the absorption signal as well as the peak half-width are observed in the presence of 02. The type of information available with the S I W and potential applications are reported.
involve many shots to obtain time-resolved absorbance signals at individual heights above the atomizer surface. Because of the uncertainties inherent in the technique, the experiment must be repeated several times a t each height of interest to obtain representative results. Even a moderate number of heights can lead to an almost unmanageable number of shots. During this period the surface characteristics of the graphite atomizer are changing irreversibly and may prohibit correlation between the first and last sets of data. With the aid of the SIW, the time and spatial information needed can be obtained much more rapidly and consistently than by the conventional procedure. To illustrate the usefulness of the SIW, the effect of O2 in the sheath gas on the absorption of Pb was examined.
T h e utility of using the astigmatism generated by a pair of spherical mirrors illuminated off-axis to improve resolution and maximize spectral throughput has been reported for spectrographic applications ( I , 2 ) and recently verified in our laboratory for use with photoelectric detection ( 3 ) . To obtain optimal spatial resolution, the field stop is placed in the tangential image plane to isolate a certain height in the spectroscopic source. With a fixed slit, a time-dependent spectral profile can be obtained only a t a single height in the source if transient phenomena are of interest. The spatial information available from a single event can be greatly increased by replacing the field stop with a set of slits continually scanning the spatial region in the tangential image plane. This set of moving slits is fabricated from a circular disk with multiple slits machined in it and serves as a spatial isolation wheel (SIW). In comparison with a static slit, the SIW has the capability of sampling the light a t several heights in t h e spectroscopic source a t regular time intervals during the course of a single event. Additionally, the use of multiple slits increases the amount of information which can be collected a t these different spatial zones. In the present study, the spectroscopic source is a West-type electrothermally heated carbon filament atomizer backlighted by a hollow cathode lamp and used for absorption measurements. In flameless atomic absorption spectrometry, interferences arise due to the sample matrix. Many of these observed interferences are believed to occur after the analyte has left the atomizer surface and are the result of gas phase reactions (4-7).T o elucidate the fundamental reactions responsible for these interferences without prior knowledge of their origins, it is important to gain a basic understanding of the actual mechanisms responsible for the observed signal enhancements or depressions. For this purpose, it is useful to follow the time-dependent analytical signal after atomization and examine where and when it is affected by added interferents. In addition to the interference problem, the development of a satisfactory model describing the atomization process itself is desirable (8-10). T h e information required to investigate these problems is best represented by a three dimensional profile of absorbance vs. time vs. distance above the atomizer surface. The large body of data necessary for such a profile would normally
Apparatus. The optical system used to obtain the astigmatic images and detect the absorption signal was described previously (3). Briefly, it consisted of two spherical concave mirrors with 800-mm focal length arranged in an under-and-over symmetric arm configuration. This focused the light from the spectroscopic source into a sagittal image on the monochromator entrance slit and a tangential image 12 mm in front of the entrance slit. The field stop was positioned in the tangential image plane. In this study, the output of the photomultiplier tube was digitized and stored using a Biomation model 805 waveform recorder which can handle 2048 eight-bit words. The carbon filament atomizer used was similar to the atomizer described by Alder and West (11). The filament was protected from ambient air by enclosing the >,idesof the atomizer with quartz windows. In the base of the atomizer, the sheath gas entered through a set of parallel channels which allowed laminar flow of Ar at gas velocities of up to 50 cm/s. All gas flow rates were controlled by precision rotameters. The atomizer power supply, developed in our laboratory, was operated in a mode which provided for a gentle drying of the sample followed by a rapid step to a preset final temperature (12). The SIW consisted of a 23-cm diameter, 1.6-mm thick aluminum disk with 60 evenly spaced, radial slits machined in it. At the position of each slit, the disk was milled down to 0.6 mm thick to prevent stray reflection from the slit edges. Each slit was 0.6 mm wide by 4.5 cm long and located 1 cm from the edge of the disk. A 100-rpm Hurst reluctance type synchronous motor was used to drive the SIW. The wheel was coupled to the motor by means of precision geared pulleys and a no-slip, positive drive belt (PIC Design Corp.) which provided rotational speeds of 50, 100, and 200 rpm by simple interchange of pulleys. These rotational velocities correspond to translational velocities of the SIW slits past the monochromator entrance slit of approximately 50, 100, and 200 cm/s, respectively. The exact velocity can be measured and varied slightly by lateral movement of the SIW. The selection of slit velocities was based on the time scale of the event being record ed . Procedure. The SIW was properly located in the tangential image plane by the procedure explained previously ( 3 ) . An aperture was positioned directly behind the wheel t o define a vertical viewing area of approximately 1 cm. To prevent light from two adjacent slits from reaching the detector at any given time, the atomizer was raised so that the light at one end of the viewing area was blocked by the image of the carbon filament. In this way, the vertical position of the filament could be adjusted until the light from only one slit at a time would reach the detector and the next slit would enter the viewing area immediately after
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Figure 1. Representation of the photomultiplier tube output for the atomization of (A) no sample and (B) a 2-pL sample of 0.6 ppm Pb
the previous slit exited. The transient waveform recorder was set to record a data point every 0.2 ms and was triggered by a comparator circuit which underwent a -13 to +13 V transition when the atomizer surface temperature reached a predetermined level. The atomizer surface temperature was measured by means of the photodiode of the power supply/temperature control circuit (12). This provided a reproducible trigger and the ability t o correlate atomization temperatures from shot to shot. A 2-pL sample of 0.6 ppm P b dissolved in triply distilled water as Pb(N0J2 was pipetted onto the carbon filament with a Hamilton syringe. The flow of Ar sheath gas was adjusted to give a vertical gas velocity of 10 cm/s. The sample was dried and atomized and the absorption signal recorded on the transient recorder. U1timately, the data were transfered to the university computation center where the data reduction was performed by the Control Data Corporation 6400/6600 system. The same procedure was repeated using a mixture of Ar and small amounts of O2 with the flow rates adjusted to maintain a 10 cm/s gas velocity.
RESULTS AND DISCUSSION Using the above procedure, time and spatially resolved absorbance data were collected for Pb. I t can be shown mathematically that t h e slight angle a t which the scanning slits sweep across the top and bottom of the viewing area accounts for an error of less than 0.2% in the spatial resolution. T h e resolution of the optical system has been determined experimentally to be 0.04 m m ( 3 ) ,therefore, the resolution achieved by the SIW arrangement is equal to the 0.6 mm width of the field stop. Figure 1 shows a characteristic trace of the photomultiplier tube output when the SIW is used. Figure 1A shows the recorded signal with no sample while Figure 1B shows the output with a P b sample. The region between any two peaks in Figure 1 represents the scan of a single SIW slit across the spatial viewing area. It is apparent from Figure 1B that the absorption is strongest in the regions closest to the filament surface. A detailed picture of a single scan from Figure 1A is shown in Figure 2. This figure also diagrammatically outlines the complete cycle of a single slit across the viewing area in front of the monochromator entrance slit. Since the optical system inverts the image, the SIW slits are swept downward in front of the monochromator entrance slit to scan the spatial zones above the filament starting a t the filament surface. I t should be noted that Figures 1 and 2 were obtained with the SIW rotating a t 25 rpm for ease of viewing and contain fewer scans than the
Flgure 2. Detailed view of a single sweep from Figure 1A with the corresponding SIW slit positions
actual outputs normally obtained. In Figure 2 , point a corresponds to 0% transmittance and occurs prior to the slit entering the viewing area. As the slit begins to enter, there is a sharp falling edge between a and b. Between b and c, the SIW slit is fully illuminated and scans the viewing area beginning with the spatial zone just above the carbon filament. As the SIW slit exits, there is a sharp rising edge between points c and d. Finally, a t point d the slit has exited and 0% transmittance is recorded again. The slits are positioned on the SIW so that the end of a cycle for one slit occurs simultaneously with the start of a cycle for the next slit. I t should be noted that the 0% transmittance reading is set by the vertical positioning of the carbon filament. The variations between points b and c in Figure 2 reflect a number of parameters such as changes in the intensity of the hollow cathode lamp at different spatial regions as well as the use of different areas of the photocathode a t different spatial viewing zones. All these variations are reproducible and are corrected for in the data reduction program by normalizing the data points a t each height to the spatially dependent output signal obtained in the absence of any absorbance signal. It should be noted that each SIW slit obtains absorbance data a t progressively later times as successive heights are examined. The data reduction program also takes this into account and the data are extrapolated so that the absorbance a t different heights is calculated for the same time. After the data reduction program has calculated the 0% and 100% transmittance levels, the absorbances, the peak half-widths, and the appearance time, a hard copy of all the information is obtained and the computer plots the data as three-dimensional profiles which give a general picture of the absorbance a t any time and a t any height above the atomizer surface, up to the limit of the viewing area. Figure 3 represents the profile obtained for the atomization of 0.6 ppm P b in pure Ar sheath gas. Figure 4 represents the profile for the same sample with the sheath gas containing 0.15% 02.As can be seen in Figure 4, the absorbance falls off at a lower height when O2 is present, which is expected because of the thermodynamically favorable reaction: P b '/*02P b O (1)
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Flgure 3. Three-dimensional profile of 0.6 ppm Pb with pure Ar sheath
gas However, there is also a shift of the peak absorbance to a later time and a narrowing of the peak half-width. Because of the reproducible triggering of the recorder a t a predetermined filament temperature, this time shift indicates a higher appearance temperature for P b in the presence of 02.These results have prompted further study of the P b system which is currently in progress. T h e profiles in Figures 3 and 4 point out the type of information available from the data. Time-dependent absorbance profiles a t several heights are obtainable as well as information on peak absorbance, total integrated absorbance, and appearance temperature of the analyte. The time resolution of the system can be improved simply by increasing the rotational speed of the SIW. The time-resolving capabilities of the SIW can be seen in Figures 3 and 4 where the peak absorbance at each successive height above the atomizer occurs a t a later time. T h e SIW has the potential of giving three-dimensional information in other spectroscopic techniques, such as atomic fluorescence, where the exciting radiation is perpendicular to the detector and better resolution of points in the source is possible. Another potential application is the use of the SIW in the temperature measurement techniques described by Bratzel and Chakrabarti (13)to obtain a three-dimensional temperature profile of the atomic vapor. There are several advantages in using the SIW to collect time and spatially resolved absorbance data. The most dramatic is the reduction in the time required for data gathering. The described procedure is many times faster than the conventional method. In addition to the saving in time, the data are also more consistent because of the ability of the SIW to obtain data on all heights above the f i e n t in a single
Flgure 4. Threedimensional profile of 0.6 ppm Pb with Ar sheath gas containing 0.15% O2
shot. All the spatial data are obtained from the same event and must, therefore, be internally consistent. The reduced number of shots required also adds to this consistency since the filament surface undergoes less degradation during a set of experiments. Finally, this system can achieve simultaneous time and spatial resolution using a standard monochromator, simple optics, and an inexpensive photomultiplier tube.
ACKNOWLEDGMENT The authors thank P. Hullman for obtaining the P b data used for the three-dimensional profiles. LITERATURE CITED (1) J. P. Walters, Anal. Chem., 40, 1540 (1968). (2) R. J. Klueppel, D. M. Coleman, W. S. Eaton, S. A. Goldstein, R. D. Sacks, and J. P. Walters. Soectrochim. Acta. Part 6.33. 1 (1978). (3) S. G. Salmon and j. A. Holcombe, Anal. Chem., 50, 1714 (1978). (4) K. W. Jackson and T. S. West, Anal. Chim. Acta, 59, 187 (1973). (5) K. W. Jackson, T. S. West, and L. Balchin, Anal. Chim. Acta, 64, 363 (1973). (6) D. Alger, R. G. Anderson, I. S. Maines, and T. S. West, Anal. Chim. Acta, 57, 271 (1971). (7) J. Aggett and T. S. West, Anal. Chim. Acta, 5 5 , 349 (1971). (8) D. J. Johnson, B. L. Sharp, T. S. West, and R. M. Dagnall, Anal. Chem., 47, 1234 (1975). (9) G. Torsi and G. Tessari, Anal. Chem., 45, 1812 (1973). (10) S. L. Paveri-Fontana, G. Torsi, and G. Tessari, Anal. Chem., 46, 1032 (1974). (11) J. F. Alder and T. S. West, Anal. Chim. Acta. 51, 385 (1970). (12) J. A. Holcombe (in preparation), Rev. Sci. Instrum. (13) M. P. Bratzel,Jr., and C. L. Chakrabarti, Anal. Chlm. Acta, 63,1 (1973).
RECEIVED for review November 7,1978. Accepted February 2, 1979. This work was supported by National Science Foundation Grant Number CHE-78-15438.
