Anal. Chem. 1980, 52, 295-300
TA
sample transmittance a t wavelength A n m
x a l b = emission window parameter, dimen-
295
Sandholm for his helpful suggestions during the theoretical of this work.
sionless
LITERATURE CITED
xalb
Parker, C. A. "Photoluminescence of Solutions"; American Elsevier: New York, 1968; pp 220-34. Winefordner, J. D.; Schulman, S. G.; O'Haver, T. C. "Luminescence Spectrometry in Analytical Chemistry"; Wiley 8 Sons: New York. 1972; pp 81-103. Demas, J. N.; Crosby, G. A. J . Pbys. Chem. 1971, 75, 991-1024. Melhuish, W. H. J . Res. Natf. Bur. Stand., Sect. A. 1972, 76, 547-60. Birks, J. 8. J . Res. Natl. Bur. Stand., Sect. A . 1976, 80, 389-99. Holland, J. F.; Teets, R. E.; Kelly, P. M.; Timnick, A. Anal. Cbem. 1977, 49, 706-10. Parker, C. A.; Barnes, W. J. Analyst, (London) 1957, 82, 606-19. Sen-Gupta, S.8. J . Indian Cbem. SOC. 1938, 15. 263-300. St. John, P. A.; McCarthy, W. J.; Winefordner, J. D. Anal. Chem. 1968, 38, 1828-35. Braunsberg, H.; Osborn, S. B. Anal. Cbim. Acta 1952, 6 , 84-95. van Slageren, R.; den k e f , G.; van der Linden, W. E. Tabnta 1973, 20, 501-12. Thomas, J. F.; Mukai, M.; Tebbens, B. D. Natl. Cancer Inst. Monogr. 1962, 9 , 127-33. Henderson, G. J . Cbem. Educ. 1977, 54,57-9. Leese, R. A,; Wehry, E. L. Anal. Chem. 1978, 50. 1193-7. Holland, J. F.; Teets. R. E.; Timnick, A. Anal. Cbem. 1973, 45, 145-53. Reference 2, pp 324-7. Kelly, P. M. Ph.D. Thesis, Michigan State University, 1976; pp 20-46.
luminescence spectral power yield, dimensionless = fraction of power absorbed by fluorophore a t wavelength X n m which is emitted at A' n m i n s t r u m e n t a l constant, dimensionless = fraction of fluorescence radiation collected as t h e emission beam secondary absorption transmission factor (see text a n d Equation 9) sample transmittance at wavelength A' n m transmission factor of cell wall and collection optics, dimensionless transmission factor of emission wavelength isolation device, dimensionless emission stray light spectral radiant flux, W nm-' detector sensitivity factor, A W-'
ACKNOWLEDGMENT One of the authors (D.R.C) would like to thank Scott
RECEIVED for review July 6,1979. Accepted October 16,1979. This work was partially supported by NSF Grant CHE 7681203.
Effects of Slow Instrumental Response on the Accuracy of Furnace Atomic Absorption Spectrometric Determinations Darryl D. Siemer" and Jon M. Baldwin Allied Chemical Corporation,
550 Second Street, Idaho Falls, Idaho 83407
The effects of slow instrumental response upon the accuracy of graphite furnace atomic absorption analyses is discussed with special emphasis on solid sample work. The effects of limiting low pass "damping" time constants at various points within the signal processing circuitry are given in theoretical examples and in experiments done with both matrix-free solutions and solid samples. Some commercially available AA spectrometers are incapable of accurately measuring either peak areas or peak height transient signals produced by the furnace atomizers with which they are commonly used.
Recent work in this laboratory has been concerned with the origins of matrix effects in graphite furnace atomic absorption spectrometry (GFAAS) and with methods for eliminating them. One source of serious error, observed in our attempts a t direct GFAAS analysis of solid samples, was strictly instrumental in origin and may be the cause of much of the grief experienced by others who are applying GFAAS to high matrix samples. The error arises from the fact that the signal conditioning electronics of some atomic absorption (AA) spectrometers are not capable of accurately responding to typical GFAAS transient signals. At first glance, it might seem that integration of the transient would ensure a response unaffected by variations in the shape of the transients. However, this is true only if the limiting time constant is introduced a t a point in the signal conditioning process where the signal is linearly related to the amount of analyte causing the response; 0003-2700/80/0352-0295$01.OO/O
Le., is proportional to absorbance. The general realization of the importance of instrumental response time in GFAAS is by no means original with us. Piepmeier and deGalan (1) presented an excellent theoretical treatment of the influence of detection system temporal response on AA signals. Others have also discussed the effect of slow instrument response on signals from matrix-free samples in various types of graphite atomizers ( 2 , 3 ) . Czobik and Matousek ( 4 ) ,in a study of chloride salt interferences on lead with the Varian CRA63 atomizer, indicated that fast electronic response was required if molecular background and analyte signals were to be adequately separated. Research on the fundamental chemical and physical processes occurring in GFAAS is now done almost entirely with fast response equipment, owing to the common realization that most commerically available AA spectrometers are not capable of accurate response to fast transient signals. The extent and subtlety of the effects of slow instrument response on the results of routine analysis is less highly appreciated and is the topic of this paper. In the following sections, we will present both simplified numerical demonstrations and experimental results on matrix-free and complex samples, showing the effect of the instrument response times on the analytical results. While the examples in this paper are taken from our work on GFAAS of solid samples, the observations and conclusions will apply to any situation where high-matrix samples are encountered. Figure 1 is a block diagram of a typical single channel AA spectrometer. In GFAAS the cloud of analyte atoms suddenly 1980 American Chemical Society
296
ANALYTICAL CHEMISTRY, VOL. 52, NO, 2, FEBRUARY 1980
4
Time Constan!
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interposed in the light path causes a change in source lamp intensity as detected by the photomultiplier. The electrical signal a t any point from the detector through the output of the demodulator or lock-in amplifier is proportional to optical intensity-not to absorbance-and therefore not to the number of atoms in the optical path. If the time constant of the demodulator or of any analogous circuit element is long relative to the width of the transient intensity signal, the absorbance signal subsequently produced will be severely distorted in shape, height, and area. Thus, any variation in
the rate of analyte atomization will cause a difference not only in the peak height but also in the integrated absorbance for a given mass of analyte.
INSTRUMENTATION Electronics. The equipment used in the present work is shown in timing pulses (square 2. The lock-in
wave, 50% duty cycle) for the lamp power supply. The analyte element hollow cathode was modulated at 570 Hz, and the hydrogen continuum source at 285 Hz. The light beams were made spatially coincident and passed through a carbon rod atomizer
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
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297
1
RC AREA
1 ABSSEC
1 SEC
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Figure 4. Effect of low-pass filtering after logarithmic conversion on
integral of absorbance output Figure 3. GraphRe cup used for solids analyses and detail of atomization geometry. A, plume of volatilized sample gas; B, optical path; C, atomizer rod; and D, burner head. E, cross section of cup showing rod placement and solid sample
(Varian CRA63) to a 1/2-meter spectrometer. The optical components were those of a Varian Model AA6 atomic absorption spectrometer. The gross photocurrent was amplified with an operational current follower. The resulting voltage signal was applied to the inputs of the two lock-in amplifiers used to isolate the signal of each lamp. The output of each lock-in amplifier was converted to a digital signal by individual Hewlett-Packard (HP) Model 3437A digital voltmeters. Control information and data were transferred between the voltmeters and an HP Model 9825A programmable calculator via an IEEE-488 bus interface. Data acquisition was initiated by a signal from the atomizer power supply at the start of the atomization heating cycle. This signal triggered an HP Model 59308A timing generator, which in turn delivered an interrupt signal to the calculator via the IEEE bus. After suitable processing the absorption transients were plotted on an H P Model 9862A x-y plotter. Alternatively, a differential logarithmic amplifier followed by an offset and gain adjustment amplifier was used to provide the input to a single digital voltmeter (Figure 2). Its output voltage, proportional to background-corrected atomic absorbance, was lowpass filtered by using suitable capacitors to give the desired time constant. The time constants of the two lock-in amplifiers were both 25 ms when the differential logarithmic amplifier was used. The preamplifier, lock-in amplifiers, differential logarithmic amplifier, and lamp power supply were all built in this laboratory. Cicruit diagrams will be supplied by the authors upon request. The atomizer power supply has been modified to incorporate a temperature feedback circuit which makes short thermal risetimes and relatively low maximum atomizer temperatures reproducibly achievable ( 5 ) . Also depicted in Figure 2 is an infrared thermometer (Thermodot Model TD2B) whose output could be interfaced to the data system, converted to absolute temperature, and plotted along with the absorption transient. Atomizer Modifications. For solid sample analysis, the stack of alternating flat and corrugated steel plates situated under the graphite rods and cup was replaced with a 2.5-cm long, 2.0-cm wide, and 0.7-cm thick aluminum plate pierced with 120 evenly spaced holes ($60 drill). The plate served as a burner head permitting use of a premixed air-acetylene flame in lieu of an inert gas sheath or the argon-hydrogen-entrained air flame commonly used to sheath the hot graphite parts during an atomization. Sample cups were machined from Poco FX91 graphite according to the design of Figure 3. The shape allowed the cups to retain a powdered sample when laid on their sides. The cups were coated with pyrolytic graphite by heating in the atomizer to 2000 ‘C for 5 s while flowing a 1 O : l argon-acetylene mixture through the burner head. For atomization of solid samples, the cups were positioned horizontally so that volatilized material would be directly exposed to and swept upward by the flame sheath gases. In this manner, analyte chemistry, temperature, and residence time are normalized (Le., made independent of matrix induced perturbations in the furnace atomization process) by the flame before the atoms enter the optical path situated 6 mm above the center of the cup. Data Processing. The digitized outputs from the two lock-in amplifiers were subjected to a 9-point Savitzky-Golay smoothing
TRUE TRANSIENT 1 ABSSEC
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function (6) and then used to calculate a background-corrected absorbance transient. The digitization rates were fast enough (60 points per second) that the smoothing operation had a negligible effect on transient shape and position. Peak integrals were calculated between the points where the absorbance signal was greater than or equal to 20% of the peak maximum. This generally corresponded to about 96% of the total peak area. When the differential logarithmic amplifier was used, only one channel of data was input to the calculator. Since the signal was in this case already converted to absorbance, the calculator was used simply to smooth and scale the data prior to displaying and integrating it.
RESULTS AND DISCUSSION The origin of the error discussed in this paper can be illustrated with a simple numerical example. Similar numerical examples of the effects of low-pass filtering on signals are developed in Malmstadt, Enke, and Crouch’s excellent electronics text (7). We will consider here the effect of a low-pass first-order (20 db/decade rolloff) filter applied either to the output of the demodulator (i.e., t o the intensity transient) or t o the output of the logarithmic amplifier (i.e.. to the absorbance transient) on the eventual analytical response when a square transient with an amplitude of 1 absorbance unit and a duration of 1 s is produced by the atomizer. With the limiting time constant, e.g., 1 s, applied a t the output of the logarithmic converter and a much shorter time constant a t the demodulator, the square pulse will be distorted as shown in Figure 4. While the relatively long limiting time constant affects the shape and reduces the height of the absorbance response, the integral is unaffected. In the second instance
298
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
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amplifier (Figure 5), the limiting time constant, again 1 s, is applied to the hypothetical square pulse in or immediately following the demodulator (before the intensity-to-absorbance conversion) and the logarithmic amplifier output is not low-pass filtered. T h e ouput transient is much more severely reduced in amplitude (peak absorbance = 0.366 as opposed to 0.632 in the previous case) and the peak area is less than half the area under the undistorted transient. A series of six experiments were run which further illustrate the effect of varying the location a t which the limiting time constant is applied. In each, 5 mL aliquots of 0.12, 0.24, and 0.48 ppm lead solutions were atomized in conventional 9-mm long atomizer tubes. A very fast (1.7 K/ms) ramp to a final atomizer tube temperature plateau of about 1670 K was used in every experiment. It must be stressed here that the actual, true atomic transients presented to the AA spectrometer’s signal processing circuitry were identical in all six sets of experiments. The differences in observed responses apparent in the following figures are not “real” - they are caused by the changes in the circuitry. In the first three experiments, the time constants of the lowpass filters at the outputs of the lock-in amplifiers (demodulators) were varied from 25, to 100, to 300 ms while the conversion to absorbance was performed digitally. In the second set, the time constants of the lock-in amplifiers were set a t the minimum value of 25 ms and the conversion to absorbance was performed with the differential log amplifier using the constants of 10, 100, or 300 ms. Figure 6 shows the results of the experiment performed with a limiting constant of 25 ms a t the lock-in amplifier outputs and digital absorbance conversion. The heavy line is the temperature-time response of the atomizer. The curvature of the temperature-time plot at the low temperature end is attributable to insensitivity of the infrared thermometer a t temperatures below 900 K when used on the range suitable
-
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Figure 9. Integrated absorbance calibration curves, differing time constants at different points in circuitry. (0)Limiting time constant, 25 ms.; no differential logarithmic amplifier. (X) limiting time constant, 100 ms; no differential logarithmic amplifier. (0)Limiting time constant, 300 ms; no differential logarithmic amplifier. (A)Limiting time constants
applied after the differential logarithmic amplifier: 10, 100, and 300 ms (superimposable) for measuring the rest of the transient. Figure 7 shows the analogous data obtained with the lock-in amplifier time constants set to 300 ms. This time constant is approximately equal to the minimum permitted with the unmodified Varian AA6 electronics. The peaks are obviously distorted, being lower, wider, and seeming to have a fundamentally different shape than that observed with the fast electronics (note the pronounced tailing). More important analytically, a calibration curve plotted from the data of Figure 7 is much less linear than one plotted with the data of Figure 6. In addition, the integrated absorbance signals are considerably larger in the first set of data. Figure 8 depicts the analytical responses obtained when the limiting 300-ms time constant was applied after the differential log amplifier (i-e.,after the absorbance conversion). Compared to Figure 7, it is obvious that a calibration curve plotted from these data will be both more nearly linear and steeper in slope. The additional step of analog signal processing done with the homemade differential logarithmic amplifier gives a slightly “noisier” signal than that observed in Figure 7 , but the response is more useful analytically. Figure 9 shows integrated absorbance vs. nanograms of lead calibration plots obtained with data from all six sets of ex-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 E '
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Figure 10. Cadmium in fly ash analysis, short lock-in constant
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periments. The slight rounding of the uppermost two plots in Figure 9 is due to the same cause, i.e., the usual negative deviation of atomic absorption analytical curves a t high absorbances (caused by stray light, fiiite source lamp line width, etx.). The shape of the two lower plots on the Figure are also somewhat affected by the same phenomena; however, the long lock-in amplifier time constants are by far the dominant cause of both the reduced slopes and the greater degree of nonlinearity as compared to the upper plots. It is clear that when varied damping time constants are applied to the same signal transient prior to the point in the spectrometer circuitry where the signal is directly and linearly proportional to the amount of analyte, widely divergent results are obtained. Of more importance analytically is the fact that if a long demodulation time constant is used, any variations in the volatilization time of an analyte peak (peak "shape") will cause a change in the integrated analytical response as well as in the peak height response. On the other hand if the "damping" is performed after the conversion of intensity values to absorbance, any amount of low pass filtering will not affect the integrated signal even though it will affect the peak height response. One of the most common effects of the presence of a concomitant material in the atomizer is that it perturbs the shape of the atomic absorption transient (8-10). For example, direct atomization of Cd from mixtures of NBS $1633 fly ash and graphite is shown in Figures 10 and 11. The samples were weighed into graphite cups and the cups placed horizontally between the rods of the atomizer. The heat program consisted of a rapid ramp to about 1600 K followed by a long period of constant temperature. After the cup had cooled, 5 FL of 0.4 ppm cadmium stock solution (0.1 F in " 0 3 ) was added to the cup, dried by passing a low current through the rods, and then atomized using the same atomizer power supply settings as for the solid sample. A stoichiometric air-actetylene flame to normalize
290
both atom residence times and atomization efficiency was used in each case, and the optical path was centered in the flame about 6 mm above the center of the cup. No "ashing" heating stage was used for either the sample or the standard. Figure 10 gives the results for the experiment done with fast lock-in amplifier time constants of 25 ms. Note that the cadmium in the standard comes off much more quickly than it does from the solid sample giving two relatively sharp peaks. The maximum peak height signal observed from the standard is approximately 60% greater than that seen from the sample even though the aliquot of standard solution contained only 70% of the cadmium that the 1.96-mg sample of the fly ash did. However, when integrals are compared and used to calculate an experimental value for the concentration of cadmium in the solid sample, good agreement (1.53 ppm as opposed to 1.45 ppm) with the certified value is obtained. Figure 11 gives the results obtained when the experiment was performed using lock-in amplifier time constants of 300 ms (representative of those of unmodified Varian-Techtron AA5 or AA6 instruments-see the schematics in the owner's manuals). In this case, the sharp "standard" atomization transient was considerably blunted by the excess degree of low pass filtering-much more so than was the broad sample atomization transient. The greater degree of downward biasing of the standard's analytical response, relative to that of the sample, gives an inaccurate (too high) experimental value for the cadmium in the solid sample based on a comparison of the integrated signals from each. It is only fair to mention here that if a much slower atomization temperature "ramp" had been used in lieu of the abrupt heating program, the results observed for the two experiments (in Figures 10 and 11) would have been more comparable. In principle a t least, any instrumental time constant could be used if the analyte evaporation rate can be slowed down enough. However, with well designed electronics, the exact shapes of the absorbance transients are not important and the analyst 'needn't worry about uncontrolled variations in them influencing the results of analyses. As is apparent upon observing the nature of the base-line signals seen in Figures 6 and 10 (with 25-ms lock-in amplifier time constants) with those of Figures 7 and 11 (with 300-ms time constants), the signals are much "noisier" when demodulation circuitry signal averaging times are short. Therefore, under conditions in which identical sample aliquots can be absolutely reproducibly pipetted into the furance and then reproducibly dried and atomized, the use of relatively long time constant signal processing circuitry will give the more precise results. However, (and this is the point of this paper) under real sample analysis conditions in which unknown amounts of a poorly characterized matrix can and will affect the shape of the atomic absorption transients observed, the analytical response of the instrument will very likely be an inaccurate indication of the amount of analyte. In solid analyses, our experience indicates that the shape and overall time response of the signals varies tremendously (depending upon the amount of sample atomized, its contact with the graphite walls of the cup, the chemical and physical nature of the matrix, the contact resistance of the graphite rods with the cup, etc.) and, in general, cannot be predicted or reproduced with any degree of precision. If good analytical results for trace elements in solid samples of widely varying gross composition are to be obtained with any degree of regularity, it is absolutely necessary that the circuitry used to process the signals does not significantly bias the integral of signals with different shapes and widths. We are aware that if careful sample-standard matrix matching is performed, carefully researched ashing stages gone through, and matrix modification resorted to, it is sometimes possible to do solid
300
Anal. Chem. 1980,
samples accurately by GFAAS using conventional atomizer geometry, sheathing gases, simple voltage or current controlled atomization power supplies, and “slow electronics” AA spectrometers. However, analysis of each new sample tends t o become a research project in “optimization” and analytical throughput is very low. The goal of analytical instrumentation designers (and of operators choosing how to set up an instrument) should not be to produce the lowest possible “detection limit” figures obtainable on simple standard solutions. The goal should be to configure the instrument to obtain an accurate, unbiased measure of the amount of analyte in real samples. If tradeoffs must be made between precision and accuracy, the nod should go to accuracy.
52,300-306
LITERATURE CITED (1) Piepmeier, E. H.; deGahn, L. Spectrochim. Acta, Part61979,31, 163. (2) Posma, F. D.; Srnit, H. C.; Roose, A. F. Anal. Chem. 1975, 47,2067. (3) Maessen, F. J. M. J.; Posma, F. D. Anal. Chem. 1974, 46, 1439. (4) Czobik. E. E. J.; Matousek, J. P. Anal. Chem. 1978,50, 2. (5) Siemer, D. Appl. Spectrosc., in press. (6) Savitsky, A.; Golay, M. J. E. Anal. Chem. 1964,36, 1627. (7) Malmstadt, H. V.; Enke, C. G.;Crouch, S.R. “Electronics Measurements for Scientists”; W. A. Benjamin, Inc.: Menlo Park, Calif.. 1974; Chapters 4-2 and 4-5. (8) Siemer, D.; Wei, Horng-Yih. Anal. Chem. 1978, 50, 147. (9) Smeyers-Verbeke, J.; Michotte, Y.; Massart, D. L. Anal. Chem. 1978, 50, 10. (IO) L’vov, B. V. Talanta, 1976, 23, 209.
RECEIVED for review May 7, 1979. Accepted November 5, 1979.
Portable Centrifugal Analyzer for the Determination of Rapid Reaction Kinetics William D. Bostick,” Martin L. Bauer, Robert McCracken, and John E. Mrochek Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
A portable centrifugal analyzer prototype ( C h . Chem. 1977, 23, 1416) is capable of rapidly initiating reactions and monitoring 17 optical channels as they rotate past a stationary photodetector. An advanced rotor drive permits transfer of discretely loaded sample and reagent into a cuvette wlthln 60 ms. Various rotor designs have been employed to ensure efflclent mixing concurrent with solution transfer, thus permitting absorbance or luminescence measurements to be made almost Immediately afler solution contact. Dyedllullon studies have been used to investigate transfer and mixing efficiencies. Rotor designs with parallel access for sample and reagent into the cuvette were found to promote efficient mixing during liquid transfer. The hypochlorite-luminol chemllumlnescent reaction served to demonstrate the utility of the system for performing rapid kinetic analyses. Appropriate adjustment of reaction conditions allows firstorder reaction half-lives as short as 0.04 s to be measured.
Approximately a decade ago, Anderson and co-workers introduced the concept of the centrifugal analyzer, in which a number of analyses are performed in parallel with a multicuvette rotor (1-3). Discrete aliquots of sample and reagent solution loaded into this rotor are transferred by centrifugal force into the optical cuvettes, establishing simultaneous starting times for the batch of reactions. Similar conditions (e.g., time, temperature) are maintained during the run. A stationary optical system monitors the cuvettes and samples the dark current during each revolution of the rotor. The analyzer is interfaced to a computer system which controls the timing and sequence of data acquisition and processes the data (4). The development of the early prototype instruments has been described in the literature (1-3, 5-10), and the instrumentation and its varied applications have been summarized in recent reviews (11-14) and a compendium (15). Centrifugal analyzers are well suited for precision kinetic photometry and are extensively used in the clinical laboratory; however, the versatile instrumentation also has general analytical utility. 0003-2700/80/0352-0300$01 .OO/O
Recently, we introduced an advanced prototype analyzer (16) which incorporates a microcomputer system to control several analyzer functions as well as to perform data acquisition and processing. The temperature of the rotor is controlled to within f0.2 “C a t one of three set points (25, 30, or 37 “C) by means of a thermoelectric heat pump located in the rotor holder. The use of an advanced rotor drive employing a clutch/brake enables discretely loaded sample and reagent to be transferred into the optical cuvette in times as short as 60 ms. Various multicuvette-rotor designs have been evaluated with the objective of attaining efficient mixing of binary solutions concurrent with their transfer. The results for the most promising of these designs are described. The combination of precise time measurements (2-MHz internal clock with a resolution of 100 bs) made during each revolution, rapid acceleration via the clutch/brake, and the use of parallel-transfer channel rotor designs permit reactions to be monitored within 100 ms of their initiation. In this study, a number of relatively rapid reactions (i.e., those persisting for a least 200 ms) were monitored by photometry or chemiluminescence to determine the performance capabilities and limits of the analyzer for times of
