LITERATURE CITED
The potential-step case reported here was included because of its significance t o photoelectrochemistry and also t o demonstrate the applicability of the theoretical model to potential-step chronoamperometry. The correction method applied here simultaneously removes both forms of charging current (produced by the applied potential change and induced by the faradaic current), resulting in corrected currents that closely satisfy the Cottrell relationship (Figures 13 and 14). Subtraction of a blank, as is the normal charging current correction procedure for this type of experiment, is not necessary. Future work (15) will involve further extension of this theoretical model in the area of photoelectrochemistry t o include studies under potentiostatic conditions with various electrolysis mechanisms (e.g., coupled first- and second-order reactions). In addition, extension to controlledpotential electrochemical techniques in general will be considered, including semi-integral analysis and staircase voltammetry.
S. S. Fratoni, Jr., and S. P. Perone, Anal. Chem., 48, 287 (1976). S. P. Perone and J. R. Birk, Anal. Chem., 38, 1589 (1966). J. R. Birk and S. P. Perone. Anal. Chem., 40, 496 (1968). G. L. Kirschner and S. P. Perone, Anal. Chem., 44, 443 (1972). R . A. Jamieson and S. P. Perone, J. Phys. Chem., 76, 830 (1972). J. E. Mumby and S. P. Perone, Chem. lnstrum., 3, 191 (1971). E. D. Schmidlin. in "Digital Logic and Laboratory Computer Experiments", by C. L. Wilkins, S. P. Perone, C. E. Klopfenstein, R. C. Williams, and D. E. Jones, Plenum Press, New York, 1975, Appendix F. (8) J. F. Eagleston and S.P. Perone. J. Chem. Educ., 48, 317 (1971). (9) J. J. Zipper, Purdue University, Lafayette, ind.. Ph.D. Thesis, 1974. (10) G. L. Booman and W. B. Hoibrook, Anal. Chem., 37, 795 (1965). (11) L. Meites, Anal. Chlm. Acta., 18, 364 (1958). (12) A. Beckett and G. Porter, Trans. Faraday SOC.,59, 2038 (1963). (13) M. Gratzel. K. M. Bansal, and A. Henglein, Ber. Bunsenges. Phys. Chem., 77, 11 (1973). (14) R. P. Baldwin and S. P. Perone, unpublished work. (15) S. S. Fratoni, Jr., and S. P. Perone, in preparation. (16) R. S. Rodgers, Anal. Chem., 47, 281 (1975). (17) S. G. Lamey, R. D. Grypa. and J. T. Maioy, Anal. Chem., 47, 610 (1975). (18) A. Henglein and M. Gratzel, Ber. Bunsenges. Phys. Chem., 77, 17 (1973).
RECEIVEDfor review May 5 , 1975. Accepted September 22, 1975. This work supported by Public Health Service Grant No. CA-07773 from the National Cancer Institute and the Office of Naval Research Contract N00014-75-C-0874. One of the authors, S.S.F., also received the Procter & Gamble Fellowship in the Department of Chemistry a t Purdue University for the 1974-75 academic year.
ACKNOWLEDGMENT The authors express their appreciation to Robert Paarlberg for his help with the data acquisition programming.
Computer-Controlled Programmable Monochromator System with Automated Wavelength Calibration and Background Correct ion R. W. Spillman' and
H. V.
Malmstadt'
Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Ill. 6 180 7
An extremely flexible computer-controlled monochromator system which is utilized in an automated AE/AF multielement flame spectrometer is described. Under computer control the monochromator can be made to quickly slew between wavelengths of analytical interest and remain at a particular wavelength until the measurement is made. The addition of a rotatable quartz plate between the exit mirror and the exit slit provides a means of automatically calibrating the monochromator, a method for accurate peak positioning, 'and the capability for background correction for emission measurements. Although developed for use in a flame spectrometer, the concepts described here could be easily extended to many other spectrometric systems. Both the entrance and exit slits are programmable, and they may be optimized for each particular measurement.
One of the goals of our laboratory has been to develop a multielement flame spectrometer capable of performing rapid analyses on a wide range of elements in many types Present address, I n t e r n a t i o n a l Divisions, Gamble Company, Cincinnati, Ohio 45224.
The Procter a n d
of samples. For reasons described elsewhere ( I ) , the instrument design incorporated both atomic fluorescence (AF) and atomic emission (AE) since the two methods are complementary and taken together offer great sensitivity for a wide range of elements. Of special concern in designing an automated system which incorporates two flame techniques into one instrument is the method of wavelength isolation and detection that should be employed. There are many approaches that could be taken and these range from simple filters with photomultiplier to more sophisticated Fourier Transform multiplex techniques and solid state arrays (2-20). A rotating filter wheel offers low cost and simplicity but suffers from spectral interferences and lack of versatility for choosing different spectral lines (3, 4 ) . A scanning monochromator has been used in some reported systems (5-7) and offers a narrow bandpass with a wide spectral range but is restricted to a small measurement time for any given wavelength element, and thereby severely reduces the amount of signal that can be obtained for a particular measurement. The same problem exists for an image-dissecting photomultiplier, in addition to its high cost and limited wavelength range. Recently, more popular approaches have centered around the use of vidicon detectors and solid state arrays (8-26). Both allow for simulta-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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-
70 A I R
SPLENOID
MINICOMPUTER
GCA/McPHERSON
I I
\ I
1
E U 700
Figure 1. Block diagram for the computer controlled programmable monochromator
neous measurements for many wavelengths and are readily adapted to computer processing techniques. They are, however, much less sensitive than a photomultiplier and a t present are still expensive, and a compromise must be made between the resolution and the desired wavelength range when conventional gratings are used. There are also systems which employ direct reading multichannel spectrometers capable of simultaneous analysis over a wide range of wavelengths (17, 18) but this method of detection is expensive, lacks versatility, and is difficult to properly align. Multiplexed methods for multielement analysis have been reported (19-23) but, in general, they have narrow applicability to only one technique or to one chemical system. In this paper, a programmable monochromator system is described which was developed for use in an automated multielement AE/AF spectrometer. This monochromator with a photomultiplier tube allows for the sequential analysis of a wide range of elements by either AE or AF and offers a number of important advantages over the previously described methods. Based in part on an earlier design by one of the authors ( 2 4 ) ,this monochromator now has computer-controlled wavelength and slit width selection and, through the use of a rotatable quartz plate, wavelength modulation is employed to ensure wavelength accuracy and to reduce background interferences during an emission measurement. In practice, this monochromator can be slewed a t about 20 nm/s between selected wavelengths, “tune” to the analytical line to within 0.017 nm, and automatically modulate the wavelength for background correction in emission measurements. Under computer control, the monochromator can be slewed to any number of wavelengths between 213 and 1000 nm and automatically recycle for a new series of measurements. The concept of making sequential instead of simultaneous measurements as in the case of a vidicon or image-dissecting tube is important in the type of analysis performed on the dual mode AE/AF system. During the time the monochromator is being slewed between the various analytical wavelengths, both the instrumental and flame parameters such as flame temperature, flame geometry, slit width, and measurement and source characteristics may be optimized for the next measurement. In practice, this individual optimization step has greatly enhanced the range and sensitivity of the analyses, and has reduced spectral and chemical interferences which would otherwise be present in a compromised system. Several criteria had to be met before the programmable monochromator could be employed in the spectrometric system. These requirements were that the wavelength selection would be fast, accurate, and reliable and that the system be relatively free of mechanical errors and maintain 304
a calibrated state throughout the analysis. T o reduce any errors that might be introduced in the mechanical wavelength drive, a feedback method involving a rotatable quartz plate was introduced. Through wavelength modulation, the plate allows for the accurate location of the maximum peak intensity. This effectively eliminates those errors which might be introduced in cases where the monochromator did not stop precisely on the peak maximum. A similar method is employed to automatically calibrate the monochromator before the start of an analysis. The addition of the quartz plate also allows for automatic background subtraction during an emission measurement. The plate can be programmed to perform a digital first or second derivative of the emission spectrum which allows for the suppression of interferences such as broad bands or continuums and eliminates the effects of instrumentally scattered radiation (25,26). Other methods of wavelength modulation such as vibrating slits (27),mirrors (28, 29), and gratings (30) have been employed in various systems. However, a vibrating slit is somewhat inflexible and requires close mechanical tolerances for precise operation. Rotating a focusing mirror in the monochromator changes the optical axis and results in the shifting of the photomultiplier incidence point (31). Because of sensitivity variations across the tube, the current output will fluctuate according to the point of incidence. The existing grating drive mechanism could be used for modulation (30); but for fast and very precise modulation, a large stepper motor would be required. The use of a quartz refractor plate connected to a motor results in a simple and yet precise modulation system and is popular with many investigators. Quartz refractor plates have been incorporated in several previously described systems, typically to obtain derivative spectra during a wavelength scan with the use of lock-in amplifiers (25, 26, 32-35). Hieftje and Sydor (36) have examined the application of wavelength modulation to minimize spectrometer misalignment. A vibrating quartz plate was used to modulate the output wavelength and several demodulation approaches were analyzed and evaluated. These same authors also proposed a computer feedback method for automatic monochromator calibration. Roldan (37)has outlined some of the errors that may be introduced when using a quartz window as a refractor plate. The application of the quartz plate in the capacity described in this paper is quite unique in its versatility and approach insofar as the authors can determine. Calculations are presented to demonstrate that none of the errors inherent in using the quartz plate seriously affect the operation described here.
INSTRUMENTATION Basic Design. The basic design of the programmable monochromator is illustrated in Figure 1. Through the counter, decoder, and motor drive circuitry, the computer controls a dc motor used for slewing the monochromator between the various analytical wavelengths. A shaft encoder is connected to the drive mechanism and generates a number of pulses proportional to the movement of the grating. These pulses, when fed back to the counters, enable the computer and decoder logic to monitor the wavelength. A simple reference device also connected to the wavelength drive is used to provide a calibration or reference point to which the drive mechanism may be initialized. When performing an analysis that requires slewing to a series of wavelengths, the monochromator is first initialized to a starting position and the computer then calculates the distance the monochromator must be slewed from that po-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
PRESETTABLE COMPUTER
COUNTERS
r
START. /N/TIAL/ZE,
DECODER CONTROL
AND DIRECTION
FREOUENCY SELECT
CONTROL Ffl INITIALIZE FFZ UP/OOWN FF3 START/STaO
Flgure 2. Block diagram for the monochromator slew control
sition in order to reach the first analytical wavelength. A coded value proportional to this distance is loaded into hardware counters and the dc motor is started. As the motor drives the sine bar mechanism which in turn rotates the grating to the new wavelength, the pulses generated by the shaft encoder are fed back into the counter circuitry. In this manner, the decoder logic is able t o monitor the grating position and, with this information, the slew speed is automatically reduced as the monochromator approaches the desired wavelength. Three slewing speeds are employed so that the wavelength selection is both rapid and precise and eliminates overshoot errors. When the chosen wavelength is reached, the motor is halted and the computer is given a ready signal. At this point, the quartz refractor plate may be employed to check the spectral alignment by “tuning” the monochromator to the maximum peak intensity. As will be descirbed in more detail later, the plate is rotated in such a manner that its refractive index causes a lateral shift of the output spectrum across the exit slit. The signal change is monitored by the photomultiplier tube and, through computer feedback, the plate can be rapidly positioned to correct any monochromator misalignment to the spectral peak. The plate can also be used to correct for background during an emission measurement by shifting the output spectrum far enough off the peak so that the resulting signal is due just to the background beside the peak. This signal can in most cases be assumed to be equal to the background immediately under the emission peak and, therefore, once measured it can be subtracted from the original measurement to give a background-corrected value. The slit width is also under computer control, and it can be set for a narrow bandpass for emission measurements or for a wide bandpass for fluorescence determinations, where less wavelength discrimination is necessary. This operation is performed during the period the monochromator is being slewed between wavelengths. Wavelength Drive Control. The monochromator employed in this spectrometric system is a GCA/McPherson Model EU-700 scanning monochromator (Acton, Mass.) with a Czerny-Turner optical arrangement; the original motor drive system has been modified, however, to allow computer control for high speed slewing between wavelengths. The basic physical modifications that are made to the wavelength drive involve the removal of the original motors and the addition of a dc motor, a shaft encoder, and a reference device (24). The shaft encoder is a Model 715 Accu-Coder encoder (Encoder Products Company, Costa Mesa, Calif.) which generates 100 pulses per shaft revolution. This figure translates into a value of 5 pulses produced for every 0.1-nm change in the monochromator. The
Figure 3. The start, initialize,and direction control circuit
reference device consists of a five-digit mechanical counter that has a hole drilled from one side to the other through the counter disks. A microbulb was placed a t one end of the counter and a photocell a t the other end so that a pulse will be produced when the holes in the disks are aligned. The counter is presently set for alignment a t 213.6 nm and serves as a mechanically-locked reference device which must be calibrated against a reference wavelength. The slew mechanism is driven by a 24-V dc motor and is controlled through three different slewing speeds. These include a fast (20 nmis), a medium (0.3 nmis), and a slow speed (0.06 nm/s) and are successuvely employed as the monochromator approaches the desired wavelength. This ensures rapid wavelength selection without overshoot. A block diagram of the interface circuitry required for the monochromator slewing mechanism is shown in Figure 2. The basic function of the circuit is to slew the monochromator from an initial point determined by the reference device to successive wavelengths as specified by the computer. The monochromator is reinitialized to the reference device a t the end of each slew cycle so that any wavelength errors incurred during the cycle will not be propagated throughout the remaining analyses. In starting up the system for analysis, the monochromator is first initialized to 213.6 nm by the start, initialize, and direction control block. Since the shaft encoder is geared to produce five pulses for every 0.1 nm, it becomes a simple matter for the computer to now calculate how many encoder pulses will be produced for the desired wavelength change. A number proportional to this value is loaded into the presettable counters by the computer and the slew is begun. As the pulses from the shaft encoder are fed back into the counters, the decoder block checks for the conditions for which the monochromator is 2.4, 0.14 and 0.0 nm from the desired wavelength. As these boundaries are crossed, the frequency select block automatically reduces the slew speed from high to medium to low and finally halts the slew motor when the wavelength has been reached. A flag is then set for the computer to indicate a completed operation. The monochromator also may be manually slewed through the use of the auto/manual circuit shown in the lower right-hand corner of Figure 2. The detailed circuitry for each of the blocks shown in Figure 2 is presented in Figures 3, 4, and 5. The detailed
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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MONOChWOMLIT(YI
,
DECODER INITIALIZE OUTPUT
~
;,:
INITIALIZE INPUTS
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Figure 4. Schematic diagrams for the counter decoder, and frequency select circuits
operation of these circuits can be found in Reference 38. The action required by the computer is to load the presettable counters, indicate the slew direction, and start the slew. The logic circuits automatically reduce the slewing rate and halt the monochromator a t the appropriate points. The positioning of the shaft encoder and the reference device in the monochromator can be seen in Figure 5. The Quartz Refractor Plate. Any monochromator slewing to preselected wavelengths involves a purely mechanical interaction between the computer and the monochromator itself. Although the unit performs consistently well in typical operation, there are times when mechanical aberrations in the slewing unit or spurious electronic pulses mistaken as encoder signals can cause error in the final wavelength setting. Most of these errors will be propagated throughout the current measurement cycle and will be corrected only after a new cycle has begun and the monochromator has reinitialized itself through the wavelength reference device described earlier. Although these errors would very rarely be of any problem in an AF measurement with its wide bandpass, it could result in rather severe errors when performing AE analyses with a much narrower slit width. Any errors in resetting to exactly the same wavelength in this case would result in a large change in the apparent emission intensity-an error which normally would be hard to detect during a typical analysis but could lead to highly inaccurate data. Originally, somewhat larger slit widths were used for the AE measurements to avoid problems of this nature, but this led to a number of problems with spectral interferences. A second important source of error centers around the operation of the reference device. This device is manually set so that a signal is produced when the monochromator passes over the 213.6-nm line. If for some reason this device is out of calibration, then all of the analyses would be affected regardless of how many times the monochromator is recycled. The slewing mechanism would consistently stop 306
Figure 5. Schematic diagrams for the auto/manual, motor driver, and monochromator circuits
From Grating
I
'I I,
Figure 6. Wavelength modulation with the quartz refractor plate
too early or too late depending on the direction of the calibration error. T o minimize this problem, the reference device was recalibrated periodically through a time-consuming manual alignment procedure. A third problem which was encountered in several investigations was the problem of background correction in AE. Occasionally cases would arise in which continuum bands of one form or another would interfere with the emission determination of several elements in various samples. The detection system originally had no capability for any type of wavelength modulation which would be useful in correcting for the variable background. In order to solve these problems, a simple modification was made to the programmable monochromator involving the addition of a rotatable quartz plate in the monochromator optical axis. The plate was placed between the exit mirror and the exit slit as shown in Figure 6 and is connected by a shaft to a stepper motor mounted on a housing on top of the monochromator chassis. The effect produced when the plate is inserted into the optical axis is demonstrated in Figure 6. As the plate is ro-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
P
ENTER
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PLATE
MEASUREMENT
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EXIT UPDATE COMPUTER CALIERATION
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Figure 7. Flow diagram for the wavelength calibration routine
tated, its refractive index will cause a lateral shift of the dispersed spectrum as it passes through the plate. The extent of this shift is dependent on the angle a t which the incident radiation strikes the plate, the thickness of the plate, and its index of refraction. By rotating the plate, the angle of incidence may be changed and the output spectrum is shifted by an amount proportional to the extent and direction of the change. Thus, the plate may be used to selectively shift the spectrum across the exit slit so that a particular wavelength or wavelengths may be quickly and precisely chosen. When connected to a stepper motor, the computer is able to exert direct control over the plate movement and now has the capability of modulating the output spectrum in a very rapid and precise manner. The output of the photomultiplier is monitored by the computer as a function of the plate position, and this feedback information is utilized in alleviating the three problems described above, those of initial calibration, wavelength slewing errors, and background correction in emission measurements. The equations describing refraction a t a surface and the resultant displacement of an incident beam of radiation are well known and can be found in most optics books (39). Taking into consideration the desired plate response and the errors which are inherent in using the plate (which are described in a later section), it was decided to use a quartz plate with a thickness of H 6 inch (1.59 mm). T o ensure minimal transmission loss over the wavelength range of interest (200-800 nm), a Suprasil 1 quartz disc (Amersil, Inc., Hillside, N.J.) with a thickness of Yls inch and a diameter of 1inch was chosen. The motor employed is a SLO-SYN stepping motor type M061-FC08 (Superior Electric Company, Bristol, Conn.) with a resolution of 400 steps per revolution when the eight-step switching sequence is used. This produces a movement of 0.9’ for each step and results in a wavelength change of 0.017 nm. A logic circuit was designed to allow both clockwise and counterclockwise indexing a t rates up to 1000 Hz. Automatic Monochromator Calibration. One of the first applications of the quartz plate was to utilize its wavelength modulation effect to calibrate the monochromator to an absolute reference. The reference may be any spectral peak for which there is a suitable source, such as the emission from the calcium 422.67-nm line. This calibration pro-
cedure requires approximately 1 minute to complete and can detect and correct errors of up to 0.5-0.6 nm to within 0.017 nm. If the calcium line is chosen, the computer will first slew the monochromator to a point that would correspond to 422.67 nm if everything were in calibration. The calcium spectral radiation is directed onto the monochromator entrance slit and, if the monochromator is out of calibration, the maximum peak intensity will not be presented exactly a t the exit slit but rather to one side or the other. By rotating the quartz plate and monitoring the photomultiplier signal, the computer automatically shifts the spectrum back and forth across the exit slit until the maximum peak intensity is found. By keeping track of how far the plate had to be moved to reach this point, the computer can calculate how far out of calibration the monochromator actually is. This information is stored and the succeeding wavelength scans are corrected by this amount so that the monochromator is now effectively calibrated. The computer flowchart for this procedure is shown in Figure 7. If a calibration is to be performed, the spectral line is specified to the computer, and the monochromator is then automatically slewed to what should be exactly that wavelength if it were in calibration. The quartz plate is then initialized to a position that is perpendicular to the optical axis so that there is no refraction of the light beam. This orientation corresponds to the “calibrated” position and the computer will now keep track of the number of steps required to move the plate from this point in order t o produce the maximum signal a t the photomultiplier. T o determine the relative position of the true peak maximum, whether it be above, below, or already present a t the exit slit, a minimum of three measurements is required. The computer makes the first measurement a t the initial plate position, the second a t five plate steps to the right, and the third a t five steps to the left of the center plate position (0.08 nm to either side of the first measurement). The relative intensities of these three points are then decoded by the computer into one of nine possible cases describing the relative peak position. The more important of these cases include the conditions of centered on the peak, off to one side, no peak seen, and located between two peaks. If the peak center has not been found, the computer will determine the direction of error and move the quartz plate accordingly. The measurement routine is reentered, and three new data points are obtained and decoded as before. This procedure is continued and, as the true peak maximum is approached, the computer will automatically narrow the search range to 4, 3, 2, and finally 1 step to either side of the first measurement of each set. When the scan has finally been narrowed to this point and the three measurements indicate that the peak maximum has been found, the computer will print out the calibration error on the teletypewriter, update the computer calibration value, and then reset the plate back to its normal perpendicular position. With the monochromator error known by the computer, the analysis wavelengths may now be selected accurately by readjusting the number of encoder pulses that should be expected in slewing to these wavelengths. The entire procedure is highly accurate and consistent and allows for a complete calibration in less than a minute. In testing the calibration routine, radiation from a calcium hollow cathode lamp was used to provide the analytical wavelength of 422.67 nm. For 70 consecutive trials, the monochromator was offset by a known amount to see if the calibrate program could detect the direction and magnitude of the error and then correct for it. For every one of the trials, the exact offset was determined and printed out on the teletypewriter. The system was checked out on sev-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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v
n
Table I. Evaluation of “Peak Alignment” Routine0
ENTER
% R S D of 10 consecutive points
MAKE PEAK MEAS.
Wavelength, nm
0-7
Monochromator scan without alignment program
Cd 228.80 4.2% Ca 422.67 12 Na 589.00 4.1 Li 610.78 6.4 0.15-nm monochromator band pass.
Monochromator scan with alignment program
0.80% 1.2 0.52 0.70
Q
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4 I--G) P RL EA TU TR EN
TO PEAK S T O R E IN PROPER
ORIGINAL POSITION
PRINT ERROR STATEMENT
DETERMINE DIRECTION ERROR
Figure 8. Flow diagram for the “Peak Alignment” routine
era1 other elemental wavelengths with both flames and hollow cathode lamps with the same results. A 10-ppm Ca solution aspirated into the flame has proved to be a satisfactory source of radiation and is presently used to calibrate the monochromator to the 422.67-nm line. The magnitude of the monochromator error that can be detected with this procedure depends on the signal intensity, the bandpass of the monochromator, and the search range that is programmed. A “Peak Find” Routine for Slewing. As was described earlier, one of the major problems with a purely mechanical wavelength drive mechanism is the possibility of slewing errors resulting in a wavelength misalignment that would seriously affect an emission measurement. The solution to this problem is the application of a “peak find” routine involving the quartz plate and computer in a manner similar to the calibration procedure. In a typical analysis, the monochromator is mechanically slewed to the analysis wavelength and the “peak find” program is then employed to check the accwacy of the wavelength setting. If a misalignment is found, the error is quickly corrected before the measurement is made. The computer flowchart for this routine can be found in Figure 8. Although similar in concept to the calibration program described earlier, this routine makes several assumptions that allow for a very rapid determination and correction for any wavelength error that might be present. The major assumption made is that the true wavelength is within 0.06 nm of the point at which the monochromator has stopped. Most likely, however, the true wavelength will be either directly on or only one step to either side of this point, When making the three measurements required for a decoding operation, the plate is therefore moved only one step to either side of the center for the initial measurements. If these data indicate a wavelength error, the plate will be moved in the direction of the error for a fourth data point. 308
T o save time, two of the three previous points are reshuffled into the proper locations, the third point (the point that was the furthest from the apparent peak maximum) is thrown out, and the new fourth measurement is put into its place. As is shown in the flowchart, this process is continued until the peak maximum is found or the error limits have been exceeded. If there is no wavelength error, the entire procedure requires approximately 1.6 s to perform. For each additional measurement that is required to locate the peak, an extra half second is needed for the determination. In practice it has been found that it rarely takes more than 2.2 s to complete the entire alignment procedure. In those cases where no discernible peak is present, as for a blank solution, the program simply assumes that the original position was the correct setting and the plate is returned to that position. If for some reason a large wavelength error has been found and that error exceeds the limits set in the computer, an error symbol will be typed out indicating the error, and the plate is automatically returned to a position closest to the peak but not exceeding the error limit. In the case that the decoded data from the quartz plate shows that the monochromator has stopped between two peaks, the plate will simply return to its original position before making the measurement. In this case, there can be no indication of which peak is indeed the correct one and an error code is again printed out on the teletypewriter. To check the operation and value of the “peak find” routine, a comparison study was made using four different wavelengths corresponding to the elements cadmium, calcium, sodium, and lithium. The results are shown in Table I. The, procedure was to first allow the monochromator to slew to the indicated wavelength and to immediately make the measurement without the use of the alignment program. After this first measurement has been made, the “peak find” routine is used to correct for any wavelength errors which might be present and a second measurement is then made. The % RSD of ten consecutive measurements of this type are shown in the table and it can be seen that there is indeed a marked improvement in the precision of the analyses when the alignment program is employed. Much of the remaining error can be attributed to flame fluctuations. It should be noted that in most cases the monochromator did mechanically slew to the correct wavelength but, because of those few measurements that were made when there was a misalignment, the overall deviation was increased to the figures shown. This routine has proved to be invaluable in operating the monochromator with the narrower slits and the reduced bandpass. It has also aided in the calibration of the sine bar mechanism. Runout on the leadscrew is 0.1 nm and must be corrected for if the wavelength drive is to be good to 0.02 nm or better. The correction for each element was rapidly determined through the use of the “peak find” routine and
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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Figure 9. Flame emission spectrum of 2 ppm lithium in the presence of 1250 ppm strontium
the computer wavelength table was then updated to correct for these errors. Automatic Background Subtraction for Atomic Emission. As was described earlier, several cases arose in which continuum background emission interfered with the emission analysis of several elements. As an example of this type of interference, experiments were attempted to measure the lithium line at 670.78 nm in the presence of a large amount of strontium. As can be seen in Figure 9, the broad SrOH bands directly interfered with the analysis, making an accurate and precise measurement very difficult. In the case where the Sr concentration will be changing unpredictably from sample to sample, an accurate analysis would be impossible without some means of subtracting out the emission background during the analysis. It can be seen that the background under the lithium peak can be approximated quite well by measuring the emission intensity slightly to the side of the peak. (Because of the response of the recorder electronics, the lithium peak appears to be much wider in the figure than is actually the case.) Moving the output spectrum off the peak maximum to one side in order to measure this background, is easily done with the quartz refractor plate. By rotating the plate 18-22 steps (0.30-0.38 nm), the spectrum can be shifted to one side so that only the background emission from the SrOH bands is detected by the photomultiplier. By alternately integrating the lithium signal plus background and then moving the plate and subtracting out the background, a signal proportional to just the lithium concentration will be obtained. In applying this background correction procedure to the analysis of the Li in Sr, the working curves shown in Figure 10 were generated. It can be seen that the technique very effectively corrects for the high background that was present under the lithium peak and is essentially equivalent to taking a first derivative of the spectrum a t one point. With a simple change of the computer program, the plate can be programmed to move to alternate sides of the peak and thereby obtain an even more accurate estimation of the background, especially in the case of a highly sloping
background. The time required to rotate the plate the 20 steps to the side is approximately 20 ms. Sources of Error in the Quartz Plate. A number of considerations had to be taken into account in order to obtain the desired performance from the refractor plate concept. There are several sources of possible error that must be evaluated in designing and operating the plate, and a few of these result in inherent limitations of the system. Several of these problems have been previously described for various systems (37, 38), and they will be briefly presented in light of the present application. Perhaps the potentially most serious problem is the change in reflection off the surfaces of the plate as a function of the plate angle. This would have the greatest effect on the automatic background subtraction routine since, in this case, the plate is rotated through a relatively large angle (18') to obtain the background emission signal. The ratio of light reflected from a surface to the light incident on the surface as a function of the incident angle is well known (39). Calculations involving both surfaces of the quartz plate show that the difference in the amount of transmitted light for the two incident angles of 0' and 18' amounts to 0.033%. This is the error that will be incurred during the background correction measurement for the case in which the background signal is on the same order as the signal intensity. Even for the case in which the background intensity is ten times the signal intensity, the error produced is only 0.33%. Neither of these figures present a serious problem in making flame measurements. The magnitude of this error increases fairly rapidly, however, as the angle is increased above 18'-20', and this represents a boundary for the plate unless computer corrections to the data are made. The introduction of the quartz plate in the optical path of the monochromator also affects the focal length of the system and therefore the resolution. Since the spectrum converges on the exit slit, the radiation does not uniformly strike the quartz plate a t only one defined angle but rather within a range of angles defined by the angle of convergence of the monochromator. Since there is a difference of angles for any particular wavelength, the extent of refraction will be different for each converging ray as stated by the law of refraction. Accordingly, the effective focal length of the monochromator is changed and, unless the exit slit is moved to correct for this aberration, the resolution will be
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
309
Slit Actuator
F
to Entrance Slit
Drive Prawl
Figure 11. The programmable slit width control
degraded. For this particular system, the beam is found to be defocused by approximately 0.015 nm, which should not cause any serious or detectable problems since bandpasses of 0.15-0.20 nm are usually employed for the emission analysis. One obvious consideration is the necessity for moving the quartz plate reproducibly between points. Any lateral uncertainty from wobbling or rotational errors will result in a misalignment and, therefore, variations in the wavelength. To eliminate or minimize such errors, the stepper motor mount was designed so that the shaft connecting the motor to the quartz plate would rotate on two precision bearings in a smooth and reproducible manner. Other somewhat minor problems include the variation of wavelength change per plate step as a function of the incident angle, the transmission properties of the quartz plate, and the change in refractive index of the quartz as a function of the wavelength. For small angles, however, this nonlinear dependence is found to be quite small and, for the purpose described here, no correction needs to be applied. To eliminate any transmission problems in the wavelength range of interest, the Suprasil 1 quartz plate was chosen for its optical quality and good transmission characteristics from 200-800 nm. Another minor problem is the change in the refractive index of the quartz as a function of wavelength. The index ranges from 1.53 a t 200 nm to 1.46 a t 650 nm. This change is small enough that it has no effect on any of the applications described in this paper. Even in the case where various spectral lines are used for the calibration step, the variations caused by differences in the refractive index are negligible. Programmable Slits. One of the requirements for the monochromator system was that it should be capable not only of slewing to each analytical wavelength, but also of selecting the optimal bandpass for the technique that is to be employed, Le., AE or AF. The bandpass requirements for each technique are quite different-whereas emission measurements require narrow slit widths to eliminate the spectral interferences, fluorescence measurements may be made with much wider slits since the method is highly specific when narrow line sources are employed for excitation. Many authors, in fact, advocate the elimination of wavelength dispersive devices for fluorescence determinations and claim greater limits of detection with the greater throughput (3, 4, 40-42). In most cases, however, these benefits can be gained only when the flame background flicker is not a major contributor to the overall noise of the measurement ( 4 3 ) . In general, this can be accomplished with rich, low-temperature flames, but these conditions give rise to chemical and matrix interferences in practical samples and are generally unacceptable. 310
As the result of S/N investigations in our own laboratory, it was decided that some form of wavelength discrimination was necessary for AF determinations, and a monochromator with a relatively wide bandpass was found to offer the greatest number of advantages. Typical fluorescence measurements are made with a slit width of 1-2 mm (2-4 nm spectral bandpass). In performing AE measurements, however, much narrower bandpasses are required to reduce the spectral interferences inherent in the technique. So that the same monochromator could be used for both fluorescence and emission measurements, some form of slit control was required so that switching between modes of operation would be easy and rapid. This was accomplished by adding an additional stop for both the entrance and exit slit actuator arms and through the plunger action of an air solenoid (Bimba Air Solenoid, Model 040-NRLSC, Samuel Harris and Co., Chicago, Ill.) the arms can be placed against either stop. A schematic diagram is shown in Figure 11. An electrical signal applied to the solenoid forces the plunger out, which in turn draws the slit actuator arms against the installed stops. This produces a slit width which is defined by the position of the fixed stop which is presently set for 2000 microns. When the solenoid is deactivated, the plunger moves back to its normal rest position, and the actuator arms return to the normal slit drive prawl through a spring action. The position of this stop is variable and can be adjusted with the existing slit control knob located on top of the monochromator chassis. Thus, the slit control allows for the automatic setting of the slits between two positions, one of which is fixed inside the monochromator and the other is set with the existing controls. The pressure for the solenoid is supplied by a compressed air cylinder and both the input and output lines are slightly pinched in order to provide smooth but quick solenoid action. The 120 VAC electrical signal required to drive the solenoid is supplied through a triac driver circuit described elsewhere ( 4 4 ) .The computer controls the state of the slits by loading the appropriate logic signal into a SN7475 data latch and using the output of the latch to run the triac driver circuit. Under normal operating conditions, the slits are typically set for 80 pm and 2000 pm (0.16- and 4.0-nm bandpass) for AE and AF, respectively.
DISCUSSION The computer-controlled wavelength drive and slit width control have been in use in this laboratory for over two years. The unit operates as part of a multielement flame AE/AF spectrometer and has been employed in the determination of Na, K, Mg, Ca, and Li in blood serum samples ( I ) . The addition of the quartz plate has extended the utility of this detection system in that the spectral interferences and monochromator misalignment problems noted in earlier studies are now no longer a serious consideration. Working curves for Na, K, Mg, Ca, and Li were obtained from the multielement analyzer utilizing this monochromator system. The results are listed in Table 11. Mg and Ca were determined by AF and Na, K, and Li by AE. The composite solution was prepared from reagent grade materials and made up in a 0.1 N HC104 and 1250 ppm lanthanum solution. The concentration ranges bracket values found for a 1:25 dilution of blood sera. The data points were obtained in a sequential manner, with the monochromator slewing to the analytical wavelengths in order from lowest to highest. The curves were prepared from six concentration points, each point being the average of two values. The precisions are quoted as typical % RSD’s for
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Table 11. Statistical Data on Analytical Curves Obtained in a Multielement Determination Element
Method
Wavelength, nm
Mg
AF AE AE AF AE
285.2 330.3 404.5 422.7 670.7
Na K Ca Li
Re1 std dev of Concn range, ppm
0.1 2-1.7
17-250 1.7-25 0.84-12
0.12-1.7
the two points in each data set. Problems were encountered in obtaining a good precision for potassium due to the fact that a weak line was used (404.5 nm) and that a relatively cool flame was employed (0pH2). Attempts to increase the sensitivity for potassium are under way and include a new burner design with an ultrasonic nebulizer. The time required for one complete cycle in which all five elements are determined and the system is recycled back to the initial wavelength setting is on the order of 80 seconds. If lithium were dropped from the analysis, the total time required would be less than a minute. The use of a quartz plate has been shown to provide a simple but reliable method of wavelength modulation. Unlike other systems that typically use a vibrating plate to produce a sinusoidal modulation effect, the technique described here utilizes a "square wave" type of modulation which allows for a time-averaging of each data point that is taken in by the computer. With sinusoidal modulation, the total amount of peak signal measured is small compared to the total measurement time, but the procedure employed here allows for efficient measurement since the plate can be indexed to a specific point and halted for signal integration. This integration time is important in both the spectral alignment procedure and the background correction cycle, particularly for weak emission lines. Hieftje and Sydor (36) concluded that signal averaging for 256 scans was essential for their sinusoidal demodulation step. Since the plate and measurement are under computer control, the appropriate integration time may be automatically implemented for each spectral line depending on its intensity. Also, unlike the previously described methods of background subtraction, the computer can automatically choose how far off the peak the background measurement should be made and in which direction. The frequency a t which the background subtraction cycle is set depends on the wavelength interval to be traveled and the integration time. Typically a modulation frequency of 10 Hz is used which is sufficient to reduce interferences from the lower frequency components of flame flicker. ACKNOWLEDGMENT The assistance of M. F. Bryant in the construction of the stepper motor driver circuit is greatly appreciated. LITERATURE C I T E D (1) R . W. Spillman and H. V. Malmstadt, submitted to Am. Lab. (2) K. W. Busch and G. H. Morrison, Anal. Chem., 45,712A (1973). (3) R. M. Dagnall, G. F. Kirkbright, T. S. West, and R. Wood, Anal. Chem., 43, 1765 (1971). (4) D. G. Mitchell and A. Johansson, Spectrochim. Acta. Part B, 26, 677 (1971). (5)J. D. Norris and T. S. West, Anal. Chem., 45, 226 (1973). (6) J. F. Skene, D. C. Stuart, K. Fritze, and T. J. Kennet, Spectrochim. Acta, Part E, 29,339 (1974).
Slope
r2
x
0.9999 0.9985 0.9979 0.9997 0.9999
2.18 8.2 2.38 2.77 2.27
104
x 10' x lo2 x 103 x 104
points, %
1-2 2-3 3-5 1-2 1-2
(7) J. B. Dawson, D. J. Ellis, and R. Milner. Spectrochim. Acta, Part E, 23, 695 (1968).
(8)M. J. Milano, H. L. Pardue. T. E. Cook, R. E. Santini, D. W. Margerum, and J. T. Raycheba, Anal. Chem., 46,374 (1974). (9) D. 0. Knapp, N. Omenetto. L. P. Hart, F. W. Plankey, and J. 0. Winefordner, Anal. Chim. Acta, 69,83 (1974). (10) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (11) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 1231 (1974). (12) Y. Talmi. Anal. Chem., 47,658A (1975). (13) Y. Taimi, Anal. Chem., 47,697A (1975). (14) K. M. Aldous, D. G. Mitchell, and K. W. Jackson, Anal. Chem., 47, 1035 (1975). (15) G. Horlick and E. Codding, Appl. Spectrosc., 29, 167 (1975). (16) D. G. Mitchell, K. W. Jackson, and K. M. Aldous, Anal. Chem., 45, 1215A (19731. (17) S. Greenfield: I. LI. Jones, H. McO. McGeachin, and P. B. Smith, Anal. Chim. Acta. 74. 225 (19751. (18) B. L. Vallee'and M. Margoshes, Anal. Chem., 28, 175 (1956). (19) S. J. Martin and H. V. Malmstadt, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1975, Paper No. 141. (20) W. C. Fuller, "The Application of Fourier Transform Techniques to Multielement Atomic Fluorescence Determinations", Ph.D. Thesis, University of Illinois, Urbana, Ill., 1974. (21) F. W. Plankey, T. H. Glenn, L. P. Hart, and J. D. Wmefordner, Anal. Chem., 46, 1000 (1974). (22) E. F. Palermo, Akbar Montaser, and S. R. Crouch, Anal. Chem., 46, 2154 (1974). (23) G. Horlick, W. K. Yuen, and K. R. Betty. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1975, Paper No. 143. (24) E. Cordos and H. V. Malmstadt, Anal. Chem., 45,425 (1973). (25) F. R. Stauffer and H. Sakai, Appl. Opt., 7, 61 (1968). (26) T. C. O'Haver and G. L. Green, Am. Lab., 7 (3). 15 (1975). (27) I. Balslev, Phys. Rev., 143,636 (1966). (28) R . T. Schneider, Am. Lab., l( 12), 8 (1969). (29) T. C. Rains, M. S. Epstein, and 0. Menis, Anal. Chem., 46, 207 (1974). (30) R . B. Timmer, "Wavelength Modulation and Noise Reduction Using a Minicomputer-Controlled UV-VIS-NIR Recording Spectrometer", Ph.D. Thesis, University of Illinois. Urbana, Ill., 1974. (31) M. F. Bryant, University of Illinois. Urbana, Ill., personal communication, 1975. (32) W. Snellrnan, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis, Anal. Chem., 42,394 (1970). (33) A. Gilgore. P. J. Stoller, and A. Fowler, Rev. Sci. lnstrum., 38, 1535 (1967). (34) R . C. Eiser and J. D. Winefordner, Anal. Chem., 44,698 (1972). (35) W. Fowler, D. Knapp, and J. D. Winefordner, Anal. Chem., 46, 601 (1974). (36) G. M. Hieftje and R . J. Sydor, Appl. Spectrosc., 26,624 (1972). (37) R . Roldan, Rev. Sci. lnstrum., 40, 1388 (1969). (38) R. W. Spillman, "The Development and Application of an Automated Multielement Atomic Emission/Atomic Fluorescence Flame Spectrometer,'' Ph.D. Thesis, University of Illinois, Urbana, Ill., 1975. (39) C. S. Williams and 0. A. Beckland, "Optics", John Wiley and Sons, inc.. New York, N.Y.. 1972. (40) R. C. Elser and J. D. Winefordner. Appl. Spectrosc., 25,345 (1971). (41) T. J. Vickers. P. J. Slevin, V. I. Muscat, and L. T. Farias, Anal. Chem., 44,930 (1972). (42) P. D. Warr, Talanta, 17,548 (1970). (43) V. i. Muscat, T. J. Vickers, W. E. Rippetoe, and E. R . Johnson, Appl. Spectrosc., 29,52 (1975). (44) E. S. iracki, M. B. Denton. and H. V. Maimstadt. Anal. Chem., 44, 1924 (1972).
RECEIVEDfor review August 18, 1975. Resubmitted November 6, 1975. Accepted November 6, 1975. The authors are grateful for partial support of this research under NSF research grant US NSF G P 18910 and MPS 74-12248.
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