Elenent : Cu kavelength 324.8 nm Magnetic field : 11 kgauss Full scale Abs. 1.0
.
’
+
Cd concentrations of 0,0.5, and 1.0 ppb. These samples were directly introduced into the cuvette and measured without any ashing process. Carrier gas was flowed at the rate of 0.05 L/min to reduce the background absorption to less than 1.5. The signals of four samples, i.e., the dilute solution, Cd added solutions, and distilled water, were observed as shown in Figure 14. The measured Cd concentration in the original urine was 0.7 ppb. Figure 15 shows the process of analysis of Cu in serum. Serum was diluted to 50% concentration with distilled water, standard solution of Cu being added to provide samples of additional Cu of 0,0.5, and 1.0 ppm. No ashing process’was employed in this measurement. The small peak or fluctuation of the baseline is due to the surface reflection of bubbles of serum formed during the drying process. The measured Cu concentration in the original serum was 0.89 ppm.
ACKNOWLEDGMENT Flgure 15. Analysis of Cu in serum
cuvette. However, no signal appeared except a small fluctuation attributed to the change of polarization by the reflection on the surface of the pipet. The temperature of the absorption cell was higher than that of bottom of the cup because of their different heat capacity and electric resistance. However, the temperature of the cup was homogeneous and the rate of vaporization was independent on the position of the sample loaded in the cup. Thus, good reproducibility was obtained without the special care of sample introduction. Next, the precision of background correction was checked by actual samples. When 10 KLof urine was directly atomized without any ashing process, a strong background absorption over 1.0 was observed. However, no signal was observed by the present method on the various lines in the case where the analyte element was absent in the sample. Figure 14 shows the process of analysis of Cd in urine. Urine was diluted to 50% concentration with distilled water. A standard solution of Cd was added to the one part of the dilute solution of the sample and we made solutions of additional
The authors express their sincere gratitude to T. Hadeishi of the University of California and to K. Ohishi, K. Fukuda, K. Uchino, K. Takahashi, and K. Moriya of Naka Works of Hitachi Ltd. for their discussions and great help.
LITERATURE CITED (1) T. Hadeishi and R. D. McLaughlin, Science, 174, 404 (1971). (2) T. Hadeishi, Appl. Phys. Lett., 21, 438 (1972). (3) H. Koizumi and K. Yasuda, Anal. Chem., 47, 1679 (1975). (4) H. Koizumi and K. Yasuda, Spectrochim. Acta, Parts, 31, 237 (1976). (5) H. Koizumi and K. Yasuda, Anal. Chem., 48. 1178 (1976).
(6) T. Hadeishi, D. A. Church, R. D. McLaughlin, 8.D. Zak, M. Nakamura, and B. Chang, Science, 187, 348 (1975). (7) T. Hadeishi and R. D. McLaughlin, Am. Lab., 7 (E), 57 (1975). (8) T. Hadeishi and R. D. McLaughlin, Anal. Chem., 48, 1009 (1976). (9) R. Stephens and D. D. Ryan, Talanta, 22, 655 and 659 (1975). 10) H. Koizumi and K. Yasuda, Spectrochim. Acta, in press. 1 1) G. Herzberg, “Molecular Spectra and Molecular Structure”, Van Nostrand Reinhold Co., New York, 1950. 12) W. Slavin, At. Absorp. News/. 24, 15 (1964). 13) S. R. Koirlyohann and E. E. Pickett, Anal. Chem., 37, 601 (1965). 14) C. Ling, Anal. Chem., 39, 798 (1967). 15) J. Kuhl, G. Marowsky, and R. Torge, Anal. Chem., 44, 375 (1972). 16) Perkin-Elmer, HGA-2100 Report L-357, Dec. 1973. 17) H. Koizumi and M. Katayama, unpublished work.
RECEIVED for review November 1,1976. Accepted March 29, 1977.
Design and Evaluation of a Random Access Vidicon-Echelle Spectrometer and Application to Multielement Determinations by Atomic Absorption Spectrometry Hugo L. Felkel, Jr. and Harry L. Pardue” Department of Chemistry, Purdue University, West La fayette, Indiana 47907
A silicon target vidicon tube has been coupled to an echelle grating spectrometer for simultaneous multielement determinations by atomic absorption. A minicomputer is interfaced to the vidicon detector to implement random access interrogratlon of selected detector elements and Is used for data processing and display. The resolution capabilities of the system are assessed for the spectral region from 3000 to 7000 A. An in-depth study of the parameters affecting the random access signal and detector linearity is presented. Quantitative data are reported for simutlaneous multielement determinations in synthetlc solutions using atomic absorptlon spectrometry.
I n a n early report from this laboratory, we described the 1112
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
adaptation of a vidicon tube for rapid scanning spectrometry ( I ) . Subsequent to that report, numerous papers have appeared from this and other laboratories which described applications to molecular (2-5) and atomic (6-10) spectrometry. While these reports demonstrate beyond any question that the vidicon and other types of imaging detectors are viable tools for analytical spectrometry, they also emphasize some serious limitations of the devices. One of the most serious limitations results from the finite length of the detector which usually forces the user into a rather severe tradeoff between spectral resolution and spectral range (2,9, 11). Most vidicon applications reported to date have been limited to what we shall call one-dimensional scanning modes
PROC-
N4 M5
~
PLOTTER
osclLLoscoP R E A L -TIME
DISPLAY
Flgure 1. Block diagram of computer controlled random access spectrometer
in which only one dimension of the two-dimensional array detector is used for spectral information. It is evident that some of the tradeoffs between spectral range and spectral resolution experienced in these systems could be overcome by using both dimensions of the array for spectral information. One approach to accomplish this, first suggested by Margoshes (12),is to employ an echelle grating spectrometer to disperse the optical spectrum into a two-dimensional pattern (13-15) and to use a two-dimensional vidicon scanning mode to interrogate detector elements corresponding to different spectral lines. By using the random access mode of interrogation in which only the line intensities of interest are measured, a reduction in computer time and space required, as well as greatly simplified data reduction, is realized for high resolution, wide spectral coverage measurements. The experimental realization of this approach has been reported by Wood and co-workers (16), in which a commercially available computer controlled camera system utilizing an SEC vidicon was employed. The work was mainly concerned with emission analyses, but the operational characteristics of the vidicon detector merit a more thorough investigation. Danielson et al. (17, 18)have described a computer controlled echelle spectrometer for emission analysis based on an image dissector, in which the detector is used in a photon counting mode. Although high sensitivities are attainable, long times must be used to have acceptable counting statistics. Others have combined an echelle spectrometer with multiple photomultiplier detectors (19). T h e versatility of a computer controlled instrument lies in the ability to easily change scanning formats by software changes rather than by hardware modifications. Many experiments not readily accomplished by hardware sequencers can easily be carried out under computer control. In addition to control functions, the computer may be used to implement a variety of data processing options for enhancing signal to noise ratios (S/N). T h e objective of this report is to present the design considerations involved in constructing a computer controlled random access vidicon spectrometer as well as to evaluate performance characteristics of the system.
INSTRUMENTATION The general layout of the random access vidicon detector and echelle grating spectrometer is shown schematically in the block diagram of Figure 1. The fundamental units composing the system are I) the optical system and dispersion devices, 2) the vidicon detector, 3) the beam control logic and signal processing module, and 4) the computer system and its associated peripherals. Although the present report describes applications of the system for atomic absorption determinations, it is also being evaluated for atomic emission spectrometry. Optical System. Energy from a hollow cathode lamp is imaged by lens 1 over the center of the 10-cm slot high solids burner for air-acetylene (No. 02-1000036-00, Varian, Louisville, Ky., 40207) and lens 2 then refocuses the radiation onto the entrance slit of
the echelle spectrometer. Unless otherwise specified, an entrance slit width of 200 pm and height of 500 pm was used throughout these experiments. Lens 1 is a 6-cm focal length quartz lens with a 4-cm diameter thht is stopped down to 1.6 cm by an iris diaphragm. Lens 2 is a 1.5-cmdiameter quartz lens with a 5-cm focal length. Various lamps were used in this work and include a multielement hollow cathode lamp containing Co, Cr, Cu, Fe, Mn, and Ni (No.JA45599,Jarrell-Ash) as well as single element hollow cathode lamps for Cr (Varian No. 2T300), Cu (Varian No. 5D874), and Fe (Westinghouse No. WL 22936). The lamps are powered by a constant current supply removed from a prototype atomic absorption spectrometer (No. 5960A, Hewlett-Packard, Avondale, Pa. 19311). A low speed shutter (S2) is used for making computer controlled dark current measurements. A high speed electromagnetic shutter (Sl, 22-8411, Ealing Corp., South Natick, Mass. 01760) is used for controlling the time interval that the vidicon is illuminated. Exposure times are entered into the shutter control module from a multiplier and a decade switch register providing shutter times of from 30 ms to 10 s. The spectrometer is a modified version of a prototype 0.75 meter Spectraspan echelle grating spectrometer (Spectrametrics, Inc. Andover, Mass. 01810) with a 73 grooves/mm echelle grating having a blaze angle of 63O 26’ and a 30’ quartz prism. The approach used to obtain an image suitable for interrogation by the vidicon array detector is to introduce auxiliary optics (M3-M5) which reduce the size of the focal plane from approximately 50 mm by 75 mm to 9 mm by 1 2 mm while maintaining adequate spatial resolution and a flat focal plane over the spectral region from 2250 to 8000 A. The auxiliary folding mirror, which is located a few inches behind the normal focal plane, generates a “white light” (Le.,all wavelengths present) image of the echelle grating just before the cassigrain mirror system, composed of M4 and M5. The auxiliary folding mirror has a focal length of 78 cm and a diameter of 10.7 cm. The focal length of the cassigrain mirror system is 13.5 cm at f/1.6. This mirror system produces a second focal plane on the vidicon target, which is a reduced image of the first spectral focal plane. At the same time, the effective flnumber experienced by the detector is changed from f / l O to approximately f / 1 . 6 so that the intensity is increased by a factor of 102/1.62or about 40 for the modified spectrometer compared to the unmodified system. This approach to image reduction circumvents the difficulties associated with short focal length optics which suffer from severe vignetting, abberrations, or limited spectral range. The formation of the two-dimensional output spectrum of the echelle spectrometer may be visualized as follows. Many wavelengths of different grating orders will simultaneously satisfy the diffraction equation and coincide because high orders of the echelle grating are used. These wavelengths may be separated by placing a prism so that its dispersion is directed perpendicular to that of the grating, thus effectively separating the orders. The resulting spectrum is a two-dimensional pattern where vertical position corresponds to the grating order, and horizontal position corresponds to wavelength within each order. For a more detailed description, the original references should be consulted (13,14). For the system used in this work, there are 72 orders in the wavelength range from 2250 to 8000 A. The reciprocal linear ANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
1113
X
- DAC
Figure 2. Block diagram of computer controlled sweep/addressing circuitry and signal processing amplifiers
dispersion varies from about 3.8 A/mm at 2250 8, to about 14.2 A/mm at 8000 A. Vidicon Circuitry. Commercial power supplies were used to power the vidicon and the associated electronics. The sweep/addressing circuitry and signal processing amplifiers necessary for random access interrogation were designed and constructed in this laboratory. The deflection coils are driven by the digitally controlled circuit represented in the block diagram of Figure 2. The two-dimensional addressing circuitry produces current in the deflection assembly to position the electron scan beam at a specific horizontal and vertical position. Additionally, the entire vidicon target may be scanned using a conventional line scan format prior to random access interrogation in order to recharge the target. With this interface, seven commands necessary for random access data acquisition have been implemented. Two bits of the 12-bit computer word are decoded into four primary functions by the command decoder. The four commands are: load the horizontal position, load the vertical position, sample the current address, and erase or recharge the target prior to random access. In addition to these primary commands, three secondary commands which control the shutters are derived from the sample function. Two of these commands are open and close the low speed shutter which is used for automated acquisition of dark current measurements. The third command is used in conjunction with the high speed shutter to expose the vidicon target for a predetermined time interval. The function and usage of these commands is best described by a typical operational scenario. At the start of a scanning cycle, the target is charged by issuing the erase command. This command activates the sweep circuitry, consisting of the presetable counters and the triangle generator, to scan the vidicon target using a conventional line scan format. After the predetermined number of erase cycles has been executed, the interface is enabled and ready to receive the next command. The next command issued is dependent upon whether the shuttered or unshuttered operational mode has been selected. In the unshuttered mode, radiant energy is allowed to impinge upon the target at all times, except during dark current measurements. In the shuttered mode, radiant energy falls on the detector only when the high speed shutter received the expose command and erasing and random access are carried out when the shutter is closed. If the shuttered mode has been selected, the expose command is issued a t this time. If unshuttered, the signal may be allowed to integrate before proceeding with random access. In either mode, the dark current level a t room temperature or serious degradation in resolution limits the integration time to less than 1 s. The readout of a given raster element is initiated by loading the parallel binary horizontal position data into the programmable counters. The horizontal DAC immediately responds and its associated coil driver positions the electron scan beam in the corresponding horizontal position. The beam is actually blanked a t this point, but the correct deflection field has been established. The vertical position is established in a similar manner. After allowing a 20-ps settling time, the sample command is issued and the current required to recharge the location is measured. This 1114
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
current is integrated for the predetermined sampling or dwell time, which can be changed from 10 ps to 150 ws in 10-w~steps. At the completion of the specified dwell time, the track and hold is gated to the hold mode and the 12-bit ADC is triggered to convert. This sequence of positioning the electron scan beam and sampling is repeated until the requisite data is acquired and then the target is erased again. The complete process may be repeated so that averaging can be used to enhance the signal-to-noise ratio. Specific details of the circuitry are available from the authors. Computer. The computer used for system control and for data acquisition and processing is a PDP-8/M minicomputer (Digital Equipment Corp., Maynard, Mass. 01754) with 28K of memory. Standard peripherals included in this system are a magnetic tape drive (TD8E DECtape), a 1.66 megabyte disk (Model 31, Diablo Systems, Inc., Hayward, Calif. 94545) with a Systems Industries controller (Model 3010, Systems Industries, Sunnyvale Calif., 94086) and a teleprinter. Several other periphrals for data display and acquisition are incorporated in this system and include an incremental plotter (No. DP-1-1, Houston Instrument, Bellaire, Texas 77401), a line printer (No. 306, Centronics Data Computer Corp., Hudson, N.H. 03051), and a 12-bit analog-to-digital converter (0-10 V input range, 20 ps conversion time). Disk memory is used for temporary data storage and processing and the tape system for permanent data storage. Software. Software was developed and run under the control of the OS/8 operating system. Data acquisition programs that interact with the vidicon interface were written in the SABR assembly language and the various support routines were written in FORTRAN 11. Wavelength Calibration. A program that predicts the locations of specified wavelengths in the normal focal plane of the echelle spectrometer was made available to us from Spectrometrics, Inc. The modified routine uses known optical constants of the spectrometer to calculate coordinates a t the vidicon focal plane in terms of millimeter displacements from a reference wavelength. Calibration of the instrument then involved development of suitable models to transform the coordinates predicted by this routine to DAC coordinates. The model selected for this purpose is given by
DAC = Bo + B,X(mm) + B,Y(mm) The same model is used for both DACs yielding six constants necessary to predict the DAC coordinates of a given wavelength. This model accounts for any rotation of the vidicon detector relative to the spectral focal plane and for the offset and gain of the deflection amplifiers. The procedure employed to initially calibrate the spectrometer was to scan the spectrum from a mercury pen lamp (No. 11SC-lC, Ultra-Violet Products, Inc., San Gabriel, Calif. 91778) using a two-dimensional raster pattern and locate the peak maxima for 1 2 of the most intense lines. The DAC coordinates of the peak maxima and the calculated coordinates are used to determine the constants in the above model using standard least-squares techniques. To hasten routine calibration, subsequent measurements involved taking about 150 points located around the previously predicted location of selected lines, locating the peak maxima, and modifying the prediction models accordingly. We found that weekly calibration was sufficient for reasonable wavelength prediction accuracy. Procedure. The symbols of the elements to be investigated and the number of horizontal and vertical points are entered into the computer from the teleprinter. A program then searches previously created tables containing the wavelengths and the vidicon focal plane locations for each element. The horizontal and vertical DAC coordinates are calculated and written onto disk memory. The current programs can handle 30 elements and six wavelengths per element. A second program then optimizes the coordinate predictions so that the raster pattern is centered around the peak maximum of each wavelength. Additionally, the optimization routine allows the user to reduce the amount of data taken down to 1 point per wavelength. For most of the data reported below, 20 horizontal points at one vertical level were acquired. Another program uses these optimized coordinates to measure the intensities a t each of the wavelengths when a start signal is issued. When the requisite number of repetitions are
Table I. Spectrometer Resolution as a Function of Wavelength Wavelength,
a
Slit width,
Ordera 72(Hg)
31 2 5.66
200
60(Fe) 56(Mn)
3131.55 3745.56 3737.13 4034.49 4033.07 5790.65 5769.59 7032.41 7024.05
wnb
Dispersion, a /mm Expected FoundC 5.26 5.36
500
6.54
6.88
6.59
7.12
Resolution, A Expected Found 0.18 0.40 (0.33)d 0.56 0.60
0.23 0.53 (0.4 1)d 39(W 200 9.63 10.2 0.33 0.76 (0. 60)d 200 12.04 32(Ne) 11.9 0.42 0.89 (0.75)d a Mercury lines from mercury pen lamp, Mn lines from Mn hollow cathode lamp, Fe and Ne lines from Fe hollow cathode lamp. Entrance slit width, slit height = 500 pm all cases. Computed as A/DAC step + 0.0125 mm/DAC step. Resolution imposed by width of electron beam. 200
completed, the raw data are stored on the disk memory and the instrument is ready for a new run. The next program allows the user to display the data and t o list values for the peak intensity or area for each wavelength. Absorbance can also be calculated and printed. The program for quantitative analysis can handle both the standard addition and calibration curve techniques for either absorption or emission measurements. The curves of absorbance or intensity vs. concentration for each wavelength may be displayed and the elemental concentrations of unknowns evaluated and printed. Nonlinear data is fitted to a second-order polynomial using the standard least-squares technique.
RESULTS AND DISCUSSION Having developed the control and measurement circuitry for random access, there were two main objectives of this study. One was to provide an evaluation of the performance characteristics of the silicon target vidicon-echelle spectrometer combination. The other was to apply the spectrometer to a limited but meaningful study of multielement determinations. Unless stated otherwise, all deviations from the mean are quoted at the 95% confidence level (mean f t s / J N and all experimental points represent the average of 32 scans. Resolution and Wavelength Accuracy. Since one of the main objectives of this work is to provide high resolution with wide spectral coverage, an investigation of the resolution capabilities of the system was initiated. The method used for determining the experimental resolution was to measure the full width at half maximum (FWHM) of several atomic lines from various hollow cathode lamps and the mercury pen lamp. The resolution is then determined by multiplying the FWHM by the reciprocal linear dispersion (RLD). The experimental RLD was evaluated by determing the number of DAC steps between the peak maxima of two wavelengths in the same grating order and dividing this into the separation (in Angstroms) of the two lines. The theoretical resolution was calculated in a similar manner, except that the FWHM is replaced by the effective slit width, where the effective slit dimensions for the spectrometer are the entrance slit dimensions divided by the image reduction factor, 5.8, and the RLD is determined by calculating the separations in millimeters of the wavelengths of interest. Table I shows the experimental and theoretical RLD and resolution as a function of wavelength. These data show that for the larger of the slit widths, experimental and theoretical resolution are in good agreement. Experimental resolution is poorer than the expected resolution for all wavelengths; however, this may be attributed to several effects such as image distortion in the output spectrum, charge blooming on the vidicon target, and the finite size of the electron scan beam. The large discrepancies observed at narrow slit widths are primarily due to the size of the electron scan beam. This
IOOOt
i
800
A- - - - - /
w
&1:11:
4 a W
0-
Figure 3. Effect of DAC step size on signal amplitude. The source used was a mercury pen lamp. (A) 5790.65 A. (B) 5769.59 A. (C) 3125.66 A. Note: One DAC step corresponds to 0.0125 mm
assertion was proven by making measurements of the beam diameter using an approach similar to that of Nieman and Enke (20). The method employed was to scan through a wavelength using different sizes of DAC steps between interrogation points. When the step size is smaller than the beam diameter, the beam overlaps a portion of the next adjacent position so that when this position is sampled a smaller signal is observed than for no overlap. The results of this experiment are shown in Figure 3. The signal reaches a maximum for a step size between points of five DAC steps. From system calibration data, it is known that one DAC increment corresponds to 0.0125 mm, yielding a beam diameter of 0.0623 mm. If this beam diameter is used with the theoretical RLDs of Table I, the theoretical resolutions shown in parentheses are calculated which are in better agreement with the experimentally observed resolutions. It should be noted that the difference between the experimental resolutions and the beam diameter limited resolution corresponds to an uncertainty of one DAC increment in determining the FWHM of the atomic line. This value for beam diameter leads to the conclusion that there are approximately 200 horizontal resolution elements or 4 X lo4 two-dimensional resolution elements on the vidicon target which corresponds to approximately 4 X 4 diodes covered a t any one position. An important factor in a random access vidicon spectrometer is wavelength position prediction accuracy. In order to ascertain the ability of the system to locate spectral lines, a preliminary wavelength calibration based on the emission spectrum of the mercury pen lamp was carried out and then the peak maxima of several atomic lines from an Fe hollow ANALYTICAL CHEMISTRY, VOL. 49, NO. 8. JULY 1977
1115
Table 11. Accuracy with Which Selected Wavelength Coordinates Are Predicted
Wavelength,
a
3581.20 3119.94 3734.81 3737.13 3745.56 3749.49 37 69.19 3815.84 3841.05 3859.91 4211.76 4301,91 4383.55 a
4000
DAC coordinatesa -Horizontal Vertical Calcd Obsd Calcd Obsd 560 804 291 314 402 443 636 473 747 254 , 705 277 201
558 806 291 314 402 443 638 473 741 254 105 277 203
709 7 31 7 34 134 735 136 738 745 748 751 797 801 808
t
t 3000-
cn
z w
k
709 731 736 736 7 31 7 36 7 34 745 748 751 795 799 806
5
2000-
W
2
tU
1 1000W
LL
O
0
5
10
15
20
25
30
ERASE CYCLES
Flgure 5. Effect of erase cycles on the unshuttered and shuttered signal amplitude. Duration of each erase cycle was 10 ms. For the shuttered data, the exposure time was 100 ms. (A, B, C) Unshuttered. (D, E, F) Shuttered. (A, D) 3737.13 A. (B, E) 3745.56 A. (C, F) 3734.87
One DAC step corresponds to 0.0125 mm.
A
3000 'Irn[
2500
g 2000 ! i
j1000 E I5O0_
,0°[0
L 0I
,,D
I
33
I
60
I
90
SAMPLING TIME
I
120
(pS)
I
150
Figure 4. Effect of sampling time on signal amplitude. Iron hollow cathode lamp used. (A) 3737.13 A. (B) 3745.56 A. (C) 3734.87 A.
(D) 4383.55 A
cathode lamp were located. Table I1 shows a comparison of the actual locations and the predicted locations of these lines. Most of the position prediction errors are zero, a few are 2 DAC steps and one is in error by 4 DAC steps out of a possible 1024. In this experiment, a DAC increment of 2 was used between interrogated points which limits the detectable errors to multiples of two. The program that was supplied to us by Spectrametrics has a wavelength position uncertainty of 50 pm in the normal focal plane or approximately 9 pm in the vidicon focal plane, assuming no distortion occurs during image reduction. This corresponds to a position uncertainty of one DAC increment. The ability to accurately predict the location of an atomic line is also influenced by the number of significant figures in the wavelength specification. Below approximatly 4500 A, six digits of wavelength information are necessary to accurately specify the location of a wavelength and five digits are necessary above 4500 A. The conclusions that may be drawn from this experiment are that the models that transform the predicted positions of wavelengths in millimeters to DAC coordinates are adequate, and geometrical distortion in the reduced image or in the deflection assembly is small. Sampling Time. Figure 4 shows the results of varying the time over which the video signal is iategrated. This integration t h e , or dwell time, determines the interval of time for which the electron scan beam is unblanked a t a particular target position and the time that the signal is allowed to accumulate 1116
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
in the integrator. The data represent the dark current corrected signal amplitudes of the peak maxima of the spectral lines. The atomic lines used for this experiment are those from an iron single element hollow cathode lamp operated a t a current of approximately 20 mA. In this and all subsequent studies, a random access scan of 20 horizontal points was taken a t each wavelength with a DAC step size of 2 between points. We have found that this data density provides the best trade-off between signal amplitude and the amount of data required to accurately locate the peak maximum of a spectral line, because the signal amplitude increases as the distance between points to the limit at which the step size is larger than the effective beam diameter. A first-order plot of the signal amplitude vs. time has a slope of 0.0528 f 0.0036 ps-' ( R = 0.9983) yielding a charging time constant per resolution element of 19 f l ks. Since the time constant of the video signal integrator is about 0.5 ps, this contribution to the observed time constant is negligible. However, the target capacitance of the vidicon is approximately 20 pf and will combine in parallel with the feedback capacitance of the preamplifier to produce an equivalent capacitance of 30 pf, which yields an equivalent preamplifier time constant of 20 ps. Therefore, the experimentally determined time constant has two contributions, the larger of which is due to the inherent characteristics of the vidicon detector and the lesser is due to the preamplifier. For the remainder of the experiments described, a dwell time of 50 ps was used. Erase Time. Because the vidicon detector is an integrating sensor, attempts to interrogate only a few isolated regions of the target will prove futile. The target areas that are not scanned will discharge because of either dark current leakage or photon induced charge neutralization, and blooming (or charge migration) will occur destroying the charge placed on the random accessed diodes so that a saturation signal will be observed that is independent of intensity. Therefore, to avoid this difficulty,the target must be scanned or erased prior to random access interrogation. Since a vidicon detector is capable of having approximately lo4to lo5resolution elements, we are forced to trade off the time required to prime the target during which no information is accumulated in order to interrogate less than the maximum riumber of possible resolution elements during rmdoin access data acquisition. This section will focus on the effects on the random access signal of the preliminary scanning step in which all diodes are recharged to initial values.
1.00
>t ln
z
loo^!^,
p0.75
G u w ?50 IL 0.25
I
J
0 00
I
1
0
I
8
I
16
1
24
32
Figure 6, Relative effect of intensity on the shuttered and unshuttered signal amplitudes. Iron hollow cathode lamp used. Wavelength observed was 3745.56 A. The erase cycle time was 10 ms. (A) 2 nA, unshuttered. (6)5 nA, unshuttered, 100-ms integration time. (C) 10 nA, unshuttered, 200-ms integration time. (D) 5 nA, shuttered, 100-ms exposure time. (E) 10 nA, shuttered, 200-ms exposure time
The lower three curves of Figure 5 show the effect of increasing erase cycles on the random access signal for an unshuttered source (i.e., radiation is constantly impinging on the vidicon target). The data for each number of erase cycles represent the dark current corrected signal amplitudes of the peak maxima of the spectral lines from an iron hollow cathode lamp operated at a current of 20 mA. The scan time for each erase cycle is 10 ms and the time required to obtain the random access information for the four wavelengths is approximately 5 ms. The amplitude of the random access signal increases up to a plateau region for increasing numbers of erase cycles. The upper half of Figure 5 shows the results of a similar experiment except that here the source radiation is shuttered. In a shuttered experiment of this type, radiation is incident on the vidicon target only when the high speed shutter is open and random access and erasing are carried out with the shutter closed. In this experiment, a shutter time of 100 ms was used. For small numbers of erase cycles, the signal is larger than that observed for larger numbers of erase cycles. The effect of intensity on the behavior of both types of experiments is represented by the data of Figure 6 for limiting signal currents of 2 nA (A), 5 nA (B, D), and 10 nA (C, E). The data are plotted as the ratio of the signal observed at a given number of erase cycles to the signal observed after 32 erase cycles. For the unshuttered experiment, the magnitude of the effect of increasing erase cycles is seen to decrease for increasing signal amplitudes in much the same manner as that for beam discharge lag in conventional scanning systems. However, for the shuttered experiment, the magnitude of the effect is smaller than for the unshuttered case and increases with increasing intensity. The effect observed for the unshuttered experiment may be rationalized by the following arguments. The instant after random access interrogation, the potential of the interrogated diodes will be very near to cathode potential. However, those diodes not accessed will have a much lower potential due to the radiation absorbed since the last erase, and blooming, as well as the incident photon flux, will cause the charge placed on the random accessed diodes to dissipate. Since the unaccessed diodes are a t a lower potential than the steadystate potential of normal scanning, their potential must increase toward the steady-state potential. However, the potential of the random accessed diodes will decrease with erase cycles toward the same steady-state potential as that of the
,
0
1
ERASE CYCLES
~
0
20
40
€0
80
ERASE TIME (MS)
Flgure 7. Effect of erase time on shuttered signal amplitude. Iron hollow cathode lamp used. (I) 3737.13 A. (11) 3745.56 A. (111) 4383.55 A. (A) 8 erase cycles. (E) 16 erase cycles. (C) 32 erase cycles
unaccessed diodes so that the random access signal is observed to increase with increasing erase cycles. The observation that the magnitude of this effect decreases with increasing signal levels is consistent with beam discharge lag and blooming considerations. At high light levels, blooming from the unaccessed diodes is more effective in reducing the charge on the random accessed diodes due to a higher potential gradient between the accessed and unaccessed diodes. However, the charging process occuring during erasing is more efficient, so that a t higher light levels, the approach of both types of diodes to the steady-state potential occurs much more rapidly relative to that of low intensities. For the shuttered experiment, a t long erase times, the potential of all diodes regardless of the charge present at the end of random access will approach cathode potential because no radiation is incident on the target. Here again we assume that random access interrogation charges the diodes to very near cathode potential. At erase times shorter than that necessary to bring the potential of the diodes to cathode potential, an additive signal contribution will exist due to charge from the adjacent unaccessed diodes blooming into the random access diodes. This signal contribution will add to that caused by photon related events causing a larger signal to be observed than for the case where enough erase cycles have been used to completely recharge the target. Since this effect may be attributed to blooming, it is logical that the magnitude of the effect increases with increasing illumination levels. All of the experiments conducted to this point have had an erase cycle time of 10 ms. Figure 7 shows the effect of changing the duration of an erase cycle for the shuttered experiment. The family of curves for each wavelength represent individual experiments with the indicated number of erase cycles for varying erase times. Several observations can be made from these data. As stated previously, the magnitude of the effect increases for increasing intensity levels. Second, these data imply that the total time required to bring the target to a given potential is a constant for a given illumination level. If we use one half the number of erase cycles, then to a first approximation, we should double the duration of each erase cycle, if we are to observe the same signal amplitude. In other words, the erase time multiplied by the number of erase cycles necessary to bring the target to a given potential is a constant for a given level of illumination for the shuttered expbriment. This experiment has an important ramification with regard to beam discharge lag in conventional scanning systems where a decreasing light level is being ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
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Figure 9, Linearity of signal for shuttered experiments. Neon lines from iron hollow cathode lamp. (A) 7032.41 A. (B) 6678.28 A. (C) 6402.25
A.
the cost of longer measurement times. For the shuttered data, the slopes a t low light levels from most intense to least intense are 1.023 f 0.017, 1.038 f 0.010, and 1.028 f 0.012, which implies that a t relatively low light levels the vidicon response is linear. However, a t higher intensities, the slopes of these curves from most intense to least intense are 1.142 f 0.015, 1.117 f 0.029, and 1.130 f 0.046. The point corresponding to this change in slope is a t a target current of approximately 3 nA which implies a linear dynamic range of 60 based on the smallest current measurable of 0.05 nA. However, this range may be extended by increasing the number of erase cycles. The conclusion to be drawn from these data is that for low intensity signals, the shuttered approach is better whereas for relatively high intensity levels, the unshuttered approach is better. In either case, to maintain a linear dynamic range for the particular analysis in question, enough erase cycles must be issued so that adequate priming is assured. The time spent priming the target need not be idle time and can be used for data processing. It must be emphasized that these data represent a limiting situation to show the effects of the preliminary erase cycles on linearity. The linear dynamic range increases by a factor of 2 to 3 for both shuttered and unshuttered experiments when 20 erase cycles are used. Multielement Experiments. The system described above has been applied to the simultaneous determination of chromium, copper, cobalt, nickel, iron, and manganese in mixtures using atomic absorption. Since a major goal of this work was to evaluate the performance characteristics of the vidicon detector, synthetic solutions containing the same relative concentrations of each metal were used to minimize matrix effects. Data from multiple scans were summed at each analytical wavelength to yield improved signal to noise ratios and normally included 32 replicate measurements. The acquisition approach used was to shutter the multielement hollow cathode lamp radiation using an exposure time of 200 ms. The target was primed between exposures by 20 erase cycles of 10-ms duration resulting in an analysis time of 13 s per run. All the data presented were corrected for dark current and stray light. The approach used for this correction was to subtract from the peak signal the average value of the background signal either side of the peak. Of the 20 points taken a t each wavelength, about two thirds contain information about the line intensity and the rest are background data. Since the variation in sensitivity and dark current over small regions of the target is very low, this approach provides an excellent means of correcting line source data. Initial
Table 111. Statistical Data for Simultaneous Determination of Cr, Cu, Fe, Mn, Ni, and Co in Synthetic Samples by Atomic Absorption 100% T
Element Cr (4254.33)
Cu (3247.54) Fe (3719.94) Mn (4030.76) Ni (3414.77) Co (3453.51) Fe (3719.94)d Fe (3020.64)d
current (io, nA) 10.5 4.7 3.5 15.1 4.1 4.3 3.52 0.65
Std dev.a (nA @ 100% 2') 0.038 0.033 0.025 0.042 0.029 0.045 0.029 0.042
Sensitivity i 2Sb Detection Intercept t 2Sb x 103 (AAlppm x l o 3 ) limit, ppmC 19.0 f 0.85 0.17 O . l ? 16 58.4 t 2 . 2 0.11 6.3 t 18 5.43 t 0.35 1.7 0.0 c 14 0.25 -6.9 t 23 9.59 f 0.59 0.55 -0.27 t 22 11.4 k 0.57 7.2 0.54 f 1.5 1.3 f 0.03 8.5 t 1.5 0.85 0.0 f 14 14.5 f 1 . 3 3.9 22 t 31
Correlation coefficient b 0.9995 0.9996 0.9989 0.9994 0.9993 0.9999 0.9981 0.9987
Based on four replicate measurements at each concentration. Computed a Based o n seven replicate measurements. Multielement hollow cathode lamp with atomization conditions optimized for Fe. using Equation la with = 2SD. attempts to correct the data by acquiring dark current spectra and subtracting these from line spectra yielded data that contained relatively high levels of stray light resulting in calibration plots with significant curvature at high concentrations. The relatively high levels of stray light encountered here are due to the neon lines from the filler gas of the hollow cathode lamp which are about 1000 times more intense than the analysis lines. The burner operating conditions used for multielement analyses are a compromise between sensitivity and elemental coverage. In multielement analyses, it is impossible to optimize the sensitivity for all elements simultaneously; instead operating conditons must be such that the best sensitivities for the most elements are obtained. Optimization studies focusing on observation height and acetylene-air ratios were carried out. We have chosen not to include these data here because they do not reveal any unexpected features. However, the point should be made that the echelle-vidicon system permitted simultaneous observation of effects of changes in each parameter on all elements being studied. The observation height used for the data reported below was 5 mm as measured from the burner top to the center of the image of the hollow cathode over the burner. The air flow rate was 4.2 L/min and the acetylene flow was 1.8 L/min which yielded a sample uptake rate of 10 mL/min. Figure 10 represents linearity plots for data for solutions containing Cr, Cu, Co, Ni, Fe, and Mn. The solid lines represent the unweighted least-squares fits to the data sets. Each data point shown is the average of four replicate determinations. Although at least two analysis lines were observed for most of the elements, only one line per element is shown here for clarity. The lines selected for this study were chosen to provide the best intensity-sensitivity trade-off. Statistical information for the data is given in Table 111. The more important data in Table I11 are the 100% T peak currents (io), the standard deviations of the peak current measurements (SD),and the sensitivity figures ( S ) for the elements because these quantities combine to determine the detection limits of the elements. The detection limit for any element is given by
where eA,, and eLO are the uncertainties in the absorbance and current measurements at 100% T expressed at any desired confidence level. Using a series expansion of the log term (In (1 x) (x - 1)- l / * ( x - 1)' ... for -1 < x < 1)and ignoring the higher order terms, it follows that the detection limit for an element is approximated by
+
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PPM CR, CU Figure 10. Linearity plots for simultaneous determination of copper, nickel, manganese, chromium, iron, and cobalt. (A) Cu (3247.54 A). (6)Ni (3414.77 A). (C) Mn (4030.76 A). (D) Cr (4254.33 A). (E) Fe (3719.94 A). (F) Co (3453.51 A)
The standard deviations of 100% currents in Table I11 vary over a rather narrow range (0.025 to 0.045 nA) and we have observed similar values at shorter and longer wavelengths. For the present system, the average standard deviation of the 100% T current is about 0.035 nA, which would correspond to a value of eta = 0.07 nA at the 95% confidence level. Thus, a reasonable representation of the 95% confidence level detection limits for the present system is DL95yo
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(IC)
For the data in Table 111, the lowest detection limits are obtained for Cu where the i d product is largest (4.7 X 0.0584 or 0.274) and the highest detection limits are obtained for Co where the ioS product is smallest (4.3 X 0.0013 or 0.0056). Equation 1 can of course be used to evaluate the lowest acceptable 100% T current for a desired detection limit for any element with sensitivity 5'. For the limited number of lines below 3250 A examined with this system (Co 2407, Cr 3579, Fe 2483, 3020, Mn 2795, and Ni 2320) with the multielement hollow cathode lamp (MEHCL), none had adequate 100% T current (io) to yield acceptable detection limits. This was surprising because Mitchell et al. (21)obtained quite reasonable detection limits for numerous elements with lines below 3000 A using single element hollow cathode lamps (SEHCL) with a silicon target vidicon system and a similar atomizer. Whether this apparent discrepancy results from lower line intensities in the MEHCL compared to the SEHCLs, differences in responses of the vidicon detectors, differences in efficiencies of the dispersion ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
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Table IV. Comparison of Sensitivities of Selected Elements Run under Different Conditions Sensitivity (Slope of calibration plot (AAlppm) x 1000) ~~
Conditionsa
Note
Cr
b b
5
30.8
i
Fe
cu
15.2 i. 1.5 8.8 f 1 . 5 66.5 i 5.5 16.6 i 2.7 8.7 i 1.7 66.0 i 2.9 C 5.4 f 0.4 58.4 i 2.2 9.6 * 0.6 a A = Single element lamp, burner conditons optimized for element of interest. B = Same as A, except multielement lamp. C = All elements studied simultaneously using multielement lamp and compromise burner conditions. Based on two replicate measurements at each concentration for each element. Based on four replicate measurements at each concentration for each element. A B C
32.2
Mn
1.1 1.0 19.0 c 0.9
optics, the smaller number of diodes included in each resolution element, differences in integration times (not specified in Ref. 21), or a combination of these and other factors is not known a t this time. The most logical reason is the smaller number of diodes included in each resolution element. However, the small output flnumber cfll.6) of the modified echelle grating should concentrate the available flux on relatively few diodes and this increased intensity should more than compensate for the smaller number of diodes. Whatever the reasons, we must conclude that additional work is needed to achieve results in this wavelength range with the present echelle system comparable to previously reported data with conventional optics. We also conclude that for elemental wavelengths above 3000 A, the echelle system yields detection limits quite comparable to those with conventional optics with the advantage of both high resolution and broad spectral range. It is desirable to discuss the effects of the compromised atomization conditions on the elemental sensitivities. Sensitivities for Cr, Mn, Fe, and Cu run under optimal and compromise conditions are included in Table IV. The data represent the slopes of the calibration plots obtained under the indicated operating conditions. The data for condition C are the same as in Table I11 but are reproduced here for ease of comparison. The confidence intervals of the slopes for conditions A and B are larger than for C because only 2 replicate measurements were made a t each concentration used in constructing the calibration plots. For Cr, Mn, and Fe, the sensitivities obtained under simultaneous analysis conditions are lower by a factor of about 1.6, than when studied individually with optimized conditions. However, the sensitivity loss for Cu is insignificant. These effects occur because the flame conditions for Mn and Cr tend to be fuel rich whereas for Cu and Fe lean flame operating conditions are favored. For the three elements with the greatest sensitivity loss, the dependence of the sensitivity on operating conditions is large whereas Cu shows only moderate variations when the fuel-air flow rates are changed. In general, for the four elements investigated, detection limits are lower by a factor of 5 to 10 when comparing single element data with multielement data, which reflects the lower line intensities obtained from the multielement lamp. However, the problem may be circumvented by using longer exposure intervals, but consequently resulting in longer analysis times. In summary, this paper identifies important characteristics and necessary operating procedures inherent in the adaptation of a silicon target vidicon detector with an echelle grating spectrometer to monitor many spectral lines simultaneously. The system yields both the high resolution and broad spectral range required for practical simultaneous multielement determinations. While the results clearly demonstrate the feasibility of multielement determinations by atomic absorption, they also raise serious questions about the suitability
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of the silicon target vidicon for wavelengths below 3000 A. This problem may be solved either by using a more intense source, a UV scintillator for the vidicon, or image detectors with improved UV response and gain. The present system should be adaptable to more efficient atomizer systems and be subject to the same improvements in sensitivities and detection limits as conventional detector systems. We have not addressed the problem of interference from emission lines in this work. We are presently exploring different approaches intended to resolve some of the problem areas identified in this study. An obvious question is whether the echelle/vidicon is applicable to continuous source atomic absorption spectrometry. We conclude that the present system has neither the resolution nor the spectral sensitivity for CSAA but are withholding judgment on extensions of the concept which would include more sensitive detectors. As a final note, we wish to encourage those who work in this area to report their data in fundamental units such as detector current, charge, etc., so that realistic comparisons can be made among different studies.
LITERATURE CITED (1) R. E. Santini, M. J. Mihno, H. L. Pardue, and D. W. Margerum, Anal. Chem., 44, 826 (1972). (2) M. J. Milano, H. L. Pardue, T. E. Cook, R. E. Santini, D. W. Margerum, and J. M. T. Raycheba, Anal. Chem., 46, 374 (1974). (3) M. J. Milano and H. L. Pardue, Anal. Chem., 47, 25 (1975). (4) M. J. Milano and H. L. Pardue, Clin. Chem. ( Winston-Salem, N.C.), 21, 211 (1975). (5) G. Horlick, E. G. W i n g , and S . T. Leung, Appl. Specfrmc., 29, 48 (1975). (6) T. E. Cook, M. J. Milano, and H. L. Pardue, Clln. Chem. ( Winston-Salem, N . C . ) , 20, 1422 (1974). (7) G. Horlick and E. G. Codding, Anal. Chem., 45, 1490 (1973). (8) K. M. Aldous, D. G. Mitchell, and K. W. Jackson, Anal. Chem., 47, 1034 (1975). (9) D. 0. Knapp, N. Omenetto, L. P. Hart, F. W. Phnkey, and J. D. Winefordner, Anal. Chim. Acta, 69, 455 (1974). (IO) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 1231 (1974). (11) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (12) M. Margoshes, Spectrochlm. Acta, Part 8 , 25, 113 (1970). (13) G. R. Harrison, J. Opt. SOC. Am., 39, 522 (1949). (14) G. R. Harrison, J. E. Archer, and J. Camus, J . Opt. SOC. Am., 42, 706 (1952). (15) P. N. Keliher and C. C. Wohlers, Anal Chem , 48. 333A (1976). (16) D. L. Wood, A. B. Dargis, and D. L. Nash. Appl. Spectrosc., 29, 310 (1975). (17) A. Danielsson, P. Lindblom, and E. Soderman, Chem. Scr., 6, 5 (1974). (18) A. Danielsson and P. Lindblom Appl. Specfrosc., 3 0 , 151 (1976). (19) P. N. Keliher, Res.lDev., 27, (6),26 (1976). (20) T. A. Nieman and C. G. Enke, Anal. Chem., 48, 619 (1976). (21) D. G. Mitchell, K. W. Jackson, and K. M. AkIous, Anal. Chem., 45, 1215A (1973).
RECEIVED for review January 25,1977. Accepted May 4,1977. This study was supported in part by Research Grant No. GM 13326-10s from the NIH USPHS. One of us (HLF) acknowledges the support of an American Chemical Society, Division of Analytical Chemistry Fellowship sponsored by The Procter and Gamble Co.
