"Methods for Chemical Analysis of Waters and Wastes", Environmental Protection Agency, Cincinnati, Ohio (1974). J. P. Haberman, Anal. Chem., 43, 63 (1971). C. 0. Huber, and D. R. Tallant, J. Hectroanal. Chem., 18, 421 (1968). B. K. Afghan, P. D. Gouiden, and J. F. Ryan, Anal. Chern., 44, 354 (1972). B. K. Afghan, and P. D. Gouiden. Environ. Sci. Techno/.,5 , 601 (1971). W. Hoyle, I. P. Sanderson. and T. S. West, J. Hectroanal. Chem., 2, 166 (1961). J. Asplund, and E. Wanninen, Anal. Left., 4, 267 (1971). H. A. Mottola, and H. Freiser, Anal. Chern., 39, 1294 (1967). H. A. Mottoia, M. S. Haro, and H. Freiser, Anal. Chem., 40, 1263 (1968). (1966). H. A. Mottola and G. L. Heath, Anal. Chem., 44, 2322 (1972). K. W. Hauck and J. W. Diiiard, "Determination of the Complexing Capacity of Natural Water", Report No. 85, Water Resources Research Institute, University of North Carolina, 1973. G. A. Rechnitz and N. C. Kenny, Anal. Left., 3 , 509 (1970). J. A. Blay and J. H. Ryland, Anal. Len., 4, 652 (1971). M. Qureshi and J. P. Rawat, Separ. Sci., 6, 451 (1971). D. G. Hill-Cottingham. J. Chromatogr.,8 , 261 (1962). Z. Masoomi and D. T. Haworth, J. Chromatogr.,.48,581 (1970). J. Vanderdeeien, J. Chromatogr.,39, 521 (1969).
E. A. Fitzgerald, Anal. Chem., 47, 356 (1975). F. J. M. Rajabalee, M. Potvin, and S. Laham, J. Chromatogr., 79, 375 (1973). J. L. Swain and J. L. Sudmeier, Anal. Chem., 40, 418 (1968). J. E. Longbottom, Anal. Chem., 44, 1972. W. A. Aue et ai., J. Chromatogr.,72, 259 (1972). S . E. Manahan and R. A. Kunkei, Anal. Chem., 45, 1465 (1973). G. F. Longman, M. J. Stiff, and D. K. Gardiner, Water Res., 5 , 1171 (1971). S. E. Manahan and D. R. Jones IV, Anal. Left., 6, 745 (1973). D. R. Jones IV and S. E. Manahan, Anal. Lett., 8 , 421 (1975). S. T. Sie and N. van den Hoed, J. Chromatogr. Sci., 7 , 257 (1969). D. R. Jones IV and S. E. Manahan, Anal. Chem., in press. K. H. Schr6der. Acta. Chem. Scand., 19, 1347 (1965). D. W. Margerum and T.J. Bydalek. lnorg. Chem., 2, 683 (1963). J. N. Done, J. H. Knox, and J. Loheac, "Applications of High-speed Liquid Chromatography", Wiley 8 Sons, London, 1974.
RECEIVEDfor review October 29, 1975. Accepted November 24, 1975. This research was supported by National Science Foundation Grant No. MPS75-03330 and USDI OWRT Matching Grant B-095-MO.
Multielement Atomic Spectrometry with a Computerized Vidicon Detector J. D. Ganjei, N. G. Howell, J. R. Roth, and G. H. Morrison* Department of Chemistry, Cornell University, lthaca, N. Y. 14853
A computerized vidicon flame spectrometer is described along with Its application to a simultaneous determination of four serum electrolytes. Data acquisition, transfer, storage, and manipulation have been greatly improved by the computer addition. Operator input has been minimized while the programming has incorporated such features as automatic least-squares polynomial curve fitting, "spectral stripplng", background corrected peak signal measurements, and internal standard corrections.
As a result of technological advances in solid state detectors, there has been a recent renewed interest in multielement analysis in atomic spectrometry (1-4). State-of-thea r t multichannel devices such as photodiode arrays, charge-coupled devices, vidicons, etc. have just begun t o be applied, so that significant system as well as detector design improvements can be expected in the near future. Recent research in our laboratory has involved the development and evaluation of one such system, the vidicon flame spectrometer (5-8). This work has involved the use of a vidicon tube coupled to an optical multichannel analyzer (OMA) operated in a manual mode for data acquisition, storage, and readout. Because the vidicon provides 500 channels of spectral data generated every 32.8 ms, the time-consuming step in the overall multielement analysis procedure has been the manual data retrieval. This study describes the development of an on-line computer system interfaced with our vidicon spectrometer for the rapid and efficient handling of the data. The interface was designed to allow both manual and computer control of the tube's data output for maximum analytical flexibility. The simple configuration of the interface board, residing in the OMA console, provides compatibility with most minicomputers. T o illustrate the quantitative capabilities of the system, a simultaneous determination of sodium, potassium, calci-
um, and lithium in 25 pl of untreated blood serum is described. While the computer system used in this study has a capability well beyond that required for this analysis, the system's power was extremely useful in both the optimization of the program structure and in enabling the design of complex system functions. These functions provide many beneficial computational features for the spectroscopist. Finally, the speed, accuracy, and flexibility of the computerized vidicon spectrometer make it a viable multichannel detection system.
INSTRUMENTAL Table I lists the apparatus used in this study. The total system layout is illustrated in the block diagram Figure 1. The flame spectrometer described in the next section is not the only system which can be used with this computerized multichannel detector. The vidicon can be mounted in the focal plane of any dispersing system which formats spectral data along a single axis. This, however, is the flame spectrometer utilized for the serum analysis application. Vidicon Flame Spectrometer. The optical components of the instrument consist of a focusing lens and 0.3-m polychromator. The modified Ebert mount monochromator, described in an earlier publication, is fitted with a 0-2000 pm variable entrance slit (8). The dispersing element is a 150 grooves-per-millimeter grating blazed for 450 nm yielding a 22 nm/mm linear dispersion. Radiation from the nitrous oxide-acetylene flame supported on a water cooled Meker-type burner is focused by the fused silica lens onto the entrance slit. A standard glass barrel hypodermic syringe delivers 25-p1 sample and standard solution volumes into the sample introduction system. This system is similar to the one used in previous studies (6, 8 ) , except a "mixing chamber" which consisted of a 9-inch length of 0.062-inch diameter polyethylene tubing, was spliced into the lead-in capillary tube. Also, the injection port was fabricated from Teflon ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
505
F RSHIFT B L A NIN K D D1* DEL B L A N K D l
O
E
B
x NEW G T DATA 40 READY L
COUNT CLOCK Di*d-]
Figure 2. OMA modification for binary data output. ( * ) OMA signal
connections.
Figure 1. Block diagram of system layout
(rather than the original Plexiglas) for better resistance to reagents. The flame source signal is detected with a uv sensitized silicon target vidicon tube fixed a t the focal plane of the polychromator. The signals are obtained from the target in a manner previously described ( 5 ) . The OMA console houses the tube sweep circuitry and the manually controlled data acquisition electronics. The tube and console are available from Princeton Applied Research Corporation. To allow for computer data acquisition an interface was constructed which did not alter the tube control circuitry nor obviate the manual use of the OMA. C o m p u t e r Interface. The OMA is interfaced to a Digital Equipment Corporation (DEC) GT40 which serves as a peripheral to the larger P D P 11/20 computer in the laboratory. The GT40 functions as the OMA buffer, graphics display unit, and the system console of the P D P 11/20. The circuitry within the OMA has been described in an earlier publication ( 5 ) .The only modification to the OMA is illustrated in Figure 2. This modification consists of adding a binary counter running in parallel to the decade counter in the OMA which is used to digitize the analog signal from the target of the vidicon tube. Each channel of data is sent from the OMA to the GT40 64 p s apart with a 768-ps time period between individual frame scans which is used by the computer to synchronize the data. The peripheral equipment associated with the P D P 11/20 includes: a 24K word memory; a 1.2-M word cartridge disk used for the R T l l operating system, program development and storage, and data storage; a nine-track magnetic tape unit for permanent data storage and system back-up; a 600 line-per-minute line printer for hard copies
of spectral data and analysis results; an incremental plotter for spectrum and calibration curve plots; and the GT40 system. A detaiied description of the interface circuitry will be furnished upon request. C o m p u t e r Programs. The majority of the programs used are written in FORTRAN and stored on disk. The P D P 11/20 computer runs all acquisition and calculation programs as well as loads the programs into the GT40. Graphics display on the GT40 is accomplished with “Picture Book,” a n assembler program supplied by DEC for the GT40, which is accessible from FORTRAN running in the P D P 11/20. Modifications to this software allow OMA data acquisition and high speed interprocessor communication. The general program structure consists of a keyboard command stream interpreter which supervises calls to the main FORTRAN routines. The commands are entered a line (80 characters) a t a time, where each line may contain several sequentially executed instructions. Individual commands may also require subsequent information which is handled in a normal operator-computer dialogue. The overall system command structure is outlined in Figure 3. There are four arrays, each capable of storing a 500-channe1 spectrum from the OMA. All system functions are carried out using the data stored in these arrays. Data are acquired from the OMA via a request to the GT40 to accumulate a designated number of frame scans into GT40 memory. After accumulation in double precision integer format, the 500 data points are transferred to the 11/20 where they are converted to real numbers and placed in the selected array. Manipulation of the data within the arrays is facilitated by separate operator commands which enable spectrum addition, subtraction, multiplication, and division. Spectrum subtraction is routinely applied to remove a background spectrum from a sample spectrum. All four functions are employed by a control routine which executes spectral
Table 1. Experimental Apparatus Syringe Flow meters Burner External optics Entrance slit Spectrometer Detector Optical multichannel analyzer Real-time readout Central computer Peripheral devices
506
Hamilton Co. constant rate microliter syringe Model 705-M. Brooks Full-view Rotometers calibrated for nitrous oxide-acetylene. Brooks Instrument Co. Varian Techtron Meker-type head designed for nitrous oxide-acetylene flame. Supracil lens with 10-cm focal length stopped down to 3.0-cm diameter; flame image was focused on the entrance slit. Jarrell-Ash Model 12-080 variable 0-2000 pm straight edged slit. Princeton Applied Research Model 1208. 0.3-m modified Ebert Mount polychromator using 150 grooves/mm grating blazed for 4500 A . Reciprocal linear dispersion 220 Aimm in first order. UV sensitive silicon diode vidicon, Model 1205F. SSR Instruments Co. Model 1205A. SSR Instruments Co. Tektronix Oscilloscope. Model 604 Monitor. PDP 11/20 with 24K memory and EAE. Digital Equipment Co. (DEC). GT40 Graphics Display and Processor with 16K memory, (DEC). RKll/RK03 Disk (DEC) TMll/TUlO Magnetic tape (DEC) Houston Instruments DP-1 incremental plotter Versatec 600 line per minute line printer
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
ACQUISITION
I M I N I PUL AT ION ADDITION SUBTRACTION MULTIPLICITION DIVISION OISPLIY
MEISURE LOCATOR HEIGHT AREA
Figure 3. Command structure of system programming
stripping (7). The procedure begins by storing in three separate arrays the sample spectrum, a stripping solution spectrum, and the solvent spectrum. The stripping solution and solvent spectra are divided by the number of frame scans for which they were accumulated to yield individual average frame scan spectra. The stripping spectrum is then multiplied by constants and subtracted from the sample spectrum until the interference is eliminated. At this point, the difference between frame scans of the sample and the stripping solution is made up by subtraction of a solvent spectrum to facilitate the complete removal of flame background and diode leakage current. Several commands provide a means of rapidly transferring spectral data to a variety of locations for use by the spectroscopist. Hard copies of the spectral data are obtained by transfer to the line printer or plotting on the incremental plotter. The GT40 is used to display any part of the spectral data requested by the operator. The four array memories are backed up by an almost unlimited storage capacity on the magnetic tape or disk. For routine analysis procedures and other experiments requiring large data storage and recall, random access disk files are provided. The spectra are labeled with four character tags which enable easy recall. The use of predetermined tags are employed in an automatic concentration calculation routine. Any spectrum in the arrays can be evaluated by peak locator and peak measurement routines. These options allow the operator the flexibility of measuring background corrected peak heights or areas for any number of the peaks in the spectrum. Background correction is accomplished by averaging five channels of background on each side of the peak, and interpolating the background under peak maximum from a line which connects those averages. If the neighboring channels contain peaks, the nearest background is taken for this calculation. A concentration calculation routine is employed for determining elemental concentrations in unknown samples. The program incorporates several of the aforementioned options and a least-squares polynomial curve fit. One element a t a time is calculated with operator input limited t o concentration of standards and location of analyte peak. Repetitions of each solution are automatically associated with the appropriate concentration through the use of the spectrum tag. This routine allows the operator to choose peak height or area signal peak measurements and automatically calculates a best fit polynomial curve. The unknown values calculated as averages for each sample are tabulated along with the standard deviation of measurement and transferred to the line printer. Contained in the routine is the option to utilize an internal standard. The analyte signal is divided by the internal standard intensity and the polynomial curve fit is subsequently performed on the signal ratio.
EXPERIMENTAL This section describes the experimental setup used to perform the simultaneous multielement serum electrolyte analysis with the computerized vidicon spectrometer.
W I V E L E N G T H . nm
Figure 4. Spectral window used in serum analysis from 650 to 880 nm
Window. The 0.3-m spectrometer system displayed 230 nm of spectral information upon the adjusted 10.5-mm active tube target. The wavelength region from 650 to 880 nm was viewed to permit the monitoring of selected analytical lines for sodium, potassium, calcium, and lithium. The spectral transitions chosen were the 670.8 nm lithium, 766.5 nm potassium, 819.3 nm sodium, and 422.7 nm calcium. The calcium signal is observed in the second order. Two absorption filters were used to reduce the emission signals from potassium and lithium at the concentrations monitored in human sera. Each of the two solutions were placed in 1-cm pathlength quartz absorption cells and situated directly in front of the spectrometer entrance slit. The potassium signal filter solution was prepared by dissolving 200 mg of bis(tetraethy1ammonium)tetrabromonickelate(II) in 25 ml of 0.05 M tetraethylammonium bromide in nitromethane. This blue solution was subsequently diluted until an absorbance of 1.20 was attained at the 766.5-nm line with respect to air for the 1-cm cell. A lithium filter was similarly prepared from 200 mg of bis(triphenylmethy1arsonium)tetrachlorocobaltate(II) dissolved in 25 ml of 0.05 M triphenylmethylarsonium chloride in-nitromethane. This solution was further diluted with the chloride containing nitromethane until the solution had an absorbance of 0.50 with reference to air at 670.8 nm. The tetrahedral complex salts were prepared using methods described by Gill and Nyholm (9); visible absorption spectra confirmed the synthesis results. Another type of spectral block was employed to eliminate the “blooming” effect caused by high concentrations of rubidium present as an ionization buffer. The radiation from the 780.0- and 794.8-nm rubidium doublet was extremely intense in the nitrous oxide-acetylene flame. The target diodes close to the strong signals were subjected to an overspillage of charge from those which were being illuminated. This “blooming” resulted in a larger noise component for analyte signals in the area, yielding poor precision. Therefore, a spectral block was used which completely prevented the rubidium signals from striking the target. The stop consisted of a 4.2-cm long black plastic strip 1.5 mm wide which was placed parallel to the spectral lines directly over the quartz faceplate at the location described. The only indication of its presence on the target was a decreased intensity for the unused 769.9-nm potassium line, and a complete absence of rubidium signals from the subsequent flame spectra. A typical serum spectrum is displayed in Figure 4. The previously described spectral block is located between the potassium and sodium signals. Cesium impurities in the carrier solution results in a net negative peak after background subtraction. The uneven baseline illustrates the necessity for baseline correction. Solutions and Samples. Multielement aqueous standards were prepared at concentrations encompassing the normal range of Na, K, and Ca in human sera. The same standards also contained Li concentrations spanning Li sera therapeutic levels. Four-element calibration solutions containing increased amounts of each element were diluted from individual stock solutions. The stock solutions were prepared from high purity metals or salts in accordance with methods described by Dean and Rains (IO) Concentration requirements of the serum standards necessitated some stock solutions to be made at 10 000 pg/ml levels. High purity rubidium carANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
* 507
Table 11. Analytical Conditions Flame Region viewed Slit width Carrier solution Sample size Integration Spectral window Filters
Fuel: C,H,, 8 psi, 3200 cm3/min Oxidant: N,O, 30 psi, 5000 cm’imin 20 mm above burner top 30 pm 20% isopropanol in water with 3400 ppm Rb 25 pl 500 frame scans 650-880 nm (Et,N),NiBr, in CH,NO, + Br(Ph,MeAs),CoCl, in CH,NO, + C1-
Table 111. Determination of Electrolytes in C.D.C. Sera Analyte
Sample No.
C.D.C., pg/ml
174-38 6.9 c O.la 374-38 13.6 i 0.1 574-38 20.1 i 0.1 K 174-38 109 ? 2 374-38 187 i 2 574-38 267 i 2 Na 174-38 2830+ 10 374-38 3160 t 1 0 574-38 3520 i 1 0 Ca 174-38 51.5 * 1.0 374-38 90.8 i 1.0 574-38 130.1 i 1.0 a Average standard deviation. three measurements.
CVFS, pg/ml
Li
b
6.9 t 13.9 * 19.9 i 106 i 184 c 266 i 2830 i 3220 i 3580 i 52.2 i 91.9 c 130.0 i Standard
Dev., 5%
0.2b 0.0 0.2 +2.2 0.1 -1.0 2 -2.8 3 -1.6 2 -0.4 120 0.0 50 +1.9 90 +1.7 2.0 +1.4 0.3 +1.2 3.0 0.0 deviation of
bonate obtained from Johnson, Matthey Co., Ltd. was used to prepare the 10 000 pg/ml rubidium stock solution. A carrier solution, 20% isopropanol (v/v) in water containing 3400 hg/ml rubidium, was used in conjunction with the micro injection system to facilitate discrete sample volume introduction. The constantly flowing solution was placed in a suspended 1-liter reservoir prior to the analysis run. Rubidium was used as a buffer to minimize cross-ionization interferences from the analytes. The buffer level was determined by the amount required to keep a potassium signal constant while sodium was varied between the extremes of concentrations found in human sera. The samples used for this analysis were experimental reference bovine sera with concentrations that bracket the levels normally encountered in clinical samples. The well calibrated samples were supplied by the Center for Disease Control (CDC) in Atlanta, Ga., and have been used in previous studies (6,B). Procedure. Table I1 lists the analytical conditions used in this analysis. The procedure was started by allocating a disk file for storage of spectra obtained during the course of the analysis. A computer control sequence was then built by entering “X”, whereupon the command string was entered as a single line: AA500, AB500, SCAB, DC, MC, FC The operator then needs only to enter “I” and the entire command string is implemented. A 25-p1 volume of solution was injected at the sample introduction port and the “I” command simultaneously entered at the keyboard. The command string sequentially performed: (AA500) acquired five hundred frame scans of the injected solution signal and placed the data in array A; (AB500) next 500 frame scans of the carrier solution spectra were acquired into array B; (SCAB) Array B was subtracted from array A with the result stored in array C; (DC) the background corrected spectrum in C (Figure 4) was displayed on the GT40 screen; (MC) along with that spectrum, all the peaks requested were located and measured; (FC) the entire spectrum was then stored in a random access disk file after the appropriate spectrum tag was assigned. Thus, three runs of each calibration standard and serum sample were acquired and stored. Only a single “I” implementation com508
*
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
mand was required for each injection, along with a brief spectral tag. These labels consisted of S1, S2, etc. denoting the calibration standards, where the three runs of S1 were tagged SlA, SlB, and S1C. Samples were similarly coded with U1, U2, etc. for the individual samples and repetitive runs were signified by UlA, UlB, and U1C. After acquiring spectra of the four standards and three samples used in the determination, the previously described polynomial curve-fit routine was used for data reduction. A single character entry called the routine. The operator first entered the analyte’s channel number of maximum intensity and the calibration solution concentrations. The analysis title and maximum order of the polynomial fit (3 for these procedures) were also designated. The output provided average concentration values for every unknown and each polynomial order curve-fit. Each of the remaining elements were calibrated in the same manner. Data reported in the analysis corresponds to first-order best-fit lines for sodium, lithium, and potassium, and a third-order curve for calcium.
RESULTS AND DISCUSSION T h e computerized vidicon flame spectrometer can obviously be used in a wide variety of applications. A simultaneous serum electrolyte analysis was used t o demonstrate the multielement capabilities of the system since the analytes are routinely determined by clinical laboratories. Our method provides several advantages: increased efficiency, no prior sample treatment or dilution, microliter sample volumes, economical aqueous calibration standards, and good precision and accuracy. Analytical Parameters. Our previous procedures for serum analysis in the manual mode involved calibration of the system using serum reference materials directly ( 6 ) , and aqueous standards together with internal standard measurements (8) t o eliminate matrix effects. Excellent results were obtained; however, in this study, it was decided t o use a more convenient dilution technique permitting the use of aqueous calibration standards. Sample dilution with direct serum introduction required the addition of a mixing chamber to the previously designed injection system (11).T o optimize the dilution and mixing conditions, the lithium signal pulse shape was monitored at slow and fast injection rates. Figure 5a shows the pulse obtained when a 25-111 volume of 10 pg/ml Li is injected as quickly as possible into the unmodified sampling system. A three-second (slow) injection of the same solution into the injection port produced the broadened pulse shown in Figure 5b. T h e incorporation of the mixing chamber resulted in the signal packet shapes shown in Figure 6. T h e fast injection signal has been reduced in peak intensity and broadened (Figure 6a). A three-second injection shown in Figure 6b resulted in a similar signal profile. T h e modified system served t o dilute the sample with a n appropriate ionization buffer and regulate the injection pulse shape by mixing. When a nonlinear calibration curve is used, this pulse shape regulation becomes extremely important. T h e vidicon system integrates the entire analyte signal pulse. Thus, a fast intense signal is equivalent in total integrated intensity t o a slow broadened pulse only if the response curve is linear throughout the range of concentrations existing in the flame a t any particular time. Nonlinearity due to self-absorption would result in lower integrated intensities for the faster pulse when compared t o broad signal packets. In particular, sample viscosity could also change the pulse shape. Experiments with calcium in both viscous and dilute serum samples have shown that the mixing chamber together with a Meker-type burner have eliminated this viscosity interference. Once the analytical conditions and spectroscopic features were optimized, the analysis was performed with considerable ease. T h e entire analysis was performed employ-
Table IV. Format of Curve Fit Routine Output Serum Lithium Analysis Data 7 / 7 / 7 5 Coeffs. in order of increasing power
-0.299087 E 00 Signal
Concn
U1A
10965.944
6.962
...
(b)
...
...
Unknown
Ave.
Std dev
RSD
1
6.884
0.151
2.195
...
..
...
Spectrum
Obsd signal
Obsd concn
Calcd concn
Dev
S1A
7993.128
5.020
4.994
0.026
...
(0)
0.662145 E-03
Unknowns
...
...
...
...
The standard deviation is 0.2906 -
Figure 5. Pulse shapes produced from fast ( a ) and slow ( b ) injec-
tions using the unmodified sample introduction system
Table V. Time of Analysis Comparison Manual
(a)
ib )
Flgure 6. Comparison of pulse shapes after addition of mixing cham-
ber ing simple keyboard commands and direct standards and sample injection into the instrument. The operator injected the solution, labeled the resultant spectra, and was ready to inject another sample again. When the analysis run was completed, the data were evaluated within the computer as directed by the analyst. The results of the simultaneous Na, K, Ca, and Li analyses of three well characterized sera are listed in Table 111. The CDC values are compared with the averages of three injections for each sample obtained with the computerized Vidicon Flame Spectrometer (CVFS). All experimental values were within 3% and most better than 2% relative deviation from the CDC values. The average relative standard deviation of all elements was 1.9%. The precision of this system was quite good considering the small sample volumes and the relatively unsophisticated microsyringe delivery system. The high relative standard deviations found for sodium reflect the low intensity signals used for the analysis. The potassium filter partially absorbed the sodium line reducing the intensity monitored by the vidicon. All of the data was directly obtained from the line printer output of the calculation routine. Table IV displays the polynomial curve-fit routine’s data format using the firstorder lithium calibration line where only the first entry under each heading is listed. As previously mentioned, sodium, potassium, and lithium values were taken from the first-order data. Calcium values were obtained from the third-order curve because of the dramatic improvement in the standard deviation of the calibration curve corresponding to the slight self-absorption. One of the benefits of the instrument, which is readily apparent, is summarized in Table V. The entire analysis can be performed with a considerable time savings over manual operation. The data displays the time spent by both methods of data handling a t each segment of the determination. The computerized system reduced the data acquisition time one minute per injection. The 40 seconds per injection allowed anough time to carefully clean the sy-
Signal averaging Instrument manipulation Measurement 4 signals Total data acquisition Data reduction Time for 21-injection, 4-element analysis
Computerized
33 siinj. 4 63 100 25 minielem.
3 3 siinj. 40 2.4 minielem.
135 min
24 min
I
...
ringe and prepare the next sample volume. Manual data reduction, using peak magnitudes and average backgrounds obtained off the OMA display was a lengthy procedure, even with the help of a desk top calculator. The interfaced system decreased the time spent calculating unknown concentrations by an order of magnitude. The computerized system’s complete four-element, three-sample analysis was performed in one fifth of the time required for manual mode operation. A more extensive example with more elements in a larger number of samples would have resulted in even greater time savings. Computerized System Capabilities. The addition of an on-line computer to the vidicon spectrometer has greatly expanded the utility of the spatial multichannel system. I t has increased the efficiency in the areas of data acquisition and processing while maintaining the inherent system flexibility, As was shown in the serum analysis, the time required for data acquisition has been considerably shortened. A single keyboard character executing the stored command line accomplished what previously entailed several manual operations. More importantly, the computer’s spectrum storage capability allowed the analyst to proceed directly to the next run. The limited memory of the OMA permitted only one analysis and blank spectrum to be stored a t a time. Data acquisition was interrupted by signal and background measurements of each difference spectrum. In contrast, the computerized system obtained data in real-time from the OMA and subsequently stored the completed spectrum in a mass storage file of essentially unlimited capacity. By virtue of the rapid data acquisition scheme, analyses required less operation time, thus were subjected to fewer instrumental fluctuations, improving the accuracy and precision of measurement. The precision of low level signal measurements can also be improved by either integration or averaging techniques ( 5 ) .With cooled vidicon tubes, extended beam delay periods can be used to integrate transient or steady-state signals on the tube face to achieve better signal-to-noise ratios. Alternatively, the averaging method, facilitated by reANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976 * 509
petitive scan storage, can be used for improving steady state measurements. However, the OMA's 5BCD memory range limited the capacity 'for signal averaging whenever weak signals were to be measured in a region containing a strong signal. In quantitative terms, a bright signal near detector saturation limited the number of repetitive frame scan accumulations to about 100. Since the improvement in signal-to-noise ratio corresponds to the square root of the number of successive additions, this memory range would limit a low-level signal improvement to a factor of 10. The interfaced system has a memory range which is capable of accumulating lo6 frame scans with high-level signals for a signal-to-noise improvement of 1000. The length of time involved, nine hours for lo6 frame scans, limits the amount of signal averaging that is experimentally feasible. However, intermediate accumulation periods expanding the original system's 3.3-second full scale accumulation limit allow the analyst a wide selection in optimizing the signalto-noise ratio for different applications. The preservation of the complete data spectrum enables sophisticated data manipulation and reduction by the P D P 11/20 system. While off-line systems have the potential to be as sophisticated as on-line in their data treatment, their scope is severely limited by the amount of information the operator can manually transfer from the OMA. Multiple array manipulations, such as the automated spectral stripping routine, must have the complete spectrum in order to perform accurately. Other complex calculation routines can be incorporated to reduce operator input and provide for computer-assisted experimental studies such as detection limits and flame profiles. By employing a command string interpreter structure, the programming has been designed to streamline operator input and facilitate software additions. The flexibility of the vidicon system is maintained by providing complete operator control with keyboard commands which can be learned with a minimal amount of practice. Standard options include the features which make the vidicon instrument attractive to analytical chemists, i.e., multielement analysis, spectral stripping, internal standardization, automatic curve-fits, and others. New functions can be easily added since the interpreter routine was structured for ex-
pansion. In effect, the system was designed primarily for dynamic operation rather than static inflexibility. Several detection system design improvements s h o d d make the vidicon an even more viable analytical tool. Computer controlled tube readout would significantly improve the real-time dynamic range and obviate the use of spectral filters. New spectral data formats as found in echelle spectrometers (12, 13), or split-grating instruments, may be more compatible with the two-dimensional vidicon target structure. New tube developments providing better ultraviolet region sensitivity, larger target areas, and better resolution should make computerized image tube systems the preferred multichannel spectroscopic detectors.
ACKNOWLEDGMENT The authors thank J. H. Boutwell and D. D. Bayse of the Center for Disease control, Atlanta, Ga., for supplying the samples of analyzed bovine serum.
LITERATURE CITED (1) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 712A (1973). (2) D. G. Mitchell, K. W. Jackson, and K. M. Aldous, Anal. Chem., 45, 1215A (1973). (3) 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). (4) Y. Talmi. Anal. Chem., 47, 658A (1975). (5) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (6) Ref. 5. D 1231 (7) Ref. 5, p 2074. (8)N. G. Howell, J. D. Ganjei, and G. H. Morrison, Anal. Chem., 46, 319 119761 - -, \
(9) N. S. Gill and R. S. Nyholm, J Chem. SOC.,3997 (1959). (10) J. A. Dean and T. C. Rains, "Standard Solutions for Flame Spectrometry." in "Flame Emission and Atomic Absorption Spectrometry," Vol. 2, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N.Y. 1971, p 327 (11) J R. Sarbeck, P. A St. John, and J. D. Winefordner, Mikrochim. Acta (Wien),5 5 (1972). (12) A. Danielsson and P. Lindblom, Phys. Scr., 5, 227 (1972). (13) D. L. Wood, A. B. Dargis, and D. L. Nash, Appl. Specfrosc., 4, 310 (1975).
RECEIVEDfor review October 22, 1975. Accepted December 12, 1975. This work was supported by the National Institutes of Health, Grant No. 5 R01 GM 19905-03.
Evaluation of a Self-contained Linear Diode Array Detector for Rapid Scanning Spectrophotometry Dennis A. Yates and Theodore Kuwana" Department of Chemistry, Ohio State University, Columbus, Ohio 432 10
The commercial availability of a linear array solid state diode detector providing large number of elements (1024), spectral sensitivity from near UV to near IR, associated electronics for a self-contained unit, and fast interrogation times (up to 10 MHz), made it attractive for evaluation as a detector for spectral acquisition in a rapid scanning spectrometer (RSS). The unit replaced the exit slit and photomultipliers of our existing RSS. The operational characteristics of the 1024-element linear diode array detector obtained from Reticon Corporation was evaluated and will be discussed. 510
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
The basic types of designs and performance characteristics of Rapid Scanning Spectrophotometers (RSS) have been reviewed by Santini, Milano, and Pardue ( 1 ) . Of these, two particular scanning methods have been quite recently discussed and are pertinent to this paper. In one (2-4), the incident angle on a grating surface was changed by an oscillating galvanometer mirror system and the dispersed spectrum was then swept across a stationary exit slit. The detector of the light intensity was considered as a single element, and it was commonly a photomultiplier tube (PMT). In the other, the dispersed spectrum was kept
