Table XII.
Identification of Se-Containing Phase in Sample U-3.6
Phase 5 d v , A. 3.269 3.079 3.482 3.061 2.767 2.100 1.988 2.308 5,001 4,430
V
24 30 21 31 36 55 59 51 8 11
PbSeOl I”
d., A.
I C
15.0 12.0 10.0 58.0 11.0 6.0 5.0 3.0 5.0 7.0
3.268 3.079 3.482 3,060 2.762 2.101 1.989 2.307 5,000 4.429
26.41 19.54 18.75 13.20 11.36 10.56 10.30 8.45 7.92 7.92
quality of diffraction analysis and provide numerical records. The prospect of programmed quantitative diffraction analysis will be feasible if the powder data are coupled with quantitative elemental analysis. Lastly, the ZRD program will free the diffractionist from the fatiguing task of searching and will allow him time to concentrate on the textural information revealed in a diffraction pattern and to attempt matching the structure of an unidentified phase by comparison with tabulated isomorphs. ACKNOWLEDGMENT
The author is grateful to Miss Carole Engbrecht of the Dow Computations Laboratory for writing the ALGOL 60
AI
44.80
program for the Burroughs 5000 digital comuuter to handle ZRD Search-Match. LITERATURE CITED
(1) Bradley, A . J., Proc. Phys. SOC.(Lond o n ) 47, 879 (1935). (2) Burbank, R. D., Acta Crust. 4, 140
(1951). (3) “Chemical Abstracts,” 56, 80N-86N (1962). (4) Claasen, A., Phzl. Mag. 9, 57 (1930). (5) Coles, B. R., J . Znst. Metals 84, 346 (1956). (6) Donnay, J. D. H., Donnay, G., Cox, E. G., Kennard, O., King, SI. V., “Crystal Data Determinative Tables,]’ 2nd Ed., ACA &lonograph N o . 5, American Crystallographic Association, 1963. ( 7 ) Frevel, L. K., IND. ENG.CHEM.,ANAL. ED. 16,209 (1944). (8)Hanawalt, J. D., Rinn, H. W., Frevel, L. K., Ibid., 10, 457 (1938).
(9) Hargreaves, A., “X-ray Diffraction by Polycrystalline Xaterials,” H. S. Peiser, H. P. Rooksby, and .4.J. C. Wilson, eds., p. 298, The Institute of Physics, London, 1955. (10) Hofmann, E., Jagodzinski, H., 2. Metallk. 46, 601 (1955). (11) Hull, A. W., J . Am. Chem. SOC.41, 1168 (1919). (12) “Index (Inorganic) to the Powder Diffraction File (1963),” ASTRI Special Technical Publication 48-M2,PhiladelDhia. Pa. (13) Johannsson, C. H., Linde, J. O., Ann. Phys. L p z . (5) 6,458 (1930). (14) Kuznetsov, V. G., Zzv. Zekt. Platzny, dkad. ?;auk. SSSR, N o . 20, 5 (1946). (15) Nial, 0..Svensk Kem Tidskr. 59. 172 (1947). (16) Paalman, H. H., Pings, C. J.,J . 9 p p l . Phys. 33,2635 (1962). (17) Smith, D. K., “A Fortran Program for Calculating Powder Patterns from Atom Coordinates,” Amer. Crvst. Assoc. Annual Meeting, March” 28-30, 1963; paper E l l . ( ( (18) Stokes, A. R., X-ray Diffraction by Polycrystalline Materials,” H. S. Peiser, H. P. Rooksby, and A. J. C. Wilson, eds., p. 409, The Instituteof Physics, London, 1955. (19) Taylor, A., Sinclair, H., Proc. Phys. SOC.57, 108 (1945). (20) The American Society for Testing and Materials, The American Crystallographic Association, The (British) Institute of Physics, and The National Association of Corrosion Engineers. (21) Warren, B. E., J . A m . Ceram. SOC. 17, 73 (1934). (22) Wyckoff, R. W. G., Posnjak, E. W., J . Wash. Acad. Sei. 13,393 (1923). RECEIVED for review December 14, 1964. Accepted February 8, 1965.
An Integrating Analog Computer for Atomic Absorption Spectrometry E. A. BOLING Medical Service and Research laboratory, Boston Veterans Administration Hospital, and the School o f Medicine, Tuffs University, Boston, Mass.
b An integrating analog computer is described which permits atomic absorption spectrophotometry in analytical situations ordinarily too noisy for satisfactory analysis. Speed and precision for routine analyses are markedly improved. The Beer-Lambert equation is solved by the computer and concentration in the undiluted specimen is given directly in digital form for each reading. O p erator fatigue is reduced. The stand= 1.0 ard deviation for the ///, setting for calcium when collecting data for 10 seconds is typically *0.0003. The relative detection limit for calcium using this apparatus is less than 0.003 p.p.m. Serum calcium analyses may be performed using as little as 5 PI. of sample. The absolute detection limit for calcium is about 1 0 - 8 gram. 482
ANALYTICAL CHEMISTRY
R
for atomic absorption measurements are usually one of two types. Meter readout in % transmission is convenient for singlebeam instruments (2). Null-point voltage dividers are typically employed for double beam applications and might be used in single beam instruments if automatic operation or digital readout were desired. Many atomic absorption analyses tend to be somewhat noisy, and for some, noise seriously limits analytical precision. For any analysis, noise increases the time needed to reach a given level of precision. This has the indirect effect of increasing the amount of sample which is required, since the sample must be atomized continuously while each decision is being made. Noise also plays a basic role in setting detection limits. EADOUT SYSTEMS
We have constructed an improved data readout system for use in atomic absorption spectrophotometry. Integrators are used for data collection, because they act to increase signal/ noise ratios and also to permit shortterm storage of the result, which conserves the sample. Servo-operated voltage dividers compute the signal ratios. I n addition, the Beer-Lambert equation is solved by a nonlinear potentiometer plus a second servodriven divider so that concentration is given directly in digital form. We have found that the device increases speed and accuracy for any analysis. Since the instrument automatically indicates the result for each sample as it is atomized, all operators analyzing a given sample should obtain identical results for any trial. Furthermore, operator fatigue is markedly re-
Table 1. Base Line / / l o f Standard Deviation Using Various Slit Widths
PI Integrator
,A Figure 1.
READOUT
READOUT
Calcium hollow cathode tube. Monochromator setting 4227 A. Slit Ill0 f standard deviation width, 2-sec. data 10-sec. d a t a mm. collection collection 0.03 1 . 0 f ,00520 1 . 0 f ,00146 0 1 1 0 f 00184 1 0 f 00061 1 0 f 00051 1 0 f 00027 0 3 10 1 0 f 00061 1 0 f 00032
Block diagram of system
duced and productivity is multiplied severalfold. For many analyses, a single working standard is sufficient. Finally, straightforward statistical evaluation of repetitive readings becomes the logical approach to such matters as detection limits, error limits, etc. EXPERIMENTAL
The logical block diagram of our system is shown in Figure 1. Our device is used in conjunction with a double-beam instrument which uses only one photodetector, a PerkinElmer Model 303. The signal separator, in addition to separating the sample and reference beam signals, restores the square wave trains which constitute the signals so that their base lines are at ground potential. This permits direct integration. The Gate circuit controls Start and Stop modes for data collection for both beams. Start is signalled by a foot switch. Data are collected until the reference integrator reaches an arbitrary preset voltage, whereupon a voltage sensitive circuit signals Stop mode. The Gate Logic.
circuitry is locked on to the 60-cycle sine wave; both Start and Stop actions are always initiated a t the same point in that cycle, regardless of when the foot button is pressed or when the reference integrator reaches the cutoff potential. This ensures the collection of an equal amount of data for both beams, since the optical chopper and signal separator are also driven a t line frequency. This feature permits the use of short data collection periods ( 2 seconds) if desired, with very little loss of accuracy, whereas without this feature such short periods are quite impractical due to random sampling error, Accuracy for longer collections is also definitely improved. A servo voltage divider divides the sample integral by the reference integral and presents % absorption on a digital counter. This divider also turns the shaft of a nonlinear potentiometer. Since shaft revolutions are directly proportional to I / I o , this potentiometer is calibrated so that its wiper voltage (at null) is the logarithm of the shaft rotation. Another servo divider then presents concentration on a second digital counter. All cycling is con-
trolled by the foot switch, which in one position resets the integrators and turns off servomotors, and in the other places Gate in Start mode and turns on servo motors. Instruments of other types-e.g., single beam or double beam with two detectors-would have somewhat different logic. Circuitry. The circuit which is used is shown schematically in Figure 2. There are many different ways in which this basic logic could be accomplished. T o achieve the performance delivered by this unit, it was necessary to control many circuit variables. The integrators are chopper stabilized. Computor - grade metalcased polystyrene capacitors are used in the integrators. All power supplies are highly regulated. h complete description of the circuit with a listing of components is available on written request to the author. Baseline I / I o Stability. The performance of this unit can be described in terms of its ability to repeat the setting for lOOyo transmission when the flame is not lit. Using a calcium hollow cathode lamp with the monochromator set a t 4227 A., measurements were performed in consecutive groups of ten. Various slit
Servo
5111
Digital Readout
Preamp.
yoConcentration
-.
-I
REFERENCE
--
yoAbsorption
Reset Figure 2.
Schematic of circuit VOL. 37, NO. 4, APRIL 1965
e
483
U"
1.0.)
Figure 3.
Comparison of signal wave-forms
widths were used. For each slit width, 10 consecutive data collections were made using each of two integrator time constants, which gave data collection periods of approximately 2 and 10 seconds, respectively. The results of this comparison are shown in Table I. Each value which is given is the standard deviation for 10 consecutive trials a t I / I o = 1.0 using the slit width listed. For each slit width, the standard deviation was smaller for the longer data collection period. Increasing the slit width also decreased the standard deviation up to a slit width of 0.3 mm., but there was no obvious decrease on further increase of slit width. The performance was tested further by simulating a very noisy analysis and comparing the reproducibility and speed of repetitive readings for both the manual readout and the computer. The calcium hollow-cathode lamp was covered on the end so that only a horizontal slit 1 mm. wide was available through which light could escape. A monochromator slit width of 0.03 mm. was also used. These changes markedly decreased signal/noise ratio. Figure 3 compares the signal wave- forms seen under these conditions with those seen when the instrument was adjusted
properly. Distilled water was then repetitively read in groups of 10 readings using both the manually nulled dynamic readout and the computer system. The mean and standard deviation are shown for each group of 10 readings in Table 11, which compares the results using both systems. Speed of analysis for equal reproducibility is markedly improved by the use of this system. Furthermore, the use of repetitive sampling with the computer for a time equal to that used for the manual null results in distinctly improved reproducibility. I n addition, it permits statistical evaluation of the results, with definition of confidence intervals. I t should also be emphasized that results for the manually nulled system were markedly dependent upon operator skill. Some operators were completely unable to obtain consistent results. I n marked contrast, the use of the computer merely required recording the result for each trial.
Table 11, / / l o Reproducibility in a Simulated Noisy Analysis
a
IluIIIuel
-I
*
f
.044
i .oe 4
t
T
,0036
O - I0
Figure 4. ANALYTICAL CHEMISTRY
1u1
.08 4
a Each datum is the mean for 10 tries a t Ill0 = 1.0 (while atomizing water) f o r a simulated noisy analysis. No data are shown for the manual readout at 10second periods, because the manual readout could not be nulled in that time. The data for the computer were averaged to give the data for 30-second periods.
484
U a L a
.le 4
.lo
Ill0 (mean f standard deviation)& Readout 10-sec. periods 30-see. periods Manual 1 . 0 f 0.0156 1 . 0 f 0.0138 Computer 1 , O f 0,0126 1,O f 0.0061 1 . 0 f 0.0140 1 . 0 f 0.0026 1 . 0 f 0.0127 1 . 0 f 0.0128 1 . 0 f 0.0138 1 . 0 f 0.0107
'CLUIU
Mean and
T
.0050
Calcium
VI
trials, compare them with background (zero absorption), and compute the statistical likelihood that the mean is different from zero. To illustrate this method, the following calcium solutions were prepared and tested: distilled water (zero calcium), 0.0025 p.p.m., 0.005 p.p.m., 0.0075 p.p.m., 0.010 p.p.m., and 0.0125 p.p.m. To test these solutions, a 10-second reading was taken while atomizing water, then a 10-second reading while atomizing one of the test solutions, then water, then calcium etc., always alternating a reading for distilled water with each sample reading. During the initial reading for water, the instrument was adjusted to read 2.0Oj, absorption, and each subsequent reading, whether for water or for a calcium solution, was merely recorded to the nearest O . O l ~ o absorption. After the completion of 10 such comparisons for each calcium solution, plus 10 comparisons done as if water were the sample-i.e., alternating distilled water with distilled water-the results were calculated. For each sample reading, the difference between the reading for the
.0076
'I
t
.0100
.0196
ppm
95% confidence limits for calcium samples
sample and the reading for the immediately previous distilled water sample was calculated to the nearest 0.0170 absorption. Each group of ten readings was begun and ended with a reading for distilled water, This permitted the calculation of 20 differences for each group of 10 calcium readings. For these 20 differences, means, standard deviations, standard errors, and 95% confidence limits were computed for each sample. Figure 4 shows the results of these tests. For distilled water and for each calcium solution, the mean is plotted with its 95yo confidence limits. The 0.0025 p,p.m. sample is different from distilled water a t the 95% level, and 4 of 5 samples are differentiated one from the other at the 9570 confidence level. The average %Yo confidence interval for the 6 calcium solutions is + ~ 0 , 0 1 7absorption, ~~ which corresponds to a concentration difference of 0.002 p.p.m. Therefore, a concentration difference of 0.005 p.p.m. should permit calcium solutions in this concentration range to be differentiated one from the other at the 95% confidence level. Sample Size. For routine analyses of blood serum, 1:20 dilutions have been employed by most workers (3,'7). This requires the use of 0.5- or 1.0-ml. samples, which is often inconveniently large. Csing the technique outlined above for trace measurements, accurate measurements of serum calcium concentration can be made using 5 pl. of sample, and averaging the results for each sample. For such an application, a 1 : 200 dilution would be appropriate, since it yields a working concentration of 0.5 p.p.m., about 250 times the relative detection limit. Five microliters would yield 1 ml. of working solution, which would suffice for 2 trials of 10 seconds each. Such a measurement has a coefficient of variation of 2Y0 or less. For routine use, it is quite likely that the usual 1:20 or 1 :40 dilution is stronger than necessary, and we are at present evaluating the use of a 1 :200 dilution for that purpose. The use of a 1 :200 dilution will decrease the sample size to 0.1 ml.
or less, a much more practical amount. It may also permit the same solution to be used subsequently in emission flame spectrometry for sodium and potassium. Absolute Detection Limits. The average standard deviation near t h e limit of detection was +o.03670 absorption, and 3 standard deviations would be 0.10870 absorption. This corresponds to a concentration of 0.013 p.p.m. A single 10-second t r y should give a significant amount of absorption for a solution of that concentration. Since this requires 0.75 ml. of solution, the absolute detection limit would be 0.013 X 0.75 X gram, or 9.7 X gram. DISCUSSION
The use of conventional analog computer integration yields results which are better than those which one can expect to obtain by the use of a conventional voltage divider readout system. Economies in time and sample size are realized, and accuracy is definitely improved. Digital techniques might well be preferable in some laboratories, with integration by voltage-tofrequency converter and digital display. Division in double-beam systems could be accomplished by halting data collection for both channels when the reference channel reached a preset Count. Under those conditions the denominator of the fraction Z/Zo would be a constant for each trial. Alternatively, data could be recorded on tape for subsequent processing by digital computer. If the computer were to be used solely for trace analyses, a much simpler approach could be employed. I n that approach, the difference between the sample and reference beam would be integrated by a chopper-stabilized integrator. The output of that integrator would be observed by a meter with multiple sensitivity ranges. The gate circuitry would be identical to that used in the present instrument, with
the exception that the integrator for the reference beam need not be chopperstabilized. Such a n instrument would give performance equally as good as the present one for trace analyses. No servo units would be needed, and only one chopper-stabilized integrator would be required. Serious problems exist in the area of atomization and flame control (3, 4). Systems such as the present one nearly eliminate instrumental variation. The need for control of the atomizer-flame combination thus becomes more acute than ever, because most of the variation in results now is due to instability in that system. Obviously, the present instrument could be used as a readout device for virtually any spectrophotometer. Advantages in accuracy and in speed of measurement would be realized if similar computers were commercially available for this general purpose. ACKNOWLEDGMENT
The author expresses his appreciation to Miss Elsie Rossmeisl, who performed the analyses reported here. LITERATURE CITED
(1) Allan, J. E., SpectTochirn. Acta 18, 259 (1962). (2) Box, G. F., Walsh, A,, Ibid., 16, 255 (1960). (3) Clinton, 0. E., Ibid., p. 985. (4) Gatehouse, B. M., Walsh, A., Ibid., p. 602. (5) Gatehouse, B. M.,Willis, J. B., Ibid., 17, 710 (1961). (6) Slavin, W., Sprague, S., Manning, D. C., Perkin-Elmer Atomic Absorption Newsletter No. 18. Februarv 1964. ( 7 ) Willis, J. B., Spectrochim. Acta 16, 259 (1960). RECEIVED for review October 26, 1964. Accepted January 26, 1965. Presented a t Eastern Analytical Symposium, New York, November 1964.
VOL. 37, NO. A, APRIL 1965
485
