(11) D. T. Coker, J. M. Ottaway, and N. K. Pradhan, Nature (London), Pbys. Sci., 233, 69 (1971). (12) J. Y. Marks and G. G. Welcher, Anal. Cbem., 42, 1033 (1970). (13) G. De Maria, R. P. Burns, J. Drowart, and M. G. Inghram, J. Cbem. Pbys., 32, 1373 (1960). (14) J. 0. Rasmuson, V. A. Fassel, and R . N. Kniseiey, Spectrocbim. Acta, Part E, 28, 365 (1973). (15) E. V. L'vov, "Atomic Absorption Spectrochemical Analysis", Adam Hilger, London, 1970. A. G. Gaydon and H. G. Wolfhard, "Flames, Their Structure, Radiation, and Temperature", 2nd ed., Chapman and Hall, London, 1960. V S. Sastri, C L. Chakrabarti, and D. E. Willis, Can. J. Cbem., 47, 587 11969). . , C. W. Dannett and H. S. T. Eliingham, Discuss. Faraday SOC., 4, 126 (1948). G. F. Kirkbright, M. K. Peters, and T. S. West, Talanta, 14, 789 (1967). Acta, Part T. G. Cowley. V. A. Fassel, and R. N. Kniseley, . Spectrocbim. . 8, 23, 771 (1968). D. T. Coker and J. M. Ottaway, Nature (London), Pbys. Sci., 230, 156 (1971). G.F. Kirkbright and T. S. West, Talanta, 15, 663 (1968). C. S. Rann and A. N. Hambly, Anal. Cbem., 37, 879 (1965); A. N. Hambly and C. S Rann in "Flame Emission and Atomic Absorption Spectrometry", Voi. I, J. A. Dean and T. C Rains, Ed., Marcel Dekker, New York. 1969, pp 249-252. C. L. Chakrabarti, M. Katyal, and D. E. Willis. SDectrocbim. Acta, Part E, 25, 629 (1970). D. M. Kemp, Anal. Cbim. Acta, 27, 480 (1962). J. J. Eisch and R . B. King, "Organometallic Synthesis", Vol. I (Transition Metal Compounds), Academic Press, New York, 1965, p 110. C. L. Chakrabarti, R. Pal, and M. Katyal, Anal. Cbem., 43, 1704 (1971). W. S. Zaugg and R. J. Knox, Anal. Cbem., 38, 1759 (1966). R. F. Browner and J. D.Winefordner, Anal. Cbem., 44, 247 (1972). A. C. West. V. A. Fassel. and R. N. Kniseiev. Anal. Cbem.. 45. 1586 (1973).
(31) C. Th. J. Alkemade in "Flame Emission and Atomic Absorption Spectrometry", Vol. I, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, 1969, pp 121-123. (32) G. F. Kirkbright, M. K . Peters, and T. S. West, Atomic Absorption Symposium, Praha, 1967 (summary); G. F. Kirkbright, A. Semb, and T. S. West, Spectrosc. Left., 1, 7 (1968). (33) J. E. Willis. J. App. Opt., 7, 1295 (1968). (34) R. W. B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra", 3rd ed., Chapman and Hall Ltd., London, 1965, p 209. (35) F. A. Cotton, A. K. Fischer, and G. Wilkinson, J. Am. Chem. Soc.. 78, 5168 (1956). (36) D. R. Bidinosti and N. S. Mclntyre, Can. J. Cbem., 45, 641 (1967). (37) D. J. Shaw, "Introduction to Colloid and Surface Chemistry", 2nd ed., Butterworths, London, 1970, pp 68-70. (38) J. 8. Willis, Spectrocbim. Acta, Part E, 25, 487 (1970). (39) S. Nukiyama and Y. Tanasawa, Trans. SOC. Mecb. Eng. Jpn, 5, 62 (1939). (40) Unpublished results of the present authors. (41) J. H. Gibson, W. E. L. Grossman, and W. D. Cooke, Anal. Cbem., 35, 266 (1963).
RECEIVEDfor review August 19,1975. Accepted December 18, 1975. The authors are grateful to the National Research Council of Canada for financial support of this project. The authors also wish to thank D. R. Wiles for the use of his facilities for the neutron activation analysis. This paper was presented a t both the 56th Canadian Chemical Conference, June 4-6, 1973, Montreal, Canada, and the 4th International Conference on Atomic Spectroscopy, October 29November 2,1973, Toronto, Canada.
Precision of Flame Atomic Absorption Measurements of Copper N. W. Bower and J. D. Ingle, Jr." Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 1
Repetitive atomic absorption measurements on copper solutions which yield absorbances in the range of 0-2 are made with a high resolution voltmeter and on-line computer and are used to evaluate the dependence of measurement precision on absorbance and instrumental variables. The experimental relative standard deviation in absorbance is compared to that predicted by a recently proposed theoretical equation. The study reveals that under the typical conditions used for copper atomic absorption measurements, noise sources associated with the lamp (Le., signal shot noise and source flicker noise), with the detection system (i.e., amplifier-readout noise, dark current noise), and with the background emission of the flame are not limltlng. Noise due to fluctuations in the transmission properties of the flame limits precision at small absorbances (i.e., A < 0.05), while fluctuations in the absorption properties of the analyte limit reproducibility for moderate and large absorbances.
varies with absorbance ( A )and instrumental variables and noise parameters. The purpose of this paper is to outline the experimental procedures for evaluation of the precision characteristics of AA measurements for a given element, on a particular AA instrument, under specific conditions. In particular, AA measurements on copper solutions ranging from 0.05 to 500 ppm were studied. The procedure involves measurement and calculation of the standard deviation of various signals and evaluation of instrumental variables and noise parameters so that theoretical calculations can be made. The net result of the procedure is that the relative contribution of the potential noise sources can be identified. For copper measurements with a 2-8, spectral bandpass, flame transmission fluctuations are limiting a t small absorbances while, at higher absorbances, fluctuations in the absorption properties of the analyte in the flame became limiting. Fluctuations in absorption properties are due to such factors as changes in the free atom population, the path length, the absorptivity of the analyte, or the sample and gas flow rates. For very small slit widths, signal shot noise and noise in the flame background emission became important.
The precision of flame atomic absorption (AA) measurements are significantly affected by the dynamic nature of flames. In a recent paper ( I ) , the previous experimental BACKGROUND and theoretical work dealing with precision and signal-toAppendix I contains the important equations and defininoise ratio (S/N) aspects of AA measurements was retions to be used in this paper. The derivation of these equaviewed and new equations were presented which indicate how the relative standard deviation in absorbance ( ~ A / A ) tions and the assumptions made in the derivations have 686
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
been previously presented (1). Equation 1 is the general equation which indicates how UAIAvaries with A and accounts for the following types of noise: shot and flicker noise in the source radiation signal, flame background and analyte emission shot and flicker noises, analyte absorption flicker noise, flame transmission flicker noise, dark current shot and excess noise, amplifier noise, and readout electronic and quantization noise. Equation 1 applies to a modern atomic AA spectrometer with direct transmittance ( T ) readout, in which the T is obtained from three measurements (reference, 0% T , and analyte signal). Similar equations are derived ( 1 ) for linear absorbance readout instruments and other variations. The equations and procedures for linear absorbance readout instruments are given in Appendix 11. I t is assumed that digital readout is employed and resolution is sufficient so that fluctuations due to noise are obvious a t all absorbances. The standard deviation in various signals may be obtained by direct calculation on repetitive measurements of a given signal or taken as 1k the peak-to-peak (p-p) noise (2). The problems of obtaining good estimates of standard deviations in signals have been discussed (3).The main problem is that drift or low frequency noise may be important over the time necessary to make a large number of measurements on a signal but not over the time to make a reference and analyte signal measurement. The following measurements and calculations are made for a given set of experimental conditions. The true experimental deviation in the absorbance ( U A ) can be calculated directly from n triplicate reference-analyte-0% T measurements, or, a simpler approach in terms of labor is to obtain the experimental U A from the standard deviations for each of the measurements separately and then combine them. The validity of this simpler approach was confirmed experimentally. This was done in the procedure below, where n measurements are made for each step. The necessary additional measurements for making the theoretical calculations are listed separately. I t is assumed that the instrumental parameters and burner positioning have been optimized for the criterion of maximum absorbance a t least approximately prior to making these measurements. If they have not been optimized, a second pass through these steps will suffice. I t is also assumed that the electronics and lamps have been warmed up adequately to allow stabilization. As an example of the use of this procedure and the equations, a representative element (Cu) was run using a modern AA spectrophotometer with a modulated light source and “lock-in” type of electronics. Ac measurements below refer to measurements made with this conventional synchronous ac system. Dc measurements refer to measurements made with a modified system in which dc electronics are connected to the photomultiplier. The term “currentto-voltage converter” will refer to all the electronics between the photomultiplier and readout which involve conversion of the photoanodic current to a voltage, as well as amplification and demodulation of the signal. EVALUATION P R O C E D U R E Ac Measurements. 1) The readout offset (usually zero) E R and its standard deviation g~ are obtained from n measurements with the leads to the readout device shorted. If no noise is observable, U R is just the quantization noise, q. However, if got (step 7) 1 U R then u~ = 0.29q ( 4 ) . 2) With the P M T supply turned off and the P M T shutter closed, the amplifier-readout system together is observed to estimate E,, and u,,. The P M T is left connected to the current-to-voltage circuitry since its impedance may affect the measurements. 3) The P M T supply voltage is turned on, and the shut-
ter is opened so that the P M T is receiving the light of the desired wavelength from the lamp. The supply voltage and gain are adjusted so that a 100% T output is obtained a t the readout. (The lamp current has been previously set. Changing it now will require another period of stabilization.) n measurements are made to estimate E,;, and its standard deviation, ur;. The voltage of the P M T should also be measured here. 4) The lamp shutter is closed and without the flame the 0% T signal is measured to estimate Eat' and uo(. 5 ) The lamp shutter is reopened, the flame is turned on, and a reference solution is aspirated and the P M T voltage is adjusted so as t o give a 100% T signal. E,, and ort are obtained from n measurements and the P M T voltage is measured again. 6) The P M T shutter is closed and n dark current measurements are made to estimate Edt and Udt. 7 ) The P M T shutter is opened and the lamp shutter is closed so that only light from the flame plus reference solution is observed and EOtand uot are obtained. It is usually important to make this measurement while the burner is clean, if memory effects are a problem of the burner system. Since memory effects will be important for real sample measurements, this measurement may be repeated after aspirating analyte solutions. 8) With P M T and lamp shutters open, the analyte solutions are aspirated and their respective signals, Est and standard deviation uSt are obtained from n measurements. A set of analyte solutions with concentrations covering the range of interest should be used. 9) With the lamp shutter closed, the most concentrated analyte solution of interest is aspirated and n measurements are made to estimate the ac emission signal, Eet,and uet. If uet is significantly greater than uot, progressively less concentrated solutions should be aspirated until uot uet. D c Measurements. For these measurements the ac electronics are replaced by a dc current-to-voltage converter and a readout device with Af and G arranged to be the same as for the ac electronics. 10) The lamp shutter is closed and the reference is aspirated (until the burner is clean) and n measurements are made to determine the dc 0% T signal, E,,* and got*. 11) With the analyte solution aspirating, Eet* and get* are obtained. 12) The P M T shutter is closed and n measurements are made to obtain E d t * and U d t * . 13) The P M T supply voltage is turned off (the P M T shutter is left closed) and the dc amplifier-readout zero, E,,*, and uar* are determined. O t h e r Variables Needed. 14) From the manufacturer’s specifications, or by measurement with a known modulated current input to the electronics, the amplification factor, G , is measured. 15) The P M T gain, m , a t the bias voltage used, is evaluated. Procedures for evaluation of the P M T gain have been discussed in detail ( 5 ) . 16) The noise equivalent bandpass, Af,is evaluated with Equations 5 or 6, and the limiting time constant or integration time. The secondary emission factor, a , may be evaluated from the dynode emission statistics (6) or assumed to be 0.3. The bandwidth constant, K , is evaluated from the above estimates of Af and CY and Equation 7 . Calculations. The experimental plot of UAIAvs. A is obtained from the measurements of urt, ust, and uot in steps 5 , 7 , and 8 respectively and Equation 2. T is calculated from Equation 3 and the experimental measurement of Ert, Est, and Eot. E, is calculated from Equation 8. T o obtain a theoretical precision plot (uAIA vs. A ) from Equation 1, it is still necessary to evaluate the flicker facANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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Table I. Components of Instrumentation Supplier and Model
Item
1. Spectrophotometer Monochromator Heath EU-700 PMT 1 RCA 1P28 P M T housing Heath EU-701-93 P M T power supply Heath EU-42A Frequency generator Heath EU-81A Power transistor Delco GM PTS 7223 Lamp Westinghouse Copper Lamp supply Lamda Model 71 Burner Jarrell-Ash Tri-Flame Heath EU-703 Flame and hollow cathode compartment and burner positioner 2. Readout PAR 128 Lock-in amplifier Analog Devices AD540J Operational amplifier Digital readout Heath EU-805 3. Computer system Digital P D P 11/20 Computer Tektronix T-4002 Graphics terminal 4. Voltage regulator Hewlett-Packard Stabiline
tors, [I, (2, [3 and the analyte emission noise ( u e ) . If the experimental measurements ur
