problems which have hindered the successful application of previous ECDs to the analysis of biologic extracts.
David King, and Linda K. Goggin is deeply appreciated.
RECEIVEDfor review April 8, 1974. Accepted August 23,
ACKNOWLEDGMENT The technical assistance of John D. Biggs, Pamela Jones,
1974. This research was supported by Grant R01 DA00729 from the National Institute on Drug Abuse, NIH, Bethesda, Md.
Computer-Controlled Monitoring and Data Reduction for Multiple Ion-Selective Electrodes in a Flowing System J. J. Zipper,' Bernard Fleet,* and S. P. Perone3 Purdue University, Department of Chemistry, La fayette, Ind. 47907
A laboratory minicomputer system has been applied to the monitoring of ion-selective electrodes in a flowing stream. Up to 5 different electrodes could be monitored slmuitaneousiy. A standard addition analytical approach was implemented with a rigorous least squares fit to the data used to obtain electrode response slope and unknown analyte concentration for each electrode. The computer was programmed so that three different modes of data monitoring were possible: (1) Operator-controlled selection of data regions to be collected for subsequent analytical computations (STATIC program); (2) computer-controlled selection of data collection regions based on a pre-selected timedelay after each addition of standard (DUMB program); and ( 3 ) selection of data collection regions based on real-time computer identification of successive voltage plateaus In the potentiometer output during a series of standard additions (SMART program). Analog Instrumentation was developed which provlded a wide bandpass (to accommodate rapid multiplexing of electrodes), low drift, and high noise rejection. The digital instrumentation provided signal averaging, data sampling wlth f0.002 mV resolution, multiplexed sampling of 5 eiectrodeshec, and real-time digital display of signals from 5 electrodes. The entire system was evaluated by performlng a large number of standard addition experiments for fluoride analysis under optimized conditions. Analytical data could be obtained with a relative error of 0.4 to 3.3% and confidence Intervals varying from f0.4 to f1.6%. A comparison of the capabiilties of the different algorithms for data collection was made.
Ion-selective electrode (ISE) potentiometry is currently one of the more rapidly expanding techniques in analytical chemistry (1-3). A wide range ( 4 ) of electrodes responsive to cations, anions, enzyme-substrate systems ( 5 - 7 ) ,organic Present address, SPEX Industries, Box 798, Metuchen, N.J. Present address, Imperial College of Science and Technology, Department of Chemistry, South Kensington, London SW7, England. Author to whom reprint requests should be sent. (1) "Ion Selective Electrodes," R . A. Durst, Ed., Nat. Bur. Stand. (U.S.) Spec. Publ., 314 (1969). (2) R. P. Buck, Anal. Chem., 44, 270R (1972). (3) R. P. Buck, Anal. Chem., 46, 28R (1974). (4) R . A. Durst, Amer. Sci., 59, 353 (1971). (5) G. G. Guilbault. R. K. Smith, and J. G. Montaivo, Anal. Chem., 41, 600 (1969). (6) J. G. Montalvo, Anal. Chem., 41, 2093 (1969). (7) M. M. Fishman and H. F. Schiff, Anal. Chem., 44, 543R (1972)
ligands ( 8 ) ,and gases (9) are available, and new designs of electrodes continue to appear. One of the newly developing areas of application of these devices is in continuous monitoring (10-12). This approach appears to be particularly promising for applications to clinical analysis (13). The analytical utility of ion selective electrodes, however, is often impaired by two inherent fundamental limitations. First, the logarithmic relationship between electrode potential and primary ion activity limits the degree of accuracy attainable for a given measurement. The second limitation is that in many cases the electrode shows only moderate selectivity toward the primary ion of interest and the influence of interfering ions becomes highly significant. Selectivity limitations can in most cases be overcome by careful selection of the sample and the nature of chemical pretreatment. The accuracy attainable, on the other hand, is primarily dependent on the analytical technique used for the measurement. Direct potentiometry, although the most widely used approach, is the least accurate. Accurate matching of sample and standard is difficult and any electrode potential drift necessitates recalibration of the electrode a t a frequency determined by the degree of precision required and the rate of drift. The use of ion-selective electrodes as end-point sensors in titrimetric processes markedly improves the degree of precision attainable, although, with few exceptions (12), at the expense of considerable loss of convenience. The most convenient method for improving accuracy in direct potentiometry is by employing a standard addition approach (14, 1 5 ) . Either a single or multiple addition of the sought ion or a reagent which complexes the sought ion (standard substraction method) are possible. The graphical procedure of Gran can be employed (16, 1 7 ) , which involves a linearization of the standard addition equation and a multiple standard addition procedure. The use of computers to further enhance the accuracy (8)C. N. Wang, P. J. Kinlen, D. A. Schoeiier, and C. 0. Huber, Anal. Chem., 44, 1152 (1972). (9) "Newsletter," Orion Research, inc., Cambridge, Mass., Vol. V, No. 2, 197.1 n 7 '-'-> r
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(10) T. S. Light, "Ion Selective Electrodes," R . A. Durst. Ed., Mat. Bur. Sfand. ( U S )Spec. Publ., 314 (1969). (11) B. Fleet and A. Y. W. Ho, "Ion Selective Electrodes," E. Pungor, Ed., Academia Kiado, Budapest, 1973. (12) 9.Fleet and A. Y. W. Ho, Anal. Chem., 46, 9 (1974). (13) G. A. Rechni!z. Amer. Lab., 6, 13 (1974). (14) "Newsletter, Orion Research, inc.. Cambridge, Mass., Vol. I, July, 1969, p 9. (15) /bid., September, 1969, p 25. (16) G. Gran. Analyst (London),77, 661 (1952). (17) A. Liberti and M. Mascini, Anal. Chem., 41, 676 (1969).
A N A L Y T i C A L CHEMISTRY, VOL. 46, N O . 14, DECEMBER 1974
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