use of the word “equivalent” be restricted to that found in the earlier of the above references ( I O ) : to describe efficiency calculated from a measurement of resolution. I also propose that the word “apparent” be used to describe efficiency calculated from a measurement of spot width and migration distance. We thus have the following definitions. For two sample spots whose capacity factors (12) (kl’ and k2’) in a given system are known, and whose resolution (R,) and average migration distance ( X ) are measured, we define the equivalent plate number, ne as: ne = 16R,2((cy/(cy-1))((1 + k 2 ’ ) / k 2 ’ )where cy = k l ’ / k 2 ’ . The equivalent plate height, he, may then be defined by: he = X / n e . For a single sample spot whose top-to-bottom width (W) and migration distance (X)are measured, we define the apparent plate number, n, by: n, = 1 6 ( X / W)’. The apparent plate height, h,, may then be defined by: h , = X / n , . In the absence of gradients, the apparent plate number and the equivalent plate number are numerically equal, and may be referred to simply as the “plate number”. The same is true for plate height. T h e terms “apparent” and “equivalent” can be extended t o other measures of efficiency or performance (e.g., the separation number (13)) in order to clearly distinguish
measurements based on resolution from those based on spot width. What are in fact different criteria for high performance in TLC can in this way become comparable.
LITERATURE CITED (1) J. Riphahn and H. Halpaap, J , Chromatogr., 112, 81 (1976). (2) S. Ebei and J. Hocke, J . Chromatogr.. 126. 449 (1976). (3) T. H. Jupille and J. A. Perry, Science, 194, 288 (1976). (4) T. H. Jupiiie, J . A m . Oi/ Chem. Soc., in press. (5) A. J. P. Martin and R. L. M. Synge, Biochem. J . , 35, 91 (1941). (6) L. S. Ettre, Chromatographia, 8, 291 (1975); 8, 355 (1975). (7) N. V. Rachinskii, J . Chromatogr., 33, 234 (1968). (8) G. H. Stewart and T. D. Gierke, J . Chromatogr. Sci., 8, 129 (1970). (9) J. A. Thorna, Anal. Chem., 35, 214 (1963). (10) T. t i Jupille and J. A . Perry, J . Chromatogr., 99, 231 (1974). (11) J. A. Perry, J . Chromatogr., 113, 267 (1975). (12) B. L. Karger, L. R. Snyder. and C.Horvath, “An Introductionto Separation Science‘ , Wiley-Interscience. New York, N.Y., 1973, pp 30-31. (13) R. E. Kaiser in “High Perfofmance Thin Layer Chromatography”, A. Zhtkis and R. E. Kaiser, Ed., Eisevier, Amsterdam, 1977.
Thomas Jupille Chemical Division BioRad Laboratories 32nd and Griffin Avenue Richmond, California 94804
RECEIVED for review February 9, 1977. Accepted June 24, 1977.
Ligand Interference Preventive Buffer in Determination of Copper(l1) by a Cupric Ion Selective Electrode Sir: In the determination of copper(I1) by means of a cupric ion selective electrode, the problems encountered are variation of activities caused by complexation of copper(II1 with ligands in samples and the abnormal response of the electrode in the presence of some ligands such as NTA and EDTA ( I ) . In addition, interferences of halide ions limit its application or practical analysis (2, 3). I n this communication, we propose to use a ligand interference preventive buffer (LIPB) to mask such interferents. In copper(I1) buffer solution consisting of polyamines and copper(I1)) the electrode shows an ideal behavior down to 10”” M of free copper(I1) or to further lower levels ( I ) . This indicates that interferences caused by complexing agents may be eliminated by adding an excess of one of the polyamines, which forms a more stable complex than any other ligand contained in the samples originally, This is a principle of
LIPB’
to the Of (4)for the determination of fluoride, the use of LIPB is based on a 1:l dilution of both standards and samdes with a solution which simultaneously has the following functions: (1)A sufficiently high level of non-interfering electrolyte is contained to fix the total ionic strength of samples by adding the solution. (2) The polyamine in the solution forms a very stable complex with copper(I1) to displace any bound ligand in samples, and total polyamine concentration is higher than that of copper(I1) in samples. (3) The solution is buffered at an appropriate pH t o complete the complexation with the polyamine.
EXPERIMENTAL The LIPB consisted of 0.40 M triethylenetetramine, 0.20 M nitric acid, and 2.0 M potassium nitrate. It was confirmed by atomic absorption analysis that copper was not contained by an amount higher than lo4 M in the LIPB. An Orion 94-29A cupric ion selective electrode was used with an Orion Model 801A Digital Ion Analyzer and a SCE with 10% potassium nitrate salt bridge. All measurements were made under ambient conditions. To 10.0 1868
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
Table I. Removal of Troubles Caused by NTA and EDTA with the LIPB Electrode potential, mV Total concn of Cu(II), Ma NTA~,~ EDTA~,~ 1.0 X -367.3 (-366.9)‘ 1.0X -400.9 (-400.6) -400.3 (-400.3)c 1.0X -433.3 (-432.7) -432.2 (-432.6) 1.0X lo-’ 1.0X 10.’
-455.3 (-451.1) -460.2 (-457.8)
-457.2 (-456.4) -465.7 (-459.4)
a All concentrations were final ones after addition of the LIPB. The concentration of each ligand was 1.0 X M. ‘Figures in parentheses are potentials for standards.
mL of standards Or t o 10.0 mL of samples containing an aPpropriate interferent, 10.0 mL of the LIPB was added. The solutions were stirred with a magnetic stirrer, and the electrode ootential was _ _read ~ after min,
RESULTS AND DISCUSSION Although the electrode showed an abnormal response such as a potential drift for a long time or a potential variation dependent on a rate of stirring in the presence of NTA and EDTA, such a problem was successfully removed by addition of the LIPB. As shown in Table I, the potential values in the presence and in the absence of NTA or EDTA were in good agreement within 1.0 mV above 1.0 X M of copper(I1). A slope of calibration curves was slightly higher than a theoretical one. However, this does not present any problem for measurements, since the reproducibility of potentials is high. As copper(I1) complexes easily with many of the anions and others, the electrode determination of copper(I1) is difficult without an appropriate pretreatment of samples in which complexing agents coexist. The results suggest that a single calibration curve can be obtained in the presence of
Table 11. Removal of Bromide Interference by Addition of the LIPB Total concn Electrode potential, mV of Cu(II), Ma 1.0x 1.0x 1 . 0 x lo-4 1.0x 10-5 1.0x
-367.6 -401.6 -433.2 -455.2 -459.8
(-367.1)b (-400.9) (-433.0) (-455.4) (-457.1)
a All concentrations were final ones after addition of the LIPB. The concentration of sodium bromide was 1.0 X M. Figures in parentheses are potentials for standards.
various ligands by addition of the LIPB. Anions such as halide ions, which form sparingly soluble silver salts, interfere greatly with the activity measurement of copper(I1). These kinds of anions convert silver sulfide at the membrane surface to silver salts of corresponding anions ( 2 , 3 ) . Since solubilities of the silver salts increase in the presence of polyamines, such conversions of the membrane surface may be depressed by addition of the LIPB. It is thus expected that the LIPB may also be effective in elimination of this kind of interference. In the presence of 1.0 X lo-* M of sodium bromide, the electrode did not show any sensitivity to cupric ion, but responded to bromide ion in weakly acidic media. As shown in Table 11, the interference of bromide
was effectively eliminated by the addition of the LIPB. Thus, using the LIPB designed in this work, one can determine copper(I1) in the presence of complexing agents and halide ions without any pretreatment of samples. The principle of the LIPB is seemingly applicable to the case of lead(I1) or cadmium(I1) electrode, by selecting an appropriate ligand.
ACKNOWLEDGMENT The authors extend their thanks to K. Abe and K. Imamura for their help in some of measurements. LITERATURE CITED (1) G. Nakagawa, H. Wada, and T. Hayakawa, Bull. Chem. SOC.Jpn.. 49, 424 (1975). (2) R. A. Durst, “Ion Selective Electrodes’’, Marl. Bur. Stand. ( U . S . ) . Spec. Pub/. 314, Washington, D.C., 1969 (3) D. J. Crombie, G. J. Moody, and J. D. R. Thomas, Tabnta, 21, 1094 (1974). (4) M. S.Frant and J. W. Ross, J r . , Anal. Chem., 40, 1169 (1968).
Akinori Jyo Takao Hashizume Nobuhiko Ishibashi* Department of Applied Analytical Chemistry Faculty of Engineering 36, Kyushu University Fukuoka 812, Japan
RECEIVED for review June 20, 1977. Accepted July 19, 1977. This work was supported by the Japanese Ministry of Education (Grant No. 011911).
AIDS FOR ANALYTICAL CHEMISTS Integrating Analog-to-Digital Converter Displaying High Signal Fidelity and Noise Immunity J. W. Frazer,’ G. M. Hleftje,” L. R. Layman,* and J. T. Sinnamon Department of Chemistry, Indiana University, Bioomington, Indiana 4740 1
In a conventional analog-to-digital conversion, the analog signal to be converted is sampled at appropriate intervals and the samples are encoded in a digital form. For this procedure to provide an accurate digital representation of the original waveform, the sampled increment should ideally be infinitely narrow in time, Le., approach a delta function. Furthermore, the analog signal must be sampled at intervals that are no more than half the period of the highest frequency component present in the signal, in order to avoid distortion ( I ) . Although these sampling criteria can be practically met by a large number of converters now commercially available, the criteria are not sufficient to ensure that the principal signals of interest will be converted with greatest noise immunity and accuracy. Electrical noise, present on all signals, can often obscure a digitized waveform of interest because of the potentially high frequency response of sampling circuitry. This situation is aggravated by the likelihood that high-frequency noise will be “aliased” ( 2 ) down into the signal frequency region. Fortunately the effect of noise on a digitized waveform can be minimized by judicious use of analog or digital filtering. Present address, Department of Chemistry, Lawrence Livermore Laboratory, Livermore, Calif. 94550. Present address, Pacific Lutheran University, Tacoma, Wash.
98444.
In particular, a hybrid system consisting of an analog filter followed by analog-to-digital conversion provides the advantages of effectiveness and programming simplicity. Unfortunately, simple analog filters can often distort a waveform to be digitized by attenuating high frequency signal components and can also alter somewhat the sampling frequency considerations. To improve this situation, the filtering action could be limited to the time between sampling operations, with each successive sample being independent of all others. Such an operation can be accomplished simply by integrating the sampled waveform for a time equal to the sampling period, after which the integral could be digitized, the integrator reset, and the process repeated. This procedure would provide maximum immunity from noise and greatest signal reconstruction capability while requiring minimal software for operation. Of course, it will be necessary to sample at a rate twice that of the signal frequency component of interest. In this paper, we will describe a new analog-to-digital conversion device which embodies the foregoing characteristics. The system, termed a variable-aperture integrating converter (VAIC) is capable of digitizing waveforms a t a maximum rate of 1000 samples per second so that most common laboratory signals can be properly digitized. T h e sample waveform is integrated between conversions for a ANALYTICAL CHEMISTRY, VOL. 49, NO. li!, OCTOBER 1977
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