both I and I1 are soluble, the absorption maxima which are observed for I1 are identical in wavelength to those reported by Brooker et al. for I (methanol, A,, = 486 nm; = 513 nm; pyridine, A,, = 605 nm). Accordethanol, A,, ingly, the methyl and hexadecyl substituted homologues together might be used to provide an extended range of solubilities for solvent polarity studies. They might also offer an interesting complementarity for the estimation of composition in certain solvent mixtures. The hexadecyl dye, 11, can be spread (from a solution in 10% ethanolln-hexane) to form a stable insoluble monoM NaOH, layer on a water surface. In such a film on the area per molecule a t a surface pressure of 5 dynes/cm is M HCL, it is 98 Az,the film being 40 A?,while on much more compressible. These values are reasonable in view of the molecular structure and ionization of the film on the acidic subphase ( 7 ) .
1.0-
A Inm) Figure 2. Absorption spectra (1 cm) of methylene chloride solutions of II, 1.48 X loF5 M, with additions of CH3S03H. Concentration of M (c)0.76 X loF5 M; (d)1.78 X lop5 acid: ( a )0; (b)0.51 X M
spondingly, the dye can be used as a sensitive indicator of acidic contaminants in these solvents. The spectra obtained by titrating a methylene chloride solution of the dye with methane sulfonic acid in the same solvent are shown in Figure 2. An interesting feature of these solution color changes is that nearly every color of the visible spectrum can be obtained by suitable mixtures to produce shades intermediate between yellow, red, and blue. Brooker et al. ( , 5 ) observed that solutions of compound I in pyridine exhibited a pronounced color change on addition of small amounts of water (from A,, = 605 nm to A,, = 550 nm with 10% H20 by volume). An analogous effect is observed with compound 11. Deep blue methylene chloride or pyridine solutions become red-violet on addition of 10% methanol. It should also be noted that in solvents in which
ACKNOWLEDGMENT I am indebted to A. G. Tweet, J. R. Ladd, D. G. LeGrand, and D. H. Wilkins for assistance and helpful discussions in the course of this work.
LITERATURE CITED A. I. Kiprianov, Russ. Chem. Rev., 29, 618 (1960). C. Reichardt, Angew. Chem., lnf. Ed. Engl., 4, 29 (1965). R. A . Mackay and E. J. Poziomek, J. Am. Chem. Soc., 94,6107 (1972). M. M. Davis and H. Hetzer, Anal. Chem., 38, 451 (1966). (5) L. G. S. Brooker. G. H. Keyes, and D. W. Hesettine, J. Am. Chem. Soc.. 73, 5350 (1951). (6) A . P. Phillips, J. Org. Chem., 14, 302 (1949). (7) G. L. Gaines, Jr., "Insoluble Monolayers at Liquid-Gas Interfaces". WileyInterscience, New York, 1966, Section 4-VI. (1) (2) (3) (4)
George L. Gaines, Jr. General Electric Corporate Research and Development Schenectady, N.Y. 12301 RECEIVEDfor review September 15, 1975. Accepted October 24, 1975.
Derivative Spectrometry with a Vidicon Detector Sir: Several recent reports have summarized applications and potential advantages of derivative spectra for atomic ( I , 2 ) and molecular (3-5) spectrometry. All of these systems use some kind of mechanical wavelength modulation to generate derivative spectra. In this report, we introduce an approach to derivative spectrometry which substitutes electronic wavelength modulation for the mechanical systems used in derivative spectrometers reported to date. Our system is based upon modifications of the vidicon detector system described earlier (6, 7 ) . Electronic wavelength modulation is achieved by superimposing a low amplitude periodic waveform on the horizontal sweep signal. The frequency of the superimposed waveform is identical to the vertical (parallel to entrance slit) sweep frequency. A lock-in amplifier referenced to the frequency and proper phase of the wavelength modulation waveform generates a signal proportional to the first derivative of the optical spectrum dispersed onto the vidicon detector. The 632.8-nm line from a He-Ne laser was used for a preliminary evaluation of the quantitative performance of the system. Figure 1 (top) represents the laser line isolated
with low resolution holographic grating dispersion optics and Figure 1 (bottom) represents the first derivative of the line generated as described above. T o evaluate the quantitative performance of the system, a linear transmission system was used to attenuate the laser line by known amounts. Figure 2 represents the amplitude of the derivative signal plotted as a function of the relative light intensity. The response is observed to be linear over about two orders of magnitude of light intensity. Numerical data included in Table I provide a measure of the precision of repeat measurements as a function of relative intensity. The uncertainties a t the lowest relative intensities include significant contributions from the quantization error of the analog-to-digital converter. Figure 1 illustrates one important advantage of the derivative mode of operation. Neither the normal nor the derivative spectra shown were background-corrected. Although there is a significant background signal in the normal spectrum due to the leakage current in the vidicon, it is relatively flat and contributes little to the first derivative. In most cases, when emission signals are processed by the ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
451
Figure 2. Plot of vidicon derivative signal vs. relative intensity
Table I. Reproducibility of Vidicon Derivative Signal for Laser Line. Relative i n t e n s i t y
derivative mode, it is not necessary to perform any background correction. I t is observed that there is no significant intercept in Figure 2 even though no background correction was used with the data. A t the time of this writing, we have obtained preliminary data for the flame photometric determination of sodium in aqueous samples. The derivative signal of the 589-nm sodium line is linear from 0.01 to 1 ppm sodium. More complete details of this and other applications to both molecular and atomic species and of the characteristics of the system will be reported in a future paper.
LITERATURE CITED (1) W. Snelleman, T. C. Rains, K. W. Yee, H. 0. Cook, and 0. Menis, Anal. Chern., 42, 394 (1970). (2) R. C. Elser and J. D. Winefordner, Anal. Chern., 44, 698 (1972). (3) R . N. Hager, Jr.. Anal. Chern., 45, 1131A (1973).
452
1.00
0.9
0.794 0.501
1.5 2.1 1.4
0.316 0.100 0.03 2 0.010
Figure 1. Normal (top) and vidicon derivative (bottom) spectra of the 632.8 line from a He-Ne laser. Each spectrum is the sum of four 100 msec scans without background correction. Resolution approximately 0.5 nm
Re1 s t d dev, 5%
4.4 3.9 9.3
(4) T. C. O'Haver and W. M. Parks, Anal. Chem., 46, 1886 (1974). (5) J. W. Strojek, D. Yaies. and T. Kuwana, Anal. Chem., 47, 1050 (1975). (6) 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). (7) M. J. Milano and H. L. Pardue, Anal. Chern., 47, 25 (1975).
Thomas E. Cook Harry L. Pardue* Robert E. Santini Department of Chemistry Purdue University West Lafayette, Ind. 47907
RECEIVEDfor review September 10, 1975. Accepted October 27, 1975. This investigation was supported in part by USPHS Research Grant No. GM 13326-09 from the National Institutes of Health.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
