Natural Abundance of Chlorine Isotopes SIR: Mass spectrometric determination of isotopic species in labeled compounds requires accurate knowledge of the natural isotopic abundances of all the elements in the compound (14). Correcting the parent peaks in lowvoltage spectra of p-chlorobiphenylordinary and deuterated-for naturally occurring isotopes (4) recently led to consistently negative residues at a mass two units above that of the most abundant species. The discrepancy suggested that the assumed distribution of the chlorine isotopes was in error. This distribution, 75.4% C135 and 24.6y0 Cln, obtained in 1936 ( f a ) , appears in four tabulations commonly referred to for such information (1, 7, 8, 10); two other such tabulations (6, 9) give 75.53% Cla6(3). The 1959 Nuclear D a t a Tables, issued b y the United States Atomic Energy Commission ( I b ) , cite the latter as the best value but also list 75.4% and a third value, 75.8%,
Table 1.
which was reported by Owen and Schaeffer in 1955 ( I S ) . Independent recalculation from the intensities of ten pairs of peaks in the mass spectra (6) of three chlorine compounds gave the abundance shown in Table I. Correction factors for CIa and HZ were taken from the tabulation by McAdams (11). TWOvalues, shown in parentheses, are considered unreliable because of the low intensities of the peaks from which they were derived. The average of the other eight values is 75.80% C135, in agreement with Owens and Schaeffer’s value. The 0.06% average deviation measures solely precision, not accuracy. Mass discrimination in the spectrometer (0,a potential source of error, seems ruled out by the lack of measurable difference among values calculated trori peaks at masses ranging from 47 to 49 to 188 to 190. Improvements in instrumentation can account for some of the discrepancy but do not seem able to account for all of it.
Natural Abundance of Chlorine-35 Calculated from Mass Spectra
Abundance, 70 75.79 c1*+ (75.54”) CCl*+ 75.81 75,76 cc13+ 75.86 75.72 (75.873 o-Dichlorobenzene* CsH&12+ 75.72 75.95 p-Chlorobiphenyl Ci2HpC1+ 75.77 Average 75.80 Average deviation 0.06 Considered b Measured at low ionizing voltage. a Measured with 70-volt electrons. unreliable; not included in average value. Compound Carbon tetrachloride”
Ion cc1+
Masses Used 47-49 70-72 82-84 84-86 117-1 19 119-121 121-123 146-148 148-150 188-190
LITERATURE CITED
(I) American Institute of Physics Handbook, pp. 8-155, hlcGraw-Hill, New York, 1957. (2) Barnard, G. P., “Modern Mass Spectrometry,” pp. 69-85, Inst. of Physics, London, 1953. (3) Boyd, A. W., Brown, F., Lounsbury, M., Can. J . Phys. 33, 35 (1955). (4) Eliel, E. L., Meyerson, S., Welvart, Z., Wilen, S. H., J . Am. Chem. SOC.82, 2936 (1960). (5) General Electric Co., Knolls Atomic Power Laboratory, Schenectady, “Chart of the Nuclides,” 5th ed., 1956. (6) Grubb, H. M., Ehrhardt, C. H., Vander Haar, R. W., Moeller, W. H., 7th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Los Angeles, May 1959. (7) Handbook of Chemistry and Physics, 41st ed., p. 453, Chemical Rubber Publ., Cleveland, Ohio, 1959-60. (8) ‘.‘Isotope Index,” 4th ed., p. 95, Scientific EouiDment co.. IndianaDob. . , Ind., 1959. (9) Lange, N. A.. ed., “Handbook of Chemistry,” 9th ed.,.p. 116, Handbook Publ., Sandusky, O h o , 1956. (10) Mattauch, “Nuclear Physics Tables” (translated by E. P. Gross and S. Bargmann), p. 111, Interscience, New York, 1946. (11) McAdams, D. R., “Isotope Correction Factors for Mass Spectra of Petroleum Fractions,’’ Esso Standard Oil Co., Baton Rouge, La., 1957,.distributed through ASTM Committee E-14 on Mass Spectrometry. (12) Nier, A. O., Hanson, E. E., Phys. Reu. 50. --.. ~.722 (1936). (13) Ow&, H.‘ RT, ’Schaeffer, 0. A., J . Am. Chem. SOC.77,898 (1955). (14) Stevenson, D. P., Wagner, C. D., Ibid.. 72, 5612 (1950). (15) U‘. S.’Atomic Energy Comm., “1959 Nuclear Data Tables,” p. 69, National Academy of Sciences-National Research Council, Washington, 1959. I
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SEYMOUR MEYERSON Research and Development Dept. American Oil Co. Whiting, Ind.
Theory of Alternating Current Polarography SIR: Works of Bauer and coworkers (4-7) have introduced into the recent literature a number of misconceptions with regard to the theory of alternating current polarography. Despite the fact that several general, and rigorous treatments of this theory predate the aforementioned papers, we fear that this erroneous work stands in danger of acceptance because of its currency and, therefore, feel compelled to point out its invalidity. Randles (19) and Ershler (11) considered the phase angle between current and potential at a faradaic impedance and derived an equation relating this angle to the parameters of the
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a
ANALYTICAL CHEMISTRY
electrode process. Other authors have extended and elaborated on this theory (9, 19-16, 18, 24). In all cases the conclusions of these theoretical studies were in agreement with the earliest work. Many of the later studies concerned themselves specifically with the validity of Randles’ simple approach under conditions extant at the dropping mercury electrode. The effects of drop curvature ( f J ) , and change of characteristics of the diffusion layer due to drop growth (16,18) have been examined and found to be negligible under the conditions assumed in the derivation of the theorynamely, alternating potentials of fre-
quencies large compared to drop time, and of amplitudes small compared with
RT/nF. Other authors ( I S , 16, 23) have considered the case in which the rate of charge transfer is so large that the species a t the electrode obey the Nernst equation, the so-called reversible case. The earlier cited works also include this situation as the limiting case when the rate constant for charge transfer approaches infinity. Again there is no disagreement in the conclusions of the studies. Concurrently with these studies Breyer and coworkers have published a number of theoretical treatments relat-
