solutions. The differences are probably caused by small variations in the quantities of t-BuC1 which have been added. The high volatility of t-BuC1 makes very accurate additions somewhat difficult. However, the variations in the pH-time response have no influence on the results since the absorbance and the pH are both continuously monitored. There are, of course, many other reactions which yield hydrogen ions as a product but upon selection of a suitable reaction, the following requirements have to be considered: generation of H + with a reasonable rate; good solubility; no complexing properties; no absorption at the wavelength where the formation of CuCy is monitored; and no reaction with other dissolved species such as buffers, the use of which is necessary to reduce the slope of the pH-time curve near pH 7. The last four requirements also hold for the reaction products. Because of the low solubility of t-BuC1 in water, it was applied in a mixture of alcohol and water. Alternatively, 2-chloro-2-methyl propanol-1 may be used in aqueous solution ( 5 ) . However, this compound is still of limited usefulness. The formation of CuCy in dilute solutions has to be monitored in the near UV (e.g., at 310 nm) but there the absorption of the aldehyde (6 = 8 1.mole-1 cm-I at 282 nm) may interfere since the aldehyde is produced in a high concentration. Oxidation of the latter with HzOz is only practicable below a pH of 6 since otherwise a brown copper-hydrogen peroxide compound is formed. However, the reaction of concentrated solutions may be followed at 700 nm with oxalate as a masking agent for copper, but at that wavelength the extinction coefficient of CuCy is rather strongly pH-dependent owing to the formation of CuHCy and CuHzCy (3). ( 5 ) H Nilsson and L. Smith, Z Phys Chem., A, 166, 136 (1933)
A third reaction which we have used is the hydrolysis of ethylchloroformate (6, 7). Because of base catalysis of the reaction (6),it can be used only below pH 8. To ensure a sufficient solubility, the addition of 10% alcohol to the aqueous solution is necessary. Two other methods for the continuous increase of [H+] were investigated, both without success: electrolytic generation of H + failed because of anodic oxidation of the metal complexes, and addition of H + by means of exchange via a cation selective membrane failed because of the inhomogeneity of the reaction mixture. Therefore the search for additional chemical reactions is being continued. In the present stage of the investigations, no evaluation of the attainable accuracy is possible. It is considered to depend in a rather complicated manner on the ratio of the rate constants of the components of the sample solution as well as on the ratio of the concentrations of the components (cf. ref. 8 for an evaluation of errors produced by analyzing a superposition of exponential curves). For mixtures which show well-separated response curves of absorbance us. pH or time, the method is in principle of the same value as ordinary spectrophotometry. In the latter case, the position of the odd points of inflection in the absorbance us. pH curve may be useful for qualitative identification of the metal ions in the sample solution. Further work on the method outlined above is in progress. Received for review July 16, 1973. Accepted January 25, 1974. (6) A . Kivinen, Acta Chem. Scand., 19, 845 (1965)
( 7 ) A. Queen, Can. J. Chem., 45, 1619 (1967). (8) B. G. Willis, W . H. Woodruff, J. R . Frysinger, D. W. Margerum, and H. L. Pardue, Anal. Chem., 42, 1350 (1970).
I CORRESPONDENCE The Boltzmann Distribution and the Detection Limits of Flame Emission Sir: Pickett and Koirtyohann ( I ) have summarized the detection limits for 62 elements by both flame emission and atomic absorption. Of the 62 elements, 24 had lower detection limits with flame emission; 21, with atomic absorption; and 17 had approximately equal detection limits with either method. These authors emphasize that the supposed higher sensitivity of atomic absorption compared to flame emission because of the Boltzmann distribution is erroneous. For example, a t 3000 "K the fraction of Cs atoms in the excited state (852.1-nm line) is only 0.007 (2). For elements with higher excitation energies and/or lower flame temperatures, this fraction can be significantly lower. In atomic absorption the signal is related to the ground state population, and in flame emission the signal is related to the excited state population. These relationships might suggest that atomic absorption is necessarily the technique with the lower detection limits. In dispelling this notion, Pickett and Koirtyohann present an argument originally suggested by Alkemade ( 3 ) .The argument assumes identical atomic absorption and flame ( 1 ) E E. Pickett and S. R. Koirtyohann. Anal. Chem., 4 1 (14), 28A ( 1969) (2) A. Walsh, Specfrochim. Acta, 7, 108 (1955). (3) C Th. J Alkernade. Appl. Opt.. 7 , 1261 (1968)
emission instrumentation and both noiseless flames and hollow cathode lamps. Though not difficult, the argument does introduce some generally unfamiliar terms. A simpler approach, assuming Alkemade's ideal conditions, is attained by considering similarities between molecular fluorescence and flame emission and between UVvisible absorption spectrometry and atomic absorption. The lower detection limits of fluorescence as compared to UV-visible spectrometry are the result of two factors (4, 5 ) . One is that the sensitivity of fluorescence can be improved by increasing the power level of the primary radiation; this is of no concern to us in the following development. The second factor involves the nature of the signal itself. In both flame emission and fluorescence, the signal is proportional to the radiant power emitted by the sample. The radiant power, in turn, is proportional to concentration. In analyzing a very dilute solution, the detector observes a faint light, as opposed to darkness when no (4) H . H. Willard, L. L. Merritt, Jr., and J. A. Dean, "Instrumental Methods of Analysis." 4th ed., D. Van Nostrand Company, Inc. Princeton, N . J . , 1965, pp 377-8. ( 5 ) D A. Skoog and D. M . West, "Principles of Instrumental Analysis," Holt, Rinehart and Winston, Inc., New York, 1971, p 240.
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sample is present. Since a signal per se is always measured, the detection limit is approached only when the signal falls to the noise level of the detector circuit. A simple gain adjustment can increase the output signal of the sample relative to the blank. It might be convenient to think of both fluorescence and flame emission as possessing inherent scale expansion. In atomic absorption and W-visible absorption spectrometry, either the ratio P/Po, the transmittance, or log Po/P, the absorbance, is measured. Po and P are the transmitted power levels of the source in the absence and presence of sample, respectively. For very dilute solutions, the familiar Beer’s law expression, P = Poe-kc, reduces to Po - P = A P = Poke. Here the signal consists of the difference, AP, between two large power levels. The detection limit is set by the minimum detectable difference in power between the incident and transmitted light. With dilute solutions, only a very small portion of the readout device is usable, and no simple gain adjustment can increase the output signal of the sample relative to the blank. There now remains the question of why atomic absorption can equal and even surpass the detection limits of flame emission in certain cases. Probably, the most important factor is the use of intense, “monochromatic” sources such as hollow cathode lamps. Without these sources, dilute solutions would remove only a very small
fraction of the power from the spectral region passed by the usual monochromator, leading to negligible A P values. Second, various methods of atomic absorption scale expansion are available. For example, one could set 100% absorption with a solution of finite concentration rather than with a blocked source. These methods of scale expansion can help to alleviate the limitation discussed in the previous paragraph. It is probably safe to conclude (in the absence of interferences), were it not for these two factors, that atomic absorption, in spite of the favorable Boltzmann distribution, could not possibly approach the detection limits of flame emission.
ACKNOWLEDGMENT The author is grateful to T. L. Nunes of this department for pertinent discussions, and to A. Bregman of the Department of Biology for his helpful comments on the manuscript. James J. Campion Department of Chemistry State University College of New Paltz New Paltz. N.Y. 12561 Received for review September 27, 1973. Accepted March 22, 1974.
Recommendations for Reporting Thermal Analysis DataThermomechanical Techniques Sir: Several techniques which measure different properties of materials are collectively known as thermoanalytical techniques. A number of these have been defined by the International Confederation for Thermal Analysis in 1968 (1). Certain special techniques, generally called thermomechanical analysis, have been developed specifically for polymers but show promise of utility in other fields. These are techniques by which mechanical and chemical properties related to viscoelasticity are measured by the change (with temperature) of deformation of the sample under load. The Committee on Standardization of the International Confederation for Thermal Analysis (ICTA) has the task of studying how and where standardization can further the value of thermoanalytical methods. One very important area is the uniform reporting of data, and recommendations have already been published for differential thermal analysis (DTA) and thermogravimetry (TG) (2) and for evolved gas techniques ( 3 ) .After study of the particular needs for reporting thermomechanical results, the Committee makes the following recommendations to guide authors, editors, and referees in presenting all necessary detail to their readers. Many of these recommendations are applicable to DTA, TG, and other thermoanalytical techniques; others are directed specifically to thermomechanical techniques. To accompany each thermomechanical record, the following information should be reported: (1) R. C. Mackenzie, Talanta, 16, 1227 (1969). (2) H. G.McAdie, Anal. Chem., 39,543 (1967). (3) H. G.McAdie, Anal. Chem., 44, 640 (1972).
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1) Identification of the sample and all materials influencing its behavior by a definitive name, and empirical formula, or equivalent compositional data. 2) A statement of the source of all materials, details of their histories, pre-treatments, and chemical purities, so far as these are known. 3) A clear statement of the temperature environment of the sample. 4) Measurement of the average rate of linear temperature change over the temperature range involving the phenomena of interest. Nonlinear temperature programming should be described in detail. 5) A statement of the method of loading (quasi-static, dynamic), type of deformation (tensile, torsional, bending, etc.) and the dimensions, geometry, and materials of the loading elements. 6) Identification of the abscissa scale in terms of time or of temperature at a specified location. Time or temperature should be plotted to increase from left to right. 7 ) Identification of the ordinate scale in specific terms where possible. For static procedures, increasing expansion, elongation or extension, and torsional displacement should be plotted upwards. Increasing penetration or deformation in flexure should be plotted downwards. For dynamic mechanical procedures, the relative modulus and/ or mechanical loss should be plotted upwards. Deviations from these practices should be clearly marked. 8) Faithful reproduction of all original records. 9) Identification of the sample atmosphere by pressure, composition, and purity; whether the atmosphere is selfgenerated, or dynamic through or over the sample. 10) Identification of the apparatus, including location of the temperature-measuring thermocouple.
