analysis (6, 7, 8) is based on Equation 1 where the semiintegral is defined
(18) The basis of this method is to let m ( t ) reach a limiting value m ( r ) when the potential a t some time 7 has caused C ( 0 , r ) = 0. A number of characteristics and advantages of this technique have been discussed in ref 7. Our general treatment is identical in formalism to the semiintegral technique and so methods based on Equation 9 will have identical characteristics to methods based on Equation 1. When C(O,T) = 0 then Equations 16, 1, and 17 become, respectively,
i(7) = ( n F A D C " ) / l
(19)
(6) K. B. Oldham, Anal. Chem., 44, 196 (1972). (7) M. Grenness and K. B. Oldham, Anal. Chem., 44,1121 (1972). (8) K . B. Oldham, Anal. Chem., 45, 39 (1973).
q ( T ) = nFVC"
(21)
These equations show that electrochemical techniques completely analogous to the semiintegral electroanalysis are based on current and charge measurement. Equation 19 is simply the limiting current for a Nernst diffusion layer in a stirred system and Equation 21 is simply Faraday's law for exhaustive electrolysis in a thin layer cell. These three methods are thus based on the same general formalism and perhaps other analogous techniques could be developed based on the general Equation 9 if suitable data treatment techniques are developed.
Received for review February 22, 1973. Accepted April 16, 1973. Ronald L. Birke Department of Chemistry University of South Florida Tampa, Fla. 33620
Comments on "Electron Probe Microdetermination of Carbon in Ferrous Alloys" Sir: A recent paper ( I ) thoroughly discusses the difficulties of light element analysis with the electron probe microanalyzer (EPA) and presents a technique for analysis of carbon in ferrous alloys. We have also been performing carbon analyses of ferrous alloys for the past several years. The majority of our alloy standards are identical to those used by Duerr and Ogilvie and we use the jet decontamination device first described by Moll and Bruno ( 2 ) . This correspondence presents the results of our analysis of these standards and some of our observations regarding carbon analysis of ferrous alloys with the EPA. The iron-nickel alloys we studied are listed in Table 1. Alloys 9 through 15 are the same as numbers 32-1 through 34-3 listed in Table I of the Duerr and Ogilvie paper. Repeated chemical analysis of specimen number 32-3 (alloy 11) gave 0.73 wt % carbon rather than 0.89% as reported by Duerr and Ogilvie. All the above alloys were made by vacuum induction melting 18-kg master heats. Each master heat was then split four ways so that four 4.5-kg, 5-cm x 5-cm ingots were successively cast from each master heat after the appropriate carbon additions had been made to it. Each ingot was heated to 1175 "C for 1 hour and then hot rolled to 1.3-cm bar stock. Thirty-centimeter long sections taken from the center of each bar, were machined to l-cm diameter rods. The rods were sealed under vacuum in Vycor (Corning) tubes and given a 7-day homogenization anneal a t 980 "C followed by a water quench. Specimens of each of the alloys were mounted in Bakelite (Union Carbide) and polished and etched to reveal their structure. After metallographic examination they were polished to a Linde B (Union Carbide) finish to prepare them for microprobe analysis. These alloys are available for use as standards from Dr. A. Chodos, Chairman, (1j J. S. Duerr and R. E. Ogilvie. Anal. Chem., 44, 2361-7 (1972). (2) S. H . Moll and G. W.Bruno, Second National Conference on Electron Microprobe Analysis. Boston, Mass., 1967.
Standards Committee, EPASA, California Institute of Technology, Pasadena, Calif. 91109. A primary standard for light element analysis should give a strong, stable, and reproducible peak intensity and have a negligible wavelength shift with respect to the alloy standards and unknowns. Attempts to use a diamond as a primary standard were unsuccessful. Carbon intensities from the diamond showed a continual drop in intensity with time. Such behavior is thought to be due to degradation of the diamond by the electron beam. A piece of hot-pressed chromium carbide (Cr3C2) was purchased from A. D. MacKay for use as a primary carbon standard. This standard gave constant and reproducible carbon intensities for a period of a t least 1 hour. High peak intensities and peak/background ratios were obtainable with it. When operating a t 10 kV and a specimen current of 0.1 FA, the Cr&2 standard gave a peak intensity of 1150 cps and a peak/background ratio of 43 for the C K, peak. Wavelength scans showed that no measurable shift of the carbon Kcu peak position occurred between the chromium carbide and alloy standards. Each alloy standard was analyzed in an Applied Research Laboratories' EMX-SM EPA operated a t 10 kV and 0.1 FA. A lead stearate deconate analyzing crystal with a d-spacing of 50.15 A was used. Counting was done in the integrated beam current mode and pulse height analysis was used. Use of the pulse height analyzer completely eliminated interference by the third order nickel L a line and fifth order iron KP line. A gas jet decontamination device using dry air as the cleaning gas was used during all analyses. The jet was made from a hypodermic needle with an inside diameter of 0.25 mm and was fastened to the final electron lens. It was placed about 150 p to one side of the beam and approximately 1 mm above the specimen. A Matheson Model 150 needle valve was used to control the air flow. The air flow through the jet was sufficient to prevent car-
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1.0
,
1
Table I. Chemical Composition of Iron-Nickel Alloys Chemical composition, Wt Oh Alloy No.
1
Nominal composition Fe-0% Ni
(Pure iron) 2 3 4 5
6 7
F e - 1 0 % Ni
a 9 10 11 12 13 14 15
Carbon
Nickel
0.02
,..
0.12 0.30
...
0.62
...
1.01 1.29 0.13 0.20 0.31
, . .
0.63
Fe-20% Ni
0.73 1.04 0.27
0.56 0.87
0.8
0.6
... 0.4
...
0.2
10.9 10.8 9.8 10.0
0
0.4
0.2
10.0
0.6
0.8
1.0
1.2
I .4
WEIGHT PERCENT CARBON IN STEEL
9.9
Figure 1. Results of
20.7 20.8 20.7
bon contamination when the operating pressure of the microprobe was kept a t 3 x 1 0 - 4 Torr using the valve. When placing the beam on a new area, several minutes were allowed to elapse before starting the analysis. This assured the removal of any residual carbon film that might exist on the specimen. Five 20-second counts were taken on the carbide standard and each of the alloy standards. Background measurements on the carbide and alloy standards were taken at 41.3 and 48.5 A. Previous spectrometer scans had shown these wavelengths to be free of interference. Background measurements taken from the 0.63% C, 10.0% Ni, balance Fe standard (alloy No. 10, Table I) were used to make the background correction for all the alloy standards with 5-20 second counts taken on each area. Each analysis was the average of two separate areas of each alloy. A correction for drift was made by averaging the carbon counts taken from the carbide standard before and after the analysis. Yickel intensity ratios were also obtained from each alloy standard. The results of the analysis of the alloy standards are shown in Figure 1, which is a plot of carbon intensity ratio us. per cent carbon in the steel. The carbon intensity ratio is the carbon K a line intensity from a given alloy standard less its background, divided by the intensity from the chromium carbide standard less its background. Each point is the average of five separate analyses. The standard deviation of these analyses is generally between 2 and 3% relative. Note that at a given carbon level, the addition of nickel to the steel decreases the carbon intensity ratio. Also note that the line through the points in Figure 3 in reference 1 does not pass through the origin. This is surprising in that the carbon intensity ratio should be infinitely small for a pure iron specimen. This behavior could be due either to erroneously low background measurements or to a very thin film of carbon remaining under the beam spot despite the air jet. Careful intensity measurements and wavelength scans on the carbide and pure iron standards do, in fact, indicate that a very thin carbon film is the
0-0
carbon analysis of alloy standards
Fe-0% Ni, U--W
Fe-10% NI: A-.-A
Fe-20% Ni
cause. The reproducibility of the carbon intensity ratios over a period of many analyses shows that the carbon film always attains the same thickness. The growth of the carbon film due to the breakdown of hydrocarbons by the electron beam is apparently just balanced by its removal rate by the air jet. Therefore, the film does not affect the accuracy of the analysis but it will raise the limit of detectability of carbon in steel. Contrary to our observations, the Duerr and Ogilvie calibration curve for the alloy standards did pass through the origin. This curve passes through the origin, despite the formation of a carbon film of equilibrium thickness, since the background was measured on pure iron with the spectrometer set on the C K a peak. The decrease in carbon intensity ratios with increase in the nickel content of the steel (Figure 1) must be due to an increase in either the absorption or atomic number corrections for carbon caused by nickel. Mass absorption coefficients for C K a X-rays in Fe and Ni were either taken directly from Shiraiwa et al. ( 3 ) or extrapolated from their coefficients using the Henke et al. ( 4 ) value 'for Kr. This gave p / p (C K a ) values of 14,300 for Fe and 16,400 for Ni. The higher p / p (C K a ) for nickel favors the absorption correction as the cause of the lower carbon intensity ratios as the nickel concentration increases. It is doubtful whether the atomic number correction for carbon would change appreciably over this nickel concentration range to cause a noticeable change in the carbon intensity ratios. G . L. Fisher G. D. Farningham The International Nickel Co., Inc. Paul D. Merica Research Laboratory Sterling Forest, Suffern, N.Y. 10901 Received for review April 16, 1973. Accepted July 2, 1973. (3) T. Shiraiwa, N. Fujino, and J. Murayama, "Proceedings of the Sixth International Conference on X-ray Optics and Microanalysis," G. Shinoda, K. Kohra, and T. ichnokawa, Ed., University of Tokyo Press. Tokyo. Japan, 1972, pp 213-18. (4) 6.L. Henke. R. L. Elgin, R. E. Lent, and R. 6. Ledingham. Norelco Rep., 14, 112-34 (1967).
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