The order of “mers” in the binary copolymer is shown in the figure above. Thus, correlation of analytical data with the copolymerization kinetics processes permits certain conclusions regarding
the structural arrangement of the acrylic copolymers containing carboxylic groups. RECEIVED for review January 23, 1967. Accepted August 25,1967.
Reproducibility of Mass Spectrographic Analyses of Steel and Aluminum Using Ion Beam Chopper P. G . T. Vossen Scientific Apparatus Division, Consultant Laboratory, GEC-AEI(Electronics) Ltd., Manchester, England
THE ANALYSIS of metals has been carried out for many years using the spark source mass spectrograph. Although this method has given excellent results for many materials under examination, the reproducibility of analyses of nonhomogeneous materials could have been better. Halliday, Swift, and Wolstenholme (1) investigated a procedure whereby the source conditions were maintained constant and relative sensitivity factors were established with a sample of the same matrix examined under the same source conditions. It was reported that for homogeneous metallic and nonmetallic materials the reproducibility on repeat analyses was normally of the order of 2O-30%, and in very isolated cases as good as 10%. Aulinger, Reerink, and Riepe ( 2 ) have shown that when applying a rotating electrode technique instead of the normal fixed electrodes, a relative standard deviation of 10 to 15% could be obtained. Despite attempts to improve the reproducibility of analysis of metals by maintaining instrumental and source parameters constant and by rotating the electrodes, no drastic improvement was made. Apart from the variable parameters mentioned above, the question arose whether a sufficient amount of the sample under examination was consumed during the analysis to obtain the best possible reproducibility for all impurities concerned. Jackson, Vossen, and Whitehead (3) have carried out reproducibility work on titanium dioxide using the ion beam chopper and quote a relative standard deviation on these analyses of 5z,which is an improvement of a factor of 2 to 3. Hence, it was decided to apply this technique to the analysis of steel and aluminum. EXPERIMENTAL
Apparatus. All spectra were taken with a solid spark source mass spectrograph Type MS702 of Associated Electrical Industries, Ltd.
(1) J. S. Halliday, P. Swift, and W. A. Wolstenholme, paper presented at the Conference on Mass Spectrometry,Paris, September 1964. (2) F. Aulinger, W. Reerink, and W. Riepe, “Methodische Untersuchungen zur Analyse mit dem Massenspektrometer,” Forschungsberichte des Landes Nordrhein-Westfalen, Westduetscherverlag, Koln und Opladen, Nr. 1793 (1967). (3) P. F. S. Jackson, P. G . T. Vossen, and J. Whitehead, ANAL. CHEM., 39,1737 (1967). 632
ANALYTICAL CHEMISTRY
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Figure 1. Variations of per cent standard deviation with chopping frequency A = Cr, 0 = Sn, X = Pb, 0 = Bi The photoplate density curves were recorded with a JoyceLoebl double-beam microdensitometer MK I11 B. The computer used for plotting the photoplate characteristic curves and calculating the exposure ratios of standard and impurities for a prefixed density, as well as the standard deviations, was the Associated Electrical Industries 1010 computer using Mercury CHLF 3 language. Only points on the linear part of the photoplate density curve were included as data to construct a straight line using the least squares method. The photographic plates utilized for this work were Ilford Type 4 2 , thin glass. Sample Preparation. The l / d n c h diameter by ’/*-inch length sample electrodes were degreased in laboratory grade ether and rinsed thoroughly in deionized water after machining to size. The steel sample electrodes were etched for 2 minutes in dilute acid (40% HCl, 6 0 x HzO), the aluminum sample electrodes for 1 minute in dilute acid (25% HCl, 7 5 x HzO). All the electrodes were thoroughly rinsed in deionized water. Procedure. The sample electrodes were mounted in the source sample clamps, and positioned as close to the first slit as possible for sparking and 24 repeat plates were recorded on each of the two samples under examination, plates one to eight in one day, and plates nine to 24 during 8 days on a two plates per day basis, For every two plates, a new pair of sample electrodes of the same bulk material was analyzed. The selected exposure ranges were for the steel sample from 0.0003 to 10 ncoulombs and for the aluminum sample from 0.0003 to 50 ncoulombs. Spark parameters were: voltage,
Table I. Reproducibility of Radiofrequency Spark Source Analysis of Steel SS53, Per Cent Standard Deviation Plates 9-10. 11-12, etc., Plate 9, 11, 13-23, Plate 10, 12, 14-24, Plates 1-8, 1 pair 8 plates, 1 daily 8 plates in 1 day 8 plates, 1 daily daily Paris paper Standard Standard Standard Standard Standard Element ICr 52Cr 66Fe 5ZCr 66Fe 5*Cr 56Fe 57Fe 18.2 26.5 17.5 20.2 9.9 17.9 10.4 ... 48Ti 12.8 30 ... 10.4 ... 15.0 52Cr 15.4 15.0 15.6 17.2 15.7 9.1 9.6 55Mn ... 14.1 13.3 6.2 12.5 8.6 8.9 40 8.1 60Ni 16.7 23.5 25.9 21.6 29.1 35 16 23.2 76As .
Q8Mo l%n Av std dev
32 30 33.4
12.5 15.8 13.2
12.8 21.3 15.1
30 kV, spark pulse repetition rate, 100 pulsesjsecond; spark pulse length, 100microseconds. Ion beam chopping frequency was 1000-1,000,000 pulsesisecond, and pulse length was 5 microseconds. The sample electrode position was maintained constant throughout the analysis. RESULTS AND DISCUSSION
Table I presents the relative standard deviation obtained for the steel SS53 of the Bureau of Analyzed Standards. It can be seen that the improvement is a factor of 2 to 3 as compared with the results in the first column (analyses on eight plates) not using the ion beam chopper technique. The relative standard deviations for aluminum standard 1791 are given in Table 11. In this case, the overall precision is somewhat better than for steel. From Table I and 11, it can be seen that there is no significant difference between the standard deviation on results for eight plates in 1 day and the standard deviation on the mean result of two plates for 8 subsequent days. An investigation was carried out into the critical settings of the ion beam chopper pulse frequency, particularly with reference to the impurity elements lead, bismuth, and tin in an aluminum matrix. These elements are known to be somewhat insoluble in aluminum and consequently they should represent a problem of nonhomogeneity. Figure 1 shows plots of relative standard deviation obtained cs. ion beam chopper pulse frequency and approximate total sparking time (obtained empirically). Figure 1 is based on five repeat exposures of 5 ncoulombs for different frequencies. The material used is A1 1791. The peaks obtained were densitometer recordings at two different cross sections of each line on the photoplate. The peak heights were then averaged. A photoplate density curve (from the previous 24 AI plates) was assumed, to obtain the corresponding charge (in nanocoulombs) at a density point on the linear portion of the curve. The relative standard deviation as in Figure 1 was then calculated on the five exposure values. For T / t ' / 2= 1.O (equal to 200 kHz), no ion beam chopping takes place; therefore the relative standard deviation is of an expected high value. By decreasing the chopping frequency to 60 kHz (T/t'/' = 1.Q the total sparking time becomes uneconomical for an exposure of 5 ncoulombs, and it would further require extremely high stability of the electronics.
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14.4 17.8 17.2
29.2 23.2 20.8
20.3 22.6 16
15.1 15.4 13.7
25.0 30.8 21.1
Table 11. Reproducibility of Radiofrequency Spark Source Analysis of Aluminum Standard 1791, Per Cent Standard Deviation 52CrInternal standard Plates Plates 10, 9-10, Plates Plate 9,
Element 48Ti 5
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56Mn 66Fe
Av std dev
1-8, 8 plates in 1 day 4.1 6.2 7.4 10.3 9.6 11.9 12.0 7.1 16.6 19.7 20.2 11.4
11, 13-23, 8 plates, 1 daily 6.2 9.4 4.5 8.7 9.6 12.0 7.1 7.2 19.3 16.1 15.1 10.5
12, 14-24, 11-12,etc., 8 plates, 1 pair 1 daily daily 7.7 5.6 10.8 6.1 7.1 4.0 7.7 6.1 9 . .. 1 -
9.0 8.6 9.8 20.1 18.2 17.4 11.4
7. . 7.
6.3 6.9 4.1 17.7 4.8 17.2 . 7.9
From the graphs it is evident that chromium is much more homogeneously distributed in aluminum than lead, bismuth, and tin, and hence a much lesser amount of sample has to be consumed to produce a good precision. The results tend to support the contention that the ion beam chopper decreases in some measure the effect of sample nonhomogeneity by consuming more sample and in addition it is evident that this technique overcomes errors due to change in excitation conditions, sample position, and gap length by allowing the use of constant source parameters. ACKNOWLEDGMENT
The author expresses his thanks to R. Brown for his encouragement and critical appraisal of the work, and to H. Somerford for his assistance in the statistical assessment of results. The author would like to express his gratitude to Aluminium Laboratories, Ltd., for providing the A1 std. 1791. RECEIVED for review July 5, 1967. Accepted November 28, 1967.
VOL. 40, NO. 3, MARCH 1968
633
