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A N A L Y T I C A L CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
contribute to a change in calibration curve slope at -100 pg/cm2 ct-quartz was based in part upon the observed changes in Ill,, (reference 2, Table 111). Having established that some degree of similar preferred orientation is present at all thicknesses of the ct-quartz used in these studies, the apparent change in slope then is not attributable to this cause. It is most likely an artifact present in wt/cm' data which extends beyond the range of our normal samples. Although theoretical efficiencies are lower for liquid suspension filtration than for air, the observed data (reference 2. Table V) do not in fact show this, and indeed indicate the better standards were prepared from liquid suspension filtration. While purist notions may lead one to adhere to air filtration for the preparation of standards, practical considerations lead one to select less time consuming procedures w h e w shoii>nt o be e x p e r i r n e n t a l l ~e~q u i i , a l ~ n (2). t In light of the magnitude of effect that particle orientation has upon the calibration curves for standards prepared from liquid filtered suspensions, use of calibration curves obtained in this manner n i t h a properly sized standard contribute less error to the analyses than does sample orientation (particle shape). over which the analyst has no control.
T h e final point of the paper (2) is the necessity for iriter-laboratory verification of methods developed to analyze for a common problem of this complexity. In a given laboratory, unknown variables have a habit of becoming constants. ACKNOWLEDGMENT The author thanks Ludo K. Frevel for reviewing the manuscript. LITERATURE CITED (1) S. Altree-Williams, Anal. Chem., preceding comment in this issue (2) J. W. Edmonds, W. W. Henslee. and R. E. Guerra. Ana/. Chem., 49, 2196-2203 (1977). (3) A. J. C. Wilson, J . Sci. Instrum.. 27. 321-325 (1950)
J. W. Edmonds Analytical Laboratories, Bldg. 574 Dow Chemical Co. Midland, Michigan 48640
RECEIVED for review October 6, 1978. Accepted November 6, 1978.
AIDS FOR ANALYTICAL CHEMISTS Determination of the Natural Abundance of Iron-58 by Neutron Activation Analysis P. F. Schmidt" Bell Telephone Laboratories, Incorporated, Allentown, Pennsylvania
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J. E. Riley, Jr. Bell Telephone Laborafories, Incorporated, Murray Hill, New Jersey 07974
T h e natural abundance of iron-58 is very low and has been reported by various investigators in the range from 0.29 to 0.33%. With F e 2 0 3samples highly enriched in 5RFebeing available from Oak Ridge National Laboratory (ORNL), a straightforward determination of the natural abundance of 58Fe is possible by co-irradiation of the enriched and natural material, and comparison of t h e 59Fephotopeak intensities. This experiment was performed with an Fe,O, sample 82.48% enriched in 5sFe, and a natural iron foil, the iron content of which had been determined to he 99.63% by mass spectroscopic examination. The relevant data and an outline of the calculations are given in the Appendix. T h e natural abundance of jsFe was found to he 0.283% h 0.010%, in good agreement with a recent recommendation by Holden ( 1 )based on older mass spectroscopic data ( 2 ) . The presently accepted thermal cross section for 58Fe is 1.14 b; in establishing this cross section, the natural abundance of 58Fewas assumed to be 0.33% (3). Since the natural abundance enters the calculation of necessity, the thermal cross section of 5sFe should be higher by the ratio 0.33 X 1.14 = 1.33 b 0.283 APPENDIX Description of the Measurements a n d Calculations to D e t e r m i n e t h e N a t u r a l A b u n d a n c e of 58Fe f r o m t h e 0003-2700/79/035 1-0306$01 .OO/O
Co-Irradiation of a 99.63% P u r e N a t u r a l I r o n Foil w i t h a n Fe203S a m p l e 82.48% E n r i c h e d i n 5sFe None of the trace impurities in the iron foil have large resonance integrals which could produce a measurable selfshielding effect. The purities of both the iron foil and of the Fe203sample were established by mass spectrometry, for the iron sample by the Analytical Chemistry Department a t Murray Hill, for the Fe,O, sample by ORNL (no impurities were detected in the latter case).
Materials Csed. Fe2*03,2.46 mg, 82.48% enriched in 58Fe, obtained from ORNL. The isotopic analysis of this material is given as follows: at.%
' 4Fe "Fe "Fo
"Fe
precision
0.46 15.57
t 0.05 to.10
1.48 82.48
*0.06
io.10
The symbol Fe* is used hereafter to refer to iron of the above isotopic composition.
21.93 mg F e foil, 99.63% pure = 21.85 mg pure iron Both the Fe2*03and the Fe foil were co-irradiated for 60 s in the "front row position" of the University of Missouri C 1979 American Chemical Society
A N A L Y T I C A L CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Research Facility at Columbia, Mo. The nominal flux in this position is stated to be 8 X 1013 n cm-2 s-l. After irradiation, the samples were each dissolved in 5.00 cm3 reagent grade HCl and were then counted in the same (rigidly fixed) position a t a distance of about 4.6 cm from the front surface of a co-axial Ge-Li detector; dead times were in the 3-570 range. T h e areas of the 59Fe photopeaks a t 1099 keV were evaluated by hand, the background on both sides of the peak being smooth and low. End of 60-s irradiation: 6/28/77 a t 1400 E D T Start of 10000-s acquisition on the 2.46 mg Fe2*O3: 7/8/77 at 1030 E D T total net count: 134945 Start of 65000-s acquisition on the 21.93 mg Fe: 7/7/77 a t 0815 E D T total net count: 39542 All decay factors were calculated according to the equation:
where t d = time elapsed between the starts of acquisition on the two samples, t,, = duration of acquisition, and h = decay constant ( = o.69315/T1/2). C a l c u l a t i o n of the N a t u r a l A b u n d a n c e of j8Fe. The molecular weight of the Fe,*O,, and the atomic weight of the Fe* of the given sample can be calculated from the atomic masses. We used the data by Mattauch, Thiele, and Wapstra in "1964 Atomic Mass Table", Nucl. Phys., 67, 1-31 (1965) in our calculation.
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The molecular weight of Fe2*03for our sample was calculated as 163.160. The molecular weight of Fe* for our sample was calculated as 57.58322. 163.160 g Fe2*03= 6.022 X loz3molecules Fe2*03 = 1.2044 X atoms Fe*, containing 1.2044 X X 0.8248 = 9.93389 X atoms of j8Fe 2.46 mg Fe2*03= 1.497755 X l O I 9 atoms "Fe 21.850 mg pure Fe = 2.356 X 10" atoms iron (MW = 55.847) 21.850 mg pure Fe, start counting 7/7/77 at 0815 for 65000 s uncorrected count rate: 0.608 338 counts/s count rate corrected for decay while counting: 0.611 869 7 counts/s 2.46 mg Fe2*03,start counting 7/8/77 a t 1030 for 10000 s uncorrected count rate: 13.494 5 counts/s count rate corrected for decay while counting: 13.506 53 counts/s count rate corrected for 26.25-h decay (from 7/7 a t 0815 to 718 at 1030): 13.7360 counts/s 2.356 X 10'' X 0'7 abundance jsFe: 0.6118697 countsls 1.497755 X 1019 atoms 5sFe: 13.736 counts/s Natural Abundance of 58Fe: 0.2832%. LITERATURE CITED (1) N. E. Holden, BNL-NCS 50605, March 1977. (2) G. E. Valley and H. H. Anderson, J . Am. Chem. SOC.,69, 1971 (1947). (3) N. E. Holden, Brwkhaven National Laboratory, personal communication.
RECEIVED for review May 17,1978. Accepted October 16,1978.
Errors in the Atomic Absorption Determination of Calcium by the Standard Addition Method J. W. Hosking," K. R. Oliver, and B. T. Sturman Department of Chemistry, Western Australian Institute of Technology, Bentley, W.A. 6 102, Australia
As part of an investigation into the use of lithium metaborate fusions as a general technique for the dissolution of materials containing silicates, the technique was applied to a variety of standard analyzed samples. The resulting solutions were analyzed for silicon, aluminum, calcium, and several other elements by atomic absorption sDectrometrv. Silicon and aluminum have been shown to cause serious interferences in the determination of calcium in the airacetylene flame ( I ) . T h e method of standard additions is often recommended as a means of correcting for interferences in atomic absorption spectrometry (2-4). T h e variation in the effect of matrix components on the absorbance a t different analyte concentrations has, in general, been neglected although it has been mentioned by several authors ( 1 , 2 ,5-8). In a n attempt to overcome this problem, Magill and Svehla ( 1 ) have suggested the use of a releasing agent in calcium determinations by standard additions with the air-acetylene flame. They also recommended the use of the nitrous oxide-acetylene flame, with potassium as an ionization buffer, for the determination of calcium in the presence of other ions. When the method of standard additions was used to determine calcium in a standard analyzed sample of portland cement after lithium metaborate fusion, the results differed significantly from the 0003-2700/79/035 1-0307$01.OO/O
known calcium content of the sample. The fact that the results were inaccurate led us to evaluate various techniques for the atomic absorption determination of calcium in the presence of dissolved silicon and aluminum, and to investigate the evident shortcomings of the method of standard additions. EXPERIMENTAL Samples. The analyzed cement samples were portland cement 24b (62.9% CaO, 20.870 SiOz,6.22% A1,OJ and British Chemical Standard No. 372 (65.870 CaO, 21.3% SiOz.5.35% AlzO,). Both of these samples were from the Bureau of Analysed Samples Ltd, Middlesbrough, England. Synthetic samples with the same calcium, aluminum, and silicon concentration as portland cement 24b were prepared from calcium carbonate, alumina, and silica. Reagents. The high purity silica was British Chemical Standard No. 313 (99.6% SiO,, 0.02% CaO). Lithium metaborate was prepared from lithium carbonate (
