ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Research Facility a t Columbia, Mo. The nominal flux in this position is stated t o 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 t h e 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 t h e peak being smooth and low. E n d 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 S t a r t 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 t h e 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.
307
T h e molecular weight of Fe2*03 for 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 a t o m s Fe*, containing 1.2044 X X 0.8248 = 9.93389 X a t o m s 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 a t 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 t o 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 t h e use of lithium metaborate fusions as a general technique for t h e dissolution of materials containing silicates, t h e technique was applied t o a variety of standard analyzed samples. T h e resulting solutions were analyzed for silicon, aluminum, calcium, and several other elements by atomic absorption sDectrometrv. Silicon a n d aluminum have been shown to cause serious interferences in t h e 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 t h e effect of matrix components on t h e 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 t o overcome this problem, Magill and Svehla ( 1 ) have suggested t h e use of a releasing agent in calcium determinations by standard additions with the air-acetylene flame. They also recommended t h e use of t h e 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 t o investigate the evident shortcomings of t h e 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 (65.870CaO, 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 (
