Anal. Chern. 1985, 57, 2889-2891
valid formula for the theoretical description of the continuum Bremsstrahlung radiation generated in a specimen of finite size, which presents one of the most serious problems in today's EPXMA.
LITERATURE CITED Llfshin, E. Proc., Annu. Conf.-Microbeam
Anal. SOC. 1964, 19,
215-220. Small, J. A.; Heinrich, K. F. J.; Newbury, D. E.; Myklebust, R. L. Scannlng Electron Mlcrosc. 1979, II 807-816. Aden, G. D.; Buseck, P. R. Proc., Annu. Conf.-Microbeam Anal. SOC.1983, 18, 195-201. Fiori, C. E.; Swyt, C. R.; Ellis, J. R. R o c . , Annu. Conf.-Mlcrobeam Anal. SOC. 1982, 17, 57-71. Heinrich, K. F. J. "Electron Beam X d a y Mlcroanalysis"; Van Nostrand Relnhold: New York, 1981. Markowlcz, A. A.; Van Grleken, R. E. Anal. Chem. 1984, 56, I
2049-2051. Armstrong, J. T.; Ph.D. Dlssertatlon, Arizona State University, 1978. Goldsteln, J. I.; Newbury, D. E.; Echlln, P.; Joy, D. C.; Fiorl, Ch.; Llfshin, E. "Scanning Electron Microscopy and X-Ray Microanalysis"; Pie-
2889
num Press: New York and London, 1981. (9) Brown, J. D. "Electron Beam Interaction with Solids"; Proceedings of
the 1st Pfefferkorn Conference; SEM, Inc.: AMF O'Hare, Chicago, IL, 1984;pp 137-144. (10) Statham, P. J. X-Ray Spectrom. 1976, 5 , 154-1138, (11) Brown, J. D.; Packwood, R. H. X-Ray Spectrom. 1982, 11, 187-193. (12) Statham, P. J. Proc ., Anno. Conf.-Microbeam Anal. Soc. 1979, 14,
247-253. (13) Love, G.;Cox, M. G.; Scott, V. D. J . Phys. D 1978, 1 1 , 7-21. (14) Markowicz, A. A,; Van Grleken, R. E. Anal. Chem. 1984, 5 6 , 2798-2801. (15) Dyson, N. A. "X-Rays In Atomic and Nuclear Physics"; Longman Group Ltd., London, 1973.
RECEIVED for review December 18,1984. Resubmitted August 8, 1985. Accepted August 8, 1985. we wish to acknowledge partial financial support from the Belgian National Foundation for Scientific Research which provided a sabbatical and from the Ministry for leave grant to Science Policy, under Contract 80-85/ 10.
Determination of Magnesium in Alumina Ceramics by Atomic Absorption Spectrometry after Separation by Cation Exchange Chromatography Tjaart N. van der Walt*' and Franz W. E. Strelow National Chemical Research Laboratory, CSIR, P.O. Box 395,Pretoria 0001, Republic of South Africa
A method Is presented for the determlnatlon of traces of magneslum In alumlna ceramlcs. After dissolution In an orthophosphoric acid-sulfuric acid mixture the magnesium Is separated from the large excess of aluminum by catlon exchange chromatography, using a 4 % cross-ilnked resin and 0.50 M oxallc acid as eluting agent. Magnesium Is flnally determlned by atomlc absorption spectrometry uslng an acetylene-nltrous oxide flame. By use of Suprapur reagents and beakers made of Teflon, contamlnatlon can be reduced to ca. 2 pg wlth a varlatlon between multlple blank runs of ca. 0.4 pg. About 3 ppm of magnesium In 1-g samples can be determlned wlth approximately the same varlatlon whlie larger amounts of magnesium (200-300 ppm in the alumlna ceramics) show a variation of only k l ppm.
Addition of small amounts of magnesium oxide to alumina below the solubility limit at sintering temperature enhances the density of the alumina ceramic and the normal grain growth rates ( I ) . Above the solubility limit magnesium oxide has a negative influence on densification and acts as a grain growth inhibitor. Accurate information on the concentration of traces of magnesium oxide in alumina ceramics is therefore important and relates to their physical properties. Atomic absorption spectrometry is probably the most attractive and simple method for the determination of small amounts or traces of magnesium. Unfortunately, aluminum causes a serious signal depression with the acetylene-air flame, even when present in low concentrations (2, 3). No interference occurs with up to 50 ppm aluminum present in s o h Present address: Isotope P r o d u c t i o n Centre, B u i l d i n g 3000, AEC, P r i v a t e B a g X 256, P r e t o r i a 0001, Republic o f S o u t h Africa.
tion when the acetylene-nitrous oxide flame is used (3). In order to determine traces of magnesium (from a few to several hundred parts per million) in alumina ceramics, very much higher concentrations of aluminum can be expected to be present in solution, up to ca. 20 000 ppm. In addition there may be even larger amounts of other elements, probably alkali metals, from fusion mixtures, should a fusion step be required. Such an aluminum and solids content will result in interference in the flame. In order to obtain reliable results, especially at low magnesium concentrations, it was therefore decided to combine a recently developed cation exchange separation of magnesium from aluminum (3) with atomic absorption spectrometry, using the acetylene-nitrous oxide flame. There remained two problems to be solved. Firstly, alumina sintered at high temperatures (ca. 1650 "C) is quite difficult to dissolve. A suitable dissolution procedure was therefore developed. Secondly, magnesium contamination introduced during the dissolution step and subsequently in the analysis had to be kept at a minimum, which was still practical for normal laboratory use and also reproducible. Methods for the dissolution of alumina previously employed in this laboratory included fusions with either potassium pyrosulfate, sodium peroxide, or sodium carbonate. Procedures using fusion with sodium hydroxide ( 4 , 5), mixtures of soda and borax (6, 7) or potassium hydrogen sulfate (8), or simply dissolution in hydrofluoric acid (9), phosphoric acid (IO), or in mixtures of acids in a PTFE bomb ( 5 , I I ) have also been described. Mendlina et al. (12) dissolved fusion alumina in a mixture of orthophosphoric acid and sulfuric acid or hydrochloric acid. Although it was felt that the alkaline fusions named above were not very suitable because of their large magnesium blank values, further investigation revealed that they were also not very effecive in dissolving magnesium-doped alumina sintered under pressure at high temperatures. The
0003-2700/85/0357-2889$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table I. Results of Magnesium Determinations in Alumina and Doped Alumina Samples
sample NBS Standard No. 699 1 g sample (first attempt) 1 g sample (second attempt) 1 g sample + 2.0 r g of Mg 1 g sample + 1 0 0 pg of Mg Revnolds RC-HP-DBM i g sample
1 g sample 1 g sample 50.0 pg of Mg 1 g sample 100.0 Fg of Mg
+ +
Alcoa A16-SG 1 g sample
Magnesium found: ppm H3P04-HZS04 K2S204 fusion fusion 2.5 f 0.2 (five runs)
2.2 & 0.2
2.8 & 0.3
1.5 f 0.4
(five runs)
(four runs)
4.9 f 0.2
2.0 & 0.4
(six runs)
(four runs) 1.5 f 0.4 (four runs)
13.0 f 0.1
(four runs) 10 f 5 10.4 f 0.7 60.5 f 0.2
(first batch) 1 g sample (second batch)
(three runs)
(four runs)
only approaches leading to a complete dissolution were fusion with 20 times the amount of potassium pyrosulfate (only for 0.2 g samples) or heating with a mixture of 15.0 g of orthophosphoric acid and 1.5 g of sulfuric acid in a large platinum crucible until most of the sulfuric acid had been evaporated, and a clear high viscous polyphosphoric acid melt had started to form (1 g samples). The second method was preferred because it did not introduce foreign cations and gave considerably lower magnesium contamination per weight of sample. In order to reduce the magnesium blank values further, reagents that had been either specially purified or were of Suprapur quality were used. As solutions containing oxalic acid attack glass slightly at elevated temperatures, Teflon beakers were used when heating such solutions. The details of the method used for the determination of trace amounts of magnesium in magnesium-doped alumina, sintered under pressure at high temperature, will be described and a few typical results are presented.
EXPERIMENTAL SECTION Reagents and Apparatus. Suprapur reagent grade chemicals were used. Water was distilled and then passed through an Elgastat deionizer. The sulfonated polystyrene cation exchange resins used were Bio Rad AG50W-X4 (for the separation of magnesium from aluminum) and Bio Rad AG50W-X8 (for the purification of oxalic acid), with a particle size of 100-200 mesh and 200-400 mesh respectively, both in the hydrogen form. A set of columns were prepared in borosilicate glass tubes (ca. 20 mm bore, 400 mm long) with a B19 female joint at the top and fitted with a fused-in no. 1 porosity glass sinter and a buret tap. at the bottom. The ion exchange columns were filled with a slurry of AG50W-X4 resin until the settled resin reached a mark at 26.0 mL volume (n6.0 g of dry resin). Three sintered alumina samples were supplied by the National Institute for Material Research (NIMR), CSIR, Pretoria. Since no sintered alumina standard was available, the alumina standard reference material no. 699, supplied by the U.S.Department of Commerce, National Bureau of Standards, Washington, DC, was used for comparison. Because the magnesium content of this material is very low and near the detection limit of the method
(3.6 + 10.0)
95 f 4 (three runs)
