X-Ray Spectrographic Determination of Cesium and Rubidium J. M. AXELROD and ISIDORE ADLER U. S. Geological Survey, Washington 25, D. C.
,An x-ray spectrographic method for the determination of rubidium and cesium was developed, using the internal-standard method and a fourchannel flat-crystal spectrograph. The sensitivity is within 0.1% for cesia and 0.0270 for rubidia; the precision is within 10% of the amount present. Results agree well with those obtained by flame photometry and by radioactivation.
Table I.
Variation of Intensity Ratio with Kilovoltage
Voltage Excitation Potential Ba cs
Kilovoltage 35 40 45 47 48.4 (max.)
0.936 1.07 1.20 1.26 1.29
Intensity a t Cs Ka: Intensity at Ba Ka!
0.972 1.11 1.25 1.31 1.33
1.29 0.820 0.548 0.527 0.524
i.0.003
zt 0.003 =!= 0.003 f 0.003 zt 0.003
At 50 kv. change in intensity ratio per kilovolt is 0.005 or about 1%. Diurnal line voltage (198 to 207 volts) corresponds to about 2 kv. at 50 kv.
I
in the distribution of rare alkalies among samples of mica, feldspar, and beryl, coupled with skepticism about results obtained gravimetrically, led to the investigation of x-ray spectroscopy for the determination of cesia and rubidia. The method was expected to give a n accuracy within 5 to 10% and a sensitivity within 0.01 to O.lyo. NTEREST
METHOD
N o new instrumentation or principles were involved, but the successful application of the method in a wave length range unfavorable for the instrument merits description.
The instrument is a flat-crystal, fourchannel air-path spectrograph with a jO-kv., 30-ma. power supply, a Machlett end window OEG-60T x-ray tube, and a current stabilizer, but no voltage stabilizer ( 1 ) . Lithium fluoride analyzing crystals and krypton-halogen-filled Geiger tubes were used. Strontium (as strontium carbonate) was used as the internal standard for rubidium, and iodine (as silver iodide) for cesium. Determination of rubidium involved no difficulties. Barium (as barium sulfate), tried first as the internal standard for cesium, proved unsatisfactory because of inadequate resolution. As intensities mere low, the use of quartz crystals to get better resolution was impractical. Silver iodide was used as the iodine carrier because soluble halides would react with the aluminum binder in the briquets. Sample briquets contained 0.940 gram of sample, 0.060 gram of internal standard mixture (5 parts of silver iodide to 1 part of strontium carbonate), and 1.00 gram of aluminum-carborundum mixture ( 3 ) . The mixtures were ground for 1 hour in
1280
ANALYTICAL CHEMISTRY
automatic mortar briquetting.
grinders
before
Table I indicates that use of the internal-standard method ( 2 ) minimizes the effect of line-voltage variation to about 2% even in an unfavorable case. This effect was made still smaller by redicate measurement: A series of samples and standards was measured and then the series of measurements was repeated until quadruplicate counts were obtained. The making of four measurements decreased the effect of sudden or erratic voltage changes by
Table II.
averaging, and taking replicates a t different times Dartlv averaged out the effects of a gradha1 voltage drift. Quadruplicate counts were for 10,000 at the internal-standard line. Counting rates were, for 1% cesium, 25 c.p.s. oyer a background of 25 c.p.s. and, for 0.270 rubidium, 125 c.p.9. over a background Of 85 c*P.s. Instead of a background correction measured on each sample, a zero correction determined from a blank was used because mica, beryl, and feldspar are of similar average atomic number, so the
Results of Analyses
cspo
RbnO S-ray
Sample Microcline Lepidolite Beryl Morganite Lepidolite Microcline Lepidolite Beryl Lepidolite Microcline Biotite
Flame RadioX-ray Flame Radio- spectrosphotometry activation spectroscopy photometry activation copy 0.07 0.10 ... 0.00 0.01 ... 0.8 0.4 0.3 0.0 0.1 7.9 0.1 0.0 0.1 0.3 0.4 0.6 6 6 4.4 3.4 0.2 0.0 1.2
0.89 0.38 0.37
