Determination of Sr-90 to Calcium Ratio

 Sr-90 in deciduous teeth will be determined under the direction of Hari D. Sharma, Professor Emeritus of Radiochemistry and president of REMS, Inc., Waterloo, Ontario, Canada, using the following procedure.

 A tooth is dried for twelve hours at 110 degrees C and then ground to a fine powder. Approximately 0.1 gram of the powder is weighed in a vial and then digested with 0.5 milliliter (ml) of concentrated nitric acid for a few hours along with solutions containing 5 milligrams (mg) Sr++ and 2 mg Y+++ carriers at about 110 degrees C on a sand bath. The solution is not evaporated to dryness. The digested powder is transferred by rinsing with tritium-free water to a centrifuge tube. Carbonates of Sr, Y, and Ca are precipitated by the addition of a saturated solution of sodium carbonate and then centrifuged. The carbonates are repeatedly washed with a dilute solution of sodium carbonate to remove any coloration from the precipitate. The precipitate is dissolved in hydrochloric acid and the pH is adjusted to the range of 1.5-2 to make a volume of 2 ml, of which 0.1 ml is set aside for the determination of calcium. The remaining 1.9 ml are mixed with 9.1 ml of scintillation cocktail Ultima Gold AB in a special vial for counting. A blank with appropriate amounts of Ca++, Sr++, and Y+++ is prepared for recording the background.

 The activity in the vial with the dissolved tooth is to be counted four times for 100 minutes each time, for a total of 400 minutes, with a low-level scintillation spectrometer. The spectrometer has special features so that the background count-rate in the 400-1000 channels is 2.25 +/- 0.02 counts per minute. The background will be counted for over 5000 minutes so that the error associated with the background measurement will be about one percent. The overall uncertainty or one sigma associated with the measurement of Sr-90 per gram calcium is +/- 0.7 picocuries per gram calcium (pCi/g Ca).

 The efficiency of counting is established by using a calibrated solution of Sr-90/Y-90 obtained from the National Institute of Standards and Technology. The calibrated solution will be diluted in water containing a few milligrams of Sr++ solution, and the count-rate from an aliquot of the solution will be recorded in channel numbers ranging from 400 to 1000 in order to determine the counting efficiency for the beta particles emitted by Sr-90 and Y-90. It will be ensured that the Y-90 is in secular equilibrium with its parent Sr-90 in the solution. In the US, the counting efficiency was found to be 1.67 counts per decay of Sr-90 with 1.9 ml of Sr-90/Y-90 solution with the amounts of 25 mg Ca++, 5 mg of Sr++, 2 mg of Y+++, and 9.1 ml of the scintillation cocktail.

 The calcium content will be determined using a Varian A-A 1475 atomic absorption spectrophotometer by flame spectroscopy at a wave length of 422.7 nanometers using acetylene plus air as fuel.