DOE Methods Compendium
RP501(a), Rev.1
Determination of Total Radioactive Strontium in High-Level Samples using Extraction Chromatography
1.0 Scope and Application
This procedure is used to determine the total radioactive strontium (89Sr and 90Sr in combination or individually) in aqueous samples of both process and nuclear waste. It is intended for low-volume (3.7 Bq/L (100 pCi/L)}. To perform this measurement, a separation and a purification of the Sr are necessary to remove radionuclides (specifically 90Y) that interfere with the subsequent beta counting. Typically, large amounts of inactive substances are also present and must be removed. This method applies extraction chromatography using an Eichrom Sr-specific column (Sr•Spec®) that absorbs Sr at high-acid concentrations, but elutes it under low-acid conditions. The Sr•Spec® can be obtained as a pre-packed column consisting of an extractant, bis-t-butyl-cis-dicyclohexano-18-crown-6, dissolved in 1-octanol and supported on an inert substrate (Amberlite). The column’s high selectivity for Sr is not affected by the large concentrations of other metal ions, such as Ca, Al, and Fe, which may be present in samples. Samples prepared by this method may be analized by either gas-flow proportional counting or liquid-scintillation counting for radioactive Sr.
2.0 Summary of Method
Two methods of analyses may be followed depending on whether 89Sr and 90Sr are to be determined together (total radioactive strontium) or separately. The sample preparation (Sr purification) is identical for both methods; however, the counting requirements and calculations differ.
2.1 Total Radioactive Strontium
2.1.1 The pre-packed column is pre-conditioned by passing a high-acid solution through it.
2.1.2 The sample is “spiked” with a known amount of 85Sr {electron capture (EC) gamma emitter} for determining percent recovery.
2.1.3 A sample aliquot, the Sr content of which will not exceed the working capacity of the column, is loaded on the column in a small amount of high-acid solution followed by a larger volume “wash” of high-acid solution. The eluent is discarded.
2.1.4 The Sr is stripped from the column with dilute acid. The eluent can then be prepared for beta counting to determine total radioactive Sr activity and for gamma spectroscopy to measure 85Sr activity.
2.1.5 Strontium recovery is based upon the measurement of 85Sr by an independent method of detection, gamma spectrometry. The 85Sr activity is subtracted from the total beta activity to determine the radioactive Sr in the sample.
2.2 Strontium-89 and Strontium-90 Determined Individually
2.2.1 A pre-packed column is pre-conditioned by passing a high-acid solution through it.
2.2.2 A sample aliquot that will not exceed the working capacity of the column is loaded on the column in a small amount of high-acid solution followed by a larger volume “wash” of high-acid solution. The eluent is discarded.
2.2.3 A second aliquot is spiked with a known amount of 85Sr (EC, gamma emitter) and loaded on a separate prepared column using the same method as discussed in 2.2.2. This is used to determine recovery of Sr while accounting for matrix effects from the sample. {Note: 85Sr or 90Sr (beta emitter) may be used to determine Sr recovery; however, 85Sr allows for this determination to be made using gamma spectrometry, an independent method of detection.}
2.2.4 The Sr is stripped from the columns with dilute acid. The unspiked sample eluent can then be prepared for beta counting by liquid scintillation or gas-flow proportional counting to determine 90Sr activity and 89Sr activity (if present). The spiked sample eluent is counted by gamma spectrometry for 85Sr recovery determination.
2.2.5 Total radioactive Sr may be determined after one beta count and after applying the recovery factor. However, if 89Sr is to be determined individually, a second count will need to be obtained after a 2- to 4-week waiting period to determine 90Y ingrowth.
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