The extractant system in Eichrom’s Sr Resin is 1.0M 4,4′(5′)-di-t-butylcyclohexano 18-crown-6 (crown ether) in 1-octanol. (See Figure 1.) A 40% (w/w) loading of this organic solution is loaded onto an inert chromatographic support. The bed density of Sr Resin is approximately 0.35 g/mL.
Figures 2 through 5 show the acid dependency of k’for strontium on Sr Resin, plotted with curves for various other elements. Horwitz, et al. reported these data from studies performed with experimental batches of Sr Resin. Eichrom’s commercial product conforms to established specifications that ensure proper performance of Eichrom issued methods. Please refer to our product specificationsfor details.
The uptake of strontium by Sr Resin increases with increasing nitric acid concentration. At 8M nitric acid, k’ is approximately 90, and it falls to less than 1 at concentrations of nitric acid less than 0.05M. The alkali and alkaline earth metals (Figures 2 and 3) show much lower affinity for the resin than strontium across the concentration range of nitric acid shown. Among the alkaline earth metals, calcium has lowest uptake and it is easy to separate Sr from Ca. Historical methods to accomplish this separation are tedious and require the use of fuming nitric acid, which is hazardous. Barium retention is somewhat high, but it’s uptake peaks at about 3M nitric acid and falls off at higher concentrations. To ensure adequate decontamination of Ba in Sr analysis, load Sr on the resin from 8M nitric acid. With adequate rinsing, Ba will be washed from the column leaving a pure Sr fraction.
The tetravalent actinides show significant retention on the Sr Resin (Figure 4). The addition of oxalic acid as a competitive complexing agent will prevent the retention of actinide elements on the column. The elution behavior of a selection of elements, in addition to those discussed above, is shown in Table 1.
The retention of Pb by Sr Resin is high across a broad range of nitric acid concentrations. (Figure 5.) It is difficult to strip Pb from this resin without complexing agents. A modified version of this resin has been developed for Pb analysis. Eichrom’s Pb Resin has a lower concentration of crown ether (to allow for easier stripping of Pb) and a less water soluble diluent (isodecanol.)
Sr Resin shows resilience to the interference of a number of cations commonly found in environmental and biological samples. Figure 6 shows the effect of various cations on strontium uptake by Sr Resin. Ca and Na cause minimal interference up to 0.5 Min concentration. The macro levels of calcium found in certain sample types, like milk, can dramatically affect Sr recoveries. In a study conducted at BNFL (formerly Magnox Electric, Berkley, UK), a Sr-85 spike was loaded out of 8M HNO3 onto a preconditioned, 2 mL Sr Resin column. The column was rinsed with 8M HNO3 and Sr was eluted with water. The effect of calcium content in the sample on % recovery of Sr-85 is shown in Figure 7. Recovery was quantitative for calcium levels up to 320mg. Above that level, however, chemical yields declined.
A similar study was conducted to measure the effect of increasing Sr content in a sample on the chemical recovery of Sr-89. The results of this study are shown in Figure 8. The chemical recoveries were quantitative when 4 and 8 mg of Sr were added, but dropped off significantly above that level.
The experimentally determined maximum capacity of the resin for Sr is approximately 21mg per 2mL column (Horwitz, et al., HP292.)Historically, in the absence of definitive studies otherwise, we have recommended working capacities for all our resins of 10-20% of maximum capacity. In this case, because of the work at Magnox Electric, we can recommend using a carrier of up to 8mg Sr.
As shown in Figure 6, potassium can cause significant reduction in Sr uptake even at relatively low levels. Additionally, the presence of 40K in the Sr fraction could cause a bias in the 90Sr measurement. For these reasons, in samples with sufficient potassium to be problematic, a selective precipitation of the divalent oxalates is recommended to eliminate the potassium interference.
Figure 9 demonstrates the different elution profiles of Sr Resin in two different particle sizes. The smaller the particle size is, the narrower the elution band. This means better chromatographic performance, but at the cost of slower flow rates, and, consequently, longer analysis times. Use of vacuum assisted flow permits the use of smaller particle size resins (20-50µ and 50-100µ) with faster flow rates than gravity flow would allow. Sr Resin (50-100µ) is available packed in cartridges for use with Eichrom’s Vacuum Box System.
Sr Resin is manufactured in three particle sizes (20-50µ 50-100µ, and 100-150µ) and is sold in bottles or ready to use in prepackaged columns (for gravity flow) and cartridges (for vacuum assisted flow.) Click here for part numbers and descriptions.