IMPROVED HPLC METHOD FOR THE SEPARATION OF RB AND SR IN CONNECTION WITH RB-SR DATING

One problem with using conventional ion-exchange chromatography for the separation of Rb and Sr in rock samples for isotopic analyses is that if the content of Rb is high, it is difficult to obtain a Sr fraction which is completely free from Rb. A much better separation of Rb and Sr can be achieved with dynamic ion-exchange high-performance liquid chromatography (HPLC). However, one disadvantage with this technique is that one of the organic compounds in the eluent will also be present in the collected fractions and may cause problems when loading the sample onto the filament. This article describes a method of trapping Sr in an on-line ion-exchange sample loop after HPLC separation, thereby avoiding collection of the organic compound. The HPLC method was compared to a low-pressure ion-exchange procedure using conventional ion-exchange resin, and the results agreed well for samples with a low and moderate Rb/Sr ratio. For samples with a high Rb/Sr ratio the described ion-exchange method could not be used due to poor separation of the two elements. The Rb content of the collected Sr fraction for all samples tested was < 10 pg when the HPLC method was used. The samples tested included one sample of lepidolite and one sample of lepidomelane with Rb/Sr ratios of approximately 300.