Application of an ion-exchange separation technique and thermal ionization mass spectrometry to 226Ra determination in otoliths for radiometric age determination of long-lived fishes

Abstract

To improve the accuracy and precision of radiometric age determination using 210Pb:226Ra disequilibria in otoliths of fishes, a technique was developed incorporating an ion-exchange procedure followed by isotope-dilution thermal ionization mass spectrometry (TIMS) to determine 226Ra. This technique counts ionized radium atoms directly; therefore, the uncertainty of the technique is superior to conventional radio-decay dependent techniques. Calcium and barium are major components of the otolith matrix that can interfere with TIMS analysis of radium. To remove these interferants, an ion-exchange separation procedure was developed. This procedure was tested by applying it to otolith samples from three fish species in three separate radiometric ageing studies. The resultant separations and TIMS determinations indicate that the procedure efficiently separates radium from calcium and barium. Measured 226Ra activities for each species were similar to previous radiometric ageing studies, with the exception of one sample. When results were compared with traditional 226Ra determination techniques, radon emanation and α-spectrometry, the separation procedure with isotope-dilution TIMS had significant advantages. Samples over three times smaller than attempted in other studies were processed with decreased uncertainty and processing time.