A HIGHLY SENSITIVE HPLC METHOD FOR THE DETERMINATION OF TH AND U CONCENTRATIONS IN GEOLOGICAL SAMPLES

A new high-performance liquid chromatographic (HPLC) procedure has bee n developed and applied to the determination of Th and U concentration s in geological materials. Rock samples are first acid-digested in scr ew-cap Savillex(R) beakers with HF-HClO4 and HCl. Th and U are then is olated from the geological matrix by a quantitative two-step ion-excha nge procedure. Determination of the elements by HPLC then follows. Th and U are separated from one another and residual matrix elements as c omplexes with alpha-hydroxyisobutyric acid (HIBA) on a reversed-phase column using gradient elution in similar to 10 min. Detection of Th an d U is accomplished by visible spectrophotometry after an on-line post column reaction with Arsenate III. Detection limits are similar to 30 and similar to 100 pg for Th and U (or similar to 0.75 and similar to 2.5 ppb using 100-mg rock samples), respectively. The HPLC technique h as been applied to the analysis of nine international and three well-c haracterized in-house standard rocks to evaluate analytical performanc e. Th and U concentrations in the analyzed samples spanned the range o f < 10 ppb to > 10 ppm. The HPLC data show excellent agreement with va lues obtained by spark source-, inductively coupled plasma- and isotop e dilution thermal ionization mass spectrometry. The 1 sigma external precision of the method is better than +/- 2% and +/- 5% for the deter mination Th and U concentrations higher than 50 and 5 ppb, respectivel y.