Ultra-trace determination of 226Ra in thermal waters by high sensitivity quadrupole ICP-mass spectrometry following selective extraction and concentration using radium-specific membrane disks

A method has been developed for the determination of Ra-226 in highly saline (TDS approximate to 6 g 1(-1)) thermal waters, using ultrasonic nebulization and a quadrupole-based instrument equipped with a high sensitivity pumping interface. A very low instrumental limit of detection of 2 fg g(-1) was achieved when the pressure in the expansion chamber was decreased from ca. 2 to 0.85 mbar. Prior to ICP-MS measurements, Ra was preconcentrated and isolated from matrix elements using various separation schemes. The conventional, single-stage cation-exchange chromatographic techniques proved to be limited, for waters with high TDS, by the fact that all the barium and the tail of Sr and Ca peaks were found in the Ra fraction and induced severe ion signal suppression. Two-step procedures involving a clean-up column based either on cation exchange in perchloric acid medium (to remove Ca and reduce Sr) or on specific extraction chromatographic material (to remove Sr and Ba) gave satisfactory results but required time-consuming sample handling. Commercially available radium-specific solid phase extraction disks, originally designed for radioactive counting methods, provided a straightforward alternative, allowing a radium fraction suitable for ICP-MS measurements to be obtained in a single step, using a simple filtration device.