PERFORMANCE OF A THERMAL IONIZATION MASS-SPECTROMETER WITH A DECELERATION LENS SYSTEM AND POST-DECELERATION DETECTOR SELECTION

An abundance sensitivity of < 10 ppb has been achieved with the redesigned retarding potential quadrupole (RPQ-II) on the Finnigan MAT 262 test instrument at the Finnigan MAT applications laboratory, and on the instrument installed at the Open University. The Open University instrument is equipped with a deflection unit after the RPQ-II to select a Faraday collector or a secondary electron multiplier (SEM) detector in addition to the standard MAT 262-RPQ-II configuration. The Faraday cup allows the evaluation of the transmission stability of the deceleration lens system independent of the effects introduced by the SEM, The yield of the RPQ-SEM has been determined with various methods and it is shown that the RPQ-II transmission error is small relative to the SEM conversion error. Using different calibration techniques, the long-term reproducibility on the Open University instrument for U-234/U-238 (NBS 0002) is < 0.7%, and for Th-230/Th-232 with an isotope ratio in the 6.1 x 10(-6) range is 1.3%. Measurement of the same Th standard with the instrument in the Finnigan application laboratory showed a reproducibility of 0.4% over a period of 1 week (10 samples). For both instruments, the achieved reproducibility is sufficient for the determination of the extreme isotope ratios required for U and Th disequilibrium studies.