A new method for calibrating the current gain of 10 13 Ω amplifiers in thermal ionization mass spectrometry.

We report a new method for calibrating the current gain of 1013  Ω amplifiers in both positive and negative mode used in thermal ionisation mass spectrometry (TIMS). This method uses any isotopic standard or sample to calibrate the gain factor as long as it can produce a stable current signal. It is simpler and more flexible than that recommended by Thermo-Fisher (the manufacture of the TIMS). In these analyses, the gains of five 1013  Ω amplifiers were assessed. The precision of gain factors was better than 100 ppm (2 RSD) in a day, and the long-term reproducibility was better than 300 ppm (2 RSD) within 2 to 8 months. After a gain was calibrated, the ratio accuracy and precision in the positive mode for 87 Sr/88 Sr of NIST 987 Sr and 143 Nd/144 Nd of La Jolla Nd were 0.710242 ± 60 (2 SD, n = 14) and 0.511842 ± 10 (2 SD, n = 22), respectively, at intensities of 88 Sr 0.3 V and 142 Nd 0.4 V, while in the negative mode for 187 Os/188 Os of Merck Os was 0.120229 ± 34 (2 SD, n = 23) at an intensity of 187 OsO3 0.01 mV. In addition, a difference in the gain factors between the negative mode TIMS (NTIMS) and positive mode TIMS (PTIMS) has been recognized. The values of the gain factor for NTIMS and PTIMS show a deviation of 0.54% on the Triton and 0.31% on the Triton Plus TIMS in this study; therefore, gain calibration should be carried out on both NTIMS and PTIMS. Moreover, a bias of ~ 1.5 × 10-5 between H and L Faraday cups for the same 1013  Ω amplifier has been detected, hinting that the efficiency of different Faraday cups may affect the gain factors, which can be eliminated through the new method of "cross-calibration" discribed in this study.