A new method for calibrating the current gain of 10¹³ 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.