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 10(13) 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 10(13) amplifiers were assessed. The precision of gain factors was better than 100ppm (2 RSD) in a day, and the long-term reproducibility was better than 300ppm (2 RSD) within 2 to 8months. After a gain was calibrated, the ratio accuracy and precision in the positive mode for Sr-87/Sr-88 of NIST 987 Sr and Nd-143/Nd-144 of La Jolla Nd were 0.710242 +/- 60 (2 SD, n=14) and 0.511842 +/- 10 (2 SD, n=22), respectively, at intensities of Sr-88 0.3V and Nd-142 0.4V, while in the negative mode for Os-187/Os-188 of Merck Os was 0.120229 +/- 34 (2 SD, n=23) at an intensity of (OsO3)-Os-187 0.01mV. 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 similar to 1.5x10(-5) between H and L Faraday cups for the same 10(13) 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.