Major factors affecting the isotopic measurement of chlorine based on the CS2Cl+ ion by thermal ionization mass spectrometry

The factors that affect isotopic measurement of chlorine based on Cs2Cl+ ion by thermal ionization mass spectrometry were studied. Graphite is essential for the emission of Cs2Cl+ ion from CsCl. No Cs2Cl+ ions are detected in the absence of graphite on the filament. The emission of Cs2Cl+ ion and the measured Cl-37/Cl-35 ratio were affected by different varieties of graphite, the pH value of the sample solution, and impurities in the solution. High-precision isotopic measurement of chlorine based on Cs2Cl+ ion is achieved only by using graphite with a perfect crystal structure. The measured Cl-37/Cl-35 ratios were much higher, and even the emission of Cs2Cl+ ion becomes impossible when pH values of the sample solution were higher than 6, corresponding to the presence of excess Cs. The measured Cl-37/Cl-35 ratios were higher when SO42- and NO3- anions were present. The results show that the measured Cl-37/Cl-35 ratios are weakly related to the amount of chlorine on filament in a range from 1 to 500 mug.