Application of Quadrupole Mass Spectrometer to the ⁴⁰Ar-³⁹Ar Geochronological Study

Abstract

A Quadrupole Mass Spectrometer (QMS) has commonly been used for qualitative analyses of gases in organic chemistry or for monitoring the vacuum conditions in industrial machines. No attempt has been made, however, to apply it to geochronological studies because of its disadvantages such as the difficulty in obtaining precise isotope ratios due to triangular peak shapes and poor reproducibility. On the other hand, there are advantages that a QMS is relatively inexpensive and gives a shorter scanning time for analysis compared with a sector type mass spectrometer. The latter characteristics is useful for ⁴⁰Ar/³⁹Ar geochronological studies, since it gives a lower background in the QMS and the possibility to obtain many more data from one sample in a limited time.
In this study, we have tried to improve a commercial QMS at many parts, such as rf-generator, quadrupole, ionization chamber, source magnet, and so on, in order to meet the requirements to use it for geochronological studies. With the use of the improved QMS equipped with an on-line microcomputer, we could obtain Ar isotope data which are sufficiently precise for the ⁴⁰Ar/³⁹Ar geochronological studies.