High-power helium inductively coupled plasma source for elemental analysis

Abstract

Summary form only given. The argon inductively coupled plasma (ICP) has been successfully used as an ion source for mass spectrometry (ICPMS) and an excitation source for atomic emission spectrometry (ICP-AES). However, the use of argon as a plasma gas has some limitations. Especially, the argon ICP does not exhibit efficient performance for higher ionization energy elements such as nonmetals, rare gases and halogens. To overcome this problem, helium ICP has been developed. Because the ionization energy of helium (24.6 eV) is higher than that of argon (15.8 eV), the use of helium ICP has a potential of enhancing the degree of excitation and ionization for every element. The reported electron number density of 1.5 kW argon and 700 W helium ICP were 3/spl times/10/sup 15/ and 0.7/spl times/10/sup 14/ cm/sup -3/, respectively. To improve the excitation and ionization ability of helium ICP, it is necessary to increase the electron number density. For this purpose, high-power drive is effective method but it was difficult to generate helium plasma with high RF power (>1 kW) because of melting of the quartz glass torch. In this study, we manufactured and tested a gas-cooled helium ICP torch. It has two coaxial quartz glass tubes and the coolant gas flows between the tubes. At the downstream, outside of the torch, the coolant gas surrounds the plasma and prevents the plasma from mixing with atmospheric gas. For the coolant gas, argon, helium and air was tested. With air cooling, it was able to generate a stable helium plasma at RF power of 2 kW. The result of spectroscopic measurements, include the electron number density and the excitation temperature, were presented at the conference.