Helium Inductively Coupled Plasma Mass Spectrometry (He ICP-MS)

Argon inductively coupled plasma mass spectrometry (Ar ICP-MS) is a multielement technique with excellent detection limits and the added ability to provide isotopic information. However, the use of argon as a plasma gas has some limitations. Major Ar background ions preclude the detection of 40Ca+, 56Fe+, 80Se+, which are the most abundant ions of Ca, Fe and Se. The argon plasma does not generate substantial quantities of elements possessing high ionization energies. To overcome these problems of Ar ICP-MS, He ICP-MS device was developed. Because the ionization energy of He (24.6 eV) is higher than that of Ar (15.8 eV), the use of He ICP as an ion source for MS has potential of enhancing the degree of ionization for every element, in particular for non-metals.

By utilizing flow visualization along with a spark-tracing method, it was found that with a conventional torch for Ar ICP an adequate helium gas vortex flow to sustain a stable plasma could not be formed. To generate stable helium plasma, we designed and manufactured the enhanced vortex flow torch. A stable doughnut-type helium plasma can be produced when using 27〜4 0 MHz RF input power that is larger than 500 W. The helium excitation temperature, OH rotational temperature and electron number density are 4100 K, 2200 K and 0.75 ×10^14 cm-3, respectively at 900 W of RF input powe r. Significant increase in electron number densities but decreases in rotational temperatures were observed, upon increased introduction of carrier gas. The experimental results suggest that the local thermodynamic equilibrium (LTE) is not consistent.

Using this He ICP source, we constructed He ICP-MS device. Sample ions that generated in the helium plasma at atmospheric pressure are introduce to the mass spectrometer in high vacuum (10^-6 Torr) attained by differential pumping and then analyzed. As a result of the effects of sampler-skimmer distance on pressure of 2nd stage and ion signal intensities, it is found that Mach disk position and most suitable position for analysis are both 7 mm. At the mass number larger than 32, no background spectrum was found except for copper that from the sampler. By this system, ion intensities of neon and krypton that have low sensitivity in the Ar ICP-MS result from its high ionization energies could be measured. The measured ion signal intensities of Ne and Kr were 36 and 24 times greater than in the case of Ar ICP-MS respectively.