Helium ICP-AES: effects of induction frequency and forward power on plasma formation and analytical and fundamental properties

Abstract

Helium inductively coupled plasmas (He ICPs), formed at 6.8, 27.1 and 40.7 MHz with a single generator, were investigated at power levels of up to 2.5 kW for atomic emission spectrometry (AES). A 27.1-MHz generator and an impedance matching network were modified for operation at the cited frequencies. Detection limits for Br I (827.2 nm), Cl I (837.5 nm), II (804.3 nm), and S I (921.2), excitation and rotational temperatures, and electron number densities were measured. In addition, spectral features of He ICPs at atmospheric and reduced pressure were compared for the wavelength range between 200 and 600 nm for the 27.1 MHz plasmas. For the reduced-pressure He ICP, the series limit of He ( 3 S- 3 P) was used to estimate electron number densities in the energy addition region. At higher operation frequencies, detection limits were improved by approximately a factor of 3 for the 1.5-kW atmospheric-pressure plasmas (2, 3 and 9 μg ml for Br, Cl and I, respectively), while no significant change in excitation and rotational temperatures and electron number densities were noted. Operation of a 27.1-MHz He ICP at 2.5 as compared to 1.5 kW, increased electron number density and rotational temperature by approximately a factor of 2 and 400 K, respectively. However, no significant change in detection limits or excitation temperature were noted at the higher power. These results were compared to data obtained for a 1.1-kW, 27.1-MHz Ar ICP.