Comparison of mass spectrometric and optical measurements of temperature and electron density in the inductively coupled plasma during mass spectrometric sampling

Abstract

Electron density ( n e) and ionization temperature ( T ion) are measured using atomic emission spectrometry (AES) from the small funnel of gas just outside the sampling orifice of an inductively coupled plasma-mass spectrometer (ICP-MS). Rotational temperature ( T rot) is measured using an OH emission band. T ion is also determined for the same elements (Zn and Cd) by using M + ion signal ratios by MS. For matrix-free solutions, typical values are n e=1.6×10 15 cm −3, T rot=3340 K, T ion (MS)≈ T ion (AES)≈7000 K. This agreement between the T ion values supports other observations that, for atomic analyte ions M + of similar m/ z values in matrix-free solutions, the relative signals in the mass spectrum reflect the corresponding relative abundances in the ICP region being drawn into the sampler. Using either MS or AES, T ion for Cd is 300–400 K higher than that for Zn, which indicates that T ion can vary for different elements in the ICP. Sodium nitrate matrix at levels up to 1000 ppm Na does not cause a measurable change in n e; 2000 ppm Na causes n e to increase to 2.1×10 15 cm −3. Sodium matrix has a large effect on the MS signal levels but does not greatly change the resulting T ion values measured optically.