Ultrasonic nebulization with an infrared heated pre-evaporation tube for sample introduction in ICP-OES: application to geological and environmental samples

Abstract

An improved sample introduction system was developed for inductively coupled plasma optical emission spectrometry (ICP-OES). The heater/condenser (HC) of a commercial ultrasonic nebulizer (USN) system was replaced by a pre-evaporation tube (PET) and a sheathing device. The aerosol exiting the USN was heated to 400 °C by infrared (IR) heating. Multivariate optimizations were conducted under robust conditions to find operating conditions improving the detection limit. Under optimum conditions and compared to conventional pneumatic nebulization, 10–25 fold improvement in sensitivity and detection limit was obtained for a range of elements using USN-PET(IR), including elements that are removed in the HC (such as Hg and B) of USN-HC. Compared to USN-PET where the PET is heated with heating tape, and to conventional USN-HC, USN-PET(IR) provided a two-fold improvement in sensitivity and detection limit. The improvement was more significant for ionic emission lines than atomic emission lines. Plasma robustness, measured by the Mg II/Mg I intensity ratio, also increased significantly, enabling the accurate multi-element analysis of geological and environmental samples using an external calibration, with an Ar emission line for internal standardization. Moreover, compared to an identical USN-PET set-up operated with the same aerosol carrier gas flow rate, but with heating tape, USN-PET(IR) significantly reduced the washout time during the analysis of geological materials, making it essentially independent of the element (with the possible exception of Hg) and analyte concentration (at least up to 400 mg L⁻¹), which suggests that it now mostly arises from the dead volume of this sample introduction system.