Ultratrace Analyses by Optical Emission Spectroscopy: The Stray Light Problem

Abstract

When atomic emission spectroscopy is employed for the determination of trace elements at concentrations near the detection limit, the spectral background will normally be a large fraction of the total measured signal. Precise background corrections are therefore required if accurate analyses are to be achieved. Experimental evidence is presented to document the fact that stray light may produce substantial shifts in the background signal measured with some spectrometers when the total composition of the sample changes. The data presented have been obtained with the inductively coupled plasma as an excitation source; similar effects can be expected with other excitation sources as well. Examples of various forms of stray light originating from grating defects (ghosts, near and far scatter) and defects in the design of spectrometers are presented. Various methods for the reduction, elimination or correction of stray light effects are also discussed.