A secondary discharge intensifies optical emission from a Mach disk extracted from an inductively coupled plasma

Abstract

The axial channel of a conventional argon inductively coupled plasma (ICP) is extracted through a circular orifice into an evacuated quartz chamber. Emission from the Mach disk region is focused on the entrance slit of an echelle spectrometer equipped with two segmented-array charge-coupled-device detectors. The background pressure in the extraction chamber is 1000 Pa (7.5 torr). At this pressure two barrel shocks and Mach disks are visible. Deliberate use of a mild secondary discharge between the plasma and the sampler enhances emission from the Mach disk for a variety of lines from typical analytes (Ca, Sr, Mg and Mn) by factors of 11 to 25. Detection limits are in the range 0.1–2 µg l^–1. Sodium chloride at concentrations up to 10 000 mg l^–1 induces only a modest loss (0–6%) of intensity for ion lines, in contrast to the much more severe matrix effects seen in ICP mass spectrometry.