The OH ‘bullet’—a promising spatial reference for the inductively coupled plasma

Abstract

In an inductively coupled plasma, the emission pattern of the OH bandhead at 306 nm can be utilized as an effective spatial reference point for analytical atomic and ionic emission measurements. This behaviour was ascertained by a monochromatic imaging spectrometer used to record two-dimensional lateral ICP analyte emission maps. The use of the OH bullet as a reference more accurately corrects for changes in applied r.f. power and matrix interferent concentration than does a fixed spatial reference (e.g., height above the load coil). The OH reference is superior also to references that are based on an atomic emission ‘bullet’ in that the OH reference is always present when an aqueous sample solution is used and because it is spatially related to both atomic and ionic emission patterns. Changes in the OH bullet position occur with increasing r.f. power but track similar movements of analyte atomic and ionic emission. The addition of lithium [an easily ionizable element (EIE)] enhances the intensity of the OH bandhead, but causes only a small vertical shift, again following the trends in atomic and ionic emission behaviour. Despite efforts to correlate the position of a lateral EIE cross-over point to the OH bullet reference, only the direction of motion (and not the magnitude) could be validated. No other spatial reference offers a better prediction of cross-over position. The OH bandhead is a readily observable, universal reference, which requires no additional sample preparation for aqueous sample solutions. A simple one-dimensional scan in the vertical direction is possible with some currently available commercial systems. However, relatively inexpensive two-dimensional tracking of this OH reference bullet would be possible with the simple combination of a narrowband interference filter and a video camera.