THE OH BULLET - A PROMISING SPATIAL REFERENCE FOR THE INDUCTIVELY-COUPLED PLASMA

In an inductively coupled plasma, the emission pattern of the OH bandh ead 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 spat ial reference (e.g., height above the load coil). The OH reference is superior also to references that are based on an atomic emission 'bull et' in that the OH reference is always present when an aqueous sample solution is used and because it is spatially related to both atomic an d ionic emission patterns. Changes in the OH bullet position occur wit h increasing r.f. power but track similar movements of analyte atomic and ionic emission. The addition of lithium [an easily ionizable eleme nt (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 lat eral EIE cross-over point to the OH bullet reference, only the directi on of motion (and not the magnitude) could be validated. No other spat ial reference offers a better prediction of cross-over position. The O H bandhead is a readily observable, universal reference, which require s no additional sample preparation for aqueous sample solutions. A sim ple one-dimensional scan in the vertical direction is possible with so me currently available commercial systems. However, relatively inexpen sive two-dimensional tracking of this OH reference bullet would be pos sible with the simple combination of a narrowband interference filter and a video camera.