Influence of time correlation on background correction using multichannel detection in inductively coupled plasma-atomic emission spectrometry

The influence of time correlation between the gross signal and different background signals on the relative standard deviation of the net signal was studied at low concentrations, using simultaneous measurements with multichannel detection ICP-AES. Both theoretical considerations and experiments were investigated to evaluate the competitive effects of R.S.D. improvement with background subtraction and R.S.D. degradation with background averaging. A dual-view Optima 3000 ICP system was used to permit the simultaneous determination of the peak (central pixel), and the various locations of the background over a set of ±1–5 pixels. Both axial and radial viewing modes were used. Two lines were selected: the Ce II 456.236 nm line in the visible region because of a high adjacent background and the Ni II 231.604 nm line in the UV region because of a low adjacent background. Concentrations were selected to be close to five and 10 times the LOD value for each viewing mode in order to have a large contribution of the background. When a time correlation was occurring between the gross signal and the background, a decrease in the R.S.D. of the net signal was observed, resulting in an improvement in the limit of quantitation. In this case, the location of background correction was not critical. This result was only obtained for the Ce II 456.236 nm line with the axial viewing mode. In contrast for this line, a time correlation between two background signals should have led to an increase in the standard deviation of the resulting background, which was actually not observed. Regardless of the background location, no time correlation was observed for the Ni II 231.604 nm line with the axial viewing mode and for both the Ni II 231.604 nm and Ce II 456.236 nm line with the radial viewing mode. The system was probably detector noise limited for the background near the Ni line. No simple explanation was found for the Ce line with radial viewing.