This work is primarily concerned with the optimization of the slit width (and thus the practical resolving power) of a new type of echelle spectrometer coupled to a 50-MHz ICP operated with a pneumatic nebulizer, as described in Part I of this article series (Spectrochim. Acta 39B, this issue (1984)). The optimization is carried out under “ICP compromise conditions” and uses detection power as criterion. With a “pure water” matrix, the effects of slit width on net line and background signals, signal-to-background ratio (SBR), relative standard deviation (RSD) of background signal and detection limit were evaluated for a set of prominent ICP lines spread over wavelengths between 190 and 500 nm. The detection limits eventually attained under optimum conditions were an order of magnitude better than “standard” values reported in the literature (winge et al., Appl. Spectrosc. 33, 206 (1979)). The optimization was extended to a Ni-Co matrix, the latter serving as an example of samples that emit line-rich spectra. In this context, a detailed analysis was made of the background enhancements associated with the presence of major elements that emit line-rich spectra. Accordingly the effects of slit width on SBR, background RSD and detection limit were differentiated in dependence on whether the background enhancement was due to quasicontinuous background, due to complete coincidence of the analysis line with a line of the matrix, or due to partial line overlap. The quasi-continuous background was attributed to the wings of strong lines of the matrix, as described in Part III ( Spectrochim. Acta 39B, this issue (1984)). It was established that with pure line wing interference the gain in detection power achieved by improving the practical spectral bandwidth from, say, 0.015–0.005 nm is approximately similar to that found for pure water, that is, a factor of 2–3. In the case of partial line overlap, larger improvements can be achieved depending on the physical widths of the lines involved and the wavelength distance between them. The paper includes a description of a computer technique for manipulating experimental spectral scans stored on floppy disks. This technique is used to simulate scans of solutions of matrices spiked with analytes using only the scans for the pure matrix, the pure analyte and the pure solvent. Various examples are detailed in connection with the question whether and to which extent high resolution does improve the accuracy of trace analysis in the case of partial line overlap. Preliminary tests led to the thesis that high resolution is capable of providing higher accuracy than medium resolution at the same ratio of the concentration present to the detection limit. If this thesis can be definitely substantiated in future work, this means that high resolution not only provides for better detection limits, but will also yield higher accuracy at lower concentrations than medium resolution, the latter advantage being associated in particular with samples than emit line-rich spectra.