Understanding the matrix effects in laser plasma spectroscopy is of considerable importance, since these effects actually limit the performance of the method. Thus, a new multifiber imaging spectrometer, coupled with an ICCD detection system, was developed and applied to this task. A special sample holder which enables simultaneous observation of single-shot plasmas through eight optical fibers was constructed. The fibers collect the emission at several locations from single-shot plasma, perpendicularly to its expansion axis. In this way, high-resolution spectra of the analyte and the matrix are simultaneously obtained with spatial and temporal resolutions. Results on investigation of matrix effects in analysis of Pb in natural soils, using this setup, are reported. The matrix effect due to the sand content in the examined samples is presented, and explanations are suggested. It was found that the optimum location in the plasma for spectral analysis depends on analyte concentration in such a way that, at higher concentration ranges, the spectra should be measured closer to the surface. A clear global optimum was observed for analyte signal as a function of location and time, indicating the best experimental conditions. The matrix dependence of that optimum is addressed. The spatial distribution of the internal plasma temperature and the related matrix effects are also discussed.