Today's state of the art silicon carbide (SiC) growth can produce semi-insulating crystals with a background doping around 5x10(15) atoms/cm(3) or lower. It is essential to have an accurate tool with low enough detection limit to measure low level nitrogen concentration. Current SIMS detection limit of low E15 atoms/cm(3) will provide accurate determination for nitrogen doping level of 5E16 at/cm(3) or higher. In order to determine the lower nitrogen concentration, it is necessary to provide better detection limit and to remove the contribution of background nitrogen properly. The "raster changing" method provides an accurate way to detennine and remove contribution of background nitrogen to the signal because secondary ion intensities and matrix ion intensities can be analyzed at same location of sample by changing primary beam raster size during a profile. We performed experiment using these techniques and were able to determine nitrogen concentration of 3.5E15 atoms/cm(3) in silicon carbide with good precision.