The inductively coupled plasma mass spectrometer (ICP-MS) is the instrument of choice for trace and ultra-trace elemental analysis. However, the ICP-MS suffers from matrix effects. Matrix effects occur when instrument response varies as the composition of the sample matrix is changed. Matrix effects, or non-spectroscopic interferences, limit the accuracy of routine analysis. Identification of the sources of matrix effects provide a basis for reducing or eliminating them. As inaccuracies in the ICP-MS are more severe than those in the ICP atomic emission spectrometer, the problem may be due, at least in part, to the vacuum interface used to couple the plasma source and mass spectrometer. The research herein is a study of matrix effect sources in the first stage of the vacuum interface. This study utilized laser-induced fluorescence of atomic species to identify factors affecting analyte transport through the sampling orifice of the vacuum interface. Several non-idealities in the performance of the interface were found. (1) Operating conditions and sample compositions can negatively affect the efficiency with which ions are extracted through the vacuum interface coupling the plasma source to the mass spectrometer. (2) The sampling cone itself was found to suppress and narrow ion distributions in the plasma. (3) Changes in the degree of ionization were identified in the first vacuum stage. The evidence of recombination and state-changing collisions was observed in the first vacuum stage at lower power settings. Matrix effects occur in the first vacuum stage, the first step of the ion extraction process. This work shows that changes in ion transport through the first vacuum stage of the vacuum interface of an ICP-MS affect the overall performance of the instrument.