The portable X-Ray Fluorescence (pXRF) instrument provides fast, real-time, non-destructive, multi-element geochemical data (i.e. elemental concentrations) on rocks and minerals at a relatively low cost and with minimal sample preparation. This study quantifies the precision and accuracy of measurements collected by the WGNHS pXRF on reference materials to characterize internal instrument error. This study also presents guidelines for identifying outliers in data collected by the pXRF.