Abstract To obtain suitable geological reference materials for microanalytical purposes, a set of eight natural glasses was prepared by direct fusion of rock chips. Multi-ion counting spark source mass spectrometry (MIC-SSMS) has been applied for trace element analysis of these reference materials. The overall analytical uncertainty of the MIC-SSMS results was determined by considering 14 possible sources of errors. It generally ranges between 27% depending on the element and its concentration. Nearly all MIC-SSMS data agree with the reference values within 010%, indicating that the estimate of the overall analytical uncertainty is reasonable.