For the analysis of biological and environmental materials by inductively coupled plasma mass spectrometry (ICP-MS), several quantification procedures can be used depending on the precision and accuracy required. Semi-quantitative methods based on the molar response curve were compared with conventional external calibration and standard additions for the analysis of waters and sewage sludges. For the analysis of biological materials, where higher quality data were required, isotope dilution analysis using enriched isotopes was applied. It was observed that the molar sensitivity for different elements in ICP-MS was a simple function of the mass of the isotopes measured after normalization for ionization efficiency which could be fitted to a third-order polynomial equation. Element ionization adjustments for the third-order polynomial, using the Saha equation, allowed the calculation of the plasma ionization temperature and electron density. For the determination of trace metals in waters and sewage sludges, the samples were spiked with different internal standards, ionization corrections were performed and the results obtained agreed with those obtained by external calibration and standard addition within a factor of 2 but, on average, the agreement was within 20%. The determination of molybdenum in biological reference materials was performed by isotope dilution analysis taking into account possible sources of error in the measurements by ICP-MS such as mass discrimination, detector dead time, isobaric interferences and random error propagation.