Absolute quantitation with PyMS and AnalyzerPro. The samples of foetal calf serum were spiked with a decreasing concentration of a mix of 17 reference compounds. Shown is the quantitation of peak areas by PyMS and AnalyzerPro for four different reference compounds (methionine, trehalose, aspartic acid, and valine), across four different GC-MS experiments, spiked with 6, 12.5, 25, and 50 μl of reference compounds (the dose level of spiked reference compounds is shown as a sample label on the x-axis). To account for metabolites naturally occurring in foetal calf serum, the areas were normalized to the lowest concentration of each compound. A good agreement in expected and observed absolute quantitation was observed for both AnalyzerPro and PyMS.