The two main approaches that can be used in metabolomics are untargeted and targeted approaches.
The approach chosen will determine how you design your experiment, prepare your samples, and what analytical techniques you use.
Untargeted (global) approach: This method measures as many metabolites as possible from a range of biological samples without any (intended) bias.
Targeted approach: This method is used when you want to measure sets of metabolites and have a specific biochemical question that you want to answer.
This approach is often used in pharmacokinetic studies of drug metabolism and when looking at the effect of therapeutics or genetic modifications on a specific enzyme.