Eluents, acidic amine phosphate buffers

Gradients of aqueous and organic mobile phases are typically used for LC-MS/MS analysis of drug compounds and metabolites. The most common aqueous solvents are water with 0.1 % formic acid or 0.1 % acetic acid (v/v) or volatile buffers like 5 mM ammonium-acetate or ammonium-formate. Often adjusted to a certain pH value with the corresponding acid or base (the pH of the eluents will have to be optimized with respect to the polarity of the analytes, since ionic species will have very low or no retention on the reversed pahse LC-columns). Other volatile buffers can be used as well. Phosphate buffers should be avoided, since they will cause suppression of the ionization and thus lead to very bad analytical performance (Venn 2000). Reagents like triethyl-amine should also be avoided as mobile phase or as part of mobile phases. They induce ion suppression as well. In terms of the organic solvents, methanol and acetonitrile are very widely used and they are very well suitable for LC-MS. Other solvents can be used as well, as long as they are compatible with the materials used in the LC-MS system. 

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