Method for determining the average degree of substitution of o-vanillin derivatized porcine somatotropin

Electrospray mass spectral observation directly on a sample of a derivatized protein, such as porcine somatotropin (pST), affords a method for evaluating the degree of substitution of this protein. Derivatization of the lysine residues and the terminal amino residue here by formation of a Schiff base with a small aromatic aldehyde (in this case, o-vanillin) affords stabilization of the protein so that it may be used in a controlled release veterinary pharmaceutical formulation. This method permits direct observation of substitutions, optimization of manufacturing procedures for producing a commercial product, and permits quality evaluation of material.