The quality control of vaccines is an important step in the release of vaccines to patients. Problems when dealing with vaccines include stability, batch-to-batch consistency and contamination. The invention provides a method of analyzing the composition of a protein-containing sample by electromagnetic spectroscopy, and derivatizing the obtained spectrum to obtain a derivative spectrum of the sample. The derivative spectrum may optionally be compared to a reference spectrum, to assess stability, contamination etc. relative to the reference. The derivative analysis technique allows the separation and discrimination between the spectral contributions of different components, thereby allowing differences in the spectra to be assigned to particular groups of the protein, or particular contaminants etc.