Although most spectra/peptide matching is done in high throughput search engines, there are times when you'd like to check the quality of competing ambiguous matches against one another, or against a completely arbitrary sequence of interest. Running the whole search engine pipeline is probably overkill, so, in this recipe, we'll look at a convenient method to run a single spectrum against a single peptide sequence and get a plot of congruence between theoretical ion sizes and those present in the spectrum.