The determination of protein, oligonucleotide and peptide molecular weights by ion-spray mass spectrometry

The mass spectra of several compounds with molecular weights in the 2500–20 000 Da range were obtained with a quadrupole mass spectrometer equipped with an atmospheric pressure ion source. Average molecular weight determinations of mellitin (2846.4 Da), a synthetic oligonucleotide (4262.8 Da), myoglobin (16 950.4 Da) and on the subunits of β‐lactoglobulin (18 277.1 Da) requiring as little as 1 pmol of material were achieved with accuracies and precisions of ± 1 Da. An ion‐spray interface was used to produce ions via the ion evaporation process, producing mass spectra containing a series of multiply‐charged molecular species. A simple method for calculating the molecular weight of unknown compounds from the spectra containing multiply‐charged ions is described.