Polyalanine - a practical polypeptide mass calibration standard for matrix-assisted laser desorption/ionization mass spectrometry and tandem mass spectrometry in positive and negative mode.

Mass calibration in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is currently obtained using mixtures of individual peptides. This procedure has several drawbacks, including laborious preparation, limited shelf life and unequal calibration mass spacing. Polyalanine, a simple to prepare polydisperse molar mass standard, alleviates these problems. Polyalanine is prepared by the typical protocols for biological sample analytics. Polyalanine is the first polymeric standard providing abundant signals in both, positive and negative polarity mode with the typical matrices DHB and HCCA. Facile MS as well as MS/MS calibration is thus enabled for the first time in both polarity modes by employing this polydisperse standard.