The use of internal calibrants (the so-called lock mass approach) provides much greater accuracy in mass spectrometry based proteomics. However, the polydimethylcyclosiloxane (PCM) peaks commonly used for this purpose are quite unreliable, leading to missing calibrant peaks in spectra and correspondingly lower mass measurement accuracy. Therefore, we here introduce a universally applicable and robust internal calibrant, the tripeptide Asn(3). We show that Asn(3) is a substantial improvement over PCM both in terms of consistent detection and resulting mass measurement accuracy. Asn(3) is also very easy to adopt in the lab, as it requires only minor adjustments to the analytical setup.