Laser desorption ionization of red phosphorus clusters and their use for mass calibration in time‐of‐flight mass spectrometry

Phosphorus clusters P(n) (n = 1-89) are easily formed from red phosphorus by laser desorption ionization (LDI) and they cover a range of up to approx. m/z 3000 in both positive and negative ion mode. The clusters are singly charged and the spectra are simple because phosphorus is monoisotopic. The mass spectra can be measured with an acceptable resolution and intensity. The use of positively charged P(n) clusters for calibration in mass spectrometry was examined and it was demonstrated that in external calibration a standard deviation of +/-0.04 m/z units can be achieved even when using a common commercial matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) instrument. When used as internal standards the P(n) clusters react with some analytes - C(60) and C(70) fullerenes and cucurbituril8, for example. It was also found that red phosphorus is a suitable MALDI matrix for peptides and proteins, illustrated by the examples of a Calmix mixture of bradykinin, angiotensin, renin, adrenocorticotropic hormone ACTH fragment 18-359 and insulin, and of insulin alone.