Desorption/ionization induced by neutral cluster impact as a soft and efficient ionization source for ion trap mass spectrometry of biomolecules

Abstract

Desorption and ionization induced by neutral clusters (DINeC) using SO2 as cluster constituents was previously shown to produce clear and fragmentation-free spectra with low background from samples prepared with standard oligopeptides. Here we demonstrate a more general applicability of this method based on examples from different classes of (bio-)molecules. In order to make better use of the ions generated during the millisecond cluster-pulse, the DINeC source was combined with an ion trap mass spectrometer. Desorption and ionization was induced by neutral SO2 clusters with a mean size of 10(3) to 10(4) molecules seeded in a pulsed He beam. The desorbed ions were accumulated in an ion trap over the whole pulse duration prior to mass spectrometric analysis. Samples were prepared by simply drop casting the respective aqueous solution of biomolecules on Si/SiO2 substrates. Clear and fragmentation-free spectra of oligopeptides were detected in single pulse operation mode. The very soft nature of the desorption process was demonstrated for phosphopeptides. DINeC spectra from bovine serum albumin samples after tryptic digest led to a clear identification of the original sequence using mass fingerprinting analysis. MS(n) capability was illustrated with two types of rhodamine dyes. Desorption and ionization induced by neutral clusters can efficiently be combined with ion trap mass spectrometry since the pulse width and repetition rate of a typical pulsed cluster beam correspond well to the discontinuous accumulation time as well as the spectral rate of the ion trap. Clear mass spectra were obtained with such a setup for a variety of biosamples demonstrating the wider applicability of the DINeC process.