Imaging with biomolecular ions generated by massive cluster impact in a time-of-flight secondary ion microscope

Imaging mass spectrometry can allow the correlation of molecular identification and spatial organization in biological samples. A useful technique would rapidly generate, from untreated samples, images of lipids, peptides and small proteins with intracellular spatial resolution. We describe the use of massive, highly charged glycerol cluster impact to produce images using ionized, intact biomolecules, with few-micrometer lateral resolution and few-minute acquisition times.

An electrospray primary ion source generating massive clusters of electrolyte-doped glycerol was coupled with a microscope-imaging time-of-flight secondary ion mass spectrometer. A continuous stream of primary cluster ions ejected secondary ions from the sample surface. The secondary ion stream was pulsed in the secondary column and either time-of-flight mass spectra or mass-selected ion images were projected onto a position-sensitive ion detector. The image acquisition times were a few minutes.

Ionized intact molecules of some common lipids, peptides and small proteins have been detected. A lateral image resolution of ~3 µm has been measured for a bradykinin ion image.

Massive cluster impact (MCI) combined with microscope-mode ion imaging allows rapid imaging using ionized intact biomolecules, with a lateral resolution acceptable for applications with biological samples.