Physical properties of the probe electrospray ionization (PESI) needle applied to the biological samples

Probe electrospray ionization (PESI) is a recently developed technique that uses a fine solid needle as a probe for sampling biological materials. In this study, we quantified the volume of liquid sample picked up by the solid needle with the tip diameter of ∼700 nm and the apex angle of ∼60°. The amounts of low-viscosity samples (rat urine) loaded on the tip of the needle by a single stroke were 0.35 ± 0.09 pl. Interestingly, the amount of liquid adhered to the tip did not significantly depend on the protein concentration, but viscosity and surface tension of the sample. Under these conditions, we successfully obtained mass spectra for each biological sample.