What is the depth of field for a mass photometry measurement with the OneMP?
Mass photometry makes use of the fact that proteins usually bind well to glass. The ratiometric contrast, is sensitive to relative changes of intensity over a small time window (usually ca. 100 msec). Only the particles, which either bind to the glass surface or unbind from it create the characteristic peaks that are detected and selected for mass measurement. Particles, which diffuse in the solution move faster than the time scale of the averaging window and their scattering contribution averages out and does not contribute to any measurable ratiometric contrast signal. Hence, those events are not detected.
Due to the fact that only particles that bind to or unbind from the glass are visible, the depth variation for binding and unbinding events is given by the height variation of the glass slide, which is on the order of 10 nm.