the present invention relates to electrospray ionization at atmospheric pressure coupled with a mass spectrometer , in particular to a special kind of micro - electrospray with liquid flows in the range of 0 . 1 to 100 microliters per minute . the invention describes the use of an off - axis pre - entrance channel in an esi ion source to prevent particulate matter with higher inertia than the gas molecules , such as droplets , from entering the mass spectrometer . the elimination of the particulate matter improves the quantitative precision of an lc / ms bioassay , minimizes the contamination of the mass spectrometer and improves the robustness for high throughput assays .