sample liquid is supplied to the ion source of an electrospray ionization mass spectrometer by capillarity induced flow through a wick element comprising a permeable porous aggregate of fibers or particles of material that is wetted by the sample liquid . this method of liquid introduction eliminates the need for pumps of pressurized gas to drive the flow . it also makes possible the convenient extraction of a representative sample from a stream of liquid flowing at any rate , no matter how large .