A method for operating a mass spectrometer (MS) includes establishing a pressure differential across a membrane wherein an upstream pressure in a calibrant gas inlet line on an upstream side of the membrane is greater than a downstream pressure in an ion source on a downstream side of the membrane; flowing a calibrant gas from the calibrant gas inlet line, through a nano-scale orifice of the membrane, and into the ion source; and maintaining the upstream pressure at a constant value. The calibrant may be flowed at a low flow rate. An MS system includes a membrane interposed between a calibrant gas introduction system and a mass spectrometer. The membrane may include an orifice of nano-scale diameter.