An electrospray ionisation source for generating charged droplets of liquid entrained within a gas flow within a vacuum chamber, comprising: a liquid insertion capillary for receiving a liquid external to the vacuum chamber and for outputting the received liquid at an output end of the liquid insertion capillary within the vacuum chamber thereby to insert the liquid into the vacuum chamber; a nebuliser part comprising one or more gas flow ducts for receiving a gas external to the vacuum chamber and for outputting the received gas at an output end of the nebuliser part comprising output end(s) of the one or more said gas flow ducts within the vacuum chamber thereby to insert a gas flow into the vacuum chamber; a charger part for charging said droplets of said liquid output by said nebulizer part; wherein the liquid insertion capillary is located within the output end of the nebuliser part so as to position the output end of the liquid insertion capillary within gas flows output by the nebuliser part, in use, to entrain charged droplets of the inserted liquid within flows of the inserted gas; a gas conduit within the vacuum chamber arranged to receive a said gas flow as a free jet in the near-field region of an under-expanded flow and to transform the free jet entrained with said charged droplets into a subsonic laminar gas flow; wherein the electrospray ionisation source is controllable to provide said charged droplets of liquid entrained within a gas in which the gas is at a source pressure, comprising; said vacuum chamber is controllable to achieve a second pressure therein lower than the source pressure, wherein the output end(s) of the gas flow ducts have a first cross sectional area (a) arranged for jetting said gas into the vacuum chamber along a predetermined jetting axis; said gas conduit housed within the vacuum chamber comprising a conduit bore having a second cross sectional area and positioned in register with the output end of the nebuliser part coaxially with the jetting axis for receiving the jet of gas; and a control apparatus operable to control the second pressure for jetting said gas to form a supersonic free jet in the gas conduit bore forming an ion guide with a jet pressure ratio restrained to a value which does not exceed the cubed ratio (A/a) 3 of the second cross sectional area and the first cross sectional area thereby with the gas conduit to restrain expansion of the free jet therein to form therealong said subsonic laminar gas flow. a liquid insertion capillary for receiving a liquid external to the vacuum chamber and for outputting the received liquid at an output end of the liquid insertion capillary within the vacuum chamber thereby to insert the liquid into the vacuum chamber; a nebuliser part comprising one or more gas flow ducts for receiving a gas external to the vacuum chamber and for outputting the received gas at an output end of the nebuliser part comprising output end(s) of the one or more said gas flow ducts within the vacuum chamber thereby to insert a gas flow into the vacuum chamber; a charger part for charging said droplets of said liquid output by said nebulizer part; wherein the liquid insertion capillary is located within the output end of the nebuliser part so as to position the output end of the liquid insertion capillary within gas flows output by the nebuliser part, in use, to entrain charged droplets of the inserted liquid within flows of the inserted gas; a gas conduit within the vacuum chamber arranged to receive a said gas flow as a free jet in the near-field region of an under-expanded flow and to transform the free jet entrained with said charged droplets into a subsonic laminar gas flow; wherein the electrospray ionisation source is controllable to provide said charged droplets of liquid entrained within a gas in which the gas is at a source pressure, comprising; said vacuum chamber is controllable to achieve a second pressure therein lower than the source pressure, wherein the output end(s) of the gas flow ducts have a first cross sectional area (a) arranged for jetting said gas into the vacuum chamber along a predetermined jetting axis; said gas conduit housed within the vacuum chamber comprising a conduit bore having a second cross sectional area and positioned in register with the output end of the nebuliser part coaxially with the jetting axis for receiving the jet of gas; and a control apparatus operable to control the second pressure for jetting said gas to form a supersonic free jet in the gas conduit bore forming an ion guide with a jet pressure ratio restrained to a value which does not exceed the cubed ratio (A/a) 3 of the second cross sectional area and the first cross sectional area thereby with the gas conduit to restrain expansion of the free jet therein to form therealong said subsonic laminar gas flow.