An interface configured to transfer ions produced in approximately atmospheric pressure conditions into a mass spectrometer for mass analysis comprising: a first conduit including an inlet configured to receive a fluid including the ions and an outlet configured to direct the fluid including the ions into the mass spectrometer, the first conduit defining a first flow path extending from the inlet to the outlet; a pump; and a second conduit including an inlet, the second conduit defining a second flow path extending from a location between the inlet and the outlet of the first conduit to an outlet of the second conduit; wherein the pump is configured to divert a portion of the fluid including the ions moving in the first flow path to the second flow path, the interface further comprising a valve located in the second flow path; wherein when the valve is in an open configuration, the valve allows a portion of the fluid including ions to be diverted from the first flow path to the second flow path by the pump; wherein when the valve is in a closed configuration, the valve does not allow a portion of the fluid including ions to be directed from the first flow path to the second flow path; wherein the mass spectrometer includes a chamber into which the fluid including the ions is directed by the outlet, wherein the pressure in said chamber is lower than the pressure at which the ions are produced; and characterised in that the interface is configured to regulate the amount of fluid including ions that is allowed to flow through the first conduit and out of the outlet into the mass spectrometer by operation of the pump and the valve without changing the cross-sectional area of the first flow path. a first conduit including an inlet configured to receive a fluid including the ions and an outlet configured to direct the fluid including the ions into the mass spectrometer, the first conduit defining a first flow path extending from the inlet to the outlet; a pump; and a second conduit including an inlet, the second conduit defining a second flow path extending from a location between the inlet and the outlet of the first conduit to an outlet of the second conduit; wherein the pump is configured to divert a portion of the fluid including the ions moving in the first flow path to the second flow path, the interface further comprising a valve located in the second flow path; wherein when the valve is in an open configuration, the valve allows a portion of the fluid including ions to be diverted from the first flow path to the second flow path by the pump; wherein when the valve is in a closed configuration, the valve does not allow a portion of the fluid including ions to be directed from the first flow path to the second flow path; wherein the mass spectrometer includes a chamber into which the fluid including the ions is directed by the outlet, wherein the pressure in said chamber is lower than the pressure at which the ions are produced; and characterised in that the interface is configured to regulate the amount of fluid including ions that is allowed to flow through the first conduit and out of the outlet into the mass spectrometer by operation of the pump and the valve without changing the cross-sectional area of the first flow path.