A mass spectrometer or ion mobility spectrometer comprising: an ion source; a first vacuum chamber; a first vacuum pump for maintaining the pressure within the first vacuum chamber lower than the pressure outside of the chamber; an inlet orifice arranged between the ion source and the vacuum chamber for allowing ions to pass from the ion source into the first vacuum chamber; a second vacuum pump and a second vacuum chamber, wherein the second vacuum pump is arranged and configured for pumping gas from the second vacuum chamber to an outlet region of the second vacuum pump, detecting means for determining when the first vacuum pump causes the pressure within the first vacuum chamber to fall below a predetermined threshold; wherein the detecting means comprises means for monitoring the value of at least one parameter that varies with the variation of pressure within the first vacuum chamber, and means for determining when said parameter reaches a threshold value that is indicative of the pressure in the first vacuum chamber being at said predetermined threshold; and signal means for indicating that said inlet orifice is at least partially blocked when said detecting means determines that the pressure within the first vacuum chamber has fallen below the predetermined threshold by determining that the parameter has reached said threshold value; characterised in that: the first vacuum pump is arranged and configured for pumping gas from the outlet region of the second vacuum pump to an outlet of the first vacuum pump so as to reduce the pressure of the outlet region of the second vacuum pump; and in that: one of said at least one parameters is the amount of power that the second vacuum pump is using or the amount of current that the second vacuum pump is drawing, and wherein the spectrometer is configured to signal that the inlet orifice is at least partially blocked when said power or current decreases to reach said threshold value; or the spectrometer further comprises a temperature sensor for monitoring the temperature of part of the second vacuum pump or a region in proximity to the second vacuum pump, wherein one of said at least one parameters is said temperature and the signal means is configured to signal that the inlet orifice is at least partially blocked when the temperature measured by the temperature sensor decreases to reach said threshold value. an ion source; a first vacuum chamber; a first vacuum pump for maintaining the pressure within the first vacuum chamber lower than the pressure outside of the chamber; an inlet orifice arranged between the ion source and the vacuum chamber for allowing ions to pass from the ion source into the first vacuum chamber; a second vacuum pump and a second vacuum chamber, wherein the second vacuum pump is arranged and configured for pumping gas from the second vacuum chamber to an outlet region of the second vacuum pump, detecting means for determining when the first vacuum pump causes the pressure within the first vacuum chamber to fall below a predetermined threshold; wherein the detecting means comprises means for monitoring the value of at least one parameter that varies with the variation of pressure within the first vacuum chamber, and means for determining when said parameter reaches a threshold value that is indicative of the pressure in the first vacuum chamber being at said predetermined threshold; and signal means for indicating that said inlet orifice is at least partially blocked when said detecting means determines that the pressure within the first vacuum chamber has fallen below the predetermined threshold by determining that the parameter has reached said threshold value; characterised in that: the first vacuum pump is arranged and configured for pumping gas from the outlet region of the second vacuum pump to an outlet of the first vacuum pump so as to reduce the pressure of the outlet region of the second vacuum pump; the first vacuum pump is arranged and configured for pumping gas from the outlet region of the second vacuum pump to an outlet of the first vacuum pump so as to reduce the pressure of the outlet region of the second vacuum pump; one of said at least one parameters is the amount of power that the second vacuum pump is using or the amount of current that the second vacuum pump is drawing, and wherein the spectrometer is configured to signal that the inlet orifice is at least partially blocked when said power or current decreases to reach said threshold value; or the spectrometer further comprises a temperature sensor for monitoring the temperature of part of the second vacuum pump or a region in proximity to the second vacuum pump, wherein one of said at least one parameters is said temperature and the signal means is configured to signal that the inlet orifice is at least partially blocked when the temperature measured by the temperature sensor decreases to reach said threshold value.