The present invention relates to apparatus and methods for controlling the gas flow between two chambers in a mass spectrometer. According to an embodiment one or both of the chambers may comprise a vacuum chamber.
Mass spectrometers often contain different regions or chambers which are at different levels of vacuum. For example, a mass spectrometer may comprise a quadrupole mass filter (“QMF”) which resides in a chamber at a pressure of approx, 1×10−5 mbar and which is followed by a collision cell at a pressure of approx. 1×10−3 to approx. 1×10−2 mbar. This in turn may be followed by a Time of Flight (“TOF”) mass analyser which may be operated at a pressure of <1×10−6 mbar.
Between these different regions there is normally an opening or differential pumping aperture which acts to limit the flow of gas from one chamber to another and through which ions must pass if they are to traverse the mass spectrometer. These openings are generally manufactured to be as thin as possible, typically 0.5 mm to 1.0 mm, so as to minimise loss of ion transmission as ions pass through the orifice. The thicker the opening is the more likely it is that some ions will strike the inner wall of the opening as they pass through the orifice and be lost.
Reducing the size of an opening (i.e. the diameter of a circular hole or the length of a slit) reduces the gas flow through it, which in turn reduces the quantity of vacuum pumping that is required to maintain the desired pressure in the different regions. This is particularly important in situations where there is a large pressure differential between vacuum chambers and hence a large gas flow, or where a small, lightweight or portable instrument is desired. However, reducing the size of an orifice makes it more difficult to focus ions through it. This can lead to ions no longer being able to pass through the orifice which in turn reduces the transmission and hence sensitivity of the mass spectrometer.
It is known to use a valve to reduce the gas flow into the initial vacuum chamber of a mass spectrometer from the atmosphere.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.