A mass spectrometer is disclosed comprising a rotatable isolation valve 1 having a curved, spherical, cylindrical or concave portion. At least a portion of an ion guide 2 is positioned so as to extend within a swept volume of the isolation valve 1 enabling the ion guide 2 to be positioned close to a second downstream ion guide 3 and for ions to be transmitted from the first 2 ion guide to the second ion guide 3 with high ion transmission efficiency.