An elemental mass spectrometer uses a mass filter to select ions from ions received from an ion source and transmit the selected ions. A reaction or collision cell receives the transmitted ions and reacts or collides these with a gas to provide product ions thereby. A mass analyzer receives the product ions, analyzes them and provides at least one output based on detection of the analyzed ions. The elemental mass spectrometer is operated to provide a first output from the mass analyzer measuring ions within a first analysis range of mass-to-charge ratios including a desired mass-to-charge ratio, M, to provide a second output from the mass analyzer measuring ions within a second analysis range of mass-to-charge ratios including a mass-to-charge ratio at least 0.95 atomic mass units lower than the desired mass-to-charge ratio, (M−i), i≥0.95 and to correct the first output on the basis of the second output.