A mass spectrometer including an entrance slit, an energy filter, a momentum filter and a detector array, the entrance slit, energy filter and momentum filter being configured to provide molecular analyte ions to the detector array at a mass resolution of about 20,000 or greater. A method for determining the isotopic composition of an analyte in a sample includes converting the analyte to molecular analyte ions, separating the molecular analyte ions using an entrance slit, separating the molecular analyte ions according to their energy levels, separating the molecular analyte ions according to their momenta, detecting two or more of the molecular analyte ions at a mass resolution of about 20,000 or greater to produce molecular analyte ion data; and analyzing the molecular analyte data to determine the isotopic composition of at least a portion of the analyte.