A method of determining the presence of isotopes in a sample, comprising (i) providing a sample beam containing the sample; (ii) causing the sample beam to travel back and forth along a single longitudinal axis; (iii) ionising the sample beam with one or more lasers, wherein ionising the sample beam comprises operating the one or more lasers at a plurality of frequencies; (iv) determining time of flight data of a first part of the sample beam relative to a second part of the sample beam; (v) determining an ionisation time using the time of flight data; (vi) obtaining data relating to the frequency of the one or more lasers at the ionisation time; and (vii) determining the presence of isotopes in the sample using the data relating to the frequency of the one or more lasers at the ionisation time.