Abstract: A method is provided of increasing the resolution in a tandem mass spectrometer having a first quadrupole Q1 to select a parent ion, a second quadrupole Q2 which contains a target gas and forms a collision cell, and a third or analyzing quadrupole Q3 which generates a mass spectrum from daughter ions from Q2. In the method, the target thickness of the target gas in Q2 is held at least at 1.32.times.10.sup.15 cm.sup.-2, preferably at least 3.30.times.10.sup.15 cm.sup.-2, and the DC offset voltage between Q2 and Q3 is kept low or zero. This greatly improves the resolution available in Q3. Q3 is therefore operated with at least unit resolution, and in some cases with resolution of 1/2 or 1/3 amu, making it possible to resolve isotopes of singly, doubly or triply charged daughter ions.