PITFALLS ON THE ROAD TO THE IDEAL TIME-OF-FLIGHT MIRROR - IDEAL TIME-FOCUSING IN THE 2ND STAGE OF TANDEM MASS SPECTROMETERS

The use of time-of-flight (TOF) mass analysers in the second stage of tandem mass spectrometers employing high energy collision-induced fragmentation is considered. The challenge of achieving detection of all fragment ions and unit mass resolution (in the mass range up to, say, 2000 m/z) cannot be successfully met by traditional reflectrons, because of the huge energy spread of the fragment ions. Quadratic-field mirrors providing independence of time of flight from initial energy and angular spreads (i.e. ideal time-focusing) are required. The limits on the mass resolutions of quadratic-field mirrors are shown to be imposed by two types of factors: (i) fundamental limitations, such as uncertainty in the coordinate of ion fragmentation inside the collision cell, metastable decay of ions inside the mirror, and the presence of small field-free and acceleration regions; (ii) imperfections in the field approximation by multi-electrode systems resulting in long-range held sag accompanied by short-range periodic field variations. The greatest challenge in practical applications of quadratic-field mirrors arises from defocusing in the transverse direction, which holds serious implications for the efficiency of transmission. It is shown that appropriate choice of mirror, detector and collision cell dimensions, coupled with correction of electrodes potentials, should be capable of providing better than unit mass-resolution in the mass range up to several thousand m/z. Comparison is made with the orthogonal TOF analysers used for the same purpose. The latter are shown to suffer more stringent limitations on mass resolution at high m/z, compared to quadratic mirrors.