Dual-field reflection time-of-flight (TOF) mass spectrometry and the Wiley-McLaren conditions

Time-of-Flight (TOF) mass spectrometry distinguishes between masses based on the time taken for ions to travel through a vacuum tube of fixed length: at the same acceleration voltage, lighter ions travel faster whilst heavier ions travel more slowly; by measuring the time taken to reach the detector, the mass-to-charge ratio can be calculated. The advantages of TOF are that, in theory, there is no limit to the mass range and full-spectrum acquisition is rapid, making it suitable for dynamic and transient analysis—which is precisely the basis for Xinceyuan’s online mass spectrometer’s ability to capture transient species such as free radicals within seconds.

‘Dual-field reflection’ refers to the placement of a reflection electrode (reflectron) at the end of the flight tube, causing ions to decelerate upon entering the reflection zone, reverse direction, and then fly back to the detector. In a single-pass TOF, ions with slightly different initial kinetic energies arrive simultaneously, causing the peak to broaden; the reflector employs a compensation mechanism whereby ‘ions with higher kinetic energy penetrate deeper into the reflection zone and travel a longer distance’, thereby ‘levelling out’ the kinetic energy distribution so that ions with the same m/z arrive at the detector almost simultaneously, thus significantly reducing peak width and improving resolution. Dual-field (two segments with different field strengths) reflection further precisely controls this compensation process.

The Wiley-McLaren condition is a classic equation used in the design of reflective TOF mass spectrometers; it provides the optimal combination of parameters for time focusing (i.e. the arrival time of ions with the same m/z being independent of their initial position and energy) under given geometries and voltages in the acceleration and reflection zones. When this condition is met, the TOF is insensitive to the dispersion of ions’ initial positions and energies, and resolution is optimised. The New Ceyuan ProC series and the mass analysers of the two imaging sources are based on a dual-field reflective TOF architecture, with a mass resolution better than 3500 (ProC-1/1C/1S) or 2000 (ProC-2) at m/z = 92, providing the foundation for distinguishing isomers in air and identifying molecules in complex biological matrices.

It is worth emphasising that the resolution of a TOF is not determined by any single component in isolation, but is the result of a precise match between the geometry and voltages of the acceleration and reflection zones. The Wiley-McLaren conditions provide precisely this set of matching relationships, enabling the dual-field reflection architecture to perform consistently – which is why, despite all being termed ‘reflection-type TOF’, the resolution of different instruments can vary by an order of magnitude. For New Ceyuan’s products, this architecture serves both ‘broad-coverage gas analysis’ and ‘high-resolution imaging identification’ requirements.