Peak profiles of ions originating from solid surfaces in time‐of‐flight mass spectrometers incorporating reflectrons

Abstract

The detected ion peak shape results from the convolution of the initial ion distributions in space, time and energy, transformed from the source to the detector by the time-of-flight (TOF) analyzer.' While the initial distributions are dependent on the action of the ionizing factor and on the conditions in the ion source, the transformation is determined by the mass spectrometer ion optical properties.Detected peak shapes were calculated for time-offlight mass spectrometers incorporating reflectrons, for rectangular time pulses of the ionizing factor and for various initial energy distributions of ions generated from solid plane surfaces. CALCULATION OF THE DETECTED ION CURRENT DISTRIBUTIONSIt is assumed that ionization occurs on a plane surface, or very close to it, during some time interval when the intensity of the primary ionizing current or photon radiation remains unchanged. For ionic processes such as plasma desorption, where ions are individually counted, the errors in the start signal are perceived as an initial time spread.The main peak structure is described by first-order packet components, aberrations being involved in higher-order details.' As reflectrons correct for the first-or even second-order energy spread, the remaining first-order packet components are those due to the time of generation t and to the longitudinal emission velocity component u (similar to half that inducing the 'turn around' time). So the arrival time t at the detector in a reflectron TOF mass spectrometer is given by the simple relation:where u = 2d/uz, d the length of the accelerating space of a single-field ion source, us the longitudinal velocity gained at the source exit grid by a reference ion having no initial velocity. The constant in (1) vanishes if the arrival time origin is taken as the time when the reference ion produced at the beginning of the ionization process is detected.The detected ion charge, q, is proportional to the (1)* Author to whom correspondence should be addressed.number of ions that reached the detector during a pre-determined time period, ions having higher longitudinal velocities arriving first. To sum their number, we integrate in the to, u plane over the domain depicted in Fig. 1. It is limited at the left by the condition (l), by the u axis and the to= T line. The ionization lasts from to = 0 to to = T, u, being the lowest initial velocity of the emitted ions. The detected charge, disregarding a multiplicative constant, can be expressed by the following combination of integralsTo determine the detected charge for 0 < t < T, only the first lower limit of integration, -t/a, must be replaced by zero in the preceding formula.I Figure 1. Integration domain in the to--v plane, that determines the number of detected ions.

Ion‐Optical solutions in time‐of‐flight mass spectrometry

“…If this condition is exactly satisfied, the reference ion flight time becomes: ( 4 u 4 (47) while the second-order aberration coefficient is: (50) where…”