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

Abstract

Time-of-flight mass spectrometers are able to cover mass ranges to around a half a million Da. The mass resolution of these instruments has been improved drastically by incorporation of electrostatic mirrors. The ionoptical solutions enabling these achievements and other promising features are discussed in terms of: First-and second-order space focusing in time for linear drift time-of-flight mass spectrometers; velocity focusing by time lag; ion-packet bunching in post-source impulse focusing; first-and second-order energy focusing in time, in single-and double-stage homogeneous electrostatic mirrors with normal and oblique ion incidence; gridless designs; perfect time focusing in parabolic potential and axial symmetry; cylindrical ion mirrors and toroidal electrostatic deflectors as time-of-flight mass analyzers; three electrostatic sector geometry applied in secondary ion microscopy and four toroidal sector systems for tandem mass spectrometry; poloidal electrostatic sectors. Estimation of time-of-flight mass spectrometer performance limits is discussed and the stable as well as the metastable peak shapes found in time-of-flight mass spectra.Time-of-flight (TOF) mass spectrometry possesses several outstanding and unique features:' an essentially unlimited mass range, the possibility of obtaining complete mass spectra from each ion packet, and potentially high sensitivity. To take full advantage of these features, the technique requires appropriate electronics for fast ion counting and a suitable design for the TOF mass analyser.The essential performance attributes of a mass spectrometer are its resolution, sensitivity and mass range. In time-of-flight mass spectrometry two groups of different mass ions are separated if they arrive at the detector at different times. Mass resolution of all types of mass spectrometer are defined in terms of the depth of the valley separating two equal intensity adjacent and overlapping peaks. Often the full width at half maximum (FWHM) definition is used where a valley is just discernable (Fig. 1). Other definitions require adjacent peaks to be resolved by deeper, typically by a 0% or 50%, valley (Fig. 2). To have the Gaussian peaks truly separated, as in Fig. 2, Am must be about 1.4 times greater than the FWHM resolution that is shown in Fig. 1. For TOFMS the ion cloud at the detector is the result of the convolution of the initial spatial and temporal ionic distributions: transformed by the spectrometer's transfer function. A simplified representation, as constant chargedensity cylinder^,^ operates with ion packets of circular outline the aberrations at the detector site being added as slices to that cylinder. In this case, two ion packets are completely resolved if one time channel is empty between their arrival. The FWHM definition corresponds to two packets just touching each other.The front of the packet of ions of mass m reaches the detector at some time t,, while a following packet of mass m + Am arrives tm+Am -tm later. The arrival of the second ion packet must leave en...

Sensitivity of recoil ion time-of-flight analysers—A comparison

“…The quantity n is defined as the ratio of the reference ion velocity when leaving the accelerating interval, divided by its velocity when leaving the extracting space interval. In the general resolution formula we substitute the expressions specific for the linear drift space TOF analyzer [13]:…”

A constant-momentum/energy-selector time-of-flight mass spectrometer

“…The mass resolution, mass accuracy, and sensitivity of the subsequent mass analysis are determined in large part by the degree to which the mass spectrometer's ion optics correct for the deviations from the ideal conditions for TOF analysis. In constant-energy mode, it has become conventional to accelerate the ions in two successive, uniform electric fields to achieve space focusing on some plane along the flight axis beyond the ion source that corrects to some degree for either an initial velocity distribution or an initial spatial distribution-corrections for both initial conditions cannot be achieved simultaneously [6,7]. In the constant-momentum mode, it is impossible to correct for an initial distribution of velocities or starting positions by accelerating the ions in a uniform electric field because all ions of the same mass gain exactly the same kinetic energy (viz.…”

“…The distance between the acceleration electrodes, the magnitudes of the accelerating potentials, and the timing and duration of voltage switching depend on whether the dual-stage accelerator is being used to correct for an initial velocity or spatial distribution. Comprehensive descriptions of the principle of dual-stage acceleration have been written [6,7].…”

Miniature time‐of‐flight mass spectrometers

“…The design parameters of two versions of miniature TOF mass spectrometers incorporating one-and twostage electrostatic mirrors with oblique and normal incidence have been determined. 8 The performance of a TOF mass spectrometer can be improved substantially by incorporating in its structure a single-stage, homogeneous electrostatic field mirror, an energy focusing in time thereby being accomplished ( Fig. 1).…”