Time-of-flight effect

The reason for the disparity between the MS/MS spectra from the two detectors originates from a time-of-flight effect, which is also observed in a full-scan MS mode and is a consequence of the spatial separation between the two mass detectors. The result is a discrimination against the lower mass region when scanning a relatively wide mass range (e.g., 85-850 Da) and is a feature on most hybrid FTMS... 

Due to its radically different design, the latest hybrid linear ion trap FTMS instrument, the LTQ-Orbitrap (Fig. 5.6), does not suffer from the time-of-flight effect. In this instrument, the superconducting magnet and the ICR cell are replaced by an electrostatic trap (C-trap) and so distances traveled by the ions from one MS device to the other are much smaller in addition a radically different ion transfer mechanism virtually eliminates any possibility for a time-of-flight effect (Makarov,... 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

A position sensitive detector (PSD) is employed, of which there are several types used effectively around the world. One type is essentially a square array of multianodes, as shown in Figure 1.6. By measuring the time-of-flight and the coordinates of the ions upon the PSD, it is possible to map out a two-dimensional elemental distribution. The elemental maps are extended to the z-direction by ionizing atoms from the surface of the specimens. The z position is inferred from the position of the ion in the evaporation sequence, so that the atom distribution can be reconstructed in a three-dimensional real space. 

Domin, M. A. Welham, K. J. Ashton, D. S. The effect of solvent and matrix combinations on the analysis of bacteria by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rapid Comm. Mass Spectrom. 1999,13,222-226. 

Different mass analysers can be combined with the electrospray ionization source to effect analysis. These include magnetic sector analysers, quadrupole filter (Q), quadrupole ion trap (QIT), time of flight (TOF), and more recently the Fourrier transform ion cyclotron resonance (FTICR) mass analysers. Tandem mass spectrometry can also be effected by combining one or more mass analysers in tandem, as in a triple quadrupole or a QTOF. The first analyzer is usually used as a mass filter to select parent ions that can be fragmented and analyzed by subsequent analysers. 

The narrowest PFI bands in the present study are 3 cm-1 FWHM, using a 0.5 V/cm extraction field with the lasers attenuated to minimize effects of space charge. We measure band positions at the intensity maxima. These are reproducible to better than 1 cm-1. The bandwidth is limited by the rotational contour and also by the ionization process. A major advantage of ZEKE-PFI over more traditional photoelectron techniques is that the energy calibration is that of the tunable dye lasers, which are quite stable from day to day. In contrast, both electrostatic analyzers and time-of-flight photoelectron spectrometers require frequent calibration. 

On the RF time scale, the transit times of electrons in long coaxial cables and the time of flight of photons in optical paths as short as a few centimeters are significant. These effects become more pronounced as the modulation frequency increases. Even simple changes made to a system will affect the resulting measurements. 

T. Tiedje, Information about Band-Tail States from Time-of-Flight Experiments Arnold R. Moore, Diffusion Length in Undoped a-Si H W. Beyer and J. Overhof, Doping Effects in a-Si H H. Fritzche, Electronic Properties of Surfaces in a-Si H CR. Wronski, The Staebler-Wronski Effect... 

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