Time of flight spectrometer

In place of the diffractometer discussed in Section 2.5.3, a spectrometer is used, which allows measurement of the energy spectrum of scattered neutrons at different scattering angles. There are four main types of spectrometers in use today, the tripleaxis spectrometer, the time-of-flight spectrometer, the back-scattering spectrometer, and the spin-echo spectrometer, each of which is briefly described in the following section. 

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In the remainder of this section, we compare EISFs and Lorentzian line widths from our simulation of a fully hydrated liquid crystalline phase DPPC bilayer at 50°C with experiments by Kdnig et al. on oriented bilayers that, in order to achieve high degrees of orientation, were not fully hydrated. We consider two sets of measurements at 60°C on the IN5 time-of-flight spectrometer at the ILL one in which the bilayer preparations contained 23% (w/w) pure D2O and another in which bilayer orientation was preserved at 30% D2O by adding NaCl. The measurements were made on samples with two different orientations with respect to the incident neutron beam to probe motions either in the plane of the bilayers or perpendicular to that plane. 

The mass spectrometer is a mass-flow sensitive device, which means that the signal is proportional to the mass flow dm/dl of the analyte, i.e. the concentration times the flow-rate. It is only now possible to realise the high (theoretically unlimited) mass range and the high-sensitivity multichannel recording capabilities that were anticipated many years ago. Of considerable interest to the problem of polymer/additive deformulation are some of the latest developments in mass spectrometry, namely atmospheric pressure ionisation (API), and the revival of time-of-flight spectrometers (allowing GC-ToFMS, MALDI-ToFMS, etc.). 

B, magnetic sector Q, quadrupole mass Hlter ToF, time-of-flight spectrometer IT, ion trap FTICR, Fourier-transform ion-cyclotron resonance. 

The ECAP incorporates an electrostatic lens in the time-of-flight spectrometer in order to improve the mass resolution by compensating for small spreads in the energies of the ions evaporated from the specimen under the pulsed electric field. A lens design by Poschenrieder or a reflectron type of electrostatic lens is used for this purpose, and is standard equipment for metallurgical or materials applications of APFIM. These typically improve the mass resolution at full width half maximum (FWHM) from m/Am 250 to better than 2000. 

Fig. 3.8. Left schematic illustration of TRPE. The IR pump pulse (hi/1) perturbs the electronic states of the sample. The photon energy of the UV probe pulse (h.1/2) exceeds the work function and monitors changes in occupied and unoccupied states simultaneously. Right experimental setup for TRPE. Pairs of IR and UV pulses are time delayed with respect to each other and are focused onto the sample surface in the UHV chamber. The kinetic energy of photoelectrons is analyzed by an electron time-of-flight spectrometer (e-TOF). From [23]...

Fig. 2. Schematic diagram of a high resolution He time-of-flight spectrometer. N-nozzle beam source, SI, 2-skimmers, Al-5 - apertures, T - sample, G - gas doser, CMA - Auger Spectrometer, IG - ion gun, L - LEED, C -magnetically suspended pseudorandom chopper, QMA-detector, quadrupole mass analyzer with channeltron.

A time-of-flight spectrometer can be used as a mass analyzer, an ion kinetic energy analyzer, and an ion reaction time analyzer. We will consider here only what factors affect the resolution of the system in mass analysis.74 The same consideration can easily be extended to find the resolution in other analyses. There are at least two kinds of mass resolution. One refers to the ability of the system to separate two ion species of nearly equal masses in the same mass spectrum. This is related to the sharpness of the mass lines, or the full width at half maximum (FWHM) of the mass lines. The other refers to the ability of the system to distinguish two ion species of nearly identical masses, but not necessarily in the same mass spectrum. This latter mass resolution is related to the sharpness of reference points in the mass lines such as the onset flight times of the ion species, and the overall long-term stability of the system. This latter resolution determined also how accurately the instrument can measure the mass of ion species. Although this latter resolution is more closely related to ion kinetic energy analysis and is as important as the former one, we will consider here only the former kind, or the conventional kind, of mass resolution. 

In general, ion reaction rates can be observed directly in a time-of-flight spectrometer with a time resolution comparable to that of the system, which is about 10-9 to 10 los. The rate measurement can achieve a much better time resolution by using an ion reaction time amplification method. With this method, very fast ion reactions can be measured with a time resolution much better than the time resolution of the system. It is with this method69 that the field dissociation reaction of 4HeRh2+ was measured with a time resolution of about 20 femtoseconds when the time resolution of the system was still only 1 ns. 

At first sight, the ion mobility spectrometer in Figure 22-16a reminds us of a time-of-flight spectrometer. In portable units, the drift tube is 5 to 10 cm long. Typically, sample adsorbed on a cotton swab is placed in the heated anvil at the left to desorb analyte vapor. 

As an example of the extreme energy resolution possible with inelastic neutron sattering, we include some recent experiments of Newbery et (17) on methane adsorbed on graphitized carbon blacks. Using a time-of-flight spectrometer and an incident... 

Figure 4. Schematic of the inelastic neutron time-of-flight spectrometer at the University of Missouri Research Reactor Facility...

Inelastic neutron scattering, on the other hand, usually employs a monochromatic neutron beam and records the intensity of the scattered neutron beam as a function of neutron kinetic energy. Such inelastic collision spectra are monitored as a function of the applied field and the (usually low) temperature. The observed peaks then represent the energy differences of thermally populated and excited unpopulated multiplet states. Inelastic neutron scattering experiments can be conducted using triple-axis, backscattering, or time-of-flight spectrometers. 

Formation of metal clusters by gas aggregation, in which metal atoms are evaporated or sputtered into a cooled inert gas flow at relatively high pressure, has been well established in last decade. By repeated collisions with the carrier gas, the supersaturated metal vapor nucleates and forms clusters. The mechanism of cluster formation can be explained with homogeneous and heterogeneous nucleation theories. The gas aggregation methods have been applied extensively to produce small clusters of metals such as zinc, copper, silver etc. [23-26]. In some cases this method was used in combination with a mass filter such as a quadruple or a time-of-flight spectrometer [27, 28], The metal vapor for cluster source can be produced by either thermal evaporation [23-28] or sputter discharge [22, 29]. 

K. Clauwaert, S. Vande Casteele, B. Sinnaeve, D. Deforce, and W. Lambert, Exact mass measurement of product ions for the strnctnral confirmation and identification of unknown compounds nsing a qnadrnpole time-of-flight spectrometer A simplified approach using combined tandem mass spectrometric fnnetions, Rapid Commun. Mass Spectrom. 17 (2003), 1443-1448. 

The QENS results were obtained at the Institute Laue-Langevin, Grenoble, using the time-of-flight spectrometers INS and IN6 (25-27). The time scale on which the motions can be observed by QENS is determined by the elastic energy resolution, which amounted to 20 and 100 /xeV for the spectrometers INS and 1N6, respectively. Based on an appropriate statistical... 

Equipment fitted with magnetic analysers has the advantage of an exceptional mass resolution (up to A//AA/ = 100000) to the detriment, however, of sensitivity. Time of flight spectrometers, coupled to pulsed primary beams, offer excellent resolution without any intrinsic limitation in terms of mass. 

In spite of these apparent limitation, the sequencing of polynucleotides, using FAB mass spectrometry (that produces only a few intense ions), looks very promising. Fast-atom bombardment has not yet reached the molecular-weight level of native DNA or RNA, and the ultimate answer might lie around the use of new bombardment atoms or perhaps even around the resurgence of the almost abandoned time-of-flight spectrometers. 

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