Drift-Tube

In the gas phase, drift tubes with electrical shutters are employed for drift velocity measurements of charge carriers. This method has also been applied for the mea- 

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Figure A3.5.7. Schematic diagram of a selected ion flow drift tube with supersonic expansion ion source.

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

There are otlier teclmiques for mass separation such as tire quadmpole mass filter and Wien filter. Anotlier mass spectrometry teclmique is based on ion chromatography, which is also capable of measuring tire shapes of clusters [30, 31]. In tills metliod, cluster ions of a given mass are injected into a drift tube witli well-defined entrance and exit slits and filled witli an inert gas. The clusters drift tlirough tills tube under a weak electric potential. Since the... 

Jarrold M F 1995 Drift tube studies of atomio olusters J. Phys. C Solid State Phys. 99 11... 

If the distance from the ion source to the detector is d, then the time (t) taken for an ion to traverse the drift tube is given by Equation 26.3. 

In a time-of-flight (TOF) mass spectrometer, ions formed in an ion source are extracted and accelerated to a high velocity by an electric field in an analyzer consisting of a long, straight drift tube. The ions pass along the tube until they reach a detector. 

In effect, the ions race each other along the drift tube, but the winners are always the ions of smallest m/z value, since these have the shortest flight times. The last to arrive at the detector are always those of greatest mass, which have the longest flight times. However, as in a race, for there to be a separation at the finish line (the detector), the ions must all start from the ion source at the same time (no handicapping allowed ). 

For example, if each bin represents 0.3 nsec and bin number 200 has been affected by an ion arrival, then the flight time must have been 200 x 0.3 = 60 nsec. Knowing the length of the drift tube, the ion drift velocity can be calculated, and from that calculation its m/z value can be deduced. 

FIG. 35. Vertical cross section of the reaction chamber equipped with the mass spectrometer system. Indicated are QMF. the quadmpole mass filter ESA. the electrostatic analyzer CD, the channeltron detector DE, the detector electronics DT, the drift tube lO, the ion optics TMP, the turbomolecular pump PR, the plasma reactor and MN. the matching network. 

An ion mobility spectrometer consists of a sample-introduction device a drift tube where ionisation and separation of ions takes place and a detector. Ionisation sources of choice include radioactive sources (e.g. a 63Ni foil), photoionisation methods, corona-spray ionisation, flame ionisation and corona discharge. The most common detection method used to measure the... 

To operate the ion TOF spectrometer in the velocity mode, we adapted a single-stage TOF spectrometer as shown in Fig. 3, which consisted of a repeller, an extractor (and guard rings, not shown) and a free-drift tube. After laser ionization, ions are extracted towards the MCP detector. For an ion with an initial kinetic energy Do, the total flight time t can be written as... 

Since the TOF spectrometer is perpendicular to the propagation axis of the probe laser, ions with a large vz could miss the MCP detector. To overcome this problem, two slots of 6 mm x 50 mm were cut on the two lateral sides of the free-drift tube and covered with a 90% T-mesh. An... 

In general, rate coefficients determined in drift tube studies, where the internal degrees of freedom are cooled, are most appropriate.133... 

In obtaining experimental information about the isomeric forms of ions, a variety of techniques have been used. These include ion cyclotron resonance (ICR),31 flow tube techniques, notably the selected ion flow tube (SIFT),32 and the selected ion flow drift tube (SIFDT)32 (and its simpler variant33), collision induced dissociation (CID),10,11 and the decomposition of metastable ions in mass spectrometers.13 All of these techniques are mentioned in the text of Section in whore they have provided data relevant to the present review. 

Although the first generation of TOF analyzers were formed by just a drift tube and had low resolution (Figure 2.10a), many improvements have been made over the years. In particular, the introduction of a reflectron , located at the end of the flight tube, improved theTOF performances a lot. In fact, once accelerated in the source, ions with the same m/z value do not have exactly the same kinetic energy. Thus they arrive at the detector at... 

Figure 2.10 Time of flight analyzer drift tube (a) and reflectron (b)...

This is used in polymer analysis, it involves the removal of solvent from a solvated polymer as it elutes down a drift tube and the isolated polymer particles then scatter fight from the light source allowing molecular weight to be calculated. 

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