SUBJECTS cyclotron

A PET scan with [ F] fluorodopa in a control subject shows that the striatum is heavily labelled whilst in a Parkinson patient with established symptoms there is little labelling. This patient s twin, whilst free of symptoms, also showed some loss of labelling and subsequently developed the disorder. Reproduced by kind permission of D Brooks, MRC Cyclotron Unit, Hammersmith Hospital, UK. 

Fourier transform mass spectrometry is made possible by the measurement of an AC current produced from the movement of ions within a magnetic field under ultra-high vacuum, commonly referred to as ion cyclotron motion.21 Ion motion, or the frequency of each ion, is recorded to the precision of one thousandth of a Hertz and may last for several seconds, depending on the vacuum conditions. Waveform motion recorded by the mass analyzer is subjected to a Fourier transform to extract ion frequencies that yield the corresponding mass to charge ratios. To a first approximation, motion of a single ion in a magnetic field can be defined by the equation... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

To date, only few very recent gas-phase studies on this subject can be retrieved from the literature, i.e., (i) a gas-phase study on the displacement of several amino acids from the chiral amido esorcinarene 9 (Scheme 9) carried out by Speranza and coworkers using an electrospray-ionization Fourier-transform ion cyclotron resonance (ESl-FT-lCR) mass spectrometer," " and (ii) Lebrilla and coworkers study on the ability of the achiral calix[4]arene 7 and calix[6]arene 8 to form inclusion complexes with natural amino acids under matrix-assisted laser... 

This quantity was the subject of several indirect attempts at calculation based on physical properties as well as ab initio computations. A direct measurement was finally achieved by ion cyclotron experiments and gave a value of 163kJmol (McMahon and Larson, 1982a) for A [HF(g)+ F (g) -> FHF (g)]. Shortly after, ab initio calculations with the basis set [(14s) (9p) (2d)/(10s) (lp)] fell in line with a value of 169 kJ mol" (Emsley et ai, 1983). Prior to this, a wide range of values for the bond energy had been canvassed, often with theoretical computations providing support (see Table 8). 

This type of mass spectrometer, which is not widely used, allows mass determination with a high precision. An ion cyclotron resonance spectrometer is basically an ion trap ions formed by electron impact, for example, are subjected to the orthogonal magnetic field B, which induces cyclotronic movement in the. rv plane (Fig. 16.8). The radius of the circular movement, which depends on kinetic energy, is given by equation (16.2). If the velocity v is small and the magnetic field B is intense, the radius of the trajectory will be small and the ions will be trapped in the ionisation... 

The classical theory for electronic conduction in solids was developed by Drude in 1900. This theory has since been reinterpreted to explain why all contributions to the conductivity are made by electrons which can be excited into unoccupied states (Pauli principle) and why electrons moving through a perfectly periodic lattice are not scattered (wave-particle duality in quantum mechanics). Because of the wavelike character of an electron in quantum mechanics, the electron is subject to diffraction by the periodic array, yielding diffraction maxima in certain crystalline directions and diffraction minima in other directions. Although the periodic lattice does not scattei the elections, it nevertheless modifies the mobility of the electrons. The cyclotron resonance technique is used in making detailed investigations in this field. 

The isotope 203Pb is produced by bombardment of thallium with protons in a cyclotron. The radioactive half-life is 52 h and decay is by electron capture. The associated 279-keV gamma ray enables the activity in the lung and other organs of volunteer subjects to be detected by external gamma ray spectrometers (Chamberlain etal., 1975). 

In a Penning trap, ions are constrained spatially by a combination of electric and magnetic fields. In the presence of a homogeneous magnetic field (B) ions that have a component of velocity perpendicular to B are subjected to the Lorentz force and subsequently follow a circular trajectory [70]. The rotational frequency of this trajectory is proportional to B and to the ion charge (q) and inversely proportional to the ion mass (m). When expressed in radians, this rotational frequency is known as the cyclotron frequency, coc, where... 

Cyclotron resonance analyser (also known as Penning ion trap) an instrument that confines ions by placing them in a static magnetic field. Inside the field, the ions are subject to the Lorentzian force that causes ions of a particular m/z to cycle at a specific frequency (cyclotron frequency). 

Often the question is asked, Is NMR harmful There is a story told by Nobel Prize winner and Professor of Physics Edward Purcell of Harvard that about the time the university s cyclotron was being constructed, he wondered whether a person could sense the tipping and forced precession of proton spins in his or her own brain. The perfect opportunity arose just as the huge cyclotron magnets had been completed but before the vacuum chamber had been inserted into the magnet gap. He and a colleague constructed an rf coil in the gap one of them placed his head in the coil, while the other connected an oscillator and tuned it to the proton Larmor frequency. Purcell reports that neither physicist could tell when his brain passed in and out of resonance. That undoubtedly marks the first NMR experiment on human subjects, and no untoward... 

The gas phase acid/base properties of molecules have been subject to equilibrium or bracketing measurements employing mass spectrometric techniques like ion cyclotron resonance (ICR) [4], Fourier transform ion cyclotron resonance (FT-ICR) [5,6], Flowing afterglow (FA) and Selected ion flow tube (SIFT) [7], and high pressure mass spectrometry (HPMS) [8]. Proton transfer between neutral molecules are then investigated by measurements of reactions... 

In the 1930 s, vinyl monomers were found to undergo chain polymerization when subjected to gamma-rays (6) or to neutrons generated by a cyclotron (7). However, the more extensive work was carried out in the late 1940 s when it became clear that powerful radiation sources would become practical tools of interest to industry. 

The basic operation of the instrument (3) consists of the Ionization of the sample in the cell by a timed electron beam which Is followed, after a short Interval, by an RF pulse applied to the plates of the cell. This pulse coherently excites all ions In the cell into cyclotron motion. The motion continues after cessation of the pulse, and the resonance is detected by the plates of the cell, amplified and the data stored In the computer. The excltatlon/detection cycle is repeated numerous times and the collected data summed. The data Is then subjected to fourler transformation and the frequency spectrum resulting converted into a mass spectrum. The spectrum Is normalized to the major peak. For quantitative work, the calibration can be based either on peak height or peak area. Here, major considerations will Include the resolution chosen and the relative concentrations of the constituents under Investigation. 

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