Double focussing

A novel HF-plasma SNMS instrument which can be combined with XPS has recently been developed [3.67]. Detection limits in the nmol/mol range have been achieved with a dedicated HF-plasma instrument attached to a double-focussing mass spectrometer [3.68]. 

ICP-MS (inductively coupled plasma mass spectrometry) is frequently used for determining ultratrace amounts of technetium [9]. In spite of the high cost of the equipment, this detection method is far superior to other radiometric methods as regards sensitivity. When a double focussing high-resolution system is used (HR-ICP-MS) and an ultrasonic nebulizer is introduced [10], the detection limit is in the order 0.002 mBq. The ICP-MS method has been successfully applied to the determination of environmental "Tc as well as to other long-lived radionuclides of neptunium and plutonium in the environment. 

In 1935 A.O. Nier designed a relatively simple but accurate mass spectrometer employing the principle of double focussing. The essential features are the same as described above but some improvements have been made which are ... 

MAT 731 double-focussing mass spectrometer, a combined EI/FI/FD/FAB ion source and an AMD Intectra direct introduction system, has been previously described in detail (12). The samples were heated linearly from 50 C to 750 C at a rate of 100 K/min. In general, 35 FI mass spectra were recorded in the m/z 50-900 mass range. 

Among other developments, a ikw collimator system composed of two double slits was designed and built at EMBL/DESY Using the double focussing X-ray EMBL instruments equipped with this collimator system, we obtained sharp, resolvable patterns with enhanced signal to noise ratio, and we were able to determine cell dimensions... 

Field desorption and electron impact ionization mass spectra were taken by using a Matsuda type double focussing spectrometer... 

Electrons from the heated tungsten filament are accelerated to the annular anode. Depending on the anticathode material a characteristic fluorescence radiation is emitted, passes through a thin Aluminum window and induces photoelectrons on the surface of the analytical sample. These photoelectrons are deflected in the spherical electrostatic analyzer, double focussed to eliminate stray electrons and finally counted by the electron multiplier. The whole system works under a vacuum of 10-s to 10 7 torr or even 10 10 torr, if surface properties have to be studied. This vacuum is generated by a Titanium... 

A few years ago, we began a research program to develop methods of analysis which would involve the use of FAB and a high performance tandem mass spectrometer. The tandem instrument was the first triple sector mass spectrometer to be designed and built by a commercial instrument company (Kratos of Manchester, U.K.). The first mass spectrometer of the combination is a double focussing Kratos MS-50 which is coupled to a low resolution electrostatic analyzer, which serves as the second mass spectrometer U). This FAB MS-MS combination has been used to verify the structures of an unknown cyclic peptide (2), a new amino acid modified by diphtheria toxin (3), and an ornithine-containing lipid (4). A number of methods have also been worked out which rely on this instrumentation. They Include the structural determination of cyclic peptides (5), nucleosides and nucleotides (6), and unsaturated fatty acids (7) and the analysis of mixtures of both anionic (8) and cationic surfactants (9). 

Although there are many attractive features of Fourier transform mass spectrometry (FTMS), there are problems with current implementations. The FT mass spectrometer is still a relatively young instrument when compared to the double-focussing mass spectrometer, and comparisons of the spectrometers and their... 

Comparison with Sector Instruments. Modern double-focussing mass spectrometers have a long history of development, which was recently reviewed O) The instrument has evolved through a long series of complex and interweaving events beginning with the early experiments of J. J. Thomson (2) and the first mass spectrographs of... 

Belshaw, N.S., Freedman, P.A., O Nions, R.K., Frank, M., and Guo, Y. (1998) A new variable dispersion double-focussing plasma mass spectrometer with performance illustrated for lead-isotopes. Int. J. Mass Spec. Ion Process, 181, 51-58. 

Rates of decomposition of the molecular ions of aminotriazoline stereoisomers have been determined over the time range tens of picoseconds to microseconds using a blade emitter and a double-focussing mass spectrometer [399]. The predominant process at the shortest times was, in all cases, loss of molecular nitrogen. The stereoisomers decomposed at very different rates, which was interpreted in terms of differing rate coefficients, k(E), arising from different amounts of steric strain. 

The rate of loss of a methyl radical from the molecular ion of diethyl ether has been measured using a single-focussing mass spectrometer [42, 825]. The time resolution here was a few picoseconds. Loss of a methyl radical from the molecular ion of di-isopropyl ether has been investigated using a double-focussing instrument [743]. 

The rates of all the various decomposition pathways of the molecular ion of n-butyl acetate have been investigated using a double-focussing mass spectrometer [743]. There appears, however, to have been either an error in the calculation of times or the calculations are simply imprecise. The times reported appear to be too short, perhaps by something like a factor of 3. This being so, the agreement between these incorrect times and decomposition times for related ions [520] would not be significant. 

Charge exchange will suffer the same problems with intramolecular kinetic isotope effects as PIPECO if single-focussing mass spectrometers are used. Charge exchange with a double-focussing mass spectrometer should, however, allow determinations of intramolecular kinetic isotope effects. 

As already discussed above, Q-ICP-MS may still require matrix removal and preconcentration because the Q mass analyzer is unable to resolve most of the spectral interferences affecting the analytical masses of PGEs. Moreover, the detection power might be inadequate for the performance of reliable measurements in some samples. PGEs determination is hampered by mass interferences and requires high analytical selectivity and sensitivity. Double-focussing SF-ICP-MS with mass resolution capabilities higher than those afforded by Q instruments (currently up to 10,000) can properly deal with most mass interferences that have... 

In practice more sophisticated double focussing arrangements are usually used, several of which are shown in figure 10.23. The Cassegrain, Gregory and Nasmyth systems are in common use, and the offset Cassegrain system has the advantage that the secondary reflector does not block the field of view of the primary dish... 

Double focussing instrumentation metastable ions sae observed at non-integer m/z values where there [Pg.262]

Double focussing, mirror-monochromator cameras are optimized for maximum flux at the sample. This type of camera is hence mainly used for real time diffraction studies on biological samples and polymers (see Sect. 4). Such a camera is shown in Fig. 23. The first optical element could only be placed at 20 m... 

Fig. 23. Schematic design of a double focussing mirror-monochromator camera at DORIS The middle of the mirror is at 20 m from the source point. A bent, triangular monochromator crystal is used for horizontal focussing and a segmented mirror (quartz) for vertical focussing. The ionization chamber is designated by-I-...

The total reflection of mirrors can be used to focus the radiation. Synchrotron radiation, while collimated in the vertical plane it spreads over the horizontal one. Wavelength resolution requirements normally restrict the vertical aperture to one mm or so, in any case. On the other hand it is desirable to condense the horizontal spread into a focal point. Double focussing with a mirror system is possible and an ideal mirror geometry has been worked out For a point source the mirror has to be shaped like an ellipsoid, and the source and the image have to be placed in the respective foci. The long distances involved in synchrotron work mean that a good approximation to shape is achieved by making use of bent cylindrical mirrors ... 

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