Mass spectrometer, double-focusing operation

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

In 1967 Liebl reported the development of the first imaging SIMS instrument based on the principle of focused ion beam scanning [24]. This instrument, the ion microprobe mass analyzer, was produced by Applied Research Laboratories (Fig. 4.5). It used an improved hollow cathode duoplasmatron [25] ion source that eliminated filaments used in earlier sources and allowed stable operation with reactive gases. The primary ion beam was mass analyzed for beam purity and focused in a two-lens column to a spot as small as 2 pm. The secondary ions were accelerated from the sample surface into a double focusing mass spectrometer of Mattauch-Herzog geometry. Both positive and negative secondary ions were de-... 

Two magnetic sector instruments have also been operated in tandem, with a collision ceil placed between the two instruments. These instruments permit high resolution selection of both parent and product ions. However, these mstruments are now rarely used as they are expensive and cumbersome to operate. Double focusing mass spectrometers can also be used as tandem mass spectrometers by a technique known as linked scanning. A product ion scan by linked scanning involves low resolution for MSI and high resolution for MS2,... 

The equipment used for this work consisted of a Hewlett-Packard 5710A GC, with on-coluitm injection, directly coupled to a VG7070E medium resolution double-focusing mass spectrometer. The GC coluitm employed was a 60 m fused silica capillary colurrm coated with a cross-linked methyl silicone stationary phase, DB-1. The initial temperature of the colurrm was 30°C, and after 4 minutes, the temperature was linearly programmed at 8°C/min to 270°C, and held at this final temperature for 15 minutes. The mass spectrometer was operated in the electron impact (El) mode, and the mass range of 20-700u was scanned once a second. The mass spectrometer was linked to an Incos data system which stored the acqnired mass spectra, and allowed these to be compared to the EPA/NIH Mass Spectral Data Base to assist with the identification of compounds detected. 

Identifrcation of components in the extracts was conducted by mass spectrometry. The sample was injected onto an HPS890 GC. The chromatographic conditions for the OV-1 column were the same as described for GC analysis. The end of the GC capillary column was inserted directly into the ion source of the mass spectrometer via a heated transfer line maintained at 280°C. The mass spectrometer was a Micromass Prospec high resolution, double-focusing, magnetic sector instrument. The mass spectrometer was operated in the electron ionization mode (El), scanning from ni/z 450 to m/z 33 at 0.3 seconds per decade. 

Fast Aloin Bombariiment (FAB-MS). This was applied to Study the lactonization of HIL to ADF FAB-MS was performed on a Finnigan MAT 8430 double focusing mass spectrometer. FAB ionisation was carried out with a saddle-field atom gun (Ion Tech. Teddington, UK) which was operated at 2 raA and 7-8 kV with xenon. Glycerol was used as matrix. The positive ions at m/z 130 (protonated molecular ion of ADF) and 148 (protonated molecular ion of HIL) were recorded. 

Positive ion fast atom bombardment (FAB) mass spectra were obtained with the JEOL HXllO/HXllO tandem double-focusing mass spectrometer with JEOL gun and collisionally induced decomposition (CID) MS/MS. The JEOL instrument was operated at +10 kV with -20 kV postacceleration at the detector. The xenon neutral beam had 6 kV acceleration from the JEOL gun. CID MS/MS was performed with 1 1000 resolution in both MS-1 and MS-2. Helium was used as the collision gas at a pressure sufficient to reduce precursor ion abundance by 75%. Samples were dissolved in methanol-glycerol (1 1, v/v) for all FABMS analyses. 

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