Mass analyzers beam-type

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). 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

We have recently investigated another type of mobile matrix - a liquid metal [100, 102]. Here, we discovered that ion bombardment of the liquid metal surface, upon which sample particles were deposited, resulted in movement of the sample species towards the primary ion beam where they were desorbed and finally detected by the mass analyzer. 

Trapping mass spectrometers can also be used as tandem mass spectrometers. Unlike beam-type mstruments, which are referred to as tandem in space, trapping mass spectrometers are tandem in time, meaning that ions are held in one region of space while the parent ion is selected and dissociated and the daughter ion analyzed sequentially in time. The ability to perform tandem mass spectrometry is inherent in the design of trapping mass spectrometers. Gen-... 

For the beam-type mass analyzers (sector, ToF, and Q), each stage of mass analysis is performed in discrete mass analyzers usually separated by a collision cell. This arrangement is called tandem-in-space. The use of multiple analyzers means that analyzers can be independently selected for the different stages of analysis based on the desired performance characteristics. 

The ions created in the ion source are focused into a beam of narrow energy dispersion. The beam is accelerated in an electric field before entering the mass analyzer. A number of different analyzers can be used, but certain types are used more often than others in combination with gas chromatography. Typically, mass spectrometers used in GC/MS are low resolution instruments. Resolution A is a measure of the resolving power, i.e., the capacity to separate signals from ions which have similar masses. It is usually defined in the following way " ... 

---