Linear TOF instrument

Fig. 4.4. Scheme of a linear TOF instrument. After being created during a laser pulse onto the sample layer, ions are continuously accelerated by a voltage U. While drifting along the field-free drift path s, they are dispersed in time. Lighter ions reach the detector first. 

In a simple linear TOF instrument (Fig. 20), ions are formed within the source region(s) in the presence of an electrical field (V). The field accelerates the ions into a field-free region (d), which the ions all ideally enter with the same kinetic energy (KE) defined by the accelerating potential. Subsequently as the ions drift... 

These distributions are the major origins of the poor resolution observed using simple linear TOF instrumentation. Resolution (R) for a TOF mass spectrometer is dependent on peak width (At) and the total flight time (t) ... 

The major deficiency of a simple linear TOF instrument is its insufficient mass resolution, resulting from flight time variations of ions of the same m/e ratio. The ionization process adds a certain amount of initial kinetic energy to the molecules before acceleration. In addition, the different spatial positions in the source from where the ions are formed lead to varying values of l, and thus to flight time variations. As a result, modern TOF instruments commonly employ a technique called reflection to enhance resolution. A reflectron TOF (RETOF) [4] (Fig. 3) is used to focus ions of the same... 

The major disadvantages of MALDI are (a) the strong dependence of the quality of data obtained on the sample preparation, (b) the low resolution of linear TOF instruments (without DE), and (c) the inability to interface the MALDI ion source directly to HPLC or CE. The advantages of MALDI are ... 

Figure 2.38 displays the scheme of a linear TOF instrument. The TOF analyser separates ions, after their initial acceleration by an electric field, according to their velocities when they drift in a free-field region that is called a flight tube. 

Only fragmentations that occur as fast as the desorption can be observed in the mass spectrum at their right m/z values in a linear TOF instrument. Dissociation that occurs later on can be observed only if the extraction from the source is delayed or if a reflection analyser is used. 

What are the causes of poor resolution in a linear TOF instrument ... 

Other types of mass spectrometer have been used to record MALDI spectra magnetic sector instruments equipped with an array detector initially offered the advantage of higher resolution than was available with the early linear TOF instruments but these spectrometers have now largely been replaced with reflectron-TOF instruments using delayed extraction ion sources. Mass spectrometers operated with higher than normal ion source pressures to... 

Fig. 11.6. MALDl mass spectra of a mixture of the small peptides angiotensin 1 (Angl) and angiotensin 3 (Ang3) from DHB matrix as obtained on a linear TOF instrument in (a) positive-ion and (b) negative-ion mode under otherwise identical conditions. Reproduced from Ref. [42] with permission. John Wiley Sons, Ltd., 2000.

Fig. 10.5. Schematic of the basic components of the linear TOF instrument. (Adapted from Ref. [31, fig. 1[.)...

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