Delayed extraction technique

To compensate for the energy spread, the first stage of the two-stage acceleration field can be activated shortly after ion production or ejection from the sample. This event typically occurs in the nanosecond to low microsecond range. The method is known as time-lag focusing or delayed extraction technique. In TOF-MS equipped with matrix-assisted laser desorption/ionization (MALDI) ion source, delayed extraction may improve the mass resolving power by a factor of two- to fivefold [9, 10]. Mass resolving power defines the sharpness of spectral features, which is explained in Chapter 5. 

State-of-the-art ToF-MS employs reflection lenses and delayed extraction [176] to improve resolution by minimising small differences in ion energies, and in these cases up to 12000 mass resolution (FWHM, m/z 600) is available. This is sufficient for most modern applications. Solid probe ToF-MS (or direct inlet high-resolution mass spectrometry, DI-HRMS) is a breakthrough. DIP-ToFMS is a thermal separation technique. Advantages of DIP-ToFMS are ... 

Applications MALDI-ToFMS is at its best as a rapid screening technique for quick identification of known additives. However, this screening is rendered slightly more complicated by the fact that MALDI-ToFMS spectra of pure additives and of additives in the presence of excess macromolecules are not always identical (matrix effect) [55]. For unknown additives, the relation MALDI-ToFMS spectrum-chemical structure is not easily established, and the use of FD or MALDI-MS/MS is then needed. As MALDI-MS shows a sensitivity difference for the various additives, it cannot easily quantify them unless the analytes are very similar. For differentiation of additives with the same mass number (e.g. Tinuvin 315 and Cyasorb UV3638 with m/z = 368) high resolution is required, as provided by delayed extraction MALDI-ToFMS. 

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

This is sometimes referred to as delayed extraction. A second technique is to use a reflectron. More recently, orthogonal introduction of ions has been introduced, in analogy to the orthogonal introduction of ions discussed above. The prOTOF 2000 marketed by Perkin Elmer Sciex is an instru-ment using orthogonal introduction. The prOTOF 2000 also uses collisional cooling in the interface between ion source and mass analyzer. 

Time-of-flight reflectron mass spectrometry with some form of time-delay extraction has become popular for large-molecule characterizations. Resolutions of up to 15,000 can be routinely achieved over very broad mass ranges, extending up to about 300,000 Da. As noted previously, time-of-flight analysers are particularly compatible with laser-based ionization techniques that produce very short bursts of ions. They are also very fast, with mass spectra often obtainable in about 25 p,s. Finally, time-of-flight analysers have been paired with a quadrupole to produce a hybrid two-dimensional mass spectrometry system that has found widespread use in protein analyses. This device will be discussed in more detail in the section on two-dimensional mass spectrometry. 

Others groups [71, 72, 73, 74] obtained an improvement of mass accuracy and peak resolution by the use of a delayed extraction system [75]. This system uses a pulsed ion extraction MALDI ionisation technique increasing the accelerating voltage from 0 up to 3 kV in 300 nanoseconds. This technique allowed an increase of peak resolution for cytochrome c (12 kDa) from 350 FWHM obtained in linear mode to 1024 with a continuous ion extraction. 

An older book that describes the basic principles of TOE, and several innovative techniques, including orthogonal extraction, post source decay, and delayed extraction. 

There are a number of options for analyzing a peptide mixture extracted from a gel by mass spectrometry. The most widely used techniques are MALDI-TOF, preferably with delayed extraction, and ESI with a triple-quadrupole analyzer or a combination of quadrupole and TOF analyzers. The combination of ESI with an ion trap is also of great interest due to its sensitivity and the possibility of performing structural studies by MS". Downsizing of the electrospray capillary such as in the nanospray technique results in another gain in sensitivity and an extension of the measuring time, which is important to optimize fragmentation conditions. 

There are two reasons for placing the cotton plug in the pipet. First, it solves a particular problem with the transfer of volatile liquids via the standard Fhsteur pipet the rapid buildup of back pressure from solvent vapors in the rubber biolb. This pressure quickly tends to force the liquid back out of the pipet and can cause valuable product to drip on the bench top. The cotton plug tends to resist this back pressure and allows much easier control of the solution once it is in the pipet. The time-delay factor becomes particularly important when the Pasteur filter pipet is employed as a microsep-aratory funnel (see the discussion on extraction techniques in Technique 4, p. 67). 

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