Delayed pulsed extraction

If a pulsed sources such as MALDI seems to be well suited to the TOF analyser, the quality of its pulsed ion beam is insufficient to obtain the high resolution and high mass accuracy. This situation is substantially improved with the development of two techniques delayed pulsed extraction and the reflectron. 

To reduce the kinetic energy spread among ions with the same m/z ratio leaving the source, a time lag or delay between ion formation and extraction can be introduced. The ions are first allowed to expand into a field-free region in the source and after a certain delay (hundreds of nanoseconds to several microseconds) a voltage pulse is applied to extract the ions outside the source. This mode of operation is referred to as delayed pulsed extraction to differentiate it from continuous extraction used in conventional instruments. Delayed pulsed extraction, also known as pulsed ion extraction, pulsed extraction or dynamic extraction, is a revival of time-lag focusing, which was initially developed by Wiley and McLaren in the 1950s, shortly after the appearance of the first commercial TOF instrument. 

Schematic description of a continuous extraction mode and a delayed pulsed extraction mode in an linear time-of-flight mass analyser, o = ions of a given mass with correct kinetic energy = ions of the same mass but with a kinetic energy that is too high. Delayed pulsed extraction corrects the energy dispersion of the ions leaving the source with the same mjz ratio. 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

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One way to reduce the kinetic energy spread is to introduce a time delay between ion formation and acceleration, referred to as delayed pulsed extraction. After a certain time delay ranging from nanoseconds to microseconds a voltage pulse is applied to accelerate the ions out of the source. 

Clusters, produced in a supersonic expansion, are ionized by laser. Delayed pulsed extraction is used to send the ions towards a lm time-of-flight mass spectrometer perpendicular to the jet axis. During the delay time between ionization and extraction, the ions spot size increases, due to their kinetic energy. The result is a broader mass peak with a width that is related to the kinetic energy released after the ionization and can be deduced after calibration of the experiment. 

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

Even slower dissociation rates can be measured by storing ions in an ion trap such as a pulsed ion cyclotron resonance (ICR) cavity (So and Dunbar, 1988) or a quadru-pole ion trap (March et al., 1992), both of which can trap the ions up to several seconds. In the ICR, ions are trapped by a combination of DC electric and magnetic fields, while in the quadrupole trap, they are stored by a combination of RF and DC electric fields. Analysis of either the depleted parent ions or the newly formed product ions is carried out by pulsed extraction of mass selected ions. Thus, the timing with respect to the photodissociation pulse is achieved by delayed ion extraction. The long time limit in this experiment is determined not by the trapping time of the instrument, but by the IR fluorescence rate of ions which is typically 10 to 10 sec > (Dunbar, 1990 Dunbar et al., 1987). 

Similarly to the extraction transients recoded at room temperature, Aj is decreasing and f ax shifts to longer times with increasing delay time at all measured temperatures. Contrary to the room temperature transients, the end of the pulse extraction current at lower temperatures (<150 K) does not reach the capacitive value j 0) indicating that some portion of the charge carriers are trapped during the applied extraction pulse. 

Laser SNMS requires the operation with properly selected duty cycles that control the delay times between the primary ion pulse, a pulsed extraction voltage for separating the secondary ions from post-ionized neutrals, and the firing of the postionizing laser pulse. Such duty cycles have, in addition, to be synchronized with the stepwise motion of the pulsed primary ion beam across the sample surface in the microprobe mode of laser SNMS. The selection of appropriate duration and decay times of the ion and laser pulses, of the laser intensity, and beam shape is important to make the photoion yields independent on the sputtered particle velocities. The detection volume must be matched to the entrance ion optics of the TOP such that it becomes independent of the individual ionization process. Usually, laser intensities in the range from 10 to lO Wcm are applied. While the particle density in the detection volume is monitored at small laser intensities, the particle flux is measured at high photon densities. 

Sequencing of modified oligonucleotides using in-source fragmentation and delayed pulsed ion extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Int. J. Mass Spectrom., 169, 331-350. 

The important instrumental development of delayed (ion) extraction (DE) or time-lag focusing (TLF) [279] has had a great impact on MALDI development [280,281]. The major limitation to the resolution provided by MALDI-MS is the initial velocity distribution of the ions. Ions with the same mass/charge ratio but different energy distributions yield a broad peak with a decrease in resolution. Correction for this by the use of pulsed extraction for time-lag focusing [282] has greatly improved the quality of the mass spectra from low masses (<100 Da) up to high masses. The term time-lag... 

FIGURE 7.28 MALDI in-source decay (ISD) mass spectrum of the oxidized B chain of bovine insulin obtained with delayed ion extraction.The extraction pulse was 1.5 kV applied 350 ns after the laser pulse. (Reprinted with permission from reference 45). 

Figure 11 Time-of-flight mass spectrum of multiphoton-ion-ized fullerenes. Upper spectrum static extraction, delayed ions give rise to asymmetric peaks. Lower spectrum pulsed extraction, delayed ions formed within a chosen time window are bunched into narrow peaks.

Cameron and Eggers introduced their velocitron, an instrument with a Nier electron impact source, a 317-cm drift tube, and oscilloscope recording. Ions were accelerated to a constant energy of 500 eV, so that their velocities would be inversely proportional to the square root of their masses. The 1955 instrument by Katzenstein and Friedland pulsed both the ionizing electron beam and the ion extraction field, using a drawout (orpushout) pulse. A pulsed extraction approach was also developed by Wiley and McLaren as ameans to improve mass resolution. Their instrument was commercialized, and their approach is the forerunner of the current delayed extraction methods used in MALDl. 

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