TOF analyser

In some respects, the time-of-tlight (ToF) analyser is the simplest of the mass separation devices. This system relies on the fact that if all of the ions produced... 

The resolution of the ToF analyser is dependent upon the ability to measure the very small differences in time required for ions of a similar m/z to reach the detector. Increasing the distance that the ions travel between source and detector, i.e. increasing the length of the flight tube, would accentuate any such small time-differences. The implication of such an increase is that the instrument would be physically larger and this goes against the current trend towards the miniaturization of all analytical equipment. 

ToF analysers are able to provide simultaneous detection of all masses of the same polarity. In principle, the mass range is not limited. Time-of-flight mass analysis is more than an alternative method of mass dispersion it has several special qualities which makes it particularly well suited for applications in a number of important areas of mass spectrometry. These qualities are fast response time, compatibility with pulsed ionisation events (producing a complete spectrum for each event) ability to produce a snapshot of the contents of the source volume on the millisecond time-scale ability to produce thousands of spectra per second and the high fraction of the mass analysis cycle during which sample ions can be generated or collected. 

Applied Biosystems Q q TOF analyser equipped with a nanoelectrospray ion source for nano ESI MS and nano ESI MS/ MS analysis... 

Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometres), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. 

Figure 15.2 shows the schematic representation of a typical ToF-SIMS device. All the system is placed under high vacuum (typically 10 7 torr) to avoid interactions between ions and air molecules. Primary ions are produced by a liquid metal ion gun and then focused on the sample to a spot with a typical size of less than 1 pm. After they impinge the surface, secondary ions are extracted and analysed by the ToF analyser. To synchronize the ToF analyser, the primary ion beam must be in pulsed mode. 

In addition to the diversity of ionisation techniques available, mass spectrometers offer a selection of mass analyser configurations. Of note are single (MS) and triple quadrupole (MS—MS) instruments, ion trap analysers (MS)n, time-of-flight (ToF) analysers, sector field analysers, and Fourier transform-ion cyclotron resonance (FTICR) instruments. 

The principle underlying time of flight (TOF) mass spectrometers is based on the relationship that exists between mass and velocity at a given kinetic energy. The instrument, which uses pulsed ionisation, measures the time taken by each mass to travel the length L of a field-free analyser tube. The basic equation (I6.l l) used in linear TOF analysers is obtained by eliminating the velocity v from equation (16.5) in conjunction with the relationship L = vt ... 

A time-of-flight (TOF) analyser measures the time t required for a particle to travel a fixed distance d. If applied to electron spectrometry, non-relativistic electrons with kinetic energy kin have a velocity v... 

Figure 10.2 Photoelectron spectra of helium at 80 eV photon energy obtained under comparable experimental conditions taken with (a) a TOF analyser (from [Lin83]) (b) a sector CMA (see [SDM82]). For details see main text.

Since pulsed laser ionization produces well defined packets of ions and electrons, TOF analysers (which essentially are magnetically shielded, electric-field-free drift tubes with apertures and an electron multiplier) can readily be used. TOF resolution for slow electrons can approach 3 meV, and throughput is similar to that of electrostatic analysers operating without an extraction field (i.e. a detection efficiency < 1%). The kinetic energy is obtained from the flight time, which is proportional to the reciprocal velocity, (KE) /2, whereas the resolution varies as (KE)/2. Thus, the resolution for 1-5 eV electrons is comparable to that for electrostatic analysers, but degrades seriously for 5-10 eY electrons. 

Contrary to most other ionization sources that yield a continuous ion beam, MALDI is a pulsed ionization technique that produces ions in bundles by an intermittent process. The pulsed nature of the MALDI source is well suited for the time-of-flight (TOF) analyser. In addition, the TOF analyser has the ability to analyse ions over a wide mass range and thus... 

ISD fragmentations lead to product ions that are always apparent in the MALDI spectra, whereas the observation of product ions from PSD fragmentation needs certain instrumental conditions. For example, a MALDI source coupled to a linear TOF analyser allows detection of fragment ions produced in the source at their appropriate m/z ratio. On the contrary, fragment ions produced after the source cannot be resolved from their precursor ions and are detected at the same apparent m/z ratio. This induces a broadening of the peaks with a concomitant loss of mass resolution and sensitivity. 

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. 

Another advantage of these instruments is their high transmission efficiency that leads to very high sensitivity. For example, the spectrum from 10 15 mol of gramicidin [44] and the detection of 100-200 attomole amounts of various proteins (cytochrome C, ribonuclease A, lysozyme and myoglobin) [45] have been obtained with TOF analysers. All the ions are produced in a short time span and temporal separation of these ions allows all of them to be directed towards the detector. Therefore, all the formed ions are in principle analysed contrary to the scanning analysers that transmit ions successively along a time scale. 

The analysis speed of TOF analysers is very fast and a spectrum over a broad mass range can be obtained in micro-seconds. So, it is possible in theory to produce in 1 second several thousand TOF mass spectra over a very wide mass range. But in practice, for most of the applications, the weak number of ions detected in each individual spectrum is insufficient to provide the required precision of mass or abundance measurement. Furthermore, it is... 

Another interesting characteristic of the TOF analyser lies in its easy mass calibration with only two reference points. As in all the mass spectrometers, the TOF mass spectrometer requires a calibration equation to relate and convert the physical property that is measured to a mass value. For the TOF spectrometer, the physical property that is measured during an analysis is the flight time of the ions. As already mentioned, the flight time of an ion is related to its mass by the following equation ... 

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