TOF magnetic

Photoelectron Analysers. Electrostatic, TOF, magnetic bottle and threshold electron analysers have all been used. The electrostatic analysers, which are similar to those in the classical PES set-up, have the advantage of an optimized design and ready commercial availability. The electron energy is a function of the analyser voltage, and the resolution... 

Standard Conditions 20 100MeV heavy ion beam (2 MeV He for hydrogen detection) ToF, magnetic, gas detector 10min per sample... 

Currently, at least four types of mass analyzer are used in mass spectrometry, namely ToF, magnetic/electric sector, quadrupole ion trap (see Figure 18.6), and FT-ICR. The ToF and magnetic/electric sector mass analyzers are most commonly used in mass spectrometry imaging, while FT-ICR analyzers have been applied only rarely to mass spectrometry imaging [117]. 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

Hybrid Magnetic-Sector Time-of-Flight (Sector/TOF)... 

In the magnetic-sector/TOP hybrid, ions produced in an ion source pass through the magnetic sector first and then might enter the TOF section, depending on how the hybrid is operated. The hybrid can be used as two separate instruments or as two instruments in conjunction with each other. 

Also in general terms, the TOF part of the hybrid is used mostly for MS/MS studies in which ions produced in the magnetic sector are collided with neutral gas molecules to induce decomposition (see Chapter 23). In this mode the instrument produces more highly resolved product ion spectra than can be attained in simple magnetic-sector instruments. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

In the following discussion, the separate use of the magnetic sector and the TOF sector are examined briefly, followed by a discussion of the hybrid uses. 

Alternatively, ions of any one selected m/z value can be chosen by holding the magnetic field steady at the correct strength required to pass only the desired ions any other ions are lost to the walls of the instrument. The selected ions pass through the gas cell and are detected in the singlepoint ion collector. If there is a pressure of a neutral gas such as argon or helium in the gas cell, then ion-molecule collisions occur, with decomposition of some of the selected incident ions. This is the MS/MS mode. However, without the orthogonal TOF section, since there is no further separation by m/z value, the new ions produced in the gas cell would not be separated into individual m/z values before they reached the detector. Before the MS/MS mode can be used, the instrument must be operated in its hybrid state, as discussed below. 

Operation of the Combined Magnetic and TOF Sectors as a Hybrid Instrument... 

The AutoSpec -TOF hybrid mass spectrometer combines the advantages of a magnetic/electric-sector instrument with those of time-of-flight to give a versatile instrument capable of MS or MS/MS at high or low resolution. 

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

Three main types point ion collectors are in use for quadrupole, magnetic-sector, and TOF instruments, and they are discussed here. The multichannel plate collector (or time-to-digital converter)... 

An added consideration is that the TOF instruments are easily and quickly calibrated. As the mass range increases again (m/z 5,000-50,000), magnetic-sector instruments (with added electric sector) and ion cyclotron resonance instruments are very effective, but their prices tend to match the increases in resolving powers. At the top end of these ranges, masses of several million have been analyzed by using Fourier-transform ion cyclotron resonance (FTICR) instruments, but such measurements tend to be isolated rather than targets that can be achieved in everyday use. 

Therefore, the orthogonal TOF mass spectrum is a snapshot of all the ions in the sampled ion beam at any one moment in time. The arrangement has advantages over magnetic sectors alone and TOF instruments alone (see Chapter 20 for further discussion). 

Scanning techniques are carried out differently with such hybrid instruments as the triple quadrupole analyzer, the Q/TOF (quadrupole and time-of-flight), and double magnetic-sector instruments. 

A newer hybrid system available commercially is the magnetic sector—TOF hybrid (38). The precursor ions can be selected with better than unit—mass resolution by msl and the product ion ions detected at high sensitivity by the TOF ms2 (39). 

Dynamic SIMS Static SIMS Q-SIMS Magnetic SIMS Sector SIMS TOF-SIMS PISIMS... 

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