Noise mass analyzers

The ion-trap mass spectrometer uses three electrodes to trap ions in a small volume. The mass analyzer consists of a ring electrode separating two hemispherical electrodes. A mass spectrum is obtained by changing the electrode voltages to eject the ions from the trap. The advantages of the ion-trap mass spectrometer include compact size and the ability to trap and accumulate ions thus increasing the signal-to-noise ratio of a measurement [534,535,551, 553]. 

Traditionally thermal ionization mass spectrometry was the instrument of choice for the isotopic analysis of metals because thermal ionization produced an ion beam with a very small kinetic energy spread ( 0.5 eV). Therefore only a magnetic mass analyzer is needed to resolve one isotope from another. Moreover, ionization of unwanted material, such as atmospheric contaminates, hydrocarbons from pump oil, or production of doubly ionized particles is almost non existent, thus background counts are minimized and signal-to-noise ratio is maximized. 

Figure 1.9. Triple-stage quadrupole Quantum mass spectrometer capable of operating under enhanced resolution conditions. To reduce the chemical noise and to improve the sensitivity, the mass analyzers were oriented in an L shape rather than the conventional straight design. An additional benefit of the L shape orientation is a smaller foot print. (Courtesy of ThermoFisher Scientific.)...

Two approaches are often used to improve the detection limit, including selected ion monitoring (SIM) and multiple reaction monitoring (MRM). In LC/MS studies, it is often desirable to increase detection sensitivity by hmit-ing the mass analyzer scan to just one ion— that is, SIM. In this mode, a single ion of interest is monitored continuously by a mass spectrometer and no other ions are detected. This results in signihcant improvement of signal-to-noise ratio. SIM trades specihcity for sensitivity. In general, the sensitivity in SIM is increased by a factor of 100 to 1000 over full-scan mass spectra. This can be quite useful in detection and quantihcation of specihc compounds at low levels. 

ESI and MALDI sources have been coupled to many different mass analyzer types. Of these mass analyzers, five will be discussed next and are shown in Figure 4.5. To achieve the highest performance from these analyzers, an experienced operator will tune the mass spectrometer to improve the resolution, sensitivity, and signal-to-noise SIN) level. Tuning is essential to achieve optimum performance, but it differs for each mass spectrometer and will not be covered in this chapter. 

Ion mobility can add an extra dimension of separation when coupled to a mass analyzer. Ion mobility spectrometry (IMS) separates ions according to their interactions with a buffer gas, in addition to differences in their m/z ratios. This can provide separation of ions (i.e., isobaric or conformational isomers), which cannot be accomplished using traditional mass analyzers. It can also be used to reduce interfering chemical noise. Ion mobility separates ions based on how long they take to migrate... 

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