Electrostatic sector

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

The electrostatic sector consists of two curved parallel plates between which is applied a potential difference producing an electric field of strengtir E. Transmission of an ion tlirough the sector is governed by the following relationship... 

Electrostatic Analyzer In magnetic-sector instruments, an electrostatic sector can be incorporated either before or after the magnet to provide energy resolution and directional focusing of the ion beam. The resolution achievable in these double-focusing instruments is sufficient to separate ions having the same nominal mass (e.g., 28 Daltons) but with different chemical formula (e.g., N2 and CO). 

Magnetic and electrostatic sectors, quadrupole, and time of flight analyzers belong to the first group, while ion trap, Orbitrap and Fourier transform ion cyclotron resonance analyzers separate ions in time. 

The first instruments used a single magnetic sector (symbol B) to effect separation of the ions. Later, the introduction of double-focusing instruments having an electrostatic sector or electrostatic analyzer (ESA, symbol E) in addition defined a standard which is still valid. 

The electrostatic sector or electrostatic analyzer (ESA) produces a radial electric field between two oppositely charged plates extending over the ESA angle ( ) (Fig. 4.21). An ion passes the ESA midway on a circular path if... 

Electrostatic sector field equilibr electrostatic force and centrifugal force e-E0 = — 2 - energy focusing 50 double-focusing sector field ms, tandem ms... 

Currently used magnetic analyser mass spectrometers constitute a logical evolution of the previously described instrument. They provide very accurate m/z values, but are limited for high mass analyses (problems produced in the magnetic sector). They also include an electrostatic sector E placed after ion acceleration and before the magnetic field, B (Fig. 16.3). 

Figure 22-12 Electrostatic sector of a double-focusing mass spectrometer. Positive ions are attracted toward the negative plate. Trajectories of high-energy ions are changed less than trajectories of low-energy ions. Ions reaching the exit slit have a narrow range of kinetic energies.

Laser matrix time of flight Electrostatic quadrupole Time of flight Electrostatic-sector... 

When bringing the magnetic sector and the electrostatic sector together, a double-... 

EB...) Electrostatic sector kinetic collisions usually coupled to ... 

B indicates a magnetic sector, E an electrostatic sector. The most common setups are double-focusing instruments with either a Nier-Johnson geometry (EB) or an inverse Nier-Johnson configuration (BE). Extended setups such as BEBE are available and can be used for up to MS experiments. 

They also imply that it is possible to discriminate between ions according to their velocity (the electrostatic sector) as well as to their mass (magnetic sector). It is this combined abihty that allows for the performance of so-called high resolution mass measurements. Mass spectrometers could be thought of as very precise balances. So precise in fact that they can differentiate between the masses of CO", N2+. and C2H4+. (respectively 27,9949, 28,0071, and 28,0313 daltons). The resolution required to differentiate these species is defined as the ratio of the mass to be measured to the incremental mass to be determined (i.e. MJ DM). In this particular case to differentiate between CO and N2+, requires a resolution of about 2 300 (from [28 / (28,0071 - 27,9949)]). The resolution is affected by several factors such as the rate at which the fields can be scanned, the difference between the masses to be determined, etc. 

---