Sector Mass Analysis

The sector mass analyser is one of the oldest types of mass spectrometer and until the last 25 years was probably the most common and familiar to the everyday scientist. In the 1950's, the first commercial mass spectrometers were sector instruments. They consist of some combination of a large electromagnetic ('B' sector), and some kind of electrostatic focussing device ('E' sector) - different manufactures use differing geometries. The figure shows a schematic of a standard 'BE' geometry double focussing instrument - that is, a dual sector instrument consisting of a magnetic sector followed by an electrostatic sector.



Ions enter the instrument from the source (bottom left) where they are initially focussed. They enter the magnetic sector through the source slit where they are deflected according to the left-hand rule. Higher-mass ions are deflected less than lower-mass ions. Scanning the magnet enables ions of different masses to be focussed on the monitor slit. At this stage, the ions have been separated only by their masses. To obtain a spectrum of good resolution - i.e. where all ions with the same m/z appear coincident as one peak in the spectrum, ions have to be filtered by their kinetic energies. After another stage of focussing the ions enter the electrostatic sector where ions of the same m/z have their energy distributions corrected for and are focussed at the double focussing point on the detector slit.

Sector instruments had huge commercial successes in the 1950's and 1960's as they were the only practical way of obtaining high-resolution data. In the last 25 years or so, with the decreasing prices of FT-MS and the development of high-resolution alternatives (for example TOF) sector instruments are in decline. They still have their applications though, and are very well suited to EI and CI ionisation and consequently GC-MS. Single sector instruments are also used in the specialist area of isotope ratio mass spectrometry (IRMS).