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).