FT-ICR mass spectrometer(1)

FT-ICR mass spectrometer is famous for its high resolution of +/- 2 parts per million. FT-MS keep the ions confined in the high magnetic field by inducing an alternating current on the metal plates. The ions circle with frequencies that are inversely proportional to the mass charge ratio. The frequency spectrum of ions is converted into mas spectrum by  Fourier transformation. Since frequency can be measured very accurately, FT-MS has very high resolution.

FT-ICR

1. important figures of MS
1.1 resolution
      m/z value divided by the peak width at half height

MS spectra

1.2 mass deviations
One important performance figure is an instruments mass resolution, typically Δm/z
eg. Δm/z=<observed m/z>-<exact m/z>=+/- 3 (3Da/charge)

High resolution instruments often report relative mass errors: ppm(Parts-per-million)
ppm=(observed - exact)/exact * 1000000

In general a |ppm| difference smaller than 5 is quite good.

1.3 monoisotopic mass and average mass
For a given compound, the monoisotopic mass is the mass of the isotopic peak whose elemental composition is composed of the most abundant isotopes of those elements. The monoisotopic mass can be calculated using the atomic masses of the isotopes.The average mass is the weighted average of the isotopic masses weighted by the isotopic abundances.  The average mass can be calculated using the atomic weights of the elements.
The exact mass/charge of an ion is calculated from the mono-isotopic mass,not the average, for each element in its elemental composition. An electron mass (or several) must be added or taken away to get the exact ion mass/charge (and divide by the charge if doubly, triply, etc. charged).