4.2.6.5 Double Focusing Systems
The magnetic sector MS described earlier is referred to as a single-focusing instrument since it only uses the magnetic component to separate ion mass to charge ratios. This can be improved by adding a second electrostatic-field based mass filter, and is referred to as double focusing. A magnetic field instrument focuses on the distribution of translational energies imparted on the ions leaving the ionization source as a means of separation. But in doing so, the magnetic sector instruments broaden the range of kinetic energies of the ions, resulting in a loss of resolution. If we combine both separation techniques by passing the ions separately through an electrostatic field (to focus the kinetic energy of the ion packet) and magnetic field (to focus the translational energies of the ion packet), we will greatly improve our resolution. In fact, by doing this we can measure ion masses to within a few parts per million (precision) which results in a resolution of ~2500. Compare this to the unit resolutions (28 versus 29 Daltons) discussed at the beginning of this section (under resolution). On the downside, these instruments can be costly.
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