MS Instruments

Mass Spectrometer Instruments

All mass spectrometers count the number of atoms of the chosen isotopes, which are differentiate on the basis of their mass to charge ratio, how the mass spectrometer (MS) achieves this varies with the type of instrument. Mass spectrometers are also additionally classified by the type of preparation unit that is attached to it. The first process in the mass spectrometer is to create the ions, then these ions will be accelerated into the analyser and then impact on to the detectors .

Ion source

Common to all MS is the ion source. To create the ions, the sample gases are injected into an ionizing chamber. The ionization is created by having a beam of electrons traversing this chamber. This chamber also contains a transverse magnetic field, to force the ions into a spiral path to make a greater path length for the ionization process to occur in and improve the efficiency. After the ions are created, they are initially propelled out the ion chamber, by the electrostatic forces created by the voltages on the ion repeller plate, and then subsequently by the focusing plates. After this point the ions coast down the flight tube. When tuning a MS, the voltages on the ion source, electron trap voltages, ion repellor plates, half plates and accelerating voltage are adjusted to optimize ionization and ion beam quality.

Mass Analysers

Magnetic Sector MS

This MS is so named because the analyser uses either a permanent magnet or an electromagnet. In this system the ions are directed into the analyser and as the ions traverse the magnetic field the radius of arc over which they travel will be proportional to the mass to charge ratio. For a given ion, the greater the charge the smaller the radius, and for a given charge the smaller the mass the smaller the radius of the arc. This system works well for lower m/z ration ions and has good mass resolution.

Quadrupole MS

A quadrupole Mass spectrometry uses a resonate cavity created by an oscillating magnetic field to selective allow an ion with a particular m/z ratio to pass through. This system has the advantage of having the ability to handle a large range of m/z ratios and is suited to large molecular masses.

Time of Flight MS

This system use an electric field to accelerate the ions through a constant potential and differentiates the ions by their travel time. The lighter the ion the quicker the travel time. These MS theoretically have an infinite mass range, and a high sensitivity.

Detectors

The detector system measures the ion current after it has passed through the analyser. Theses currents are small (pico amperes) and the detector may employ some form of ion multiplier or use a Faraday cup with a charge sensitive amplifier to get the signal to usable levels. To increase the range of ions (isotopes) that can be collected the MS may use multiple Faraday cups in the detector stage.

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