The Mass Spectrometer

Posted Aug 29, 2012 at 12:00 am

The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle. It converts the ions so they can be moved and worked by external electric and magnetic fields. It has four stages: ionization, acceleration, deflection and detection.

Figure 1 

Ionization

In this process the atom is ionized by striking one or more electrons off to give a positive ion. Mass spectrometers always work with positive ions. The particles in the sample atoms are therefore shelled with a stream of electrons, and some of the collisions are active to knock one or more electrons out of the sample particles to make positive ions.
Most of the positive ions shaped will carry a charge of positive 1 since it is much more difficult to eliminate electrons from a positive ion. These positive ions are converted out into the rest of the machine by the ion propeller which is a metal plate carrying a slight positive charge.

Figure 2 

Accelerator

The positive ions are resisted away from a positive ionization chamber and pass through three slits, the final one of which is at 0 volts. The middle slit coveys some intermediate voltage. All the ions are enhanced into a finely focused beam.

Figure 3 

Deflection

Different ions are bounced by the magnetic field by different amounts. The amount of deflection depends on the mass of the ion wherein the lighter ions are deflected more than heavier ones. Also, ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.
Most of the ions passing through the mass spectrometer will have a charge of 1+, thus, the mass/charge ratio will be the same as the mass of the ion.

Figure 4 

Detection

When an ion strokes the metal box, its charge is neutralized by an electron moving from the metal on to the ion which leaves a space between the electrons in the metal, and the electrons in the wire shuffle along to provide it.

Figure 5 

A flow of electrons in the wire is identified as an electric current which can be increased and recorded. The more ions attained, the greater the current.

Applications of Mass Spectrometers

Mass spectrometers are complex detectors of isotopes founded on their masses. They are commonly used in carbon dating and other radioactive processes. The combination of a mass spectrometer and a gas chromatograph creates a powerful tool for the recognition of trace quantities of contaminants or toxins. Mass spectrometers are also used for a number of satellites and spacecraft identification of the small numbers of particles interrupted in space. Like, the SOHO satellite uses a mass spectrometer to examine and study the solar wind. It isalso used for the analysis of residual gases in high vacuum systems.Mass spectrometers are also widely used in space missions to measure the composition of plasmas. For example, the Cassini spacecraft, which measures the mass of ions in Saturn’s magnetosphere.

Figure 6