Mass Spectrometer

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve – deflected out of its original path by the lateral force. A bowling ball is moving sideways and it is attempted to be deflected with an air hose.

Well, this is not going to have much effect on the mass of a bowling ball. What if you replaced it with a ping pong ball? Now the mass is low and the ball is deflected with no problem. If you knew the speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of curved path it was deflected through. The less the deflection, the heavier the ball.

You can apply these same principles to atomic-sized particles. Magnetic fields can deflect atoms and molecules – provided the atom or molecule is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren’t.

The 4 stages of what occurs in a mass spectrometer are Ionization, Acceleration, Deflection, and Detection.

The history of mass spectrometry has its roots in physical and chemical studies regarding the nature of matter. The study of gas discharges in the mid-19th century led to the discovery of anode and cathode rays, which turned out to be positive ions and electrons.

Improved capabilities in the separation of these positive ions enabled the discovery of stable isotopes of the elements. Mass spectrometers were used in the Manhattan Project for the separation of isotopes of uranium necessary to create the atomic bomb. In 1815, the English chemist William Prout observed that the atomic weights that had been measured were integer multiples of the atomic weight of hydrogen.

Jöns Jakob Berzelius in 1828 or Edward Turner in 1832 appeared to have disproved Prout’s hypothesis. Canal rays, also called anode rays, were observed by Eugen Goldstein, in 1886. Goldstein used a gas discharge tube that had a perforated cathode. In 1913, as part of his exploration into the composition of canal rays, J. J. Thomson channeled a stream of ionized neon through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path.

Thomson observed two patches of light on the photographic plate (see image on left), which suggested two different parabolas of deflection. Thomson concluded that the neon gas was composed of atoms of two different atomic masses.

Thomson’s student Francis William Aston continued the research at the Cavendish Laboratory in Cambridge, building the first fully functional mass spectrometer that was reported in 1919. In 1921, F. W. Aston became a fellow of the Royal Society and received a Nobel Prize in Chemistry in the following year.

In 1918, Arthur Jeffrey Dempster reported on his mass spectrometer and established the basic theory and design of mass spectrometers that are still used to this day. In 1932, Kenneth Bainbridge developed a mass spectrometer and verified the equivalence of mass and energy, E = mc2.

Now let’s try to understand what is going on in the MS. It’s important that the ions produced in the ionization chamber have a free run through the machine without hitting air molecules.

The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate. Bombarded with a stream of electrons, some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions.

Most of the positive ions formed will carry a charge of +1 because it is much more difficult to remove further electrons from an already positive ion. These positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge.

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

Different ions are deflected by the magnetic field in different amounts. The amount of deflection depends on two factors which are combined into the mass/charge ratio. The mass/charge ratio is given the symbol m/z.

Most of the ions passing through the mass spectrometer will have a charge of 1+ so that the mass/charge ratio will be the same as the mass of the ion. The other ions collide with the walls where they will pick up electrons and be neutralized. Eventually, they get removed from the mass spectrometer by the vacuum pump. When an ion hits the metal box, its charge is neutralized by an electron jumping from the metal onto the ion.

That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it. A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current.

The output from the chart recorder is usually simplified into a “stick diagram”. This shows the relative current produced by ions of varying mass/charge ratios.

The vertical scale is related to the current received by the chart recorder – and so to the number of ions arriving at the detector: the greater the current, the more abundant the ion. As you will see from the diagram, the commonest ion has a mass/charge ratio of 98.

Other ions have mass/charge ratios of 92, 94, 95, 96, 97, and 100. That means that molybdenum consists of 7 different isotopes. Assuming that the ions all have a charge of 1+, that means that the masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98, and 100.