Topic 2.2: The mass spectrometer

2.2.1 Describe and explain the operation of a mass spectrometer

Mass spectrometer is used to measure different isotopes and their relative abundances.

The mass spectrometer has five basic operations:

Vaporization: A vaporized sample is injected into the instrument. This allows the individual atoms of element to be analysed

Ionization: The atoms are hit with high-energy electrons which knock out electrons, thus producing positively charged ions.

Acceleration: The positive ions are attracted to negatively charged plates. They are accelerated by an electric field and pass through a hole in the plate.

Deflection: The accelerated positive ions are deflected by a magnetic field placed at the right angles of their path. The amount of deflection is proportional to the charge/mass ratio. Ions with smaller mass are deflected more than heavier ions. Ions with higher charger are deflected more as they interact more efficiently with the magnetic field.

Detection: Positive ions of a particular mass/charge ratio are detected and a signal sent to a recorder. The strength of the signal is a measure of the number of ions with that charge/mass ratio that are detected.

2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12C scale

Relative atomic mass uses the 12 C scale because using the actual mass of individual atoms are beyond our experiences. For example hydrogen atom is 1.67 x 10^-24 g in mass or 1.001 in RAM.

As carbon is a very common element which is easy to transport and store because it is a solid, its isotope, 12 C, was chosen as the standard in 1961.

2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.

This is a simple Mass Spectrum. The horizontal axis shows the mass/charge ratio of different ions on the carbon-12 scale and the relative abundance is shown on the vertical axis. Normally the relative abundances come in percentages.

So the percentages should be 75% and 25%.

(75 x 35) + (25 x 37) = 3550 = total mass

3550/100 = 35.5 = average mass

There are normally more m/z than 2 therefore you should really practice using a variety of numbers.