WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

There are four main steps: 1. Ionisation  The vaporised sample passes into the ionisation chamber.

Exam tip You do not need to be able to draw a diagram of a mass spectrometer but you may be asked to label one and describe the function of each part.

The particles in the sample (atoms or molecules) are bombarded with a stream of electrons, and some of the collisions are sufficiently energetic 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. 2. Acceleration  An electric field accelerates the positive ions to high speed. 3. Deflection  Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on: ▪ The mass of the ion. Lighter ions are deflected more than heavier ones. ▪ The charge on the ion. Ions with two (or more) positive charges are deflected more than ones with only one positive charge. These two factors are combined into the mass/charge ratio m / z . (Unless otherwise stated, mass spectra given will only involve 1+ ions, so the mass/charge ratio will be the same as the mass of the ion.) 4. Detection  The beam of ions passing through the machine is detected electrically. Only ions with the correct mass/charge ratio make it all the way through the machine to the ion detector. (The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by the vacuum pump.) The signal is then amplified and recorded. For the ions with an incorrect mass/charge ratio to reach the detector, the strength of the magnetic field can be varied. For example, in the diagram on page 33, the beam of ions with the black dotted line is the most deflected. For the ions to reach the detector, they need to be deflected less so a smaller magnetic field would be necessary. It’s important that the ions produced in the ionisation chamber have a free run through the machine without hitting air molecules so a vacuum is needed inside the apparatus.

Stretch & challenge

During the detecting stage, when an ion hits the metal box, its charge is neutralised by an electron jumping from the metal to 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.

2 Knowledge check State how positive ions are produced in a mass spectrometer.

Calculating relative atomic mass As stated, data from mass spectrometry can be used to calculate relative atomic mass, e.g. on the right is the mass spectrum of lead The relative atomic mass is a 1.4 Relative percentage abundance

52.4

Knowledge check

3

A mass spectrum of a sample of hydrogen showed that it contained 1 H 99.20% and 2 H 0.8000%. Calculate the relative atomic mass of the hydrogen sample, giving your answer to four significant figures.

24.1

weighted average of the masses of all the atoms in the isotopic mixture, therefore: relative atomic mass = DRAFT (1.40 × 204) + (24.1 × 206) + (22.1 × 207) + (52.4 × 208) 100 = 207.2 Other uses of mass spectrometry include: ▪ Identifying unknown compounds, e.g. testing athletes for prohibited drugs. ▪ Identifying trace compounds in forensic science. ▪ Analysing molecules in space. 204 22.1 205 206 207 208 Mass/charge ratio 34