AQA AS Chemistry Unit 1 Chapter 1 - Atomic Structure

What are mass spectrometers?

• Instruments that measure relative atomic and molecular masses accurately on a scale on which the mass of a 12-Carbon atom is defined as exactly 12.
• They determine the mass of separate molecules or atoms.

Uses / What for?

• Forensic Scientists use them to determine illegal drugs!

How it works The instrument is kept under high vacuum to stop air molecules colliding with the ions which will inhibit them from being detected. The sample must be investigated in a gaseous state.

Ionisation: The electron gun is a heated cathode that sends a beam of electrons across the instrument. An electron is then knocked off the atom or molecule. Most of the time this leaves a +1 charge ion but about 5% of time an ion with a 2+ charge is formed. Acceleration: The positive ions are attracted to negatively charged plates which accelerate them to very high speeds. The amount of speed that an ion has is governed by mass. Lighter ions travel faster. There are some slits in the negatively charged plates which the ions go through making a beam. Deflection: The positive ions then travel through a magnetic field which acts at right angles towards their direction of travel. The ions are deflected through an arc. The magnitude of deflectition is dependent upon the mass:charge ratio (m/z). Detection: The strength of the magnetic field is increased so that ions of increasing mass are detected. When the ions hit the detector they accept electrons and set up small current proportional to ion abundance. The strength at which the ion hits the detector causes readings for the m/z on a mass spectrum.

The different isotopes that make up an element may be identified as this spectrometer detects individual ions which have different masses.

High Resolution mass spectrometry: The masses may be measured up to five decimal places.

Relative atomic masses are weighted averages of the mass of isotopes of the element, taking into account both masses and abundances relative to an atom of 12-Carbon isotope which is exactly 12.