THE MASS SPECTROMETER WHAT IS A MASS SPECTROMETER An instrument to accurately determine the relative atomic mass Relative atomic mass (Ar) is the average mass of atoms of an element relative to an atom of carbon – 12 Separates atoms or molecules according to their charge and mass. This can be used to identify substances  e.g. illegal drugs

THE MASS SPECTROMETER THE LAYOUT 5 key stages: Vaporisation Ionisation Acceleration Deflection Detection ---- = heavy ions ---- = ions reaching detector ---- = light ions

THE MASS SPECTROMETER SUMMARISING WHAT HAPPENS: 1. Ionisation: Atoms are converted to ions 2. Acceleration: Ions are accelerated 3. Deflected: Deflected according to their mass & charge 4. Detection: They arrive at a detector CONDITIONS: Vacuum  so ions do not collide with air molecules (might stop them reaching the detector) Gaseous State  solids are vaporised before being injected

THE MASS SPECTROMETER LOOK IN MORE DETAIL: Stage 1: Ionisation Beam of electrons knocks electrons from atoms or molecules in the sample. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Nearly all lose just one electron (~5% will lose two) Mass spectrometers always work with positive ions!!

THE MASS SPECTROMETER Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection The ions are then deflected by a magnetic field according to the ratio of their mass to charge (m/z), where z is the charge (usually +1) Heavier ions are deflected less than light ones 2+ ions are deflected twice as much as 1+ ions

THE MASS SPECTROMETER Stage 4: Detection Magnetic field is gradually increased  increases deflection This allows ions of increasing mass to enter the detector On striking the detector ions accept electrons, lose their charge and create a current Current created is proportional to the abundance of each ion

Mass spectra of zirconium THE MASS SPECTROMETER MASS SPECTRA OF ELEMENTS From the strength of the magnetic field at which a particular ion hits the detector the value of the mass to charge ratio (called m/z) is calculated A graph is produced (mass spectra) showing the relative abundances of each ion type Mass spectra of zirconium

THE MASS SPECTROMETER MASS SPECTRA OF ELEMENTS We can use the mass spectrometer to identify the different isotopes making up an element Each isotope is detected separately because they have different masses To calculate an elements relative atomic mass (which is given in the periodic table) you must take account of the relative abundances of each isotope Zirconium has 5 isotopes!

THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS This is the mass spectra for chlorine We have 2 isotopes with relative isotopic masses of 35 and 37, detected in a ratio of 3:1 (or 75%:25%) To calculate the relative atomic mass of chlorine: (35 x 75) + (37 x 25) 100 = 35.5 Check the Ar of Chlorine in your periodic table

THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS RAM of Cl = 35.5 Notice there is no line at 35.5 on the mass spectra. No atoms of Cl actually have this mass. It is the average of all the isotopes and their abundances! STEPS: Multiply the m/z value by the relative abundance % for each peak Add results for each peak together Divide by total relative abundance

THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS Calculate the RAM of the element from its mass spectra: Boron Zirconium 100 51.5 23 17.1 17.4 11.2 2.8 Most abundant assigned 100 Use the percentage detected OR

THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS (10 x 23) + (100 x 11) 123 RAM = 10.8 Boron 100 23

THE MASS SPECTROMETER CALCULATING RAM OF ELEMENTS RAM = 91.3 (90 x 51.5) + (91 x 11.2) + (92 x 17.1) + (94 x 17.4) + (96 x 2.8) 100 RAM = 91.3 51.5 17.1 17.4 11.2 2.8

THE MASS SPECTROMETER DOUBLY CHARGED IONS: It’s not always as simple as we’ve just made it During ionisation a small number of ions with a 2+ charge are created These are deflected more than singly charged ions (behave as if they have half the mass) A closer look Cl shows peaks at m/z 17.5 and 18.5. Because double ionisation is rare these peaks have very small abundances

THE MASS SPECTROMETER CHLORINE GETS TRICKIER… Chlorine can form a molecule  Cl2 This can also be ionised, creating molecular ions Its spectra will therefore show 3 other peaks: - At m/z 70, due to 35Cl 35Cl - At m/z 72, due to 35Cl 37Cl - At m/z 74, due to 37Cl 37Cl

THE MASS SPECTROMETER SPECTROMETERS IN SPACE Space probes will carry spectrometers to identify elements in rocks E.g. Giotto Space Probe passing close to Halley’s comet Remember: we need a vacuum for a mass spectrometer to work. This is easy for a space probe as space is a vacuum!!

THE MASS SPECTROMETER DATA FOR HALLEY’S COMET Greenburg J. M (2002). Cosmic Dust and Our Origins. Surface Science. 500: 793-822