Starter: Spec links 1.06–1.10

Green pens out! Spec links 1.06–1.10

Learning objectives analyse and interpret data from mass spectrometry to calculate the relative atomic mass from the relative abundance of isotopes and vice versa predict the mass spectra for diatomic molecules, for example chlorine understand how mass spectrometry can be used to determine the relative molecular mass of a molecule Spec links 1.06–1.10

Mass spectrometry? What did you learn? Spec links 1.06–1.10

WHAT IS A MASS SPECTROMETER An instrument to accurately determine the relative atomic mass THE MASS SPECTROMETER 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 first mass spectrometer was built in 1918 by Francis W Aston, a student of J J Thomson, the man who discovered the electron. Aston used the instrument to show that there were different forms of the same element. We now call these isotopes. In a mass spectrometer, particles are turned into positive ions, accelerated and then deflected by an electric or magnetic field. The resulting path of ions depends on their ‘mass to charge’ ratio (m/z). Particles with a large m/z value are deflected least those with a low m/z value are deflected most. The results produce a mass spectrum which portrays the different ions in order of their m/z value. MASS SPECTROMETRY USES Mass spectrometry was initially used to show the identity of isotopes. It is now used to calculate molecular masses and characterise new compounds Francis Aston Spec links 1.06–1.10

A mass spectrometer consists of... an ion source, an analyser and a detector. ION SOURCE ANALYSER DETECTOR A MASS SPECTROMETER PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTED Spec links 1.06–1.10

HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONISATION and VAPOURISATION gaseous atoms are bombarded by electrons from an electron gun and are IONISED sufficient energy is given to form ions of 1+ charge Spec links 1.06–1.10

HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONISATION gaseous atoms are bombarded by electrons from an electron gun and are IONISED sufficient energy is given to form ions of 1+ charge ACCELERATION ions are charged so can be ACCELERATED by an electric field Spec links 1.06–1.10

HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONISATION gaseous atoms are bombarded by electrons from an electron gun and are IONISED sufficient energy is given to form ions of 1+ charge ACCELERATION ions are charged so can be ACCELERATED by an electric field DEFLECTION charged particles will be DEFLECTED by a magnetic or electric field Spec links 1.06–1.10

HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONISATION gaseous atoms are bombarded by electrons from an electron gun and are IONISED sufficient energy is given to form ions of 1+ charge ACCELERATION ions are charged so can be ACCELERATED by an electric field DEFLECTION charged particles will be DEFLECTED by a magnetic or electric field DETECTION by electric or photographic methods Spec links 1.06–1.10

HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONISATION gaseous atoms are bombarded by electrons from an electron gun and are IONISED sufficient energy is given to form ions of 1+ charge ACCELERATION ions are charged so can be ACCELERATED by an electric field DEFLECTION charged particles will be DEFLECTED by a magnetic or electric field DETECTION by electric or photographic methods Spec links 1.06–1.10

THE LAYOUT THE MASS SPECTROMETER 4 key stages: Ionisation Acceleration Deflection Detection ---- = heavy ions ---- = ions reaching detector ---- = light ions

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 THE MASS SPECTROMETER CONDITIONS: a)Vacuum  so ions do not collide with air molecules (might stop them reaching the detector) b)Gaseous State  solids are vaporised before being injected (Gas chromatography)

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 THE MASS SPECTROMETER

THE LAYOUT THE MASS SPECTROMETER 4 key stages: Ionisation Acceleration Deflection Detection ---- = heavy ions ---- = ions reaching detector ---- = light ions

Mass spectrometry cut and stick Spec links 1.06–1.10

Mass spectrometry guide for dummies Your task is to create a how to guide about mass spectrometry You must include: A labelled diagram The name of each stage What happens at each stage Spec links 1.06–1.10

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 THE MASS SPECTROMETER Mass spectra of zirconium positions of the peaks gives atomic mass

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 THE MASS SPECTROMETER Zirconium has 5 isotopes!

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 THE MASS SPECTROMETER Check the A r of Chlorine in your periodic table

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! THE MASS SPECTROMETER STEPS: 1.Multiply the m/z value by the relative abundance % for each peak 2.Add results for each peak together 3.Divide by total relative abundance

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

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

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

Mass spectrometry guide for dummies To your guide add: What a mass spectra shows How you work out RAM using mass spectrum Spec links 1.06–1.10

Mass Spec of a compound When the molecule passes through the mass spectrometer it is broken into fragments which form ions and are detected. This process is called fragmentation. The pattern of peaks is called a fragmentation pattern. The last peak is an ion made from the complete molecule. It is called the molecular ion peak The biggest most stable peak is called the base peak Joint Chemistry Sixth form

MASSSPECTRUMOFC 2 H 5 Br MASS SPECTRUM OF C 2 H 5 Br The final peak in a mass spectrum is due to the molecular ion. In this case there are two because Br has two main isotopes. As each is of equal abundance, the peaks are the same size. molecular ion contains Br 81 Br Spec links 1.06–1.10

Past paper questions… Spec links 1.06–1.10

REVISION CHECK What should you be able to do? Recall the four basic stages in obtaining a mass spectrum Understand what happens during each of the above four stages Understand why particles need to be in the form of ions Recall the the meaning of mass to charge ratio (m/z) Explain how the mass/charge value affects the path of a deflected ion Interpret a simple mass spectrum and calculate the average atomic mass Understand how mass spectrometry can be used to calculate molecular mass CAN YOU DO ALL OF THESE? YES NO Spec links 1.06–1.10