Presentation on theme: "MASS SPECTROMETRY AP Chemistry Big Idea 1"— Presentation transcript:
1 MASS SPECTROMETRY AP Chemistry Big Idea 1 Allows for detection of atomic weightsSupplementing slide 22 from chpt 2 powerpoint
2 MASS SPECTROMETRYThe first mass spectrometer was built in 1918 by Francis W Aston, a student of J J Thomson, the man who discovered the electron.Francis Aston
3 MASS SPECTROMETRYAston used the instrument to show that there were different forms of the same element. We now call these isotopes.Francis Aston
4 Particles with a large m/z value are deflected least MASS SPECTROMETRYIn 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 leastParticles with a low m/z value are deflected most.
5 PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED A MASS SPECTROMETERDETECTORION SOURCEANALYSERA mass spectrometer consists of ... an ion source, an analyser and a detector.PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED
6 HOW DOES IT WORK? Step 1: IONIZATION DETECTORION SOURCEANALYSERStep 1: IONIZATIONgaseous atoms are bombarded by electrons from an electron gun and are IONIZEDsufficient energy is given to form ions of 1+ charge
7 HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTORION SOURCEANALYSERStep 1: IONIZATIONStep 2: ACCELERATIONions are charged so can be ACCELERATED by an electric fieldSome type of voltage
8 HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTORION SOURCEANALYSERStep 1: IONIZATIONStep 2: ACCELERATIONStep 3: DEFLECTIONcharged particles will be DEFLECTED by a magnetic or electric fieldUsually passed through an evacuated tube where they come in contact with electric or magnetic fieldThe magnetic field causes the ions to be deflected by different amounts depending on their mass. The ions are then detected and shown in a graph called a mass spectrum.
9 HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTORION SOURCEANALYSERStep 1: IONIZATIONStep 2: ACCELERATIONStep 3: DEFLECTIONStep 4: DETECTIONby electric or photographic methods
10 HOW DOES IT WORK? IONIZATION ACCELERATION DEFLECTION DETECTION DETECTORION SOURCEANALYSERIONIZATIONACCELERATIONDEFLECTIONDETECTION
11 HOW DOES IT WORK? - Deflection 20Ne21Ne22NeHEAVIER ISOTOPES ARE DEFLECTED LESSthe radius of the path depends on the value of the mass/charge ratio (m/z)ions of heavier isotopes have larger m/z values so follow a larger radius curveas most ions are 1+charged, the amount of separation depends on their mass
12 HOW DOES IT WORK? - Deflection 20Nem/z valuesABUNDANCE1+ ions2+ ions20Ne22NeDoubling the charge, halves the m/z valueAbundance stays the same21Ne22NeHEAVIER ISOTOPES ARE DEFLECTED LESSif an ion acquires a 2+ charge it will be deflected more; its m/z value is halved
13 WHAT IS A MASS SPECTRUM? MASS SPECTRUM OF NEON 20Ne %21Ne 0.26%22Ne 8.82%MASS SPECTRUM OF NEONIn early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes Ne, 21Ne and 22Ne.positions of the peaks gives atomic masspeak intensity gives the relative abundancehighest abundance is scaled to 100% and other values are adjusted accordingly
14 CALCULATING AVERAGE ATOMIC MASS Calculate the average relative atomic mass of neon using data on the previous page.Out of every 100 atoms are 20Ne , are 21Ne and are 22NeAnswer: 20.18ATIP In calculations of this type... multiply each relative mass by its abundanceadd up the total of these valuesdivide the result by the sum of the abundances;(100 in this case)* if the question is based on percentage abundance, divide by 100 but ifit is based on heights of lines in a mass spectrum, add up the heightsof the lines and then divide by that number (see later).
15 CALCULATING AVERAGE ATOMIC MASS Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is 39.1.Assume that there are n nuclei of 39K in every 100;there will then be (100-n) of 41K.SoAnswer: % 39K and 5% 41K
16 Closest Element: Zirconium (Zr, Z=40) IDENTIFY THE ELEMENTFirst, calculate average atomic massThen find the element with the closest mass on periodic tableAverage Atomic Mass = 91.32Closest Element: Zirconium (Zr, Z=40)