MASS SPECTROMETRY AP Chemistry Big Idea 1 Allows for detection of atomic weights Supplementing slide 22 from chpt 2 powerpoint

MASS SPECTROMETRY The 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

MASS SPECTROMETRY Aston used the instrument to show that there were different forms of the same element. We now call these isotopes. Francis Aston

Particles with a large m/z value are deflected least MASS SPECTROMETRY 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 Particles with a low m/z value are deflected most.

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

HOW DOES IT WORK? Step 1: IONIZATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION gaseous atoms are bombarded by electrons from an electron gun and are IONIZED sufficient energy is given to form ions of 1+ charge

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION ions are charged so can be ACCELERATED by an electric field Some type of voltage

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION Step 3: DEFLECTION charged particles will be DEFLECTED by a magnetic or electric field Usually passed through an evacuated tube where they come in contact with electric or magnetic field The 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.

HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION DETECTOR ION SOURCE ANALYSER Step 1: IONIZATION Step 2: ACCELERATION Step 3: DEFLECTION Step 4: DETECTION by electric or photographic methods

HOW DOES IT WORK? IONIZATION ACCELERATION DEFLECTION DETECTION DETECTOR ION SOURCE ANALYSER IONIZATION ACCELERATION DEFLECTION DETECTION

HOW DOES IT WORK? - Deflection 20Ne 21Ne 22Ne HEAVIER ISOTOPES ARE DEFLECTED LESS the 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 curve as most ions are 1+charged, the amount of separation depends on their mass

HOW DOES IT WORK? - Deflection 20Ne 0 4 8 12 16 20 m/z values ABUNDANCE 1+ ions 2+ ions 20Ne 22Ne Doubling the charge, halves the m/z value Abundance stays the same 21Ne 22Ne HEAVIER ISOTOPES ARE DEFLECTED LESS if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

WHAT IS A MASS SPECTRUM? MASS SPECTRUM OF NEON 19 20 21 22 23 20Ne 90.92% 21Ne 0.26% 22Ne 8.82% MASS SPECTRUM OF NEON In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes ... 20Ne, 21Ne and 22Ne. positions of the peaks gives atomic mass peak intensity gives the relative abundance highest abundance is scaled to 100% and other values are adjusted accordingly

CALCULATING AVERAGE ATOMIC MASS Calculate the average relative atomic mass of neon using data on the previous page. Out of every 100 atoms... 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne Answer: 20.18 A TIP In calculations of this type... multiply each relative mass by its abundance add up the total of these values divide the result by the sum of the abundances; (100 in this case) * if the question is based on percentage abundance, divide by 100 but if it is based on heights of lines in a mass spectrum, add up the heights of the lines and then divide by that number (see later).

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. So Answer: 95% 39K and 5% 41K

Closest Element: Zirconium (Zr, Z=40) IDENTIFY THE ELEMENT First, calculate average atomic mass Then find the element with the closest mass on periodic table Average Atomic Mass = 91.32 Closest Element: Zirconium (Zr, Z=40)