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MASS SPECTROMETRY MASS SPECTROMETRY CONTENTS • Prior knowledge • Background information • The basic parts of a mass spectrometer • The four stages of obtaining a spectrum • How different ions are deflected • Calculating molecular masses using mass spectra • Example questions • Test questions • Other uses of mass spectrometry • Check list 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. 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 Francis Aston 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. USES Mass spectrometry was initially used to show the identity of isotopes. It is now used to calculate molecular masses and characterise new compounds A MASS SPECTROMETER DETECTOR ION SOURCE ANALYSER A mass spectrometer consists of ... an ion source, an analyser and a detector. PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTED HOW DOES IT WORK? DETECTOR ION SOURCE ANALYSER IONISATION • gaseous atoms are bombarded by electrons from an electron gun and are IONISED • sufficient energy is given to form ions of 1+ charge HOW DOES IT WORK? DETECTOR ION SOURCE ANALYSER 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 HOW DOES IT WORK? DETECTOR ION SOURCE ANALYSER 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 HOW DOES IT WORK? DETECTOR ION SOURCE ANALYSER 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 HOW DOES IT WORK? DETECTOR ION SOURCE ANALYSER 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 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 2+ ions 22Ne 1+ ions 20Ne ABUNDANCE 21Ne 22Ne HEAVIER ISOTOPES ARE DEFLECTED LESS 0 4 8 12 16 20 m/z values Doubling the charge, halves the m/z value Abundance stays the same • 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 • if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved WHAT IS A MASS SPECTRUM? 20Ne MASS SPECTRUM OF NEON 90.92% 21Ne 0.26% 22Ne 19 20 21 22 8.82% 23 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 EXAMPLE CALCULATION (1) Calculate the average relative atomic mass of neon using data on the previous page. Out of every 100 atoms... Average = TIP 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are (90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179 100 In calculations of this type... 22Ne Ans. = 20.18 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). EXAMPLE CALCULATION (2) 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 x nuclei of there will then be (100-x) of 41K. so thus 39x + 41 (100-x) = 39.1 100 - 2x = - 190 39K in every 100; therefore 39 x + 4100 - 41x = 3910 so x = 95 Ans. 95% 39K and 5% 41K 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 CAN YOU DO ALL OF THESE? YES NO