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 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

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

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

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

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

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

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

HOW DOES IT WORK? - Deflection 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 20 Ne 21 Ne 22 Ne HEAVIER ISOTOPES ARE DEFLECTED LESS

HOW DOES IT WORK? - Deflection 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 20 Ne 21 Ne 22 Ne HEAVIER ISOTOPES ARE DEFLECTED LESS m/z values ABUNDANCE 1+ ions2+ ions 20 Ne 22 Ne Doubling the charge, halves the m/z value Abundance stays the same

In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes Ne, 21 Ne and 22 Ne. 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 MASS SPECTRUM OF NEON Ne 90.92% 21 Ne 0.26% 22 Ne 8.82% WHAT IS A MASS SPECTRUM?

Calculate the average relative atomic mass of neon using data on the previous page. EXAMPLE CALCULATION (1) Out of every 100 atoms are 20 Ne, 0.26 are 21 Ne and 8.82 are 22 Ne Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) = Ans. = 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).

EXAMPLE CALCULATION (2) Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is Assume that there are x nuclei of 39 K in every 100; x there will then be (100-x) of 41 K. xxx so 39x + 41 (100-x) = 39.1 therefore 39 x x = xx thus - 2x = so x = 95 Ans. 95% 39 K and 5% 41 K

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