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Isotopes & Mass Spectrometry

Isotopes & Mass Spectrometry. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. A. X. Mass Number. Element Symbol. Z. Atomic Number. Isotopes. Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei. Atomic number (Z) = number of protons in nucleus.

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Isotopes & Mass Spectrometry

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  1. Isotopes & Mass Spectrometry 2008 SPECIFICATIONS KNOCKHARDY PUBLISHING

  2. A X Mass Number Element Symbol Z Atomic Number Isotopes Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons 2.3

  3. Charge (if ion) Atomic Mass Symbol Atomic Number

  4. Hydrogen Protons: 1 Neutrons: 0 Electrons: 1 H 1 1

  5. Sodium Na+1 23 11 Protons: 11 Neutrons: 12 Electrons: 10

  6. 133 55 EXAMPLE How many protons, neutrons and electrons are found in an atom of Cs Atomic number = protons and electrons There are 55 protons and 55 electrons Mass number = sum of protons and neutrons 133 – 55 = 78 There are 78 neutrons

  7. Micro World atoms & molecules Macro World grams Atomic mass is the mass of an atom in atomic mass units (amu) By definition: 1 atom 12C “weighs” 12 amu On this scale 1H = 1.008 amu 16O = 16.00 amu 3.1

  8. 7.42 x 6.015 + 92.58 x 7.016 100 Natural lithium is: 7.42% 6Li (6.015 amu) 92.58% 7Li (7.016 amu) Average atomic mass of lithium: = 6.941 amu 3.1

  9. 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. Francis Aston USES Mass spectrometry was initially used to show the identity of isotopes. It is now used to calculate molecular masses and characterise new compounds

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 2+ ions 1+ ions 20Ne 22Ne ABUNDANCE Doubling the charge, halves the m/z value Abundance stays the same 0 4 8 12 16 20 m/z values 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 • if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

  18. 20Ne 90.92% 21Ne 0.26% 22Ne 8.82% 19 20 21 22 23 WHAT IS A MASS SPECTRUM? 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

  19. EXAMPLE CALCULATION (1) 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 Average =(90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179Ans. = 20.18 100 TIPIn 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 * 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).

  20. 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 xnuclei of 39K in every 100; there will then be (100-x) of 41K. so 39x + 41 (100-x) = 39.1 therefore 39 x + 4100 - 41x = 3910 100 thus - 2x = - 190 so x = 95Ans. 95% 39K and 5% 41K

  21. Example (3) A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions. Calculate the average relative atomic mass. 100% 60% 40% ABUNDANCE 0 10 20 30 40 m/z values ANSWERS ON NEXT PAGE

  22. Example (3) A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions. Calculate the average relative atomic mass. 100% 60% 40% ABUNDANCE 0 10 20 30 40 m/z values Average = [ (60 x 38) + (40 x 40) ] / 100 = 38.80

  23. OTHER USES OF MASS SPECTROMETRY - MOLECULAR MASS DETERMINATION - Nowadays, mass spectrometry is used to identify unknown or new compounds. When a molecule is ionised it forms a MOLECULAR ION which can also undergo FRAGMENTATION or RE-ARRANGEMENT to produce particles of smaller mass. Only particles with a positive charge will be deflected and detected. The resulting spectrum has many peaks. The final peak (M+) shows the molecular ion (highest m/z value) and indicates the molecular mass. The rest of the spectrum provides information about the structure. IONISATION MOLECULAR ION FRAGMENTION RE-ARRANGEMENT FRAGMENTION

  24. MASSSPECTRUMOFC2H5Br 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...79Br 81Br

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