AQA AS Physical Chemistry - Mass Spec PDF

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE TIME OF FLIGHT MASS SPECTROMETER 1 2 3 4 5...

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE TIME OF FLIGHT MASS SPECTROMETER 1 2 3 4 5 e- e- + + e- e- e- 1 Vaporisation The sample is “vaporised” so all so all atoms / molecules are completely separated. These are injected into the spectrometer. All done in a vacuum to prevent interference from other substances. 2 Ionisation The atoms/molecules are “ionised” to give a 1+ charge. This is so that they can be accelerated by the charged plates and create a current in the detector. Two ways this can be achieved: A) Electron Impact: Electrons are fired at the sample by an electron gun. These remove an electron from the atoms / molecules to create a 1+ ion. X(g) + e- → X+(g) + 2e- B) Electrospray Ionisation: Sample is dissolve din a polar solvent and is passed through a needle with a high voltage across it. This causes them to GAIN a proton (H+) to create a 1+ ion. This causes the m/z value to increase by 1! X(g) + H+ → XH+(g) 3 Acceleration The positive ions are accelerated by a negatively charged plate. All ions are given the same kinetic energy. This means that lighter ions travel faster, heavier ions travel slower! 4 Ion Drift The flight tube is very long, so causes the ions to drift apart. The lightest ions get to the detector first, then second lightest etc, and the heaviest ions get there last. Hence they are separated by mass so we can measure them. 5 Detection The 1+ ions hit the negatively charged plate. They attract electrons towards them to create a current. The current is measured so that we are able to quantify two things: A) The time it took to reach the detector (and therefore calculate the m/z value) B) The abundance (how many there were). The more there are, the greater the current. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE MASS SPECTRUM e.g. silicon 100 92.2% Si has 3 stable isotopes: 28Si, 29Si, 30Si Everything that comes out of Relative a mass spectrometer has a 50 Abundance 1+ charge. (%) So, m/z = isotopic mass of 4.7% 3.1% the isotope since 0 m/z = mass/charge. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 m/z (Mass:charge ratio) CALCULATING RELATIVE ATOMIC MASS (Ar) Ar Si = (28 x 92.2) + (29 x 4.7) + (30 x 3.1) = 28.1 g.mol-1 100 This calculation finds the weighted average for a silicon atom. HINTS | TIPS | HACKS We divide by 100 as, most of the time, abundance is given as a percentage! Sometimes abundance is given as a relative value and NOT a percentage. In which case you replace the “100” in the equation for the sum total of the abundances. How To Tackle Ar Isotopic masses are always whole numbers (integers). Calculation Questions Ar values tend to give to 1d.p. In exam questions, you may be given the Ar value and asked to find the isotopic mass of missing isotopes. Be prepared to rearrange the equation to do so! When asked “which species” causes a peak, ALWAYS be sure to put a “+” sign on it! e.g. Which species causes a peak at m/z 28? …. answer 28Si+ AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE DIATOMIC ELEMENTS e.g. chlorine Diatomic elements such as chlorine show both 9 individual Cl+ ions: 3 35Cl+ & 37Cl+ 6 Relative and molecular Cl2+ ions that Abundance contain different 1 combinations of Cl isotopes: 1 (35Cl-35Cl)+ m/z 70 (35Cl-37Cl)+ m/z 72 10 20 30 40 50 60 70 80 90 (37Cl-37Cl)+ m/z 74 m/z (Mass:charge ratio) THE 9:6:1 RATIO The 35Cl isotope is approximately 3 x more abundant than the 37Cl isotope. i.e. they exist in a 3:1 ratio. Therefore, the chances of a 35Cl being in a Cl2 molecule is 3 in 4. the chances of a 37Cl being in a Cl molecule is 1 in 4. 2 The Statistics The chances of creating a 35Cl-35Cl molecule = 3 x 3 = 9 The chances of creating a 35Cl-37Cl or 37Cl-35Cl molecule = (3 x 1) + (1 x 3) = 6 The chances of creating a 37Cl-37Cl molecule = 1 x 1 = 1 Therefore the ratio of the peaks is 9:6:1 Watch out for this in your exam! Also…. bromine 79Br -81Br 2 79Br 81Br Relative 11 1 1 Abundance 79Br -79Br 81Br -81Br 80 90 100 110 120 130 140 150 160 170 m/z (Mass:charge ratio) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE VELOCITY CALCULATIONS When accelerated, ALL ions gain the SAME KINETIC ENERGY. This is key here. However, as they have different masses, their VELOCITY will be different. Where mass is low, velocity is high and vice versa. K.E. = ½ m.v2 K.E. = Kinetic Energy m = mass of a single ion (kg) v = velocity (m.s-1) Also…. v=d/t v = velocity (m.s-1) d = distance (length of flight tube) (m) t = time taken to reach detector (s) You may be asked to calculate any one of the following: Kinetic Energy, Mass of isotope (via m), Velocity, Length of flight tube (Distance) or Time! HINTS | TIPS | HACKS To find velocity… v2 = 2K.E. … then square root your answer m To find mass… m = 2K.E. v2 d If asked to find distance (length of flight tube) or time taken….. v t How To Tackle Velocity Calculations AQA www.chemistrycoach.co.uk © scidekick ltd 2024