2 Spectrometry

The Time of Flight Mass Spectrometer The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements. The following are the essential 4 steps in a mass spectrometer. 1. Ionisation The sample can be ionised in a number of ways. Two of these techniques are electron impact and electrospray ionisation Electron impact •A Vaporised sample is injected at low pressure •An electron gun fires high energy electrons at the sample •This Knocks out an outer electron •Forming positive ions with different charges E.g. Ti  Ti+ + e– Electro Spray Ionisation • The sample is dissolved in a volatile, polar solvent • injected through a fine hypodermic needle giving a fine mist or aerosol • the tip of needle has high voltage • at the tip of the needle the sample molecule, M, gains a proton, H+ , from the solvent forming MH+ • M(g) + H+  MH+ (g) • The solvent evaporates away while the MH+ ions move towards a negative plate Learn all these steps carefully! It needs to be under a vacuum otherwise air particles would ionise and register on the detector There are various models for atomic structure  1 Ionisation area Acceleration area Ion drift area Detection area Ion detector Heavy ions Light ions Time measurement Electron impact is used for elements and substances with low formula mass. Electron impact can cause larger organic molecules to fragment Electro Spray Ionisation is used preferably for larger organic molecules. The softer conditions of this technique mean fragmentation does not occur 2. Acceleration •Positive ions are accelerated by an electric field •To a constant kinetic energy KE = ½ KE = kinetic energy of particle (J) m = mass of the particle (kg) = velocity of the particle (ms–1 ) 3. Flight Tube •The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster. •The heavier particles take longer to move through the drift area. •The ions are distinguished by different flight times = / t = time of flight (s) d = length of flight tube (m) = velocity of the particle (m s–1 ) 4. Detection •The ions reach the detector and generate a small current, which is fed to a computer for analysis. The current is produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the abundance of the species You don’t need to learn these equations but may be asked to use them in a calculation Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity. Combining the two equations gives you Rearranged gives Example A sample of Nickel was analysed and one of the isotopes found was 59Ni. The ions were accelerated to have 1.000 x 10-16 J of kinetic energy and travelled through a flight tube that was 0.8000 m long. How long would one ion of 59Ni+ take to travel along the flight tube? The Avogadro constant L = 6.022 × 1023 mol–1 Mass of one ion of 59Ni+ = mass of one mole of 59Ni+ The Avogadro constant = 59/ 6.022 × 1023 = 9.797X10-23 g = 9.797X10-26 kg t= 0.8000 √( 9.797X10-26/(2x 1.000 x 10-16)) t=1.771X10-5 s