AQA A-level Chemistry Topic 1.4 - TOF Mass Spectrometry Questions and Answers

AQA A-level Chemistry Topic 1.4 - TOF Mass Spectrometry Questions and Answers What are the two types of ionisation for a mass spectrometer? Electron impact Electrospray ionisation Describe the stages of electron impact. 1. High energy electrons are fired at the sample using an electron gun. 2. This knocks off an electron from each atom/molecule to form 1+ ions. Describe the stages of electrospray ionisation. 1. The sample is dissolved in a volatile solvent (e.g. methanol or water) and injected through a fine hypodermic needle as a spray into the ionisation chamber. 2. The needle is attached to the positive terminal of a high voltage power supply. 3. As the spray emerges from the end of the needle, the particles gain a proton from the positive charge of the supply. 4. The solvent evaporates, leaving the 1+ ions. When would you use the two types of ionisation? Electron impact: used for elements and low Mr compounds. Electrospray ionisation: used for high Mr compounds. Outline the four main stages in mass spectrometry 1. Ionisation: electron impact or electrospray ionisation - forms 1+ ions 2. Acceleration: the ions are attracted to a negatively charged plate and accelerate towards it - lighter ions and more highly charged ions achieve a greater speed. 3. Ion drift: Ions pass through a hole in the negatively charged plate, form a beam and travel along a flight tube. 4. Detection: Lighter ions arrive at the detector first as they have higher velocities. Flight times recorded and positive ions pick up an electron from the detector, causing a current to flow. What does the mass spectrometer determine? m/z and relative abundance Why is the TOF mass spectrometer kept under a high vacuum? to prevent the ions that are produced colliding with molecules from the air Why do some ions travel faster? 1. lighter in mass 2. more highly charged This means they have a higher velocity High resolution mass spectrometry Low resolution mass spectrometry 5 decimal places 1 decimal place How do you calculate the average relative atomic mass? (abundance x mass number)/total abundance What do mass spectrometers on space probes identify? - the elements in rock samples - amounts of the gases in the atmosphere On a mass spectra what is the relative molecular mass (Mr)? the molecular ion peak (the one with the highest m/z value) In organic molecules why is there a small peak at Mr+1? small peak due to isotopes What does the mass spectrometer measure? - m/z - relative abundance of isotopes. Why is electron impact only used for elements and low Mr compounds? Electron impact can cause larger organic molecules to fragment. If asked to give the species for a peak in a mass spectrum then give ... charge and mass number e.g. 24Mg+ For the mass spectra of chlorine, 35.5Cl, why are there peaks at Mr 70, 72 and 74? due to the diatomic molecules (e.g 70 is due to Cl-35 and Cl-35) Which method is most likely to lead to the break up of the ions into fragments? Electron impact Explain why different ions take different times to travel through the flight tube. time of flight depends on mass of ions lighter particles travel faster Appropriate number of significant figures means... lowest number of significant figures in the question