7:Modern Analytical Techniques 1
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What are the four stages of mass spectrometry?
1) Ionisation 2) Acceleration 3) Deflection 4) Detection
What happens during ionisation?
1) The vaporised sample passes through the ionisation chamber, where they are bombarded by electrons from the electrically heated metal coil. 2) These electrons will knock off the electrons from atoms of the sample making them positively charged ions. 3) These positively charged ions are then pushed out by repulsion from another positively charged plate (known as the ion repeller)
What happens during acceleration?
The ions are accelerated into a finely focused beam and to have the same kinetic energy
What happens during deflection?
1) The ions get deflected by a magnetic field. - The lighter they are the more they are deflected. - The higher the charge on the ion, the more it gets deflected.
What happens during detection?
1) Positive ions hit the negative plate and an electron jumps from the plate onto the ion. 2) This produces a flow of charge. - The greater the current produced the greater the abundance of that particular ion.
What are the three functions a mass spectrometer carries out?
• creates positive ions from a neutral sample • separates the ions according to their mass/charge ratio (m/z) • measure the relative abundances of the ions and their relative atomic mass
Why is a vacuum required for a mass spectrometer?
To allow for ions to move fast and freely without collision of air molecules
What does spectrometry measure?
Mass
What does spectroscopy measure?
Energy
What is the molecular ion peak?
The peak with the highest m/z value -> the relative molecular mass of the molecule
Why is there always a small m/z line towards the right of then molecular ion peak?
It’s caused by the carbon-13 isotope in the molecule. - carbon -13 is a stable isotope of carbon and makes up 1.11% of all carbon atoms
What is fragmentation?
When a molecular ion is broken into 2 smaller products - one positive ion and one radical
What does the base peak indicate?
The peak with the greatest abundance (the tallest peak)
What is infrared radiation?
Part of the electromagnetic spectrum with the frequencies below that of red light.
When does stretching occur?
When a bond absorbs infrared radiation and uses it to alter the length of a bond
What is the transmittance value (in the IR spec)?
Represents the amount of radiation absorbed at a particular wavenumber.
What is the wavenumber?
Represents the frequency of IR radiation absorbed by a particular bond in a molecule
What is intensity of infrared absorption?
The amount of infrared radiation absorbed.
What are the three types of bonding vibrations?
• symmetrical stretch • asymmetrical stretch • bending (when the bond angles and decrease)
What does stretching vibration depend on?
• the mass of the atoms on either side of the bond • the bond strength • the bond length
Why don’t simple non-polar molecules (e.g H2 and Cl2) absorb infrared radiation?
They don’t have a dipole. -when the molecule stretches due to a vibration there is no change in the dipole, so there is no absorption of radiation.
What does a greater mass mean?
Lower frequency
What does a stronger bond mean?
Higher frequency
What does higher energy lead to?
Greater amplitude