July 7, 2021
Mass Spectroscopy
- the process is similar to that used to gather information about isotopes
- gaseous molecules are ionised, accelerated and deflected
- the amount of deflection depends on m/z - mass/charge ratio
- each molecule undergoes fragmentation into smaller particles
- each peak in the spectrum is due to a charged fragment with a certain m/z value
- the final peak is due to the molecular ion which has the largest m/z value
- the m/z value of the molecular ion is used to calculate the relative molecular mass
In AS/A-level Chemistry exam, you will be asked the process of Time-of-flight, make sure you get the BOLD texts in your answers! 👩🏫
Time-of-flight
1. Electrospray Ionisation- Useful for substances with a high formula mass
- Sample is dissolved in volatile solvent and injected through a fine hypodermic needle to give a fine mist.
- Tip of needle is attached to positive terminal of a high - voltage power supply.
- Particles are ionised : they gain a proton from the solvent as they leave the needle producing positive ions
X(g) + H+ = XH+
- The solvent evaporates while the XH+ ions are attracted towards a negative plate. Here they're accelerated.
or Electron Impact- Used for substances with a low formula mass
- Sample being analysed is vaporised and high energy electrons are fired at it.
- The high energy electrons are from an electron gun : a hot wire filament with a current running through it emitting electrons.
- This usually knocks off 1 electron from each particle forming a 1+ ion.
X(g) + e- = X+(g) +2e-
X(g) = X+(g) + e-
- The 1+ ions are then attracted towards a negative electric plate where they're accelerated.
2. Acceleration
- The positive ions are accelerated using an electric field so they have the same kinetic energy.
- As V=¬2KE/m, the velocity of each particle depends on it's mass.
- Lighter particles have a faster velocity and heavier particles have a slower velocity.
3. Flight tube/ Ion drift
- The positive ions travel through a hole in the negatively charged plate into a tube.
- The TOF of each particle through this tube depends on it's velocity so it depends on it's mass.
- TOF is directly proportional to the mass of ions.
4. Detection
- Positive ions hit a negatively charged plate.
- When they hit the detector plate the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current that is measured.
- The size of the current gives a measure of the number of ions hitting the plate.
5. Data analysis
- A computer uses the data to produce a mass spectrum.
- This shows a m/z ratio and abundance of each ion that reaches the detector.
References:
- https://www.semanticscholar.org/paper/Some-Thoughts-on-Electrospray-Ionization-Mechanisms-Crotti-Seraglia/14aea399c70c9ee04b980eef66081dcc3c2c27dd/figure/0
- https://www.creative-proteomics.com/support/electron-ionization.htm
- https://www.researchgate.net/figure/MALDI-analyses-with-a-time-of-flight-mass-analyzer-modified-according-to-10_fig2_259396248
- https://www.chemguide.co.uk/analysis/masspec/elements.html
This is the end of the topic!
Drafted by Cherry (Chemistry)