Mass Spectrometry

Question 1

What is mass spectrometry?

Answer 1

This technique weighs molecules. A sample is introduced into the instrument, vaporised, and ionised. By focusing the ions onto a detector using a magnetic and electric fields, it is possible to determine the masses of the ions present.

Question 2

Stages of elucidating a molecular structure

Answer 2

1. Isolate and purify the sample (precipitation, distillation, recrystallisation, chromatography)
2. Determine molecular formula (mass spectrometry)
3. Identify functional groups and conjugation (Infrared and UV/Vis spectroscopies)
4. Assemble structural formula (Correct bond connectivity and stereochemistry - NMR spectroscopy)

Question 3

General idea behind a mass spectrometer

Answer 3

• Molecules are ionised.
• Ions are accelerated through an electric field.
• Trajectory of molecules is bent by a magnetic field. Degree of bending determined by the ion’s
  charge (z) and mass (m).
• Ions are separated out according to their “charge to mass ratio” (m/z) and detected.

Question 4

Types of chromatography in mass spec

Answer 4

• Gas Chromatography-Mass Spectrometry
• Liquid Chromatography-Mass Spectrometry

Question 5

Ways to ionise a molecule

Answer 5

• Electron Impact (EI)
• Electrospray Ionisation (ESI)
• Matrix Assisted Laser Desorption Ionisation (MALDI)
• Chemical Ionisation (CI)

Question 6

Electron Impact (EI)

Answer 6

• Electrons produced by thermionic emission
• Electrons are accelerated through the ionisation space towards an anode
• They interact with analyte molecules in the gas phase and cause them to ionise a radical ion
• Dissociative result: produces charged and neutral fragments plus electrons
• Non-dissociative result: produces an ionised parent molecule and electrons

Question 7

Electrospray Ionisation (ESI)

Answer 7

• Formation of aerosol, with N2 nebuliser, gentle heat.
• Include mild acid/ buffer, so ions formed as MH+ (not M+) i.e. just like protonation.
• Ions enter mass analyzer as before.
• Very gentle – can use to analyze proteins.

Question 8

MALDI - Matrix-Assisted Laser Desorption/Ionisation

Answer 8

• Ideal for large molecular analysis
• Based on the bombardment of sample molecules with a laser light
• The sample is pre-mixed with a highly absorbing matrix compound (e.g. CHCA, DHB)
• The matrix transforms the laser energy into excitation energy for the sample, which leads to sputtering of analyte and matrix ions from the surface of the mixture.
• No fragments or decomposition.
• Single charge

Question 9

The Mass Analyser : Quadrupole

Answer 9

• Quadrupole mass analyzers use oscillating electrical fields to selectively stabilize or destabilize ions passing
  through a radio frequency (RF) quadrupole field.
• It acts as a mass selective filter (ion trap)
• A common variation of the quadrupole
  is the triple quadrupole when you have big molecules to analyse.

Question 10

Time of Flight (ToF)

Answer 10

• Measure the time it takes for an ion to reach a detector while travelling over a known distance.
• Ions are accelerated by an electric field of known strength.
• The velocity of the ion depends on the mass-to-charge ratio.
• This time will depend on the mass-to charge ratio of the particle (heavier particles reach lower speeds).

Question 11

Detector

Answer 11

• When the molecular ion reaches the end it strikes a metal box
• An electron from the metal box neutralises the molecular ion
• The movement of the single electron is detected as an electric current, which can be amplified and recorded
• Discrete dynode and continuous dynode

Question 12

Mass spectrometry of larger molecules

Answer 12

m/z = (M+ nH+)/n
Peak moves left as the charge increases