Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
7 – MODERN ANALYTICAL TECHNIQUES I
7A: MASS SPECTROMETRY
1. be able to use data from a mass spectrometer to:
i. determine the relative molecular mass of an organic compound from the
molecular ion peak
ii. suggest possible structures of a simple organic compound from the m/z of
the molecular ion and fragmentation patterns
Mass spectrometry
- Mass spectrometry is a technique used by chemists to determine the formulae and
structure of molecules.
o It helps to determine relative atomic mass, relative molecular mass and molecular
structures of organic compounds.
o In this way, it can be used to identify unknown compounds.
- In modern chemistry, the use of instrumental techniques such as mass spectrometry for
analysis is more important that chemical analysis.
o Although the instruments may be expensive to purchase, analysis using them is
quick to perform and extremely accurate.
o Chemical analysis also destroys the sample by reacting it; instrumental analysis
uses a very small sample and, in most cases, does not destroy it.
- Note: this is not spectroscopy! A mass spectrum is obtained as a result of chemical
reactivity whereas spectroscopic techniques use electromagnetic spectrum (unlike
techniques we will see later).
- A mass spectrometer (the instrument) has three basic functions:
o i) To vaporise and ionise molecules to produce charged species.
o ii) To separate these charged species according to their mass (mass-to-charge
ratio, m/z).
o iii) To detect these charged species and record them on a spectrum.
The mass spectrometer (don’t need to memorise):
- 1. Injection – the molecule of interest is injected into the spectrometer.
- 2. Vaporisation – the molecule of interest is vaporised into the gas-phase. Lower
molecular weight molecules vaporise in the vacuum. Higher molecular weight molecules
also require heating (as they have higher boiling temperatures).
- 3. Ionisation – the gas-phase molecules are bombarded with high-energy electrons (from
an electron gun) in a technique called electron impact ionisation.
o This causes the molecule to lose an electron and form a positively-charged
species known as a radical cation.
o However, it may also split the molecule into fragments. As a result, the mass
spectrum consists of a ‘fragmentation pattern.’
- 4. Acceleration – charged species are accelerated by an electric field from a repeller
plate. Only one of the species (the charged cation) from fragmentation is detected in the
mass spectrometer.
- 5. Deflection – the charged ions pass through a magnetic field which causes them to be
deflected. The amount that a particular ion is deflected is dependent on two factors:
o i) The mass (m). Larger masses are deflected less.
o ii) The charge of the ion (z). Higher charges are defected more.
o Together, the ions are deflected depending on their mass-to-charge (m/z) ratio.
, - 6. Detection – as the ions reach the detector a signal is generated.
o By changing the strength of the magnetic field, ions of different masses are able
to fully travel through the magnetic field and reach the detector.
o This allows us to detect ions of different masses.
A spectrum is produced plotting the relative abundance of each ion against the mass-to-
charge ratio (the mass with z=1).
- The ion that is the most abundant results in the peak with the highest intensity is set to
100%. The height of all other peaks are measured relative to this.
- The peak with the greatest relative abundance is called the base peak.
- The peak that corresponds to the molecular ion (radical cation) is called the molecular
ion peak, M peak or parent peak.
o In A-level simplified diagrams, this is usually the peak on the mass spectrum with
the highest m/z value.
- More stable ion fragments produce peaks with a higher intensity (a higher relative
abundance).
- Molecules also break up more readily at weak bonds.
o Positive ions with the charge on a secondary or tertiary carbon atom are more
stable than ions with the charge on a primary carbon atom, so they will appear in
greater relative abundance.
- The pattern of peaks on a mass spectrum is known as the fragmentation pattern.
In mass spectrometry, we measure the mass of the individual ions as opposed to the
measuring the average mass of all the ions (as practiced in the topic of moles).
- The mass of an atom is approximately the sum of its protons and neutrons (as its
electrons have negligible mass).
- The atoms present in each individual ion are of a specific isotope.
- Isotope – an atom of the same element containing the same number of protons but a
different number of neutrons.
How do organic molecules typically fragment?
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