Mass Spectrometry Notes

Mass Spectrometry

Learning Intentions

• Identify the main components of a mass spectrometer.

• Label the function of each component.

• Determine what a spectrograph looks like upon process completion.

• Calculate relative isotopic mass from a series of sample readings.

Mass Spectrum

• The mass spectrum is essentially a fingerprint for a molecule.

• Molecules can be identified by computer using a spectral database.

Uses of Mass Spectrometry

• Monitoring breath of patients under anaesthesia.

• Detecting traces of toxic chemicals in contaminated marine life.

• Identifying unknown compounds.

• Detecting banned substances such as steroids in athletes.

• Determine the abundance of each isotope of an element.

• Gain further info about structure and chemical properties of molecules

• Analysing molecules in space

Uses of Mass Spectra (in context)

• To calculate relative atomic mass.

• To investigate the structure of molecules.

Stages in Mass Spectrometry

Stage 1: Ionisation

• The atom is ionized by knocking one or more electrons off to give a positive ion.

Stage 2: Acceleration

• The ions are accelerated so that they all have the same kinetic energy.

Stage 3: Deflection

• The ions are then deflected by a magnetic field according to their masses.

• The lighter they are, the more they are deflected.

• The amount of deflection also depends on the number of positive charges on the ion.

  • The more the ion is charged, the more it gets deflected.

Stage 4: Detection

• The beam of ions passing through the machine is detected electrically.

Mass Spectrometer Components

1. Vaporization Chamber

• The sample (element or compound) is vaporized.

2. Ionization Chamber

• Positive ions are produced from the vapor.

• $X(g) + e^- \rightarrow X^+(g) + 2e^-$

  • $X$ can be an atom or molecule resulting in simple or molecular/polyatomic ions.

3. Accelerating Electric Field

• Positive ions are accelerated by a known and fixed electric field.

4. Deflecting Magnetic Field

• Positive ions are then deflected by a known and variable magnetic field.

5. Ion Detector

• Ions are detected.

6. Recorder

• The mass spectrum is traced out by the recorder.

Mass Spectrum of Rb

• X-axis: Mass/charge ratio

  • For singly charged ions, $e = 1$. Therefore mass/charge = mass of atom = isotopic mass (relative to C-12) ≈ mass number (whole number).

• Y-axis:

  • Relative abundance,

  • Ion intensity, or

  • Detector current.

Relative Isotopic Mass

• The relative isotopic mass of a particular isotope of an element is the relative mass of one atom of that isotope on the $^{12}C = 12.0000$ scale.

Isotopic Abundances Example (Magnesium)

• The mass spectrum of magnesium shows three isotopes:

  • 79% is magnesium-24

  • 10% is magnesium-25

  • 11% is magnesium-26

• Relative atomic mass calculation:

  • $\frac{(24 \times 79) + (25 \times 10) + (26 \times 11)}{100} = 24.32 \text{ g/mol}$

Example Question (Lithium)

• The mass spectrum of lithium showed two peaks:

  • m/z = 6, abundance = 7.4%

  • m/z = 7, abundance = 92.6%

• Calculate the relative atomic mass of the lithium sample.

More Definitions

• Molecular ion ($M^+$): The positive ion formed in mass spectrometry when a molecule loses an electron.

• Fragmentation: The process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion.

The Process of Mass Spectrometry

• An electron is knocked off during ionization, forming a positively charged molecule.

  • $C<em>2H</em>5OH \rightarrow C<em>2H</em>5OH^+ + e^-$

• The mass of the lost electron is negligible.

• The molecular ion has a molecular mass equal to the molecular mass of the compound.

• This molecular ion can be detected and analyzed.

Fragmentation

• The high energy electrons can either produce the molecular ion or cause the molecule to fragment.

  • $C<em>2H</em>5OH^+ \rightarrow CH<em>3 + CH</em>2OH^+$

• This results in a positive fragment ion and a neutral species.

What does Mass Spectrometry tell us?

• We can determine the molecular mass of a molecule by using mass spectrometry by locating the $M^+$ peak.

• This produces the peak with the highest m/z value in the mass spectrum. (Ignore ${}^{13}C$)

Beyond the Syllabus: Fragmentation patterns

• We can use the fragments to determine the structure of an unknown compound to (in most cases) give its precise identity.

• Although the molecular ion peak will have the same m/z value, the fragmentation patterns will be different.

• Organic compounds produce a unique mass spectrum, which is used as a “fingerprint”.

Identifying Fragment Patterns