Chemistry (9701)
Topic 18 of 20Cambridge A Levels

Analytical Techniques

Using spectroscopy to identify and determine the structure of organic molecules.

Analytical techniques are fundamental tools in modern chemistry, allowing scientists to elucidate the structure of unknown compounds. For the Cambridge A-Level syllabus, a combination of three key techniques—mass spectrometry, infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy—provides a powerful method for piecing together the molecular puzzle.


### Mass Spectrometry (MS)


Mass spectrometry is used to determine the relative molecular mass (Mr) and to gain structural information from the fragmentation of a molecule. The process involves several key stages:

  • Ionisation: A sample is vaporised and bombarded with high-energy electrons, knocking an electron off the molecule to form a positive ion, known as the molecular ion (M⁺).
  • Acceleration: The positive ions are accelerated by an electric field so they all have the same kinetic energy.
  • Deflection: The ions are deflected by a magnetic field. The amount of deflection depends on the mass-to-charge ratio (m/z). Lighter ions are deflected more than heavier ions.
  • Detection: The deflected ions are detected, and a computer generates a mass spectrum—a plot of relative abundance against m/z.

    • Interpreting the Spectrum:

    The peak with the highest m/z value is the molecular ion peak (M⁺), and its m/z value is equal to the Mr of the molecule. For example, the M⁺ peak for ethanol (CH₃CH₂OH) would be at m/z = 46. You may also see a very small peak at M+1 (e.g., m/z = 47 for ethanol). This M+1 peak is due to the presence of the carbon-13 isotope (¹³C) in the molecule.


    The high-energy ionisation process often causes the molecular ion to break apart into smaller, charged fragments. This is called fragmentation. The pattern of fragment ions provides clues about the molecule's structure. Common fragment losses seen in spectra include:

    * m/z = 15: Loss of a CH₃⁺ group

    * m/z = 29: Loss of a CH₃CH₂⁺ group

    * m/z = 43: Loss of a CH₃CH₂CH₂⁺ or (CH₃)₂CH⁺ group


    ### Infrared (IR) Spectroscopy


    Infrared spectroscopy is used to identify the functional groups present in a molecule. The principle is that covalent bonds are not static; they vibrate (stretch and bend) at specific frequencies. When IR radiation is passed through a sample, the bonds absorb energy at frequencies that match their natural vibrational frequencies. An IR spectrum is a plot of transmittance against wavenumber (in cm⁻¹).


    Interpreting the Spectrum:

    Dips in the graph, called absorptions, indicate that energy has been absorbed. We can identify functional groups by matching the wavenumber of these absorptions to known values from a data booklet.

    * O-H bond (alcohols): Appears as a broad absorption in the range 3200–3600 cm⁻¹.

    * O-H bond (carboxylic acids): A very broad and strong absorption from 2500–3300 cm⁻¹.

    * C=O bond (carbonyl group): A sharp, strong absorption in the range 1680–1750 cm⁻¹. Its exact position helps distinguish between aldehydes, ketones, esters, and carboxylic acids.

    * N-H bond (amines/amides): A sharp absorption (or two peaks for -NH₂) around 3300–3500 cm⁻¹.

    * C-H bonds: Absorptions are always present around 2850-3100 cm⁻¹ but are less useful for distinguishing between compounds.


    The region below 1500 cm⁻¹ is called the fingerprint region. It contains many complex absorptions unique to a particular molecule. While difficult to interpret directly at this level, it can be used to confirm the identity of a compound by matching its fingerprint region to that of a known sample.


    ### Nuclear Magnetic Resonance (NMR) Spectroscopy


    NMR spectroscopy provides the most detailed information about the carbon-hydrogen framework of a molecule. We focus on proton (¹H) NMR. The principle relies on the spin of the ¹H nucleus. In a strong external magnetic field, these nuclei align with or against the field. By absorbing energy in the radio frequency range, they can 'flip' between these two energy states.


    Interpreting a ¹H NMR Spectrum:

    A ¹H NMR spectrum provides three key pieces of information:

  • Chemical Shift (δ): The position of a signal along the x-axis (measured in parts per million, ppm) indicates the chemical environment of the protons. The scale is referenced against Tetramethylsilane (TMS), which is assigned a value of δ = 0 ppm. Protons near electronegative atoms (like O or Cl) are deshielded and appear at a higher chemical shift (downfield). Protons in electron-rich environments are shielded and appear at a lower chemical shift (upfield).
  • Integration: The area under each peak is proportional to the number of protons in that specific environment. The spectrum often shows an integration trace or provides a ratio, e.g., 3:2:1, indicating the relative number of protons creating each signal.
  • Spin-Spin Splitting: A signal for a set of protons can be split into multiple peaks by the influence of non-equivalent protons on an adjacent carbon atom. The n+1 rule is used for interpretation: if a proton has 'n' non-equivalent protons on the next carbon, its signal is split into 'n+1' peaks. Common patterns include:

    • * Singlet: 0 adjacent protons.

    * Doublet: 1 adjacent proton.

    * Triplet: 2 adjacent protons.

    * Quartet: 3 adjacent protons.


    By combining the chemical shift, integration, and splitting pattern, the precise structure and connectivity of the molecule can be determined. These three techniques—MS, IR, and NMR—are used together to provide a complete and unambiguous structural identification of an organic compound.

    Key Points to Remember

    • 1Mass spectrometry determines the relative molecular mass (Mr) from the molecular ion peak (M⁺) and structural clues from fragmentation patterns.
    • 2Infrared (IR) spectroscopy identifies functional groups by detecting the absorption of IR radiation corresponding to specific bond vibrations.
    • 3Key IR absorptions include the broad O-H stretch (~3200-3600 cm⁻¹) and the sharp, strong C=O stretch (~1680-1750 cm⁻¹).
    • 4¹H NMR spectroscopy provides detailed information about the hydrogen environments within a molecule.
    • 5In ¹H NMR, chemical shift (δ) indicates the proton's environment, integration gives the proton ratio, and spin-spin splitting (the n+1 rule) reveals adjacent protons.
    • 6Tetramethylsilane (TMS) is the standard reference compound in NMR, with its signal set at δ = 0 ppm.
    • 7Combining data from MS, IR, and NMR is essential for the unambiguous determination of a complete molecular structure.

    Pakistan Example

    Quality Control in Pakistan's Pharmaceutical Industry

    In Pakistan, major pharmaceutical companies like Getz Pharma, Searle, and Ferozsons Laboratories rely heavily on these analytical techniques. They are essential for quality assurance and regulatory compliance with the Drug Regulatory Authority of Pakistan (DRAP). For instance, when synthesizing a batch of an active ingredient like paracetamol, ¹H NMR and IR spectroscopy are used to confirm its molecular structure is correct and free from impurities. Mass spectrometry is then used to verify that its molecular mass is exactly as expected, ensuring the purity and safety of the final medicinal product sold in pharmacies across the country.

    Quick Revision Infographic

    Chemistry — Quick Revision

    Analytical Techniques

    Key Concepts

    1Mass spectrometry determines the relative molecular mass (Mr) from the molecular ion peak (M⁺) and structural clues from fragmentation patterns.
    2Infrared (IR) spectroscopy identifies functional groups by detecting the absorption of IR radiation corresponding to specific bond vibrations.
    3Key IR absorptions include the broad O-H stretch (~3200-3600 cm⁻¹) and the sharp, strong C=O stretch (~1680-1750 cm⁻¹).
    4¹H NMR spectroscopy provides detailed information about the hydrogen environments within a molecule.
    5In ¹H NMR, chemical shift (δ) indicates the proton's environment, integration gives the proton ratio, and spin-spin splitting (the n+1 rule) reveals adjacent protons.
    6Tetramethylsilane (TMS) is the standard reference compound in NMR, with its signal set at δ = 0 ppm.

    Formulas to Know

    IR absorptions include the broad O-H stretch (~3200-3600 cm⁻¹) and the sharp, strong C=O stretch (~1680-1750 cm⁻¹).
    TMS) is the standard reference compound in NMR, with its signal set at δ = 0 ppm.
    Pakistan Example

    Quality Control in Pakistan's Pharmaceutical Industry

    In Pakistan, major pharmaceutical companies like Getz Pharma, Searle, and Ferozsons Laboratories rely heavily on these analytical techniques. They are essential for quality assurance and regulatory compliance with the Drug Regulatory Authority of Pakistan (DRAP). For instance, when synthesizing a batch of an active ingredient like paracetamol, ¹H NMR and IR spectroscopy are used to confirm its molecular structure is correct and free from impurities. Mass spectrometry is then used to verify that its molecular mass is exactly as expected, ensuring the purity and safety of the final medicinal product sold in pharmacies across the country.

    SeekhoAsaan.com — Free RevisionAnalytical Techniques Infographic

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