Analytical Techniques (HSC SSCE Chemistry): Revision Notes

Analytical Techniques

Introduction to analytical techniques

Understanding the structure of organic molecules is crucial for chemists. It helps them learn how chemical bonds form and how different molecules interact with each other. This knowledge is particularly important for understanding biological molecules such as DNA, enzymes, and ribosomes. When we know molecular structures in detail, down to individual atoms, we can develop new medicines and improve early diagnosis of diseases like cancer.

The key to success in these medical advances lies in the specialised instruments that allow chemists to determine molecular structures. Each analytical instrument provides unique information about a substance. By combining data from multiple techniques, chemists can build a complete three-dimensional picture of the molecule's structure. The chemist's role is to interpret this information and translate it into a meaningful structural representation.

Overview of organic spectrometry

The analytical techniques used to study organic compounds are collectively known as organic spectrometry. These techniques work by monitoring how molecules respond when exposed to different types of energy. Each spectrometer uses a specific energy source to probe different aspects of molecular structure.

The four analytical techniques

There are four main analytical techniques used in organic chemistry:

Mass spectroscopy

This technique uses high-energy electrons to analyse substances. It provides information about the molar mass of molecules and can detect different isotopes of elements. Mass spectroscopy is highly sensitive, requiring only small amounts of sample material while providing detailed structural information.

Nuclear magnetic resonance (NMR) spectroscopy

NMR spectroscopy uses radio waves as its energy source. The data obtained from NMR helps chemists determine the carbon-hydrogen backbone of molecules, revealing how atoms are connected within the structure.

Infrared (IR) spectroscopy

This technique uses infrared waves (heat energy) to examine substances. IR spectroscopy identifies the types of chemical bonds and functional groups present in molecules. The spectra produced can act as unique "fingerprints" that help identify specific compounds.

Ultraviolet-visible (UV-vis) spectrophotometry

UV-vis spectrophotometry uses ultraviolet and visible light to analyse substances. This technique measures the concentration of organic compounds and can also use absorption spectra as "fingerprints" for compound identification.

The following table summarises the energy sources and data outputs for each technique:

Technique	Energy source	Data output
Mass spectroscopy	High-energy electrons	Peaks correlating to the $m/z$ ratio that indicate the molar mass and isotopic abundances
NMR spectroscopy	Radio waves	Peaks that are useful for determining the C—H backbone of a molecule
IR spectroscopy	Infrared waves	Bands used to determine the type of bonds and functional groups present; may identify compounds using spectra as 'fingerprints'
UV-vis spectrophotometry	UV-visible waves	Absorption spectra that show the concentration of organic compounds; may identify compounds using spectra as 'fingerprints'

The electromagnetic spectrum

The electromagnetic spectrum refers to all the different types of electromagnetic radiation that the Sun emits. This radiation travels as waves with varying energies and wavelengths. We can only see a small portion of this spectrum called the visible spectrum, which contains the colours of the rainbow.

The electromagnetic spectrum ranges from radio waves (lowest energy, longest wavelength) to gamma rays (highest energy, shortest wavelength). The visible portion can be split into different colours, with red light having less energy and a longer wavelength than violet light.

Key relationships in the electromagnetic spectrum

Three important properties of electromagnetic radiation are related:

• Wavelength: The distance between successive wave peaks, measured in metres (m) or nanometres (nm, where $1 \text{ nm} = 10^{-9} \text{ m}$)
• Frequency: The number of waves passing a point per second, measured in hertz (Hz) or megahertz (MHz, where $1 \text{ MHz} = 10^{6} \text{ Hz}$)
• Energy: The amount of energy carried by the radiation

How analytical techniques use the electromagnetic spectrum

Each analytical technique relies on a specific part of the electromagnetic spectrum. The energy range used corresponds to the type of molecular information the technique can reveal:

• Mass spectroscopy uses the highest energy (high-energy electrons)
• UV-vis spectrophotometry uses ultraviolet and visible light
• IR spectroscopy uses infrared radiation (lower energy than visible light)
• NMR spectroscopy uses radio waves (lowest energy)

By using different parts of the spectrum, these techniques can examine different aspects of molecular structure, from overall molecular mass to specific bond types and arrangements.