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Close up of a spectometer

Infrared spectroscopy: an overview

Infrared spectroscopy of food samples is generally carried out using one of two wavelength ranges: the near-infrared (NIR) or the mid-infrared (MIR).

The visible and infrared parts of the electromagnetic spectrum showing the usual definitions of visible to far-infrared wavelength ranges The visible and infrared parts of the electromagnetic spectrum, showing the usual definitions of the near-infrared and mid-infrared wavelength (or frequency) ranges. Also shown are the relationships between wavelength, frequency and wavenumbers

In some NIR experiments, it is useful and easy to collect data from the red part of the visible wavelength range too, in which case the spectroscopy may be referred to as Vis-NIR. It is usual to describe near-infrared (and visible) radiation by its wavelength measured in nanometres (nm, 10-9m) or microns (mm, 10-6m).

In contrast, mid-infrared radiation is generally described by its frequency. However, rather than using Hertz as the unit of frequency, mid-infrared spectroscopists have historically expressed frequency in terms of wavenumbers (with units cm-1, which indicates wave cycles per centimetre) for reasons of convenience. Frequency measured in Hertz is converted into wavenumbers through division by a constant value - the speed of light.

An infrared spectrum is a graph that shows the detected intensity of infrared radiation on the vertical axis, plotted against the wavelength (or frequency) along the horizontal axis. It generally contains a number of features - absorption bands - which arise from the interaction of the radiation with vibrating molecular bonds.

NIR Spectrum of rapeseed oil This is a typical visible/near-infrared reflectance spectrum of an edible oil. The strongest feature is an absorption band at around 670nm (approximately the boundary of the visible and NIR regions).

Absorption bands are bell-curved (Gaussian) in shape: centred at specific frequencies, but relatively wide. Depending on the experimental setup, they can appear as downwards troughs (transmission spectra, reflectance spectra) or upwards peaks (absorption spectra).

Most infrared spectra contain multiple, overlapped absorption bands. The overall pattern of bands is determined by the nature of the different chemical entities in the sample (carbon-hydrogen bonds, carbon-oxygen bonds, and so on) and of the different modes of bond vibration (stretching, bending, rocking).

MIR Spectrum of Ethanol A typical mid-infrared spectrum, of ethanol, showing the “fingerprint” region where the strongest absorption bands occur. Note that a convention in the mid-infrared is for the x-axis scale to be reversed, i.e. with values decreasing from left to right.

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This article is from the free online course:

Identifying Food Fraud

UEA (University of East Anglia)

Course highlights Get a taste of this course before you join:

  • Becoming a food fraud detective
    Becoming a food fraud detective

    Watch this introduction to the course by Dr Kate Kemsley which gives an overview of food fraud from ancient to modern times

  • Near-infrared spectroscopy
    Near-infrared spectroscopy

    Watch this video in which Dr Kate Kemsley introduces the analytical technique of near-infrared spectroscopy

  • Mid-infrared spectroscopy
    Mid-infrared spectroscopy

    Watch this video showing how FTIR can be used to glean detailed information on chemical components present in ground roast coffee using coffee beans

  • Introducing NMR spectroscopy
    Introducing NMR spectroscopy

    In this video Dr Kate Kemsley introduces the concepts behind Nuclear Magnetic Resonance spectroscopy

  • Benchtop NMR spectroscopy
    Benchtop NMR spectroscopy

    Will Jakes an MChem graduate of the University of East Anglia describes the principles and virtues of the benchtop NMR spectrometer