FOOD ANALYSIS: SPECTROSCOPY
Diagram
Two ways to read molecular vibrations
FTIR spectroscopy
Infrared light shines on the sample and the bonds absorb it at their own frequencies. The missing light is the spectrum.
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Raman spectroscopy
A laser shines on the sample and a tiny part of the light is scattered with a changed frequency. The shifted light is the spectrum.
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The pair
Both read the same molecular vibrations, but by different rules, so each sees what the other misses. They complement each other.
Figure 1: FTIR measures absorption of infrared light; Raman measures inelastic scattering of laser light. Drawn in our own style from the spectroscopy unit of the FTN Instrumental Techniques of Analysis notes, Unit 3, and from Food Quality Analysis: Applications of Analytical Methods Coupled With Artificial Intelligence, chapters 2 and 4.
Introduction
Fourier transform infrared spectroscopy, written FTIR, and Raman spectroscopy are the two vibrational spectroscopy methods of food analysis, and both read the vibrations of molecules to identify what a food holds. The difference is how the light is read: FTIR measures the infrared light a sample absorbs, while Raman measures laser light a sample scatters with a changed frequency. Because the two methods answer to different rules, a vibration strong in one is often weak in the other, so they give complementary information about the same sample.
The FTN Instrumental Techniques of Analysis notes, Unit 3, explain infrared spectra and the FTIR instrument, and Food Quality Analysis, Elsevier, 2022, applies FTIR to fats and oils in chapter 2 and Raman to food and pharmaceutical materials in chapter 4. This article follows those sources.
Quick comparison
Table 1: FTIR compared with Raman spectroscopy
| Aspect | FTIR | Raman spectroscopy |
|---|---|---|
| What is measured | Absorption of infrared light by molecular bonds | Inelastic scattering of monochromatic laser light by molecular bonds |
| Light used | Infrared light, across the mid-infrared range | Visible or near-infrared laser light |
| Selection rule | A vibration is seen when it changes the dipole moment of the molecule | A vibration is seen when it changes the polarisability of the molecule |
| Instrument | An FTIR spectrometer: a light source, an interferometer, and a detector, with a Fourier transform turning the signal into a spectrum | A Raman spectrometer: a laser source, the sample, a monochromator to sort the scattered light, and a detector |
| Spectrum axis | Wavenumber, showing where light was absorbed | Raman shift, the difference in frequency between the incident light and the scattered light |
| Water | Water strongly absorbs infrared light, so it can hide the signal of the food and interfere with the reading | Water is a weak Raman scatterer, so it interferes little, and wet samples can be read |
| Sample preparation | Minimal or none: solids, liquids and gases can be analysed, often without destroying the sample | Minimal or none: solids, liquids and gases can be analysed, often without destroying the sample |
| Strong for | Vibrations that change the dipole moment, such as O-H, N-H and C=O stretches, and the functional groups of a molecule | Vibrations that change the polarisability, such as the vibrations of aromatic rings and carbon backbones |
| Food use in the books | Quality analysis of fats and oils, with multivariate analysis, in chapter 2 of Food Quality Analysis | Quality analysis of food and pharmaceutical materials, with multivariate analysis, in chapter 4 of Food Quality Analysis |
| Limit | Water interference, and a sample must be thin enough or prepared so the infrared light can pass or reflect | The scattered light is very weak, a tiny fraction of the incident light, so the instrument must be sensitive |
Scientific explanation
Definition
FTIR spectroscopy is a method that records the whole infrared spectrum of a sample at once through an interferometer and a Fourier transform: infrared light shines on the sample, the bonds absorb it at the frequencies of their own vibrations, and the pattern of absorption identifies the functional groups present. Raman spectroscopy is a method in which a sample is lit with a strong monochromatic laser and the scattered light is read: most photons scatter with the same energy, called Rayleigh scattering, while a very tiny part scatter with a lower frequency, called Stokes scattering, and the difference in frequency, the Raman shift, gives the same kind of vibrational information by a different rule.
Working principle
In FTIR, infrared spectra identify functional groups by their characteristic group frequencies: each bond absorbs at its own frequency, and the intensity of the absorption can be used for the calculation of the relative concentration in the sample. The instrument records the whole spectrum at once through an interferometer, and a Fourier transform turns the measured signal into the spectrum. In Raman, the incident laser light interacts with the molecules, and the inelastically scattered light loses energy equal to the energy given to the molecular vibrations. The selection rule is that the incident light must induce a change in molecular polarisability, and the induced dipole then radiates the scattered light. The molecular vibration information in a Raman spectrum is also found in an infrared spectrum, but with different selection rules and intensity patterns, so the two methods provide similar but complementary information about the molecular structure of a food material.
Apparatus
FTIR needs an infrared light source, an interferometer, a sample holder, and a detector, with a Fourier transform to turn the signal into a spectrum. Raman needs a monochromatic laser source, a sample holder, a monochromator to sort the scattered light, and a detector to count the photons as a function of the Raman shift.
Measurement procedure
Both methods read the sample with little or no preparation, and both are quantitative through calibration: the FTIR absorbance is read against a calibration graph, as in the Beer-Lambert treatment in the FTN notes, and the Raman intensity is read against standards for the material being measured. In food work FTIR is applied to the quality analysis of fats and oils, and Raman to the quality analysis of food and pharmaceutical materials, each combined with multivariate analysis in the book chapters cited below.
Factors affecting readings
For FTIR, water is the largest factor: water strongly absorbs infrared light, so a wet sample can hide the bands of the food, and the measurement must allow for it. Temperature and the wavelength of the light also matter, as they do for refractive index work. For Raman, the weakness of the scattered light is the largest factor: the inelastically scattered light is only a very tiny fraction of the incident light, so the instrument must count very few photons, and fluorescence from the sample can swamp the signal. The choice between the two therefore turns on the sample: a dry, fatty sample suits FTIR, while a wet sample suits Raman.
Formulas and calculations
The books used here give no worked food example with numbers for either method, so none is given here. For FTIR, quantification rests on the Beer-Lambert law stated in the FTN spectroscopy unit, A = ebc, where A is the absorbance, e is the molar absorptivity, b is the path length, and c is the concentration: the absorbance at a band is read from a calibration graph made from standards. For Raman, the books give no formula, so none is stated: the Raman shift is read from the spectrum, and the amount is read against standards.
Industrial applications
- FTIR: quality analysis of fats and oils, the subject of chapter 2 of Food Quality Analysis.
- FTIR: identifying functional groups in a food by their characteristic group frequencies.
- FTIR: rapid, non-destructive checking of food quality and authenticity, with minimal sample preparation.
- Raman: quality analysis of food and pharmaceutical materials, the subject of chapter 4 of Food Quality Analysis.
- Raman: reading wet samples and samples through an optical fibre for remote monitoring of a food process.
- Raman: giving the complementary half of the vibrational picture where FTIR bands are weak or hidden.
Advantages and limitations
- FTIR, advantage: it records the whole spectrum at once, identifies functional groups directly, and is a mature method for fats and oils.
- FTIR, limitation: water strongly absorbs infrared light and can hide the signal, so wet samples are hard to read.
- Raman, advantage: water interferes little, samples need almost no preparation, and readings can be taken at a distance through an optical fibre.
- Raman, limitation: the scattered light is very weak, so the instrument must be sensitive, and fluorescence from the sample can swamp the signal.
FAQ
What is the main difference between FTIR and Raman spectroscopy?
FTIR measures infrared light absorbed by the sample, while Raman measures laser light scattered by the sample with a changed frequency.
Why are FTIR and Raman called complementary?
Both read the same molecular vibrations, but by different selection rules: a vibration strong in FTIR is often weak in Raman, and the reverse, so each sees what the other misses.
Which method suits a wet sample?
Raman, because water is a weak Raman scatterer and interferes little, while water strongly absorbs infrared light and can hide the FTIR signal.
What does FTIR stand for?
Fourier transform infrared: the instrument records the whole spectrum at once through an interferometer, and a Fourier transform turns the signal into the spectrum.
What is the Raman shift?
The difference in frequency between the incident laser light and the inelastically scattered light, measured in wavenumbers, which carries the vibrational information.
References and further reading
- FTN subject: Instrumental Techniques of Analysis, Unit 3: Spectroscopic Techniques.
- Food Quality Analysis: Applications of Analytical Methods Coupled With Artificial Intelligence, Elsevier, 2022 (Google Books page read)
- Adriana S. Franca and Leo M. L. Nollet (eds.), Spectroscopic Methods in Food Analysis, CRC Press, 2017 (Google Books page read)
