Ensuring value with mid-infrared

In the previous video, we saw how pattern recognition can be used to identify different oils from their Vis NIR spectra. We also learned that whilst this method can handle single species oils quite well, it’s not so good at dealing with mixtures. This is because absorption bands, in the visible and near infrared, are broad, and often very similar for different substances. These problems can be solved by moving into the longer wavelengths of the mid infrared. In this region, spectral bands are sharper, and more distinct patterns are obtained from different compounds. The standard instrument for carrying out mid infrared spectroscopy is the Fourier Transform Infrared or FTIR spectrometer.

These come in a range of sizes and prices, from small, ruggedised machines that can be placed in quite demanding environments, to large multifunction instruments better suited to an analytical lab. This is a research grade instrument that requires liquid nitrogen to cool and stabilise the detector, and a purge of dry air that flows continuously through the sample chamber to reduce moisture. The main components inside the instrument are the source, which produces broadband mid infrared radiation, a Michelson interferometer, which uses a moving mirror to modulate each wavelength slightly differently, the sample chamber, which is equipped with a device which allows the radiation to interact with the sample, and the detector.

The final component of the system is the computer, which applies a mathematical decoding method called a Fourier transform to undo the modulation and extract the information from each wavelength separately. Here we are collecting a spectrum of sunflower oil, using a sampling technique called Attenuated Total Reflectance, or ATR. The sample is placed upon the surface of a polished diamond crystal, within which infrared radiation is trapped by the mechanism of total internal reflectance. A quantum effect causes some leakage of radiation at the interface. This effectively produces a very short transmission path through the sample of the order of the few microns in length. This tiny path length is needed in the mid infrared, because many materials have extremely strong absorption bands.

If the path length is too long, all incoming radiation would be absorbed and no spectrum seen at all. Acquiring data is fast. Good quality spectra can be collected in just a few seconds. Because of the very high stability of FTIR spectrometers, we normally make hundreds of repeated measurements, one after another, on the same sample. These are then added together to give a spectrum with better signal to noise. Once acquisition is finished, the data is presented as an absorption spectrum, for which a measurement of the clean, dry crystal is used as a background. Positive bands in the spectrum indicate wavelengths at which the samples absorbed infrared radiation.

Notice that in the mid infrared, the convention is to present spectra on a frequency scale with units of wave numbers. Compared with Vis NIR, there is much more detail in the mid infrared, especially in the fingerprint region shown here. Some of the bands can be attributed to specific molecular bonds present in the sample. For instance, the very large feature at 1744 wave numbers, arises from stretching vibrations of the double bond in the carbonyl functional group. At 910, there’s a small peak that arises from polyunsaturated fatty acids. It’s easily seen in sunflower oil as this is quite a high concentration of these compounds. Olive oils, however, contain much lower concentrations of polyunsaturates.

This gives us a way of checking whether olive oil has been adulterated with sunflower oil, and it is much more sensitive than the Vis NIR method. Samples adulterated at 15% weight-for-weight level are easily detected. Elsewhere in the spectrum, other groups of bands are also very sensitive to the exact fatty acid composition, making FTIR coupled with pattern recognition, a useful tool for verifying oil composition, across a wide range of oil types. In the next video, we will see how FTIR can be used to examine coffee, but not in liquid form, but rather as solid samples. We will be looking at ground roast coffee, another high risk product with regards to fraud.