Infrared spectrometers: NIR and MIR compared

Infrared spectrometers
Infrared spectrometers can broadly be divided into two types: dispersive and Fourier transform instruments. Although it is possible to configure each type of instrument to work across both ranges, in general, dispersive spectrometers operate in the NIR, and Fourier transform instruments in the MIR.
Near-infrared and mid-infrared compared
During the early development of infrared spectroscopy for food analysis, the focus was on the use of near-infrared. NIR spectroscopy is now firmly established in the food sector, utilised for a wide range of applications. However, as MIR instrumentation has developed, becoming simpler to use and more accessible in terms of cost, applications to food analysis are increasingly being reported. Here, the advantages and disadvantages of each technique in the context of food analysis are compared.Dispersive NIR spectroscopy | Fourier transform MIR spectroscopy (FTIR) |
Advantages: | |
Cheaper instrumentation costs than MIR Much more robust components – easier to make rugged instruments, involving no moving parts Very flexible sampling arrangements – interaction between radiation and sample can take place via reflection or transmission, from a wide range of sample types and geometries | Fundamental vibrations of molecular bonds occur in the “fingerprint” region of the MIR, making the spectral profiles very sensitive to chemical composition – even very similar molecules can produce quite distinctive spectra Very accurate frequency scale due to internal laser reference beam – facilitates co-addition of data to improve signal-to-noise ratio. High optical throughput – plenty of radiation available to interact with sample Detector receives all wavelengths at once giving a signal-to-noise benefit ((i.e. multiplexing) |
Disadvantages: | |
Significant absorption bands only arise from bonds that include a hydrogen atom. This means that functional groups such as C=O (carbonyl, found in oils and fats) and C-N (found in proteins) do not produce distinct, identifiable bands Absorption bands are very broad and overlapped, so that many chemically different samples give rise to almost indistinguishable spectral profiles, and assignment of features to individual chemical entities is difficult | Some absorption bands are so strong in the infrared (O-H in water, for instance) that sample thicknesses of more than a few microns lead to complete absorption and no spectrum is obtained. This makes transmission sampling practically impossible for many samples, and reflectance methods must be used instead. Attenuated total reflectance (ATR), which has an effective sample pathlength of <10microns, is the best way to analyse food samples, but a disadvantage is that very little of the total sample volume is actually inspected – this can be problematic for samples that are not homogenous on the micron scale |
Our purpose is to transform access to education.
We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.
We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.
Learn more about how FutureLearn is transforming access to education