Near Infrared (NIR) analysis is a spectroscopic technique that makes use of the naturally occurring electromagnetic spectrum.
The NIR region is the area of the spectrum defined by wavelengths between 700nm and 2500nm. Near Infrared is an accurate and rapid analysis method that is well suited for quantitative determination of the major constituents in most types of food and agricultural products.
The NIR advantage
The overall advantages of using NIR analysis is that it provides rapid analysis data for better decision making in food and agri production processes. Compared to traditional analysis methods it requires little or no sample preparation and no chemicals or consumables. It is non-destructive, operator friendly, fast (30-60 seconds), reliable and precise.
How it works
The working principle can be defined as follows:
- Near Infrared light is directed onto a sample
- The light is modified according to the composition of the sample and this modified light is detected (see transmission and reflectance below)
- The spectral modifications are converted to information regarding the composition of the sample
- These conversion algorithms are called ”calibrations”
Transmission and reflectance
An infrared spectrum can be obtained by passing infrared light through a sample and determining what fraction is absorbed by the sample (transmission). Alternatively, light can be reflected from the sample and the absorption properties can be extracted from the reflected light (reflectance). NIR reflectance is a subject in itself that will not be covered in detail here.
NIR reflectance and transmittance methods can be chosen according to the analysis job, for example, transmittance is good for measuring cheese to obtain a representative measurement throughout the sample. For homogenous samples such as milk powder, reflectance is ideal.
For both methods, the NIR wavelength range is an important consideration. For instance, a Short Wave NIR range (850 – 1050nm) gives good sample penetration with NIR transmission.
An infrared spectrum can be obtained by passing infrared light through a sample (transmission) or the light can be reflected from the sample (reflectance).
What’s your wavelength?
Any discussion about NIR wavelength must start with the spectrometer that converts the infrared light into a usable signal. A common consideration across all spectrometer technologies is the wavelength range measured in nanometers (nanometers nm or reciprocal centimetres cm-1).
Without getting too deep into a technical discussion it can be said that certain wavelengths are better for measuring certain samples and parameters than others. The wavelength required to measure protein in grain for example is different from that required to measure amino acids in feed ingredients and so on. Another consideration is the signal to noise ratio offered by different instruments at certain wavelengths as this indicates the quality of the signal generated.
Monochromator, FT-NIR and DDA
For quantitative measurements of feed, a so-called scanning grating monochromator instrument is a proven choice. It is ideal for quantitative measurements across a broad spectrum of applications with a broad wavelength range for a wide range of parameters including those such as colour in fish food or similar requiring the visible region in addition to the NIR region. Alternatively, when using NIR transmis¬sion for measuring inhomogeneous samples such as grain, it is an advantage to use the Short Wave NIR range (850 – 1050nm) where the light penetration is good and the premium signal to noise ratio offered by a scanning grating monochromator is essential.
On the minus side, an internal wavelength standard is required to achieve good wavelength accuracy. However, like all other aspects of NIR technology, scanning grating instruments are constantly evolving and many are using this in¬ternal wavelength standard to demonstrate good wavelength accuracy.
For qualitative measurements in the laboratory where narrow instrument bandwidth is needed, FT-NIR technology has advantages. It is ideal for pin-point qualitative measurements of substances with narrow absorption bands or for some quantitative measurement ap¬plications for samples having closely spaced narrow absorption bands. Plus, it is possible to adjust the resolution to obtain the best tradeoff between wavelength resolution and the signal to noise ratio.
Disadvantages include a lower signal/noise ratio than a monochromator instrument, particularly at short wavelengths and the omitted visible wavelength range (below 850 nm). FT NIR is also a vibration-sensitive technology and the design must take this into account for applications in a production environment.
On the subject of production, the use of NIR analysis instruments close to the production line or even for continuous measurements of material in the production line is increasingly widespread. For these measurements, fixed grating Detector Diode Array (DDA) is the best option. It is ideal for robust and vibration-tolerant instruments and the simultaneous measurement of the full spectrum also makes it tolerant to sample movement. On the downside, a tradeoff between wavelength range, resolution and signal to noise ratio must be made.
NIR or FTIR?
NIR and FTIR are, in principle, very similar. The main difference is that FTIR works across longer wavelengths. Additionally, many special optical components and materials are used with FTIR. The benefit of the longer wavelengths is that more specific chemical information is typically obtained from the samples.
A comparison of NIR and FTIR is provided in this video in relation to applications in the dairy industry. Called ‘Directions in dairy analysis’, the video includes interviews with experts from FOSS who explain the technology and the considerations to be made when selecting an infrared analytical instrument.
