We’ve put together some commonly asked questions to give you more information about NIR Spectroscopy
What is NIR Spectroscopy?
NIR spectrometers consist of a light source, a tool to split the light into separate wavelengths, and a detector. NIR spectra create a “fingerprint” of the sample by the NIR energy transmitting through it (often but not exclusive to liquids) or reflecting off it (solids) and the subsequent NIR radiation interaction. The instrument detector collects the light after interaction with the sample. These spectra include data related to the physical and chemical properties of molecules in the sample by absorption of chemical bonds. However, there is often overlap between the wavelengths of chemical bonds in the NIR spectral range (750 to 2500nm), so the spectra can be difficult to interpret visually. A single peak absorption change cannot be attributed to the change in one chemical or physical property because of the vibrational energy overlap between different bonds. Because of this, mathematical pre-processing and chemometrics must be used to extract relevant information from spectral data. Mathematical models are created that are used for either classification or a quantitative measurement that correlates the NIR spectra to reference data for a parameter of interest. Once such models are created and validated, they can use the NIR spectra for direct measurement.
In some cases, a process known as local modeling is used where classification is first performed, and then a subsequent quantitative model is chosen to measure a parameter of interest-based on the classification analysis. Multiple models can be created from one set of NIR spectra if there are reference methods for all parameters of interest. It is also essential to use a calibration data set that covers the entire range of the parameters of interest to be measured.
What is the difference between NIR and FT-NIR Spectrometers?
FT-NIR (Fourier Transform-NIR) is a technique used to simultaneously collect high-resolution spectral data over the entire range of interest. There are significant advantages to this method over a dispersive spectrometer, which measures intensity over a very narrow range of wavelengths over time. This technique requires repeated measurements to obtain absorption spectra over the full range of interest. FT-NIR spectrometers use a light beam containing multiple frequencies which is modified very quickly in a short period of time. This occurs by shining the light into a Michelson interferometer, a configuration of two mirrors. One mirror is fixed and the other moves to transmit and block different wavelengths of light. The raw data is the light absorption for each mirror position and processing is required to change the raw data into the desired spectrum showing the light absorption at each wavelength. The processing is known as a Fourier Transform and it converts displacement of the mirror (measured in cm and called an interferogram) into the inverse domain and resulting absorbance spectrum (wavenumber in cm-1). FT-NIR spectrometers collect spectra in wavenumber (an inverse measure of distance) as opposed to dispersive NIR spectrometers collecting spectra in wavelength (a measurement of frequency).
There are numerous advantages to using FT-NIR spectrometers over dispersive instruments. Both resolution (WHY) and signal-to-noise ratio (SNR) tend to be higher for FT-NIR instruments. Higher SNR is the result of simultaneous wavelength collection as well as higher throughput. Wavelength accuracy is higher as well because wavelength is calibrated in FT-NIR instruments by passing a laser beam of known wavelength through the interferometer. Wavelength calibration in dispersive instruments is dependent on the mechanical movement of diffraction gratings and this can lead to inaccuracy. Imperfections in diffraction gratings and accidental reflections can also lead to stray light in dispersive instruments, which is the radiation of one wavelength appearing in another wavelength in the spectrum. Because the wavelength is determined by the modulation frequency of the interferometer in FT-NIR spectrometers, there is no direct equivalent to the stray light that can be found in dispersive instruments.
The primary advantage of FT-NIR spectrometers is that instruments are inherently reproducible, which eliminates the need for method transfer of calibration models needed to analyze the spectra. The process of collecting spectral data and correlating reference values of the parameter of interest to the spectra using chemometrics can be time-consuming and reference tests can be expensive. When a calibration model is created using one dispersive spectrometer, it may not work on another spectrometer of the same type because of differences in the dispersive element. One good example of this is AOTF spectrometers, which use a crystal modulated at different frequencies to disperse light into different wavelengths. Such instruments can never be reproduced due to the inability to duplicate crystals. Calibration models can be transferred from one instrument to another using mathematical algorithms and software standardization. This process is known as method transfer. Method transfer can be difficult, time-consuming, error-inducing, and sometimes may not work at all depending on the type of spectrometer, especially when transferring to multiple instruments. FT-NIR spectrometers require no method transfer between instruments, even when using calibration models made by a different vendor as long as the original instrument is also an FT-NIR spectrometer. This results in saving of time and resources, especially when deploying multiple instruments in a production environment.
What are the Advantages of Using NIR Spectroscopy Over Traditional Testing Methods and Other Spectroscopic Methods?
NIR spectrometers offer a fast, non-invasive, and cost-effective method for testing parameters of interest in numerous industries. There is little if any sample preparation required and no sample destruction. No chemicals or solvents are required for use. While NIR spectroscopy requires the creation of calibration models using chemometrics to correlate spectra to reference values, once these models are constructed the benefits are enormous. Multiple parameters can be measured with a single scan after models are created. Reference testing in the food and beverage industries often consists of expensive and time-consuming tests like HPLC and wet chemistry tests. These tests require skilled technicians and if not performed on-site, the results can take a week or longer to obtain. Performing such tests in a real-time process setting is often impractical if not impossible. Advances in the technology of NIR spectrometers and the Process Analytical Technology (PAT) initiative have led to the implementation of NIR spectroscopy as a real-time process control tool.
Various spectroscopic methods exist but NIR spectroscopy has proven to be the most suitable technology in many industries for measuring parameters of interest in both laboratory and process settings. NIR offers the advantages of little to no sample preparation, deep light penetration into the sample, and no sample destruction. The deep light penetration is especially advantageous in the food industry when samples are often not homogenous. Some methods are not suited to measure parameters of certain organic molecules as well.
How is NIR Spectroscopy Used in the Food and Beverage Industries?
NIR spectroscopy is used in numerous verticals in the food and beverage industries for testing of raw materials, intermediates, and finished products. Depending on the type of instrument, it can be used as a portable handheld mobile tool, a laboratory instrument, or a process control tool to optimize the manufacturing process and reduce waste. Industries that use NIR spectroscopy for testing include alcoholic and non-alcoholic beverages, edible oils, dairy, meat, seafood, and spices. Identification of ingredients and additives in one application that is used in many of these industries. Conformity testing is another example of an application that can used for identification of a “good” or “bad” batch of samples. In these two cases, NIR spectra are often matched to a library of known spectra to identify the constituent of interest, eliminating the need for construction of quantitative calibration models. This is similar to identification of raw materials in the pharmaceutical industry, which is usually the first step in the manufacturing process. Composition analysis requires the construction of calibration models to measure specific concentrations of parameters. Fat, protein, moisture, and carbohydrates are examples of organic parameters that have been measured in many verticals in the food and beverage industries. More specific parameters have been measured as well, such as fatty acid profiles, sugar content, alcohol content, acidity, and even salt concentration. While salt is not an organic molecule, it often has an indirect effect on water molecules in a sample. An indirect correlation is acceptable when constructing chemometric models for use by NIR spectroscopy, but such models must be carefully analyzed to ensure that the analysis is legitimate. Color analysis is another example of an indirect correlation which has successfully been measured using NIR spectroscopy.
Food fraud and adulteration is a tremendous problem in the food and beverages industries and NIR spectroscopy has been used as a tool to identify and monitor adulteration in products. Adulteration can constitute many different forms and methods. Misidentification of a product is one common form of adulteration. Adding cheaper ingredients or dilution is another method of adulteration. In some cases, this can be dangerous and present a health hazard, such as adding melamine to products containing protein. Melamine mimics protein in standard wet chemistry tests and in this case, NIR spectroscopy not only presents a faster and cheaper method for adulteration identification, it actually presents a solution when standard testing will not work. If an adulterant is identified in a food or beverage product using NIR spectroscopy, the sample can be sent off for further testing. Large-scale testing is often impractical using standard methods and using NIR spectroscopy can be a powerful screening tool for adulterant identification.
The use of NIR spectroscopy as a real-time process control tool has become more prevalent as advances in technology have moved the focus of new instruments from the laboratory to the process. Process Analytical Technology (PAT) is a framework for innovative process manufacturing and quality assurance. Critical points and parameters during manufacturing of a product are defined and the process is designed in a way that such points and parameters can be measured using analytical tools and instruments for real-time process feedback and control. Such instruments must be able to measure on-line and in a non-invasive manner. Many vendors have developed instruments that are able to measure multiple points in a process with a single instrument, usually using optical fibers and probes. PAT has become an important part of pharmaceutical as well as chemical manufacturing and is beginning to acquire a hold in the food & beverage industry. Real-time feedback using NIR spectroscopy can optimize the use of materials as well as reduce or eliminate the production of material that does not meet specifications.