An Introduction on Near-Infrared (NIR) Spectroscopy

Near-infrared (NIR) spectroscopy is a powerful analytical technique that uses the near-infrared region of the electromagnetic spectrum, typically from around 780 to 2500 nanometers, to probe the molecular composition of samples. One of its major advantages is the rapidity with which measurements can be made (Blanco & Villarroya, 2002). It has found widespread applications across various industries including pharmaceuticals, agriculture, food and beverage, biomedical research, environmental monitoring, and much more.   In this post, we will briefly explore the principles of NIR spectroscopy, its instrumentation, applications, advantages, and limitations.

Principles of NIR Spectroscopy

NIR spectroscopy operates on the principle of molecular overtone and combination vibrations, which occur due to the stretching and bending of molecular bonds such as C-H, N-H, and O-H bonds. These vibrations result in absorption of light in the near-infrared region, leading to characteristic absorption spectra for different compounds. NIR spectroscopy measures the absorbance of light at multiple wavelengths within the near-infrared range, generating a spectrum that represents the sample’s molecular composυition.

The vibration of molecules are described using the harmonic oscillator model. This is by which
the energy of the different, equally spaced levels can be calculated from the formula:

E_vib  = (υ + 1/2) * (h/2π)* √ (k/μ)

where υ is the vibrational quantum number, h is the Planck constant, k is the force constant and m is the reduced mass of the bonding atoms. Only those transitions between consecutive energy levels (Δυ =1) that cause a change in dipole moment are possible,

ΔE_vib = ΔE_rad = hυ

where υ is the fundamental vibrational frequency of the bond that yields an absorption
band in the middle IR region.

The methods for exploiting these bond stretches and vibrations have been understood for many years but the analytical technology continues to be a continuous source for new understanding.

Instrumentation

The basic components of a NIR spectrometer include a light source, a sample holder, a dispersive element, such as a grating or prism, a detector, and a data processing unit. Common light sources for NIR spectroscopy include tungsten-halogen lamps, deuterium lamps, and more recently, light-emitting diodes (LEDs) and laser diodes. Detectors commonly used in NIR spectrometers include photodiode arrays and charge-coupled devices (CCDs). Fourier-transform NIR spectrometers are also widely used, employing interferometer-based techniques for high-speed and high-resolution spectral acquisition.

How NIR Reflectance Probes Work

NIR reflectance probes operate by emitting near-infrared light onto a sample and then measuring the amount of light that is reflected back. The interaction between the NIR light and the sample’s molecules results in specific absorption and scattering patterns, which are characteristic of the sample’s chemical composition. The reflectance data collected by the probe is then analyzed using chemometric models to quantify various components within the sample.

In a typical setup, the NIR light source is directed at the sample through optical fibers connected to the probe. The reflected light is captured by the same or different fibers and transmitted to a detector. The intensity of the reflected light at various wavelengths is measured, producing a spectrum that can be used to infer the sample’s properties.

Applications of NIR Spectroscopy

1. Pharmaceuticals: NIR spectroscopy is used for raw material identification, quantification of active pharmaceutical ingredients (APIs) and excipients, monitoring of drug manufacturing processes (e.g., blending, granulation), and quality control of finished products.
2. Agriculture: In agriculture, NIR spectroscopy is employed for soil analysis, determination of nutrient content in fertilizers, assessment of crop quality (e.g., moisture, protein, fat content), and monitoring of grain storage conditions.
3. Food and Beverage: NIR spectroscopy finds applications in food and beverage industries for analysis of ingredients, detection of adulteration, prediction of nutritional properties, and quality control of final products. A number of specific projects have been developed in recent years to develop the applications. One such example is NIR4Dairy.
4. Biomedical Research: In biomedical research, NIR spectroscopy is used for non-invasive analysis of biological samples, such as blood, tissues, and biofluids, for disease diagnosis, monitoring of metabolic processes, and drug delivery studies.
5. Environmental Monitoring: NIR spectroscopy is employed for environmental monitoring applications, including analysis of water quality, detection of pollutants in air and soil, and monitoring of industrial emissions.
6. Fermentation Monitoring: One of the most interesting applications is real-time monitoring of analytes in fermentation media. There is this need for rapid and robust analytical characterization of analytes and NIR spectroscopy fits the bill. 

Advantages of NIR Spectroscopy

1. Non-destructive: NIR spectroscopy is a non-destructive technique that requires minimal sample preparation, making it suitable for rapid analysis of samples without altering their integrity.
2. Multicomponent Analysis: NIR spectroscopy can simultaneously analyze multiple components within a sample, providing comprehensive information about its composition.
3. Real-time Analysis: NIR spectroscopy enables real-time or online monitoring of processes, allowing for rapid decision-making and process optimization.
4. Versatility: NIR spectroscopy is applicable to a wide range of sample types, including solids, liquids, and gases, across various industries.
5. Cost-effective: NIR spectroscopy offers cost advantages over traditional analytical techniques, as it eliminates the need for expensive reagents and consumables.

Limitations of NIR Spectroscopy

1. Limited Depth of Penetration: NIR radiation has limited penetration depth, restricting its application to the surface or near-surface analysis of samples.
2. Interference from Water: The presence of water in samples can interfere with NIR spectra, particularly in the O-H stretching region, requiring careful sample handling or spectral pre-processing techniques.
3. Complex Data Analysis: Interpretation of NIR spectra often requires sophisticated multivariate data analysis techniques, such as chemometrics, due to the complex and overlapping spectral features.
4. Instrumentation Sensitivity: Achieving high sensitivity and resolution in NIR spectroscopy requires advanced instrumentation, which may be costly and not readily available in all settings.
5. Limited Quantitative Accuracy: While NIR spectroscopy is well-suited for qualitative and semi-quantitative analysis, achieving high precision and accuracy in quantitative measurements may be challenging due to factors such as sample heterogeneity and matrix effects.

Suppliers

The main suppliers of equipment all have specialisms in various food categories.

Here are some of the main suppliers:

1. FOSS Analytical: FOSS is a leading provider of NIR solutions, specializing in food analysis. They offer a range of instruments for measuring parameters such as moisture, fat, protein, and total solids in dairy, meat, grains, and other food products. Their instruments are designed for both laboratory and production line environments, ensuring flexibility and reliability​ (FOSS Food Solutions).
2. Thermo Fisher Scientific: This company provides a variety of NIR analyzers, including the Antaris II FT-NIR Analyzer, which is used for quality monitoring throughout the food production process. Their equipment is suitable for analyzing liquids, powders, and other food products, allowing for rapid, non-destructive testing without sample preparation​ (Thermo Fisher Scientific – US).
3. Galaxy Scientific: Galaxy Scientific focuses on advanced FT-NIR technology for food industry applications. Their analyzers can inspect raw materials, perform quick lab analyses, and monitor processes on production lines. They emphasize the advantages of their instruments in improving process efficiency and product quality​ (Galaxy Scientific).
4. Bruker Corporation: Bruker offers a range of NIR spectrometers designed for food safety and quality applications. Their solutions are suitable for analyzing various food matrices and provide high-resolution spectra for accurate analysis.
5. Agilent Technologies: Known for their analytical instruments, Agilent provides NIR solutions that are used in food analysis for moisture, fat, and protein measurements, contributing to quality assurance in food manufacturing ​.
6. PerkinElmer: This company provides a range of NIR instruments designed for food and beverage analysis, helping to ensure product quality and compliance with regulations. Their NIR solutions are integrated into food manufacturing processes for real-time monitoring.

These manufacturers produce equipment which can measures material both in contact and without contact.

Equipment For Off-Line Sampling

The equipment needed for off-line sampling (also applicable to inline monitoring) in a more detailed sense are the following:-

 When it comes to inline monitoring the following equipment considerations are needed.

1. NIR Spectrometer

• Type: Compact, ruggedized, and often portable for industrial environments.
• Design: These are designed for in-line or at-line integration, often smaller than lab-based systems.
• Configuration: Usually fiber-optic-based systems to connect the spectrometer to the measurement point in the production line.
• Wavelength Range: Typically covers the near-infrared region (750 nm to 2500 nm).

2. Fiber Optic Probes

• Purpose: Fiber optic probes are essential for in-line NIR monitoring as they allow remote measurement, transporting NIR radiation from the spectrometer to the sample and back.
• Types:
  • Transmission probes: For monitoring liquid samples or solutions in a flow-through cell. These use mono fibers.
  • Diffuse reflectance probes: For monitoring powders, solids, or slurries. Use fiber bundles to guide the light from and to the spectrometer.
  • Contactless probes: For non-contact measurements of surfaces (common in the food or textiles industry). Also use fiber bundles.
• Probe Materials: Probes must be designed to withstand harsh industrial environments, made of materials like stainless steel or sapphire windows.

3. Flow Cells

• Purpose: Flow cells are the alternative to immersion probes and widely available for process monitoring. They are placed directly into a pipe or bypass so that the sample flows through the cell. These vary in size, shape and diameter depending on the p[ipe diameter. Analysis is similar to an immersion probe. A fibre optic cable transfers the light beam from the source to sample. This is collected by a second fibre and returned to the detector.
• To ensure the cabling for the fibre optics is kept rigorously in position, they are usually fixed with bracketing. It means the cells are remounted with greater precision than would normally be available.

4. Process Interface (Sampling Point)

• Flow Cells: These are used for liquid or gas streams, allowing the NIR light to pass through the stream for real-time measurement.
• Inline Windows: In the case of solid or powder materials, a window or port is built into the process equipment (like a chute or pipeline) where the probe can make contact for reflection measurements.
• Immersion Probes: Used in tanks or reactors for real-time liquid monitoring. The immersion probes are divided into three basic types. These are transmission probes for clear liquids, reflection probes for solid materials and transflection probes for suspensions or emulsions.

5. Light Source

• Type: The NIR spectrometer uses an internal or external light source, such as a tungsten-halogen lamp or LED, that sends NIR light through the fiber optics to the sample.
• Durability: The light source must be robust for continuous operation in an industrial environment.
• Wavelength Range: Covers the NIR range (typically 750–2500 nm).

6. Detector

• Type: Common detectors for in-line NIR applications include InGaAs (Indium Gallium Arsenide) detectors, which provide high sensitivity and stability in the near-infrared range.
• Purpose: To capture the NIR light that has interacted with the sample, measuring the intensity of light absorption or reflection.
• Temperature-Stabilized: Industrial detectors are often temperature-controlled for stability in fluctuating environmental conditions.

7. Process Control System (PLC Integration)

• Purpose: The NIR spectrometer needs to communicate with the plant’s process control system (e.g., a PLC, DCS, or SCADA system).
• Function: The spectrometer sends real-time data back to the control system, which uses this information to monitor and adjust the process parameters (e.g., adjusting temperature, pressure, or feed rates).
• Communication Protocols: Common protocols include Modbus, OPC, Ethernet/IP, or Profinet.

8. Data Processing Unit (PC or Embedded System)

• Purpose: To control the spectrometer, collect the spectral data, and analyze the results in real-time.
• Software: Specialized software is used for spectral data processing, chemometrics, and multivariate analysis. The software typically offers:
  • Real-time monitoring.
  • Predictive modeling for product quality.
  • Alarm systems for out-of-spec measurements.
• Calibration Models: Pre-built calibration models specific to the application (e.g., moisture content, chemical composition) are used to interpret the spectral data.

9. Calibration Standards

• Purpose: Regular calibration is needed to ensure the accuracy of in-line NIR measurements. Reference standards with known NIR absorption properties are used.
• Types: Standards can include reference materials like polystyrene, moisture calibration standards, or specific materials depending on the monitored parameter (e.g., a known sample of the production material).

10. Environmental Protection Housing (for harsh conditions)

• Purpose: In industrial environments with dust, moisture, or high temperatures, NIR instruments and probes need to be protected.
• Types:
  • Explosion-proof housings for hazardous environments (e.g., in chemical processing).
  • Dustproof or waterproof enclosures in food or pharmaceutical plants.

11. Temperature Control System (Optional)

• Purpose: For processes where temperature fluctuations can affect the accuracy of NIR measurements, temperature control might be required for the sample or the equipment.
• Type: Flow cells or probes might be jacketed or have integrated cooling/heating mechanisms.

Sensor Heads

Bruker have developed sensor heads for non-contact analysis. This is a design intended for contactless measurement of moving solid material. It exploits the diffuse reflection mode. It is claimed to have advantages over other reflection probes.

This design uses two NIR light sources which illuminate a sampling spot of approximately 10 mm in diameter. This is said to be 20 times greater than for a conventional diffuse reflectance probe. It uses monofibres transmitting light back to spectrometer. A reference standard is integrated for automatic background measurement without any need to disassemble the sensor during the process as in most solid probes.

The head can be attached to a viewing window in reactors and pipelines. It is also mounted over conveyor belts so as to measure solid materials

Fouling

Fouling of the probe is achieved using a variety of devices. For liquids, a vortex or tubulator is installed around the probe to encourage turbulent flow. Bruker for example have developed a system of vortex cooling to account for high temperatures. In areas where dust or particulates can stick to the probe or the flow cell, air jets have been developed that continually blow binding material away. Similarly anti-adhesion coatings can now be found and which offer a suitable alternative to physical methods of cleaning.

GEA have developed the Lighthouse Probe® which is a DAD sensor and has found application in moisture determination. Axiom Analytical, Inc. developed the FPT-850SCN Self-cleaning Near-Infrared Transmission Probe back in 2012. This was a probe intended for probes that suffered from a build up of particulates on the probe window or on other exposed surfaces. The device used an integral spray nozzle that periodically directed high pressure streams of liquid solvent or water or vapour at the window. It was designed for extremely challenging environments..

Software Packages

To complement the hardware, we now see suppliers of software packages to exploit the spectral data coming from various probes. Back in 2000, NDC Infrared Engineering, (Irwindale, Calif., USA) had developed a data capture system for process analysis and calibration. In keeping with what is now a standard offer, the system was analysing process trends, collecting process data and then displaying this as a graphical plot in real time. It was used to identify rogue measures. The data mirrored events in the system.

Near-infrared (NIR) spectroscopy is a versatile analytical technique with broad applications across various industries. Its non-destructive nature, ability for multicomponent analysis, real-time monitoring capabilities, and cost-effectiveness make it an invaluable tool for research, quality control, and process optimization. Despite its limitations, ongoing advancements in instrumentation, data analysis techniques, and sample handling methodologies continue to enhance the capabilities and utility of NIR spectroscopy in diverse fields, driving innovation and progress in analytical science.

References

Blanco, M., & Villarroya, I. N. I. R. (2002). NIR spectroscopy: a rapid-response analytical tool. TrAC Trends in Analytical Chemistry, 21(4), pp. 240-250.

Nicolai, B. M., Beullens, K., Bobelyn, E., Peirs, A., Saeys, W., Theron, K. I., & Lammertyn, J. (2007). Nondestructive measurement of fruit and vegetable quality by means of NIR spectroscopy: A review. Postharvest Biology and Technology, 46(2), pp. 99-118

Porep, J. U., Kammerer, D. R., & Carle, R. (2015). On-line application of near infrared (NIR) spectroscopy in food production. Trends in Food Science & Technology, 46(2), pp. 211-230