Near-infrared spectroscopy (NIRS) is a non-destructive analytical method using light from the near-infrared spectrum (780 to 2500 nanometers) to examine materials, including biological tissues. The technique provides a window into a sample’s composition by measuring how light that passes through or reflects off it is modified.
This technology is valued for its ability to perform rapid analyses with little to no sample preparation. These features make it useful for real-time monitoring in production processes and for measurements in sensitive environments across many scientific and industrial fields.
How NIRS Sees the Unseen
The principle of NIRS involves illuminating a sample with near-infrared light and detecting how the light changes after interacting with the material. Molecules within the sample absorb and scatter this light in predictable ways based on their molecular bonds. These unique patterns of absorption and scattering create a spectral signature for specific chemical constituents.
Molecules that interact with near-infrared light, known as chromophores, include water, fats, proteins, and hemoglobin. For instance, NIRS can distinguish between oxygenated hemoglobin (oxyhemoglobin) and deoxygenated hemoglobin because they absorb light at slightly different wavelengths. An NIRS device measures the intensity of the light transmitted through or reflected from the sample.
By analyzing this resulting light, scientists create a spectral signature of the sample. This signature acts as a fingerprint, revealing the concentration of different molecules after algorithms convert the raw data into quantitative measurements.
Diverse Applications of NIRS Technology
In medicine, NIRS is used to monitor physiological processes non-invasively. Functional NIRS (fNIRS) measures brain activity by detecting changes in blood oxygenation in the cerebral cortex for neuroscience research and clinical studies. The technology also monitors muscle oxygenation in sports science, providing insights into how muscles use oxygen during exertion.
The food and agriculture industries use NIRS for quality control, assessing the composition of products like grains, meats, and fruits. For example, it can determine the moisture in cereals, fat in meat, or ripeness of fruit. This allows producers to make quick decisions to ensure product consistency.
The pharmaceutical industry employs NIRS for quality assurance during drug manufacturing. It is used to verify the identity and purity of raw materials and to monitor the formulation process in real-time, helping ensure products meet regulatory standards.
Understanding NIRS Devices
NIRS instruments are designed in various forms, from portable units to large laboratory systems. Wearable NIRS devices, such as headbands for brain monitoring or patches for muscle oxygenation, allow for data collection in natural environments. These systems provide continuous, real-time physiological information during activities like athletics or daily life.
For more precise analyses, benchtop NIRS instruments are used in laboratory settings. These larger systems are common in pharmaceutical and food quality control labs where high accuracy is required. They can perform complex measurements on liquids, solids, and powders.
Handheld NIRS devices balance portability and performance for on-site analysis in fields like agriculture. A farmer can use a handheld scanner to assess forage nutrition in the field, or an environmental scientist can analyze soil composition on-site. These devices provide immediate data, facilitating quick decisions without transporting samples to a lab.
Strengths and Limitations of NIRS
A primary advantage of NIRS is its non-invasive and non-destructive nature, allowing analysis without altering or damaging the sample. The technology uses non-ionizing radiation, making it safe for repeated use on humans and enabling continuous monitoring.
Many NIRS devices are portable, provide real-time results, and are often more cost-effective than techniques like magnetic resonance imaging (MRI). The speed of analysis, often taking less than a minute, is beneficial in time-sensitive situations.
However, the technology has limitations. The penetration depth of near-infrared light into tissues is shallow, typically a few centimeters, which restricts its use to superficial layers and prevents examination of deeper organs. NIRS measurements are also highly sensitive to movement, which can create data artifacts and complicate interpretation.
The measurements are indirect, requiring that spectral data be correlated with the property of interest through mathematical calibration models. Developing these models can be complex and time-consuming, and the signal can be influenced by factors like temperature and light scattering, which may reduce accuracy.