Electromagnetic energy is used when you tune your radio, watch TV, send a text message, or make popcorn in a microwave oven. Every hour of every day, you rely on this energy. The world you know would not exist without it. The electromagnetic (EM) spectrum encompasses all forms of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays.
The human eye can only detect only a small portion of this spectrum called visible light. Today we will try to understand the basics of near-infrared spectroscopy and its industrial application.
Introduction to NIR Spectroscopy
NIR is an abbreviation for Near Infrared Spectroscopy, and it refers to the analytical technique of analysing materials for compositional or distinctive traits using near-infrared light.
When you reach your palm out to a burning fire, you “feel” the heat it emits, but what is going on? The fire emits light and infrared (IR) radiation, the majority of which is near-infrared (NIR) radiation.
William Herschel, a musician and a successful amateur astronomer (he found Uranus), discovered this radiation in the year 1800 because he wanted to know if any specific colour was connected with heat from sunlight. He found that the heat maximum was beyond the red end of the spectrum.
Near-IR spectroscopy belongs to the group of vibrational spectroscopy techniques, along with mid-infrared spectroscopy (primarily FT-IR) and Raman spectroscopy. The first near-IR spectrometers were developed in the 1980s, initially designed for industrial applications and chemical analysis.
Near-infrared (NIR) spectroscopy is an adaptable form of analysis that may be used in a variety of research and industrial process applications. Long used in remote sensing, NIR spectroscopy has gained popularity in the industrial sector as a low-cost method for assessing materials in order to optimise operations and manage expenses. NIR radiation is present in optical fibre, TV remotes and, of course, near-infrared spectroscopy.
How does it work?
The main principle behind the different methods of spectrophotometry, including NIR spectroscopy, is the Beer-Lambert Law. According to this law, the concentration of a certain chemical compound in a solution determines how much light, whether visible or infrared, this solution will absorb.
NIR spectroscopy (NIRS) is an absorption spectroscopy method that helps detect the chemical composition of a substance or solution by measuring how much near-infrared light the compound or solution absorbs. Near-infrared radiation waves are slightly longer than those of visible light and make use of the near-infrared region of the electromagnetic spectrum (from about 700 to 2500 nanometres). By measuring light scattered off of and through a sample, NIR reflectance spectra can be used to quickly determine a material’s properties without altering the sample.
Absorptions in the NIR region are generated from fundamental vibrations by two processes; overtones and combinations. Overtones can be thought of as harmonics. So every fundamental will produce a series of absorptions at (approximately integer) multiples of the frequency (frequency is the reciprocal of wavelength). Combinations are rather more complex. Combinations arise from the sharing of NIR energy between two or more fundamental absorptions. NIR absorptions are at a higher state of excitement so they require more energy than a fundamental absorption. Combinations arise from the sharing of NIR energy between two or more fundamental absorptions.
We can think of chemical bonds as weak springs holding together two or more atoms, these springs will vibrate naturally and when energy is added to the system then they will vibrate more energetically. When exposed to near-infrared radiation, a molecule absorbs the electromagnetic photons and starts the process known as the vibrational transition — stretching, shrinking, bending, rocking back and forth, and so on. Because of this mechanism, NIR spectroscopy is commonly referred to as vibrational absorption spectroscopy.
Because each different material is a unique combination of atoms, no two compounds produce the exact same near-infrared spectrum. Therefore, near-infrared spectroscopy can result in a positive identification (qualitative analysis) of each different material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present. With modern software algorithms and statistical treatments, NIR spectroscopy is an excellent tool for quantitative analysis, offering a practical alternative to time-consuming wet chemical methods and liquid chromatographic techniques
Molecular vibration helps determine the chemical composition of matter through NIR spectroscopy. Let’s consider a molecule of water as an example. A molecule of water consists of two partially positively charged atoms of hydrogen connected to a partially negatively charged atom of oxygen. When exposed to specific frequencies of infrared radiation, the water molecule is excited to one of the following higher-energy vibrational modes:
Asymmetric stretch, during which one of the hydrogen bonds shrinks while the other extends;
Symmetric stretch, during which both hydrogen bonds shrink or stretch;
Scissoring bend, during which both hydrogen atoms rock back and forth towards each other as if they were impaled by a pair of scissors.
NIR requires little or no sample preparation. It can also be used to analyze multiple constituents in a single scan. NIR spectroscopy is commonly used to quantify organic functional groups, particularly O-H, N-H, and C=O.
Instrumentation
To record the intensity at different wavelengths, a source, a detector, and a dispersive device (such as a prism or, more typically, a diffraction grating) are used. Fourier transform NIR equipment with an interferometer are very popular, particularly at wavelengths above 1000 nm. FT-NIR hardware is generally more complex but advances in electronics, methods, and manufacturing have significantly improved the detector sensitivities, resolution, and immunity from vibrational effects. Though, according to the literature, FT-NIR and NIR instruments were comparable in predictive performance and there seemed to be no advantage of either method over the other for some of the constituents measured. The spectrum can be measured in either reflection or transmission depending on the sample.
Tungsten halogen light bulbs or quartz halogen light bulbs or light-emitting diodes (LEDs) are most often used as broadband sources of near-infrared radiation for analytical applications.
The type of detector employed is mostly determined by the wavelength range to be monitored. Silicon-based CCDs are appropriate for the shorter end of the NIR range but are insufficiently sensitive for the majority of the range (over 1000 nm). For wavelengths over 1100 nm, InGaAs and PbS devices are more suited and have better quantum efficiency.
Benefits of NIR Testing
- Simple to use – standard operation is loading a sample cell and turning on the device.
- Little sample preparation – most samples may be analysed as-is (non -destructive) or with minor grinding or particle size reduction.
- No hazardous chemical waste – no chemicals are used at all.
- Fast analysis – typical analysis times are 10 seconds – 2 minutes.
- Simultaneous results for multiple parameters – multiple constituents are predicted with one sample analysis.
- High accuracy and sensitivity – Gratings on a NIR spectrophotometer aid in the accurate selection of certain wavelengths throughout the testing procedure
- Reliable results – for most analyses, NIR instruments have a prediction accuracy within 1.5 times of the reference method error with much better precision.
- Cost-effective – one analyst can typically analyse several hundred samples in a day with no consumable costs.
Some of the Applications:
- Vegetables & fruit: Water content, carbs, calories
- Dairy products: Calories, fat, proteins, water
- Food/Meat: Protein, carbohydrate, moisture, fat, food oils and different polymers
- Grain & fruit: best time for harvesting
- Pharmaceutical: Active pharmaceutical ingredients (APIs) solid, liquid and pharmaceutical forms, process monitoring or in QC laboratories.
- Agriculture: detection of plant diseases
Conclusion:
NIR can often penetrate a sample far deeper than FTIR and, unlike Raman, is unaffected by fluorescence. As a result, while NIR spectroscopy is not as chemically specific as Raman or FTIR, it may be quite effective in investigating bulk material with little or no sample preparation.
NIR is a very reliable technique for the positive identification and quantitation of any sample, thus making it an invaluable tool for quality control and quality assurance applications.
