This article leads a discussion on the differences between near-infrared (NIR) and infrared (IR) spectroscopy, which is also referred to as mid-infrared (mid-IR) spectroscopy. This leads to the idea that NIR spectroscopy might be better suited to address analytical concerns in the laboratory and industrial manufacturing processes.
Differences in Wavelengths and Energy
NIR spectroscopy concerns the analysis of how NIR light and matter interact. In spectroscopic analysis, light is determined by the wavelength, not by the applied energy.
The wavelength of light is inversely proportionate to its energy. In other words, the smaller the wavelength, the greater the energy.
The electromagnetic spectrum is displayed in Figure 1. The NIR region ranges from 780 to 2500 nm and sits between the visible region (higher energy) and the infrared region (lower energy).
Figure 1. The electromagnetic spectrum. Image Credit: Metrohm Middle East FZC
Molecules exposed to light from the electromagnetic spectrum's IR and NIR regions will often vibrate in certain regions. These are known as functional groups. IR and NIR spectroscopy are categorized and placed into the vibrational spectroscopy group.
Figure 2 displays several functional groups and molecules active in the NIR region. The variances in the vibrations brought about by IR radiation or NIR light are a result of greater energy wavelengths in the NIR region in contrast to those in the IR region.
Figure 2. Major analytical bands and relative peak positions for prominent near-infrared absorptions. Most chemical and biological products exhibit unique absorptions that can be used for qualitative and quantitative analysis. Image Credit: Metrohm Middle East FZC
Vibrations in the infrared region are categorized as fundamental, which indicates a shift from the ground state to the first excited state. Conversely, vibrations in the near-infrared region are classified into two categories: combination bands (excitation of two vibrations combined) or overtones.
Overtones can be classed as those vibrations moving from the ground state to a level of excitation above the first state (Figure 3). Combination bands and overtones occur less frequently than fundamental vibrations. As a result, there is a visible reduced intensity of the peaks or absorption bands in the NIR range than in the IR region.
Figure 3. Schematic representation of the processes occurring with fundamental vibrations and with overtones. Image Credit: Metrohm Middle East FZC
To put it simply with an analogy, most people climb stairs one step at a time, but some hurriedly climb two or three stairs at once. This is similar to the contrast between IR and NIR: IR (fundamental vibrations) is akin to one step at a time, which is more common than the act of climbing two or more stairs at a time, as is the case with NIR (overtones and combination bands).
The probability of observing vibrations in the NIR region is lower than observing those in the IR region. Therefore, the corresponding absorption bands demonstrate a reduced intensity.
Advantages of NIR Spectroscopy Over IR Spectroscopy
The theoretical outline as discussed draws the conclusion that NIR has several advantages over IR spectroscopy, as discussed in the following:
Lower Intensity of Bands with NIRS, Therefore Less Detector Saturation
For solids, pure samples can be used without preparation in a vial suitable for NIR analysis. With IR analysis, preparation in the form of a KBr pellet or carefully administering the solid sample to the Attenuated Total Reflectance (ATR) window is required. These practices mean there is a clean-up process added post-analysis.
For liquids, including viscous substances, NIR spectra can be measured and inserted in disposable 4 mm (or 8 mm) diameter easy-to-fill vials. However, for IR analysis, very short pathlengths (<0.5 mm) are required in combination with expensive quartz cuvettes or flow cells, neither of which are user-friendly.
Higher Energy Light with NIRS, Therefore Deeper Sample Penetration
Unlike IR spectroscopy, NIRS transmits information relative to the bulk sample and not just information regarding surface traits.
NIRS can be Used for Quantification and Identification
Scientists often utilize infrared spectroscopy to see if certain functional groups are present in a molecule (identification only). In fact, quantification is a benefit that NIR spectroscopy offers (see below).
NIRS is Versatile
NIR spectroscopy is useful for various tasks such as the quantification of chemical substances (e.g., moisture, API content), identifying chemical parameters (e.g., hydroxyl value, total acid number) as well as determining physical parameters (e.g., density, viscosity, relative viscosity, and intrinsic viscosity).
NIRS is Compatible with Fiber Optics
Fiber optic cables are compatible with infrared radiation due to physical limitations. However, with NIR you can easily transfer the laboratory method directly into a process environment using an analyzer with a long, low-dispersion fiber optic cable and a robust probe.
NIR ≠ IR Spectroscopy
As described, NIR can be differentiated from IR, although both are types of vibrational spectroscopy, NIR is faster and more user friendly compared to IR.
NIR does not require preparation of the samples and can offer information regarding the bulk material.
NIR spectroscopy’s versatility facilitates the quantification of a variety of different chemical and physical parameters in the laboratory which can be transferred to a process environment.
Watch the video below to learn about the main differences between IR and NIR spectroscopy.
To discover more about NIR spectrometers for lab and process analysis, as well as Raman solutions, click here.
This information has been sourced, reviewed and adapted from materials provided by Metrohm Middle East FZC.
For more information on this source, please visit Metrohm Middle East FZC.