In the first article in our series about near-infrared (NIR) spectroscopy, we have explained what NIR spectroscopy is. This article focuses on the difference between near-infrared and infrared (IR) spectroscopy, which is often also called mid-infrared (mid-IR) spectroscopy. We will also discuss why NIR spectroscopy might be more suitable than IR spectroscopy for your analytical challenges in the laboratory and in industrial manufacturing processes.
Differences in wavelengths and energy
We have defined NIR spectroscopy as the analysis of the interaction between NIR light and matter. In spectroscopic analysis, light is defined by the wavelength (and not by the applied energy). If this sounds new, you can refer to the first blog article of this series:
The wavelength of light is inversely correlated to its energy. Therefore, the smaller the wavelength, the more energy there is. The electromagnetic spectrum is shown in Figure 1. The NIR region lies between the visible region (higher energy) and the infrared region (lower energy) at 780 to 2500 nm.
Light from both the IR and NIR region of the electromagnetic spectrum induces vibrations in certain parts of molecules (known as functional groups). Thus, IR and NIR spectroscopy belong to the group of vibrational spectroscopy. Figure 2 shows several functional groups and molecules that are active in the NIR region.
The difference in the vibrations induced by IR radiation or NIR light is due to the higher energy of NIR wavelengths compared to those in the IR region.
Vibrations in the infrared region are classified as fundamental – meaning a transition from the ground state to the first excited state. On the other hand, vibrations in the near-infrared region are either combination bands (excitation of two vibrations combined) or overtones. Overtones are considered vibrations from the ground state to a level of excitation above the first state (Figure 3). These combination bands and overtones have a lower probability of occurring than fundamental vibrations, and consequently the intensity of peaks or absorption bands in the NIR range is lower than peaks in the IR region.
This can be better understood with an analogy about climbing stairs. Most people climb one step at a time, but sometimes you see people in a hurry taking on two or three stairs at once. This is similar to IR and NIR: one step (IR – fundamental vibrations) is much more common compared to the act of climbing two or more stairs at a time (NIR – overtones and combination bands). Vibrations in the NIR region are of a lower probability than IR vibrations. Therefore, the corresponding absorption bands have a lower intensity.
Advantages of NIR spectroscopy over IR spectroscopy
The theoretical outline above lets us derive the following advantages of NIR over IR spectroscopy.
Lower intensity of bands with NIRS, therefore less detector saturation
For solids, you can use pure samples as-is in a vial suitable for NIR analysis. With IR analysis, you either need to create a KBr pellet or carefully administer the solid sample to the Attenuated Total Reflectance (ATR) window, not to mention cleaning everything thoroughly afterwards.
For liquids, NIR spectra should be measured in disposable 4 mm (or 8 mm) diameter vials which are easy to fill, even in the case of viscous substances. IR analysis requires very short pathlengths (<0.5 mm). Either costly quartz cuvettes or flow cells are needed, neither of which are easy to fill.
Higher energy light with NIRS, therefore deeper sample penetration
This means NIRS provides information about the bulk sample and not just surface characteristics, as with infrared spectroscopy.
NIRS can be used for quantification and for identification
Scientists often use infrared spectroscopy to detect the presence of certain functional groups in a molecule (identification only). In fact, quantification is one of the strong points of utilizing NIR spectroscopy (see below).
NIRS is versatile
NIR spectroscopy can be used for the quantification of chemical substances (e.g., moisture, API content), determination of chemical parameters (e.g., hydroxyl value, total acid number) or physical parameters (e.g., density, viscosity, relative viscosity, and intrinsic viscosity). You can click on these links to download our free Application Notes for each example.
NIRS works with fiber optics
This means you can easily transfer a method from the laboratory directly into a process environment using an analyzer with a long, low-dispersion fiber optic cable and a rugged probe. Fiber optic cables are not possible to use with infrared radiation due to physical limitations.
NIR ≠ IR spectroscopy
In summary, NIR is a different analytical method than IR, although both are types of vibrational spectroscopy. NIR is faster and easier to handle than IR. It does not require sample preparation and can provide information about the bulk material. It is also versatile. NIR spectroscopy allows for the quantification of different kinds of chemical and physical parameters and can also be implemented in a process environment.
Watch our video to learn about the major differences between IR and NIR spectroscopy.
In the next installment of this series, we focus on the process of implementing a near infrared spectrometer in your laboratory workflow, using a specific example.