NIR Versus Mid-IR: How to Choose

Apr 01, 2006
By Spectroscopy Editors
Volume 21, Issue 4

Table I: A random listing of samples amenable to infrared analysis
Over the past twenty years, the use of the near infrared (NIR) region of the spectrum for a wide variety of the analytical procedures has grown precipitously. There are a number of instrument manufacturers providing a range of instruments from sophisticated costly spectrometers to recently introduced, lower cost, portable models. There are frequent seminars devoted to NIR use in different industrial areas. And because of the nature of NIR analytical data, a great deal of effort has gone into developing methods to sort out useful information from the generally weak and overlapping absorption material typical of NIR data.

While NIR spectroscopy has found many broad applications in industrial process management, NIR advocates have sometimes tended to become overzealous in applying NIR to all industrial analyses, while in some cases, other procedures such as Raman and mid-IR spectroscopy could provide more useful analytical data more readily. After all, it is the mid-IR spectral region where most of the fundamental structural information is produced and when measurements in that region can be made on a particular class of samples, it may well be the method of choice. It is the purpose of the following sections to describe the overall characteristics of both spectral regions — the minuses as well as the plusses — to help the perspective user to select the spectral region best suited to his or her perspective uses.

NIR and Mid-IR Strengths and Weaknesses

NIR Strengths

1. Higher energy levels because radiation levels from black body emitters peak at shorter wavelengths.

2. High sensitivity photo conductive detectors function in the NIR but not in the mid-IR.

3. Water is reasonably transparent in this region, making it possible to use it as a solvent for some applications.

4. Perhaps most important, low-cost materials such as glass and quartz transmit NIR radiation and can be used as cell windows, focusing lenses, and optical fibers.

NIR Weaknesses

Figure 1: A portion of the electromagnetic spectrum showing the relationship between NIR and mid-IR.
The major weakness of the NIR region is that the absorption bands occurring there are the overtones of the fundamental bands residing in the mid-IR region. As a result, they are relatively weak and not clearly delineated. This makes quantitative calculations complex and calibration procedures quite laborious and not transferable from one instrument to another.

NIR Applications

In spite of the above limitations, NIR equipment is becoming widely used in such applications as diffuse reflection measurements on foods, monitoring pharmaceutical processes, in-situ biomedical applications, to name a few.

Mid-IR Strengths

1. Organic functional groups have characteristic and well delineated absorption bands in this spectral region.

2. Because molecules differ from each other by having different combinations of functional groups, their mid-IR spectra can be used to identify them and characterize their structure.

3. Mid-IR spectra of mixtures are additive. Absorption bands associated with individual components in a mixture are frequently isolated from other bands and can be used to quantify the individual components by the strength of their absorption.

4. Calibration data in the mid-IR is much more generic than that in the NIR and thus is more readily transferable from instrument to instrument.

Mid-IR Weaknesses

1. As a result of the aforementioned black-body radiation curve, available energy in the mid-IR decreases substantially with wavelength.

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