Chapter 9
Use of Near Infrared
Reflectance Spectroscopy
J.B. Reeves III
Nutrient Conservation and Metabolism Laboratory,
Livestock and Poultry Sciences Institute,
Agricultural Research Service, USDA, Beltsville, Maryland, USAIntroduction
Most of us are familiar with the use of
spectrometers for quantitative determina-
tions. In these applications, one generally
runs a set of standards containing various
concentrations of the analyte, for example
protein, and then compares the results of
unknowns with the standard results using
a standard curve. In such cases, the
assumption is made that only the concen-
tration of the analyte of interest (protein, P,
etc.) is changing, and that the absorbance
changes linearly with the changing concen-
tration of the analyte according to Beer’s
Law, which states that:
A= abc (9.1)where A= absorbance or (log 10 (1/transmit-
tance)), b= the cell pathlength, c= concen-
tration and a = the molar extinction
coefficient for the analyte at the wavelength
used (constant telling how strongly the
analyte absorbs at the wavelength chosen).
In such determinations, the wavelengths
used are generally in the ultraviolet (UV) or
visible portion of the electromagnetic
spectrum (EMS), and the wavelength
needed for each determination is known.
The near infrared (NIR, 1100–2500 nm)
portion of the EMS occurs at wavelengths
longer than the visible (VIS, 400–760 nm)
and shorter than the mid-infrared (MIDIR,
2500–25,000 nm) regions. Although the
MIDIR is again very familiar, being used to
identify unknown materials by comparing
their MIDIR spectra with published works
on group absorptions, i.e. ketone carbonyl at
5800–6100 nm, etc. (Colthup et al., 1990),
the NIR region has only gained prominence
in the last 20 years or so. There is also a
region (760–1100 nm) between the visible
and NIR which has sometimes been
included in the NIR and is used similarly.
This region will be considered as the short-
wave-NIR (SWNIR) for the discussion at
hand. It should be noted that this region is
often at least partially available on many
UV–VIS or VIS spectrometers designed to
use test tubes and cuvettes; however, these
are not NIR spectrometers as used to per-
form feed analysis.
Near infrared spectroscopy (NIRS) is
based on the absorptions of C-H, N-H and
O-H groups found in organic constituents.
These absorptions are the overtones and
combination bands of the much stronger
absorptions found in the MIDIR spectral© CAB International2000. Farm Animal Metabolism and Nutrition
(ed. J.P.F. D’Mello) 185