"Radiation interacting with a sample may be absorbed,
transmitted or reflected. In the classical spectroscopy
experiment, reflection is eliminated so that the proportion
of radiation attenuated by the sample may be measured as
transmittance. Beer’s law then defines a proportionality
between transmittance and the product of concentration
of the absorbing species and path length. For a clear
transparent liquid sample such as beer, hot starch melts,
wine or vegetable oil, the path length may be fixed by
means of a static or flow-through sample cuvette or a pair
of fiber-optic probes and a calibration developed using
samples of known concentrations. For example, Halsey.2/
used standard solutions of ethanol in water to develop
a calibration for alcohol content of beer. It should be
noted that, owing to the relatively weak intensities of
NIR absorption bands, samples such as vegetable oils
may be analyzed without dilution in a solvent.
Beer’s law is only valid in the absence of light scatter
in the sample. Scattering changes the path length through
which the radiation passes and, because the amount
of scattering varies from sample to sample, the path
length cannot be defined. This type of experiment is
known as diffuse transmittance (Figure 3), the most well known
example of which is liquid whole milk. The
fat globules in the milk scatter light in the manner
shown and invalidate Beer’s law. Diffuse transmittance
measurements are usually carried out in the 800–1100nm
region of the spectrum where the weak absorptions enable
useful data to be obtained using thicknesses of 1–2 cm
of samples such as meat, cheese or whole grain. Near infrared
transmittance (NIT) instruments are particularly
applicable to the analysis of whole grains and a typical
apparatus is shown schematically in Figure 4. A sample
of grain is placed in a hopper from where aliquots are
dispensed into the measurement chamber. When analysis
is complete, the sample is discharged into a collection
tray."
(NEAR-INFRARED SPECTROSCOPY IN FOOD ANALYSIS 1
Near-infrared Spectroscopy
in Food Analysis
Brian G. Osborne
BRI Australia Ltd, North Ryde, Australia)
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