2. NIR Spectrometry
The NIR method may be regarded as "applied spectroscopy",
which means that it falls under the physical analysis procedures (spectral
analysis). It is a quick method by which individual organic components of a
mixture of substances can be determined both qualitatively and quantitatively
on the basis of physical measurements. Identification, receiving and final
inspections and in-process analysis – carried out non-destructively in seconds,
without any great preparation of the samples – are making this method
increasingly important. NIR spectrometry is not a method for trace analysis.
The limits of detection and quantitation are in the percent range.
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| Fig. 79: Electromagnetic spectrum |
The near-infrared (NIR) region is the wavelength range between 800 nm
and 2,500 nm (12,500 cm-1 to 4,000 cm-1) of the electromagnetic spectrum (Fig.
79). This range is between the mid-infrared region at longer wavelengths and
the visible region at shorter wavelengths.
In the near-infrared region, molecular groups such as -OH, -NH, -CH
and -CO are caused to vibrate very strongly. The -OH vibrations are
characteristic of compounds such as alcohols, phenols, acids, keto alcohols and
hydroxy esters; the group of -NH vibrations represents both primary and
secondary amines and amides. Aromatic compounds, olefins and paraffins are
reflected by the -CH vibrations. The -CO bands occur with carboxylic acids,
carboxylic acid esters, ketones, alcohols and also aldehydes. Tab. 65 gives
examples of typical band positions for these molecular groups.
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| Tab. 65: Wavelengths of typical bands of molecular groups |
Since these groups are typical components of food ingredients such as
water, protein, fats and carbohydrates, the NIR method is particularly suitable
for characterizing foods. As we have already mentioned, quantitative
information from the NIR technique is based on the Lambert-Beer Law.
In principle there are two ways of determining the absorption
properties of a sample to be analyzed: by detecting the diffuse reflected light
(NIR instruments) or by detecting the transmitted light (NIT instruments).
The difference between reflection and transmission techniques lies in
the physical position of the detector. In the case of the reflection technique
the detector receives the light reflected by the sample and is therefore in front
of the sample chamber, whereas in the transmission technique the detector is
situated behind the sample compartment (Fig. 80).
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| Fig. 80: Splitting of light into a reflected part and a transmitted part by a solid, opaque sample |
In the case of solid, opaque samples it is usually the energy
diffusely reflected by the sample that is measured rather than the transmitted energy.
This is done by calculating the ratio of the energy reflected by the sample to the
total reflection of an internal standard.
The values for reflection are shown as a common logarithm of the
quotient 1/reflected energy. Further subdivisions can be made according to the
different optical systems. For example, a distinction should be made between
singlebeam and double-beam systems.
Besides solid-filter systems, which have long been used for standard
applications in the food industry, monochromator systems are gaining significance
since the greater volume of data generated by them provides more information
for characterizing ingredients. In the monochromator, light is split up into
its spectrum and a narrow wavelength range is then isolated by means of a
slit. In Fourier transform spectrometers a modulator is used instead of the
monochromator. This modulator uses one modulating frequency for each
wavelength, thus allowing a definite allocation of wavelengths to frequencies.
This dispenses with the need for spatial dispersion of the electromagnetic radiation
and allows simultaneous measurement of the information for all wavelengths.
A further advantage of NIR/NIT spectroscopy besides the small amount
of work required for sample preparation is doubtless that it permits quick, non-destructive
analysis of the materials to be tested. The possibility of online use in routine
procedures and the fact that chemicals are not usually required make NIR spectroscopy
a user-friendly method.
Various techniques of sample analysis such as whole grain measurement,
cell measurement and measurement by means of glass-fibre probes in containers
of various kinds have contributed to an increased use of this method. By using
light-conducting fibre-optic probes it is possible to examine the sample at a
considerable distance from the measuring instrument. For example, analyses of
transparent and opaque solids can be carried out with fibre lengths of 1 m to 50
m. This means that the sample no longer has to be conveyed to the site of the equipment
(e.g. the laboratory); the measuring cell comes to the sample – a great advantage
in many sectors of routine analysis. Moreover, the method permits
"in-line" measurement for continuous characterization of materials
and process control.
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| Tab. 66: Suitable measuring techniques for various applications and types of sample |
The choice of the instrument and the measuring technique to be used
depends on the frame of reference and the planned application (Tab. 66). Examples
of the uses of such techniques are to be found in the publications of
Zwingelberg, et al. (1987), and Bolling and Gerstenkorn (1987). These studies
show how the NIR method can be used to determine quality parameters of products
made from wheat and wheat flour.
The information derived from the NIR measurements depends to a very
large extent on the parameters chosen for sample preparation and on the measuring
parameters. To ensure reproducibility of the measurements and the results derived
from them it is essential to maintain corresponding conditions (similar conditions
for preparation and measurement).
The evaluation of scatter spectra makes it possible to assess the
hardness of grain and the lightness of its colour. However, when determining quality
parameters a distinction has to be made between values that can be determined directly
and those that must be determined indirectly. The protein, water and starch
content of a sample and also its fat content are variables that can be
ascertained directly from NIR data, since their spectroscopic signals are attributable
to defined molecular vibrations (vibrations of the amide and hydroxyl group).
Values determined indirectly include the sedimentation rate, damage to
the starch, water absorption and baked volume. These are derived from the IR
vibration spectra by calculation.
Specific uses of NIR transmission spectroscopy for ascertaining the
quality of individual grains in respect of moisture and protein content and grain
hardness are the subject of various publications.
As we have already said, one of the great advantages of the NIR
applications is their speed. Constituents that contribute to the value of a
product can be determined simultaneously within a few minutes and called up in
sequence if the spectrometer is suitably calibrated.
NIR Methods and Procedures
The choice of the sample preparation method (whole grain or powder
sample) is of central importance for the conduct of NIR analyses. For both
measuring procedures the homogeneous condition of the material is the most important
parameter. If the material is comminuted, the absorption behaviour of the
sample depends on the degree of grinding. The light from the source is altered
by the sample and then recorded by detectors; a computer then converts it to a
figure that shows the amount of the constituent present.
The routine use of this method does, however, require extensive
calibration for the specific constituents or parameters that are to be determined.
The ratio of reflection to transmission is determined by measuring samples with
known levels of the constituents concerned, and this ratio is stored as a
calibration constant. Comprehensive calibration requires a large number of
samples varying as widely as possible; the amount of the desired constituent must
be determined in each sample by reference analysis.
The influence of variability is taken into account by the fact that
spectra determined over a long period are stored in special reference libraries.
These are used to establish mean spectra that provide information on possible
deviations. For standard applications with ground products (water, protein, starch
and fat content), filter instruments have been used successfully for years.
Their construction makes them especially suitable for the production sector.
However, the filter instruments only cover those wavelength ranges
which have been set in order to determine the constituents to which they
relate. Besides the filter instruments there are also scanning instruments, which
are used chiefly to analyze complex substances. Since these scanning instruments
use the whole NIR wavelength range for calibration they generate a larger
volume of data and thus enable much more information on the samples to be
collected and evaluated. Many aspects of quality assurance can be covered
quickly and at low cost with the very accurate FT-NIR spectrometers. FT-NIR and
AOTS (acusto-optical-tunable scanning) systems are further developments that
make systematic use of the possibilities offered by modern microelectronics in
the fields of signal generation, control engineering and data processing.
In recent years IR spectroscopy has become an important tool, with
applications that range from routine identity and purity testing through qualitative
and quantitative analysis of reaction mixtures to the identification of unknown
compounds. One reason for this development is the enhanced use of IR technology
in conjunction with microelectronics by the manufacturers and the systematic
practical implementation of the theoretical benefits of these IR methods. A
further reason is the flood of new regulations and the demands they make on
quality control and quality assurance, which has speeded up the use of IR
spectroscopy. The introduction of the ISO 9000 series of standards has resulted
in a tremendous increase in the analytical work accompanying the production process.
This applies both to the speed of analysis and the number of analyses to be
performed. In particular near-infrared spectroscopy (NIR) has gained importance
in this sector.
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| Fig. 81: NIR spectrum (reflection) of a Type 550 flour |
The future will doubless see an increase in the importance of networks
and questions of the administration of near-infrared networks under practical
conditions in the field of spectral analysis. Fig. 81 shows an NIR spectrum of
a Type 550 flour measured by the reflection method. In this spectrum the
various group vibrations are allocated to the relevant wavelength range as an
example.
The NIR bands occurring are seen to correlate with the functional
molecule groups. The -CH, -CH2 and -CH3 vibrations occur in region 1; region 2
represents the vibrations of the R-CONH2, R-CONHR, R-CNH2 and R-CNHR groups;
region 3 shows the -OH vibrations. When evaluating the spectra it should be taken
into account that various spectral regions often overlap, making a definite allocation
of of wavelengths to molecular groups virtually impossible. In the above example
the regions 2 and 3 overlap in the wavelength range around 1,450 nm. Only when
further regions of the spectrum are considered is it possible to allocate them
with certainty.
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