2. Attenuated total reflectance (ATR)
– ATR accessories are especially useful for obtaining IR
spectra of difficult samples that cannot be readily
examined by the normal transmission method.
– They are suitable for studying thick or highly
absorbing solid and liquid materials, including films,
coatings, powders, threads, adhesives, polymers, and
aqueous samples.
– ATR requires little or no sample preparation for most
samples and is one of the most versatile sampling
techniques.
3. Theory of ATR
– ATR occurs when a beam of radiation enters from a more-
dense (with a higher refractive index) into a less-dense
medium (with a lower refractive index).
– The fraction of the incident beam reflected increases
when the angle of incidence increases. All incident
radiation is completely reflected at the interface when the
angle of incidence is greater than the critical angle (a
function of refractive index).
– The beam penetrates a very short distance beyond the
interface and into the less-dense medium before the
complete reflection occurs.
– This penetration is called the evanescent wave and
typically is at a depth of a few micrometers (μm).
– Its intensity is reduced (attenuated) by the sample in
regions of the IR spectrum where the sample absorbs.
4.
5. Method
– The sample is normally placed in close contact with a
more-dense, high-refractive-index crystal such as zinc
selenide, thalliumbromide thalliumiodide, or
germanium.
– The IR beam is directed onto the edge of the ATR
crystal and internally reflected through the crystal
with a single or multiple reflections.
– Both the number of reflections and the penetration
depth decrease with increasing angle of incidence.
– For a given angle, the higher length-to-thickness ratio
of the ATR crystal gives higher numbers of
reflections.
6. A variety of types of ATR accessories are available:
such as 25 to 75° , vertical variable-angle ATR, horizontal
ATR, and Spectra-Tech Cylindrical Internal Reflectance
Cell for Liquid Evaluation (CIRCLE®) cell.
The resulting ATR-IR spectrum resembles the
conventional IR spectrum, but with some differences:
- Identical absorption band positions but with different
relative intensities
7.
8.
9.
10.
11.
12. NEAR-INFRARED SPECTROSCOPY
The near-infrared (NIR) region of the spectrum extends
from the upper wavelength end of the visible region at
about 770 nm to 2500 nm (13,000 to 4000 cm-1).
– Absorption bands in this region are overtones or
combinations of fundamental stretching vibrational
bands that occur in the region of 3000 to 1700 cm-1 .
– The bonds involved are usually C—H, N—H, and O—
H.
– Because the bands are overtones or combination, their
molar absorptivities are low.
16. Spectra of water
MIR / NIR
0
1
2
3
4
5
Extinction
9000 8000 7000 6000 5000 4000 3000 2000
Wavenumber (cm-1)
1000
1st overtone
Combination
Fundamental
stretching
Fundamental
bending
17. NIR Evaluation of spectra
•Requires comparison against known
data
*Qualification:
comparison against a set of spectra
of known good quality
*Quantification:
using spectral information of
samples of known concentrations
to predict concentrations of new
unknown samples. PLS(Partial
least squares) is used for this type
of evaluation.
Analysis values
Reference values
18. Instrumentation
– Four different types of instruments are available for the NIR
region.
1- Sophisticated but useful for fixed, well-characterized samples.
– Grating instruments are similar to those used for UV-visible
absorption spectroscopy.
– Discrete filter instruments usually containing filter wheels for
selecting different wavelengths.
– Fourier transform spectrometers: are also available
commercially for NIR spectrometry.
2- Simple Speed and ruggedness
– Acoustooptic tunable filter (AOTF) instruments are available.
– The acoustooptic filter is a solid state device that diffracts
radiation at wavelengths determined by a radio frequency
signal applied to the crystal.
20. Some useful solvents for NIR spectroscopy.
Solid lines indicate satisfactory transparency for use with 1-cm cells.
21. Interpretation of NIR Spectra
• In Raman and mid-IR spectroscopy, the
interpretation of spectra is possible and valuable
information is gained in regards to the presence
of functional groups.
• This type of interpretation is very difficult in NIR
spectroscopy, due to significant overlapping of
bands.
• In NIR spectroscopy we are usually interested in
observing differences between spectra, and not in the
interpretation of NIR spectra.
22. – NIR shows some similarities to UV-visible spectrophotometry
and some to mid-IR spectrometry.
– Indeed the spectrophotometers used in this region are often
combined UV-visible-NIR ones.
– The majority of the absorption bands observed are due to
overtones (or combination) of fundamental bands that occur
in the region 3 to 6 m, usually hydrogen-stretching
vibrations.
– NIR is most widely used for quantitative organic functional-
group analysis.
– The NIR region has also been used for qualitative analyses and
studies of hydrogen bonding, solute-solvent interactions,
organometallic compounds, and inorganic compounds.
Similarities with Uv-Vis and Mid-IR
23.
24. Applications of NIR Absorption Spectrometry
– In contrast to mid-IR spectroscopy:
NIR absorption spectra are less useful for identification and more
useful for quantitative analysis of compounds containing functional
groups made up of hydrogen bonded to carbon, nitrogen, and
oxygen.
– Such compounds can often be determined with accuracies and
precisions equivalent to ultraviolet-visible spectroscopy, rather than
mid-IR spectroscopy.
Some applications include:
– The determination of water in a variety of samples, including:
glycerol, hydrazine, organic films, and fuming nitric acid.
– The quantitative determination of phenols, alcohols, organic acids,
and hydroperoxides based on the first overtone of the O-H
stretching vibration that absorbs radiation at about 7100 cm-1 .
– Near-infrared spectroscopy is widely applied in agriculture for
determining the quality of forages, grains, and grain products,
oilseeds, coffee, tea, spices, fruits, vegetables, sugarcane,
beverages, fats, and oils, dairy products, eggs, meat, and other
agricultural products. It is widely used to quantify the composition
of agricultural products because it meets the criteria of being
accurate, reliable, rapid, non-destructive, and inexpensive
25. – NIR spectrophotometry is also a valuable tool for
identification and determination of primary and secondary
amines in the presence of tertiary amines in mixtures.
– The analyses are generally carried out in carbon
tetrachloride solutions in 10-cm cells.
– Primary amines are determined directly by measurement
of the absorbance of a combination N-H stretching band
at about 5000 cm-1.
– Neither secondary nor tertiary amines absorb radiation in
this region.
– Primary and secondary amines have several overlapping
absorption bands in the 3300 to 10,000 cm-1 region due to
various N-H stretching vibrations and their overtones,
whereas tertiary amines can have no such bands.
– Thus one of these bands gives the secondary amine
concentration after correction for the absorption by the
primary amine.
