3
Infrared Spectroscopy
Revision
The following questions cover the important concepts that you should have understood in the first
year instrumentation subject.
1.
What molecular or structural features give rise to absorption of infrared (IR) radiation?
2.
Give an example of an inorganic species that would not absorb IR radiation.
3.
Give an example of an organic species that would not absorb IR radiation.
4.
What method of sample preparation would be appropriate for the following?
(a) a pure organic liquid
(b) a pure organic solid, soluble in most solvents
(c) a pure insoluble organic solid
3.2 Regions of the infrared
The infrared region stretches from the red end of the visible spectrum (12500 cm-1, 800 nm) to the
microwave region (20 cm-1, 500 µm). The range with which you are most familiar is 4000-600 cm-1.
This is known as the mid infrared, and is most useful for structural determination of organic species.
The high wavenumber sub-region, closest to the visible, is the near infrared (used for moisture
determination and polymer analysis, outlined in more detail in this chapter), and the low end, the far
infrared (limited use, but does shows characteristic absorptions due to different isomer forms).
3.3 Radiation sources
The most common infrared source is an inert solid, electrically heated to temperatures approaching
2000K. Materials used include silicon carbide (Globar), mixed rare earth oxides (Nernst glower) and
nichrome wire.
3. Infrared spectroscopy
3.4 Monochromators
Reflection gratings were the basis for all commercially infrared spectrophotometers, but since the
advent of Fourier transform instruments (see section 3.6), monochromator-based IR instruments have
gone into decline, and are probably not even able to bought anymore.
3.5 Detectors
Infrared radiation is heat, and requires a detector to sense the increase in temperature by an absorbing
material with a very small heat capacity, i.e. one that gives a significant temperature rise for small
amounts of heat energy absorbed. The mass of the absorbing material is kept as small as possible to
further increase the temperature increase. Even so, the temperature rises that occur are in the range
of a few thousandths of a degree.
Other than the difficulty in measuring such small temperature changes, the other problem is
heat from the surroundings, i.e. stray radiation. The detector must be well insulated (in a vacuum in
fact), because not only will the laboratory but also the other components of the instrument potentially
cause problems. The beam from the source is always chopped so that surrounding levels of heat can
be compensated for.
The most widely used detector is a thermocouple, which is composed of a piece of blackened
gold foil, which absorbs the beam. The foil is welded to two thin wires, of different metals. An electric
potential develops between the two junction points, the magnitude of which is due to the difference
in temperature. One junction is exposed to the beam, the other bonded to a heat sink, and thus
remains at the temperature of the surroundings. A common pair of metals is platinum and silver.
Thermocouples are most useful across the mid-IR range,.
The Golay detector works by expansion of gas on absorption of heat. The gas is sealed within a
cylindrical chamber, which at one end is exposed to the beam. The heat absorbed by the conducting
end of the cylinder is transferred to the gas, which on expansion, causes a thin, flexible metal
diaphragm at the other end to bend. The distortion of the plate is measured optically or by including
as part of a capacitor circuit. The Golay detector is most useful for the far infrared region and in nondispersive instruments (see below).
A bolometer or thermistor relies on electrical resistance changes due to temperature variations,
and employs a Wheatstone bridge. Two matched sensing elements are included in the bridge, one
exposed to the beam, the other is insulated.
In the near infrared region, photon detectors, using semiconductor devices are common. They
have a faster response time and more sensitive, but are restricted to this range.
3.6 Instrument configurations
Until the last decade, conventional dispersive mid-IR spectrophotometers existed, based around a
scanning monochromator. Now they are no longer, replaced by a very differently designed instrument
known as a Fourier transform (FT) spectrometer. These are superior in every single way to dispersive
scanning instruments, and with much cheaper computing power (an essential for FT instruments), the
cost differential that did exist when FT instruments first became available in the 1980s has gone.
Fourier transform instruments
The nature of common IR detectors doesn’t lend them to multi-array, such as in the UV/VIS region with
photodiodes. The advantages of multi-channel instruments are obtained in another way in the IR.
A mathematical technique known as the Fourier transform (FT) has been used since the advent
of cheap, powerful computers to separate the complex waveform that is the continuous spectrum of
radiation into its components individual wavelengths and intensities. Thus, no dispersing system (i.e.
monochromator) is needed - the Fourier transform takes the instantaneous detector response and
“disperses” the wavelengths itself.
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While the instrument has no monochromator, it has an additional component, called an
interferometer, whose exact workings we will examine in Advanced Instrumentation Techniques. The
throughput of an FTIR spectrophotometer is increased by the absence of a monochromator, meaning
greater sensitivity, added to the other advantages of multi-channel instruments.
Non-dispersive Instruments
Many important commercial and polluting gases are strong IR absorbers, e.g. carbon dioxide, water,
carbon monoxide, methane. However, a conventional IR instrument could not cope with the
environmental demands placed by an industrial workplace. The delicate optics in the monochromator
would be affected by the harsh conditions.
A non-dispersive instrument is one that does not use a monochromator. This may be a filterbased instrument, or one without any wavelength selector at all. While filters do exist for IR
spectroscopy, the main non-dispersive instruments are of the latter type. A number of different types
exist, but we will look at the so-called “positive-filter” instrument. Figure 3.1 shows a typical NDIR
instrument.
flow-through
sample cell
flexible metal
source
gas-filled boxes
reference cell
FIGURE 3.1 Schematic diagram of a non-dispersive IR instrument
The gas inside the Golay-type detector boxes expands due to heating from the IR beam. Differences
in the amount of heat, as a result of different amounts of IR absorption between sample and reference
cells causes a difference in the pressure inside the two detectors. The causes the metal foil to flex
towards the cooler box, and the degree of flexing is determined by the metal being part of an electrical
circuit (acting as a condenser).
The reference cell can be filled with a non-absorbing gas (e.g. nitrogen), or with a mixture
containing possible interferences (i.e. absorbing gases that may be in the sample other than the
analyte). For example, a CO-measuring instrument would contain the typical atmospheric
concentration of CO2 in the reference cell to compensate for its absorption in the sample. The sample
cell would typically be a flow-through type, allowing continuous measurements.
3.7 Mid IR Cells
You should be familiar with the use of crystalline NaCl plates as the main material used in the
construction of cells for the mid IR region. NaCl and similar salts, such as KBr and CsI, are used because
of their complete transparency to IR radiation, because of the absence of covalent bonds. They are
used as loose plates for liquid film spectra, where the surface tension of the liquid holds the pair
together, or in fixed-pathlength cells, where a plastic spacer separates the plates by typically 0.1 mm,
and they held by metal plates on wither side which are screwed together. These are often called
sandwich cells. Figure 3.2 shows the construction of a typical sandwich cell.
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FIGURE 3.2 Normal and expanded views of a sandwich cell
(from www.brukeroptics.com & www.mccarthyscientific.com)
The pathlength of such a cell is not strictly defined by the thickness of the spacer, because it can be
squashed by tightening of the screws. The pathlength of the cell can be determined by measuring the
spectrum of the empty cell. The spectrum will not simply be a straight line, but will exhibit a sine wave
curve, due to interference effects between reflected parts of the beam off each wall of the cell and the
transmitted beam. Equation 3.1 is used to measure the pathlength.
d=
5n
f1 − f 2
Eqn 3.1
where d in pathlength of the cell in mm, n is the number of fringes (complete cycles) in the
wavenumber region f1 to f2. This can also be applied
to thin films of materials with refractive indices
significantly differ to that of air, e.g. plastic, with a
modification to the equation to account for the RI
value of the sample.
The IR spectra of gases can be measured in the
normal way, but because of the low number of gas
molecules, a longer pathlength is required (see Figure
3.3).
Where the sample contains moisture or is an
aqueous solution, soluble salt cell materials are
obviously not suitable. A range of non-soluble
materials are used including AgCl (all regions) and fused silica (near IR).
FIGURE 3.3 IR gas cell (www.brukeroptics.com)
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EXERCISE 3.1
Calculate the pathlength of an IR cell with the following spectrum.
2752 cm-1
2286 cm-1
3.8 Mid IR solvents
One of the revision questions established that there is no such thing as an organic compound, and
therefore, solvent, that doesn’t absorb in the mid IR, because all liquids (at room temperature) are
covalently bonded compounds. Therefore, there is no perfect IR solvent – they all will have some
absorption peaks. The choice of solvent then depends on what you are wanting to do:
•
qualitative – one that has few peaks which can be subtracted out using matched sandwich cells
•
quantitative – one that doesn’t absorb strongly where the analyte does
In general, it is the simple and small compound that has the simplest, and therefore, least detailed IR
spectrum.
For qualitative work where as much as possible of the analyte spectrum should be unaffected
by the subtraction of the solvent peaks (never perfect), trichloromethane (chloroform) has been
commonly used (see spectrum in Figure 3.4). Tetrachloromethane has an even simpler spectrum, but
is highly carcinogenic, and is no longer used. However, it is probably better to persevere with a KBr
disc or use an ATR unit (see below).
For quantitative work, it is matter of looking for something that will dissolve the sample but
not have any peaks near the analyte peak. For polar compounds, it isn’t easy as water and ethanol
(see Figure 3.5), whilst simple, have substantial absorption across the range, and though propanone
(Figure 3.6) isn’t much better, it does at least have some gaps. If its peaks coincide with the analyte
peak, then further searching is required.
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FIGURE 3.4 Spectrum of trichloromethane
FIGURE 3.5 Spectrum of ethanol
FIGURE 3.6 Spectrum of propanone
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For non-polar analytes, things are a little easier. Trichloromethane is often suitable, as are hexane
(Figure 3.7) and cyclohexane (Figure 3.8). Pentane has too low a boiling point to be useful. In principle,
benzene’s simple spectrum makes it useful, but its toxicity eliminates it from general use. Methyl
benzene (toluene) is significantly more complex.
FIGURE 3.7 Spectrum of hexane
FIGURE 3.8 Spectrum of cyclohexane
3.9 Attenuated total reflectance spectroscopy
Measuring the spectrum of a solid in IR by KBr disc is simply a pain in the you know where. Also, some
materials simply can’t be made into KBr discs, because they can’t be ground into a powder, eg fabric,
plastic. Wouldn’t it be good if someone thought of a way that allowed you to run any sort of solid or
liquid without need to make KBr discs or buy AgCl plates?
Someone has! It is called attenuated total reflectance (ATR) and not surprisingly, uses
reflectance spectroscopy principles to record the spectrum. ATR uses the property of total internal
reflection where a beam of infrared light is passed through a crystal in such a way that it reflects at
least once off the internal surface in contact with the sample. This reflection extends into the sample,
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typically by a few microns, and the characteristic absorption frequencies are absorbed. The beam is
then collected by a detector as it exits the crystal. The effect works best if the crystal is made of an
optical material with a higher refractive index than the sample being studied (see Figure 3.9).
sample
crystal
IR beam
FIGURE 3.9 ATR schematic diagram
In the case of a liquid sample, pouring a shallow amount over the surface of the crystal is sufficient. In
the case of a solid sample, it is pressed into direct contact with the crystal. Because the transmission
of the beam into the solid sample is improved with as complete contact as possible, solid samples are
usually firmly clamped against the ATR crystal, so that trapped air is not the medium through which
the beam travels, as that would distort the results.
Crystals commonly used in ATR units include diamond (the best but most expensive) and ZnSe
(limited pH range and relative soft).
The IR spectrum produced is basically identical to that which would be generated by in a
transmission cell (or KBr disc), but far more conveniently. However, the intensity of the higher
frequency (>1000 cm-1) peaks are distorted, making them less intense, and so the spectrum does not
look like a “normal” transmission one. This makes it difficult to correlate with library spectra recorded
in transmission mode, unless a correction (available in the control software) is made.
3.10 Near IR spectroscopy
Absorption of near IR radiation is due to overtone and combination frequencies from the strong peaks
in the mid IR. This means that the intensities are quite low, and the absorptions tend not be able to
be assigned to one particular bond. These seeming disadvantages meant that that the area was not
widely studied for many years. However, in the last twenty years, near IR spectroscopy, particularly in
reflectance mode, has become widely applied to the analysis of a range of commercially important
materials, such as cereal grains, oils and plastics for analytes such as protein, water, sugar.
The most significant absorptions are due to O-H, N-H and C-H bonds, but at distinctive
frequencies from different groups of molecules. Thus, water can be distinguished from ethanol,
methanol and sucrose by the absorption peak due mainly to O-H at around 1400-1900 nm. However,
the presence of different levels of salt has been shown to affect the absorption by water. The protein
content of many foods, especially wheat and related grains, has been determined, due to the large
number of similar N-H bonds in proteins.
One of the main advantages of NIR spectroscopy is the minimal sample preparation required.
In reflectance mode, grains of wheat, for example, can be analysed directly. Furthermore, the optics
required are not salt-based, but can be silica, which is far more resistant to environmental condition.
The semi-conductor detectors available for NIR are also far more sensitive than those described above
for mid IR instruments.
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The main disadvantage of near IR relates to its quantitative use. There is a substantial amount of peak
overlap, meaning that simple absorbance measurements for calibration graphs are rarely possible.
Complex mathematical procedures on reference standards (of that type of sample) are required to
create a calibration.
What You Need To Be Able To Do
•
•
•
•
•
•
•
•
•
•
define important terminology
list and describe common radiation sources used in IR spectrophotometers
list and describe common detectors used in IR spectrophotometers
explain why monochromator-based IR instruments no longer exist
describe common IR cells
determine the pathlength of a IR cell
discuss the choice of solvent
outline how ATR works
list applications of ATR
list applications of near IR
Revision Questions
1.
Why is it not strictly correct to call an FTIR spectrophotometer a multi-channel instrument, in
the same way that a diode array UV/VIS is?
2.
How could atmospheric moisture be compensated for in NDIR analysis?
3.
The spectrum of an empty sandwich cell exhibits 27 cycles between 2100-2750 cm-1. Calculate
the pathlength.
4.
For qualitative purposes, why is it better to record the spectrum of a solid as a KBr disc or using
the ATR unit, rather than as a solution?
5.
For quantitative purposes, why is it only acceptable to record the absorbance of a solid or liquid
sample in a sandwich cell?
Answers to these questions on following page.
Answers to class exercises can be found in the Powerpoint file provided on the website.
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Answers to Revision Questions
1.
The FTIR has only one detector, so while it has the speed capability of a multi-channel
instrument, this is achieved in a different way.
2.
Add water vapour at a similar concentration to the reference cell.
3.
Pathlength = (5 x 27) ÷ (2750 - 2100) = 0.21 mm.
4.
No extra peaks.
5.
The sandwich cell has a fixed pathlength, necessary to maintain Beers Law.
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