Fourier Transform Infra-red spectrophotometers
Objectives
After this session
students will be able to
• Explain
the construction and working of Fourier Transform Infra-red spectrophotometers
• Outline the advantages of FTIR over dispersive
type of instruments
Fourier Transform IR
v Fourier
Transform Infrared (FT-IR) spectrometry was developed in
order to overcome the limitations encountered with dispersive instruments
mainly the slow scanning process.
v A
solution was developed which employed a very simple optical device called an interferometer.
The interferometer produces a unique type of signal which has all of the
infrared frequencies “encoded” into it. The signal can be measured very
quickly, usually on the order of one second or so.
FTIR systems
- Mechanical
operation
•
Encode
(modulate) the spectral information using a Michelson Interferometer.
- Mathematical
operation
•
Computer
processing of encoded information to produces the spectrum (Decoding).
Optical Diagram Michelson Interferometer
Interference is superimposing of waves
Relationship between light source spectrum and the
interferogram (signal output from interferometer)
Note that the time domain signal, even after
modulation, contains the same information as in the frequency
domain.
Michelson Interferometer (Mechanical operation)
v Most
interferometers employ a beamsplitter which takes the incoming infrared
beam and divides it into two optical beams. One beam reflects on a flat
mirror which is fixed in place. The other beam reflects on a flat
mirror which is on a mechanism which allows this mirror to move a
very short distance (typically a few millimeters) away from the beamsplitter.
v Because
one beam travels is a fixed length and the other is constantly changing as its
mirror moves, the signal which exits the interferometer is the result of these
two beams “interfering” with each other. The resulting signal is
called an interferogram which has the unique property that every
data point which makes up the signal has information about every
infrared frequency which comes from the source.
Fourier transform (Mathematical Operation)
Because the analyst requires a frequency spectrum (a
plot of the intensity at each individual frequency) in order to make an
identification, the measured Interferogram signal can not be interpreted
directly. A means of “decoding” the individual frequencies is
required. This can be accomplished via a well-known mathematical technique
called the Fourier transformation. This transformation is
performed by the computer which then presents the user with the desired
spectral information for analysis.
FT-IR summary
Background Spectrum
Ø A
background spectrum (with no sample in the beam) must be
collected for all IR measurements. This can be compared to the measurement with
the sample in the beam to determine the “percent transmittance.”
A single background measurement can be used for many sample measurements
because this spectrum is characteristic of the instrument itself and its environment.
Ø The
strong background absorption from water and carbon dioxide in the
atmosphere can be reduced by purging the optical bench with an inert gas
or with dry carbon dioxide – scrubbed air .
Schematic illustration of FTIR system
FT-IR Advantages
1- Fellgett’s (multiplex) Advantage
- Fast:
All frequencies of the source
reach the detector simultaneously (all of the energy is on the detector
all of the time), instead of analyzing a sequence of small wavebands
available from the monochromator in dispersing IR instruments.
– Get data for the entire spectrum in one
second or less.
• Improve
signal to noise ratio (S/N ratio):
Fast scans enable recording and averaging
many scans.
2- Connes Advantage (Frequency accuracy advantage)
• Why
is there a laser in FT-instruments?
Interferogram is not
recoded continuously, but sampled at discrete intervals to give different data
points. The closer the spacing between data points, the greater the wavenumber
range of the spectrum.
Monochromatic visible
He-Ne laser beam is passed along with the polychromatic IR light to provide a single
wavelength interferogram that oscillates much more quickly than
anything in the IR (shorter wavelength).
The laser is used as
an internal clock to trigger data point’s acquisition events.
IR data points might
be taken at every zero point of the laser interferogram.
FTIR instruments employ a He-Ne laser as an internal
wavelength calibration standard. These instruments are self-calibrating
and never need to be calibrated by the user.
The precise
reproduction of wavenumber positions from one spectrum to the next will increase
the resolution of the spectrum, and make it easy to differentiate
between adjacent peaks too close to each other (high resolving power).
Frequency accuracy
makes signal averaging highly precise and thus adds further
improvement in S/N ratio.
3- Jacquinot (throughput) Advantage
•
Few optical elements and no slits (greater
throughput of radiation)
–
The detector receives up to 50% of the energy of
original light source (much larger than the dispersion spectrometer)
–
This will enhance the sensitivity of
measurement and causes further improvement in the S/N ratio.
4- No stray light
•
Because
the FT experiment modulates the source radiation and then detects only
modulated radiation, there is essentially no stray light problems as
there are with scanning instruments.
•
Any stray light that reaches the detector is not
incorporated into the spectrum since it is unmodulated. Thus
there is no possibility of errors occurring during measurement (accurate
quantitative analysis).
Summary of FT-IR Advantages
• Speed Because all of the frequencies are
measured simultaneously.
• Sensitivity is dramatically
improved with FT-IR ; detectors are much more sensitive, the optical throughput
is much higher, higher signal to noise ratio.
• Mechanical Simplicity The moving
mirror in the interferometer is the only continuously moving part in
the instrument. Thus, there is very little possibility of mechanical breakdown.
• Internally Calibrated These
instruments employ a He-Ne laser as an internal wavelength calibration
standard .These instruments are self-calibrating and never need
to be calibrated by the user.
Analytical information obtained using IR techniques
I)
Qualitative
a)
Structural Elucidation through
interpretation of functional group region (4000- 1300 cm-1),
fingerprint region (1300- 910 cm-1), aromatic region (910- 650 cm-1).
b) Compound
Identification to find a reference IR spectrum that
matches that of the unknown compound.
c) IR mostly
used for rapid qualitative but not quantitative analysis.
II ) Quantitative
A = a b c
- The
intensity of an absorption band is linearly proportional to the concentration
of analyte of interest at a certain frequency. - Quantification parameters include peak
height, peak area; integration of band area should be done carefully to
ensure maximum accuracy, near IR region is better suited for
quantitation.
Applications of Infrared Analysis
Ø Analysis
of petroleum hydrocarbons, oil and grease content (detection of Freons).
Ø Determination
of air contaminants.
Ø Determination
of protein, starch, oil, lipids and cellulose in agricultural products.
Ø Far-
Infrared region is particularly useful for inorganic studies (crystals and
semiconducting materials).
General Applications of Infrared Analysis
- Pharmaceutical
research. - Forensic
investigations. - Polymer
analysis. - Lubricant
formulation and fuel additives. - Foods
research. - Quality
assurance and control. - Environmental
and water quality analysis methods. - Biochemical
and biomedical research. - Coatings
and surfactants.
Summary
• Michelson
interferometer is an important component in FTIR instrument
• The
spectrum obtained is an interferogram
• By
using a mathematical algorithm, Fourier transform, interferogram is converted
into dispersive IR spectrum
