Instrumentation of IR and Raman Spectrophotometers

1. INSTRUMENTATION OF IR
AND RAMAN SPECTROSCOPY
by
Sangeeth P S
MSc. Chemistry (Specialization in Energy Science)
School of Energy Materials

2. • PART-1: INSTRUMENTATION OF IR SPECTROPHOTOMETER
• PART-2: INSTRUMENTATION OF RAMAN SPECTROPHOTOMETER
PARTS

3. • Different types of sample preparation in IR spectroscopy
• Types of IR spectrophotometers
• Instrumentation
PART-1: CONTENTS

4.  For solids, liquids, gases and in solution phase
require different types of preparation
Obeyed materials are dissolved in a transparent
matrix
 Water is dried because, water absorbs near
3710 cm-1
and 1630 cm-1
TYPES OF SAMPLE PREPARATION IN IR
SPECTROSCOPY

5.  Finely ground solid sample in mixed with
powdered KBr and made into a transparent
pellet.
KBr doesn't interfere in IR region
Main disadvantage is KBr may absorb water
and interfere in the spectra.
As Solid Samples – KBr Pellet Method

7.  Finely ground liquid sample with an oily
mulling agent typically ‘Nujol’ using mortar and
pestle.
Thin film of mull is placed between NaCl flat
plates and spectrum is measured.
Main disadvantage is Nujol has absorption
bands in 2924-2860, 1462, 1360 cm-1
As Liquid Samples – as a mull or paste.

9. A series of plates indicating various forms of physical damage with a comparison to a good plate
(Copyright: Colorado University-Boulder).

10.  Non-hygroscopic solvents like DCM or Carbon
tetrachloride is used as a matrix
 A drop of sample is deposited on KBr or NaCl
plate, solution is evapourated to dryness and
film formed on KBr disc is directly analyzed to
obtain spectra
Film made should not be opaque
As a film

11.  Neat liquid sample in an appropriate solvent is
placed between two plates of NaCl or KBr
 Neat spectrum can be obtained by this
method
As solution phase samples

12. Introduce gas into a special long-path-length cell (usually ≥10 cm).
Cells have NaCl windows.
Vapor phase technique is limited by low vapor pressure of most organic compounds.
As gaseous sample

13. • Dispersive infrared spectrophotometers were
developed in the 1940s and were traditionally used
to obtain infrared spectra.
• In the 1960s, a new technique called Fourier-
transform infrared (FTIR) spectroscopy was
introduced.
• Initially, FTIR instruments were expensive and
primarily used in advanced research settings.
• Today, FTIR spectrometers are widely used due to
their superior speed, sensitivity, and efficiency
compared to dispersive instruments.
INSTRUMENTATION OF IR SPECTROSCOPY

14. INSTRUMENTATION OF DISPERSIVE IR
SPECTROSCOPY

15. INSTRUMENTATION OF DISPERSIVE IR
SPECTROSCOPY - PARTS
1. Radiation Source
2. Sample and reference cells
3. Monochromator
4. Detectors and Amplifier
5. Recorder

16. 1. Radiation Source
Inert solids heated electrically to a range of 1000 to 1800 °C to promote thermal
emission of radiation. The most common sources are:
• Nernst filament (composed of rare-earth oxides such as zirconium, cerium and
thorium)
• Globar (composed of silicon carbide),
• Nichrome coil

17. 2. Sample and reference cells in IR spectroscopy
• Glass or quartz cuvettes cannot be used in IR spectroscopy because they
absorb strongly in most of the IR region, interfering with the spectrum.
• Alkali metal halides such as:
• Potassium chloride (KCl) [High Purity]
• Sodium chloride (NaCl) [High Purity]
are commonly used to make sample and reference cells.
• These materials are transparent to IR radiation, allowing accurate measurement
of the sample’s spectrum.

18. 3. Monochromator
• It consists of two main components:
o Rapidly rotating chopper:
 Alternates the IR beam between the sample and reference paths.
o Slowly rotating diffraction grating:
 Selects and varies the frequency or wavelength of IR radiation.
 Sends individual frequencies to the detector.

19. 4. Detectors and amplifiers.
The detector records the ratio of the intensity of transmitted light (I) to the incident light
(I₀).
1. Thermal Detectors
• Use multiple thermocouples connected together for higher sensitivity.
• Detect infrared radiation by measuring the heat it produces.
• This heat causes a flow of current, which is directly proportional to the intensity of the
incoming IR radiation

20. 2. Photon Detectors
• Use semiconductor materials.
• Detect IR radiation through interaction with photons.
• IR photons excite non-conducting electrons to a conducting state.
• This process generates a small current or voltage.

21. 4. Recorder
• The IR spectrum is recorded as a plot of frequency (in wavenumber, cm⁻¹) vs.
intensity of absorption (in transmittance)
• Unlike UV-visible spectroscopy (which uses wavelength), IR spectroscopy uses
wavenumber (cm⁻¹) as the unit of frequency.
• The spectrometer measures how much infrared radiation passes through the
sample compared to a reference (blank).

22. Transmittance Formula:
• T = I / I₀
Percentage Transmittance:
• %T = (I / I₀) × 100
• A lower % transmittance means more absorption at that frequency (peak in the
IR spectrum).

23. • Instead of viewing each component frequency
sequentially, as in a dispersive IR spectrometer, all
frequencies are examined simultaneously in Fourier
transform infrared (FTIR) spectroscopy.
• Infrared radiation from the source first passes through
an interferometer.
• Then it is passed through detector, amplified signal is
converted from analog to digital using an analog-to-
digital converter (ADC).
• Fourier Transform is carried out by softwares to
convert the raw data into an interpretable IR
spectrum.
FTIR SPECTROPHOTOMETER

24. INSTRUMENTAION OF FT-IR SPECTROPHOTOMETER
Michaelson
Interferometer

25. MICHAELSON-INTERFEROMETER (COMPONENTS)
1. Moving mirror
2. Fixed mirror
3. Beam splitter
• The two mirrors are positioned perpendicular to each other, and the semi-reflective beam
splitter bisects the plane between them, splitting the incoming radiation into two separate paths
• It is made by coating a thin film of germanium or iron oxide onto an IR-transparent substrate,
typically potassium bromide (KBr) or cesium iodide (CsI).
• Infrared energy from the source strikes the beam splitter, which splits the beam into two parts—
one directed toward the moving mirror and the other toward the fixed mirror; after reflecting
off their respective mirrors, with the moving mirror traveling back and forth at a constant
velocity, the beams recombine at the beam splitter.

26. • The beam from the moving mirror travels a different optical path than the one from the
fixed mirror, and when the two are recombined, some wavelengths interfere
constructively while others interfere destructively, creating an interference pattern
known as an interferogram.
• Interferogram is passed through the sample, then spectrum is analyzed and all IR
information is read simultaneously unlike dispersive instruments.

28. EXAMPLE OF A SAMPLE IR SPECTRA
Background Spectra;
• Carbon dioxide (CO₂): Doublet at ~2360 cm⁻¹ and a sharp spike at 667 cm⁻¹.
• Water vapor (H₂O): Broad irregular bands near 3600 cm⁻¹ and 1600 cm⁻¹.

29. • Sample preparation in Raman
spectroscopy
• Types of Raman spectrophotometers
• Instrumentation
PART-2: CONTENTS

30. Sample Preparation (Traditional Aspect)
• In a specimen chamber, the sample is fixed with the help of lens laser light is
incident.
• Normally, liquids as well as solids samples are tested in a capillary tube made up of
Pyrex.
• Powder and pellet are examined directly, no sample preparation is required in this
case.

31. Important points to note while sample preparation
• There should be no loss of analyte during sample preparation.
• In the sample preparation process, best chemical form of the analyte is used.
• During sample preparation, there should be removal of interferents in the matrix.
• Other important factor is that there should be no addition of any new interferent
while preparation.
• Dilution and concentrating the analyte should be done carefully.

32. Basic Instrumentation of a conventional
Raman Spectrophotometer

33. Parts of conventional Raman Spectrophotometer
1. Excitation Source (Laser)
• Provides a monochromatic light (usually in the visible region).
• Common lasers: Argon-ion (488 & 514.5 nm), Krypton-ion, Helium–Neon (632.8
nm).
• Function: Excites the sample and induces Raman scattering.
2.Optical System (Lenses and Mirrors)
• Lenses are used to:
• Focus the laser light onto the sample.
• Collect the scattered Raman light.
• Mirrors or beam splitters may also be used for directing light efficiently.

34. 3.Sample Holder / Specimen Chamber
• Solids: Can be examined directly (powder/pellet form).
• Liquids: Usually placed in Pyrex capillary tubes.
• No sample preparation is required for many solid samples.
4. Monochromator
• Function:
• Disperses the scattered light based on wavelength.
• Removes Rayleigh scattering (elastic scattering).
• Acts as the dispersive element to isolate the Raman signal.
• Adjustable slit widths allow control over spectral resolution.

35. 5. Detector
• Converts the optical signal into an electrical one.
• In conventional systems, the common detectors are:
• Photomultiplier Tube (PMT) – used in scanning instruments.
• Charge-Coupled Device (CCD) – used in multi-channel systems.
• Silicon photodiode arrays – used for fast spectral acquisition.
6. Data Processing Unit
• Receives the electrical signal from the detector.
• Processes and displays the Raman spectrum.
• Allows storage, analysis, and interpretation of the data.

36. 8.Filters (Laser and Edge Filters)
• Placed in the optical path:
• Band-pass filter: Purifies laser beam before it reaches the sample.
• Long-pass filter: Blocks Rayleigh scattering before light enters the detector.

37. Basic Instrumentation of a FT Raman
Spectrophotometer

38. Parts of FT Raman Spectrophotometer
1. Laser Source
• Nd:YAG laser (1064 nm) – emits in the near-infrared (NIR) region.
• Purpose: Reduces fluorescence in organic samples and provides stable, high-intensity
radiation.
• Continuous wave (CW) type is typically used.
2. Interferometer
• Usually a Michelson interferometer.
• Modulates the light to generate an interferogram.
• Enables all wavelengths to be measured simultaneously

39. 3. Sample Holder/Stage
• Solid, liquid, or slurry samples can be analyzed.
• Often used with fiber-optic probes for remote or in-situ measurements.
4. Optical Filters
• Rayleigh scattering filters (e.g., notch or edge filters) block intense elastic scattering.
• Band-pass filters ensure only the desired Raman scattering reaches the detector.

40. 5. Detector
• Detects the interferogram and converts it into an electrical signal.
• Typical detectors used:
• Germanium (Ge) detectors
• Indium Gallium Arsenide (InGaAs) arrays
• Operated at cryogenic temperatures to reduce thermal noise and enhance signal-to-noise
ratio.
6. Data Processing Unit
• Applies Fourier Transform (FT) to convert the interferogram into a Raman spectrum.
• Provides high-resolution spectral output and allows detailed analysis.

41. Types of Raman Spectrophotometers
• Dispersive Raman Spectrophotometers
• Equipped with gratings or prisms.
• More common in conventional setups.
• Non-dispersive Raman Spectrophotometers
• Use an interferometer (e.g., in FT-Raman).
• Suitable for near-IR applications and fluorescence-prone samples.

43. References
• Larkin, P. (2011). Infrared and Raman spectroscopy: Principles and spectral interpretation. Elsevier.
• Colthup, N. B., Daly, L. H., & Wiberley, S. E. (1990). Introduction to infrared and Raman spectroscopy
(3rd ed.). Academic Press.
• Le Ru, E. C., & Etchegoin, P. G. (2009). Principles of surface-enhanced Raman spectroscopy and related
plasmonic effects. Elsevier.
• McCreery, R. L. (2000). Raman spectroscopy for chemical analysis. Wiley-Interscience.

44. THANK YOU