FTIR- Fourier Transform Infrared Spectroscopy

Principle, Methods, Applications, Tools, and Chemical Identification

1. Principle of FTIR

Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic, polymeric, and, in some cases, inorganic materials. The principle is based on the absorption of infrared radiation by a sample, resulting in vibrational transitions in molecular bonds.

• IR Radiation is passed through a sample.
• Molecules absorb specific frequencies of IR light that match their natural vibrational frequencies.
• The absorption pattern (spectra) is like a molecular fingerprint.
• The interferogram is converted into a spectrum using a Fourier Transform algorithm.

2. Methods of FTIR

There are two main modes of FTIR analysis:

a. Transmission Mode

• Sample is placed directly in the path of the IR beam.
• Common for liquids and transparent solids.

b. Attenuated Total Reflectance (ATR)

• IR light reflects internally through a crystal in contact with the sample.
• Ideal for solids, powders, and films without preparation.

c. Diffuse Reflectance (DRIFTS)

• Suitable for powdered or rough-surfaced samples.
• IR beam is reflected and scattered by the sample.

d. Reflectance/Specular Reflectance

• Used for thin films or surface coatings.

3. Applications of FTIR

FTIR is widely applied in several fields:

• Pharmaceuticals: Identifying functional groups in drug compounds.
• Polymers: Analyzing polymer structures and degradation.
• Environmental Science: Detecting pollutants or contaminants.
• Food Industry: Identifying food adulterants or quality testing.
• Nanotechnology: Monitoring surface modifications and nanoparticle functionalization.
• Botany and Biotech: Identifying phytochemicals and metabolites in plant extracts or callus culture.

4. Tools and Components Used in FTIR

• IR Source: Typically, a Globar (silicon carbide) or Nernst glower.
• Interferometer: Michelson interferometer to modulate the IR signal.
• Beam Splitter: Divides the IR beam (commonly made of KBr or CaF₂).
• Sample Holder/Stage: Holds solid, liquid, or powdered samples.
• Detector: Converts IR signal to an electrical signal (DTGS or MCT detectors).
• Computer + Software: For Fourier transform and spectrum analysis.

5. Common Chemical Names Detected in FTIR

FTIR identifies functional groups by detecting bond vibrations. Examples:

Functional Group	Wavenumber (cm⁻¹)	Typical Compounds Detected
–OH (hydroxyl)	3200–3600	Alcohols, Phenols
–NH (amine)	3300–3500	Amines, Proteins
C=O (carbonyl)	1650–1750	Aldehydes, Ketones, Esters
–CH (alkane/alkene)	2800–3100	Hydrocarbons
–COOH (carboxylic)	1700–1725 & 2500–3300	Carboxylic acids
–C≡C, –C≡N	2100–2260	Alkynes, Nitriles
–C–O–C– (ether)	1000–1300	Ethers, Glycosides

These are often used to identify:

• Flavonoids
• Phenolics
• Alkaloids
• Terpenoids
• Carbohydrates
• Proteins/Peptides

FTIR Procedure

To launch the FTIR program, double-click the OMNIC icon.

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From the Collect pull-down menu, select Setup, and select the following:

• No. of scans = 8
• Resolution = 2 cm⁻¹
• Zero filling = None
• Final format = Absorbance

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From the Edit pull-down menu, select Options. Click on the Collect tab at the top. Click the “Collect to New Window” box to deselect it. Click OK to close the window.

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Select Background from the Collect pull-down menu (Repeat background collection every 30 minutes to 1 hr. if the humidity is high).

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From the Collect pull-down menu, select Collect. Sample: Place the sample in the holder in the IR, then select Yes. Add to window = Yes

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From the Process pull-down menu, select: Automatic Baseline. Correct old spectrum shown in red and the new spectrum shown in blue. Click on the red one. From the Edit pull-down select Clear from the Process pull-down menu, select % Transmittance.

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From the View pull-down menu, select Display Limits (to change the axes of the graph).

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During spectrum display, click on T (for text) at the bottom of the page. Click on peaks of interest to label.

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From the File pull-down menu, select Print.

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To Run the Next Sample: from the Window pull-down menu, select New Window (more than 7 open windows will crash OMNIC).

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Repeat steps 4–8 above.

KBr Pellet Procedure for Solid Samples

• Take about 1/8th of the solid sample on a micro spatula and about 0.25–0.50 teaspoons of KBr.
• Mix thoroughly in a mortar while grinding with the pestle. If the sample is in large crystals, grind the sample separately before adding KBr.
• Place just enough spl. to cover the bottom in the pellet die.
• Place in press and press at 5000–10000 psi.
• Carefully remove the pressed sample from the die and place it in the FTIR sample holder.
• The pressed disc should be nearly clear if properly made. If it is translucent, regrind and repress.