Here’s a detailed overview of Fourier Transform Infrared (FTIR) Spectroscopy for structural fingerprinting:
1. Introduction
FTIR spectroscopy is an analytical technique used to identify functional groups and characterize molecular structures based on the absorption of infrared light.
- It is widely used for structural fingerprinting of natural products, polymers, resins, and essential oils.
- Each molecule has a unique IR absorption pattern, often called its “fingerprint region” (usually 400–1500 cm⁻¹).
- In agarwood research, FTIR can detect sesquiterpenes, chromones, and other bioactive compounds in resin or extracts.
2. Principle
- Molecules vibrate in specific modes (stretching, bending, twisting) when exposed to infrared radiation.
- IR light is absorbed at frequencies corresponding to these vibrational modes.
- The instrument records an absorbance vs. wavenumber (cm⁻¹) spectrum.
Key Regions:
| Region (cm⁻¹) | Vibrational Type | Example Functional Groups |
|---|---|---|
| 4000–2500 | X–H stretching | O–H, N–H, C–H |
| 2500–2000 | Triple bonds | C≡C, C≡N |
| 2000–1500 | Double bonds | C=O, C=C |
| 1500–400 | Fingerprint region | Complex skeletal vibrations unique to molecules |
3. Sample Preparation
FTIR is versatile and can analyze solids, liquids, and gases.
- Solid samples
- KBr pellet method: Mix sample with KBr powder and press into a transparent disc.
- ATR (Attenuated Total Reflectance): Direct contact of solid with ATR crystal.
- Liquid samples
- Drop placed on ATR crystal or sandwiched between IR-transparent windows (NaCl, KBr).
- Gaseous samples
- Contained in gas cells with IR-transparent windows.
Advantages of ATR-FTIR: Minimal preparation, rapid, non-destructive.
4. Data Acquisition and Interpretation
- Obtain FTIR spectrum: Absorbance (or transmittance) vs wavenumber (cm⁻¹).
- Identify functional groups:
- Compare peaks with known standards or reference tables.
- Example: O–H stretch around 3400 cm⁻¹, C=O stretch around 1700 cm⁻¹.
- Fingerprint analysis:
- The region 1500–400 cm⁻¹ provides a unique pattern for a compound.
- Used for sample authentication, quality control, or comparison of extracts/resins.
5. Applications
- Natural products & phytochemistry: Identifying sesquiterpenes, chromones, flavonoids.
- Essential oils & resins: Structural fingerprinting to detect adulteration or quality.
- Polymers & biomaterials: Confirming chemical modifications.
- Pharmaceuticals: Functional group verification of active ingredients.
- Food & flavor analysis: Detecting key aromatic compounds.
6. Advantages
- Rapid and non-destructive analysis.
- Minimal sample preparation (especially with ATR).
- Provides functional group information and unique fingerprint region.
- Can be combined with chemometric analysis for sample classification.
7. Limitations
- Limited sensitivity compared to GC-MS or HPLC.
- Cannot directly quantify complex mixtures without calibration or chemometrics.
- Overlapping peaks may complicate interpretation.
- Requires some expertise to assign peaks accurately.
Workflow Summary for FTIR Structural Fingerprinting
Sample collection → Sample preparation (solid/liquid/gas) → FTIR measurement → Spectrum acquisition → Functional group analysis → Fingerprint region comparison → Structural characterization