Fourier Transform Infrared Spectroscopy
Recently, a report explains how experts use the Fourier Transform Infrared Spectroscopy (FTIR) to analyse post-blast forensic investigations.
About Fourier Transform Infrared Spectroscopy
Fourier Transform Infrared (FTIR) spectroscopy is an analytical technique used to obtain an infrared spectrum of the absorption or emission of a solid, liquid, or gas to identify its chemical composition. It works by measuring how molecules absorb infrared light at specific frequencies, which correspond to the vibrational modes of their chemical bonds, producing a unique "molecular fingerprint".
Principle of Operation
The core component of an FTIR spectrometer is a Michelson interferometer, which uses a beam splitter, a fixed mirror, and a moving mirror to modulate the infrared light beam. This process simultaneously collects information from all infrared frequencies.
- Source: A broadband infrared light source emits a beam of radiation.
- Interferometer: The beam enters the Michelson interferometer, where the beam splitter divides it into two paths. One beam goes to a stationary mirror, and the other to a mirror that moves back and forth. The beams reflect and recombine at the beam splitter, creating an interference pattern called an interferogram.
- Sample: The recombined beam passes through or reflects off the sample, which absorbs specific frequencies of light characteristic of its molecular structure.
- Detector: The detector measures the intensity of the light after interacting with the sample, producing a raw signal (interferogram) that is a function of the mirror's position (or time).
- Computer: A mathematical process called a Fourier transform converts the interferogram from the time/spatial domain into an interpretable spectrum (intensity versus frequency, typically in wavenumbers, cm⁻¹).
Advantages over Dispersive IR
FTIR has largely replaced older dispersive IR spectrometers due to several key advantages:
- Speed (Fellgett's Advantage): It measures all infrared frequencies simultaneously, providing significantly faster scan times.
- Sensitivity (Jacquinot's Advantage): It has higher optical throughput (allows more light to pass through) because it uses an aperture instead of narrow slits, resulting in a better signal-to-noise ratio.
- Accuracy (Connes's Advantage): A built-in laser (e.g., Helium-Neon) is used to calibrate the wavelength scale precisely with each scan, providing high wavenumber accuracy.
- Versatility: A wide range of sampling techniques (transmission, attenuated total reflectance (ATR), reflectance) allow analysis of solids, liquids, gases, powders, and pastes with minimal or no sample preparation.
Common Applications
FTIR is a versatile tool used across many industries and research fields:
- Chemical Analysis: Identifying and characterizing organic and some inorganic compounds based on their functional groups and molecular structure.
- Quality Control: Verifying raw materials, ensuring batch consistency in manufacturing, and detecting impurities or contaminants.
- Materials Science: Analyzing polymers, coatings, and composites to understand their composition and properties.
- Forensics: Identifying trace evidence, unknown substances, and illicit narcotics.
- Biomedical & Pharmaceutical: Studying protein structures, analyzing tissues and cells for disease detection (e.g., cancer diagnostics), and ensuring drug quality.
- Environmental Monitoring: Detecting pollutants and analyzing microplastics in environmental samples.
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