Fourier transform infrared spectroscopy

A topic from the subject of Spectroscopy in Chemistry.

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Fourier Transform Infrared Spectroscopy

Introduction

Fourier transform infrared (FTIR) spectroscopy is a powerful analytical technique that provides information about the molecular structure and composition of materials. It is based on the absorption of infrared radiation by molecules, which causes them to vibrate and rotate. The resulting changes in molecular energy are detected and used to generate a spectrum that can be analyzed to identify and quantify different functional groups and chemical bonds in the sample.

Basic Concepts

Infrared radiation: Electromagnetic radiation with wavelengths ranging from 0.78 to 3000 µm.

Vibrational and rotational motion: Molecules absorb infrared radiation and undergo vibrational and rotational movements.

Vibrational modes: Specific frequencies of infrared radiation that correspond to specific vibrational modes of the molecule.

Fourier transform: A mathematical technique used to convert the time-domain signal collected by the spectrometer into a frequency-domain spectrum.

Equipment and Techniques

FTIR spectrometer: Contains a source of infrared radiation, a sample holder, a detector, and an interferometer.

Interferometer: Generates a modulated beam of infrared radiation.

Sample preparation: Depends on the physical state of the sample (e.g., solid, liquid, gas).

Types of Experiments

Transmission: IR radiation passes through the sample.

Reflection: IR radiation reflects off the sample surface.

Attenuated total reflection (ATR): IR radiation penetrates a small distance into the sample through total internal reflection.

Diffuse reflectance: IR radiation is reflected and scattered by the sample.

Data Analysis

Peak identification: Identify peaks in the spectrum that correspond to different functional groups or chemical bonds.

Peak integration: Measure the area under the peaks to quantify the concentration of specific functional groups.

Spectral subtraction: Remove common interferences by subtracting one spectrum from another.

Applications

Functional group identification: Determine the presence of specific chemical groups, such as alcohols, ketones, or aldehydes.

Structural elucidation: Determine the molecular structure of organic and inorganic compounds.

Polymer characterization: Analyze the composition and structure of polymers.

Material identification: Identify unknown materials, such as plastics, fibers, or coatings.

Forensic analysis: Identify drugs, explosives, or other trace evidence.

Medical diagnostics: Detect diseases or other medical conditions by analyzing biological samples.

Conclusion

FTIR spectroscopy is a versatile and powerful analytical technique that provides valuable information about the molecular structure and composition of materials. It is widely used in various fields of science, including chemistry, biology, materials science, and medicine.

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR is a powerful analytical technique used in chemistry to identify and characterize organic and inorganic compounds. It involves the interaction of infrared radiation with a sample, resulting in the absorption or transmission of radiation at specific frequencies. This absorption or transmission is then measured and plotted to generate a spectrum, which provides valuable information about the sample's chemical composition and structure.

Key Points:

Principle: Measures the absorption or transmission of infrared radiation as a function of frequency (or wavenumber).
Sample Analysis: Can identify functional groups, determine molecular structure, and quantify sample composition. It can be used for both qualitative and quantitative analysis.
Data Representation: Spectra plot absorbance or transmittance versus wavenumber (cm^-1). Wavenumber is inversely proportional to wavelength.
Characteristic Bands: Specific functional groups exhibit characteristic absorption bands in the infrared region, providing information about their molecular structure. For example, the presence of a C=O stretch is indicative of a carbonyl group.
Interpretive Techniques: Absorption intensities, band positions, and peak shapes (e.g., sharp vs. broad) assist in compound identification. Spectral libraries and databases are commonly used for comparison.
Applications: Widely used in fields such as organic chemistry, polymer science, pharmaceutical analysis, environmental monitoring, materials science, and forensic science.

Main Concepts:

FTIR utilizes a Michelson interferometer to obtain a Fourier transform of the infrared spectrum. This technique, unlike older dispersive methods, offers advantages such as improved signal-to-noise ratio and faster scan times, leading to higher resolution and sensitivity. The interferometer splits a beam of infrared light, sends the two beams down different paths, recombines them, and measures the resulting interference pattern. A mathematical process called a Fourier transform then converts this interference pattern into the familiar infrared spectrum.

The infrared spectrum of a compound provides information about its molecular structure because different functional groups and bonds vibrate at characteristic frequencies. The absorption of infrared radiation occurs when the frequency of the radiation matches the vibrational frequency of a bond or group within the molecule. The position (wavenumber) and intensity of absorption bands correspond to the vibrational frequencies and bond strengths of the functional groups present.

FTIR is a versatile and non-destructive technique (in most cases) that finds applications in various scientific disciplines, allowing researchers to gain insights into the chemical composition and molecular structure of both organic and inorganic compounds. Sample preparation methods vary depending on the sample's state (solid, liquid, gas) and the desired information.

Experiment: Fourier Transform Infrared Spectroscopy (FTIR)

Materials:

FTIR spectrometer
IR-grade KBr powder
Sample (e.g., solid, liquid, gas)
Mortar and pestle (for solid samples)
IR-transparent windows (e.g., NaCl, KBr) and holder (for liquid samples)
Gas cell (for gas samples)

Procedure:

1. Sample Preparation:

Solid Sample: Grind a small amount of the sample with IR-grade KBr powder (typically a 1:100 sample-to-KBr ratio) using a mortar and pestle until a fine powder is obtained. Press the mixture into a thin pellet using a hydraulic press.
Liquid Sample: Place a small drop of the liquid sample between two IR-transparent windows. Ensure a thin, uniform film is formed to avoid excessive absorption. Gently clamp the windows together using a suitable holder.
Gas Sample: Fill a gas cell with the sample gas to the desired pressure. Ensure the cell windows are clean and free of contaminants.

2. Instrument Setup:

Turn on the FTIR spectrometer and allow it to stabilize.
Calibrate the instrument according to the manufacturer's instructions. This usually involves running a background scan with a clean reference (e.g., air or an empty sample holder).
Select the appropriate resolution and scanning range (e.g., 4 cm^-1 resolution, 4000-400 cm^-1 range). Higher resolution provides more detail but increases scan time.

3. Data Acquisition:

Carefully place the prepared sample in the spectrometer's sample compartment.
Start the scan. The instrument will collect the infrared spectrum, which plots absorbance or transmittance versus wavenumber.

4. Data Analysis:

The resulting spectrum will show characteristic absorption bands at specific wavenumbers. These bands correspond to the vibrations of functional groups within the molecule.
Identify the absorption bands using a suitable reference database (e.g., NIST Chemistry WebBook) or by comparing the spectrum to literature data. Assign these bands to specific functional groups (e.g., O-H, C=O, C-H).
Interpret the data to determine the molecular structure and composition of the sample.

Significance:

FTIR spectroscopy is a powerful technique for identifying and characterizing organic and inorganic compounds. It provides valuable information about the molecular structure, specifically the presence and types of functional groups. It is a non-destructive technique, requiring only a small amount of sample. FTIR finds widespread applications in various fields including chemistry, materials science, environmental science, forensics, and pharmaceutical analysis.

Key Procedures:

Careful sample preparation is critical for obtaining high-quality and reproducible results. Contamination or improper sample handling can lead to inaccurate or misleading spectra.
Proper calibration and background correction are essential for reliable measurements. A well-calibrated instrument ensures that the collected data is accurate and meaningful.
The accurate interpretation of FTIR spectra requires a thorough understanding of vibrational spectroscopy principles and the correlation between absorption bands and functional groups. Consultation with reference databases and/or experienced spectroscopists may be helpful.

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