(LEC) MODULE 3.2: INFRARED SPECTROMETRY (IR)

IR Spectroscopy

A technique based on the vibrations of the atoms of a molecule.

IR Spectrum

is basically a plot of transmitted (or absorbed) frequencies vs. intensity of the transmission (or absorption).

IR Spectroscopy

Frequencies appear in the x-axis in units of inverse centimeters (wavenumbers), and intensities are plotted on the y-axis in percentage units.

NIR (NEAR INFRARED)

Interest in this has grown in recent years particularly in the food and feedstuff industry where it is routinely used for quantitative analysis

1. NIR

2. MIR

3. FIR

What are the IR Spectrum Regions? (3)

MIR (MID INFRARED)

Most widely used region with molecular vibration typically involved in organic molecules, it provides a wealth of structural information as well as quantitative data.

FIR (FAR INFRARED)

Principally concerned with rotational spectral and crystal lattice vibrations.

1. Strong

2. Medium

3. Weak

Classifications of IR Bands (3)

1. Strong

2. Medium

3. Weak

1.  covers most of the y-axis.

2. falls to about half of the y-axis.

3. falls to about one third or less of the y-axis.

Infrared (IR)

most useful in providing information about the presence or absence of specific functional groups.

Infrared (IR)

can provide a molecular fingerprint that can be used when comparing samples. If two pure samples display the same IR spectrum it can be argued that they are the same compound.

Infrared (IR)

It does not provide detailed information or proof of molecular formula or structure. It provides information on molecular fragments, specifically functional groups.

Infrared (IR)

Very limited in scope, and must be used in conjunction with other techniques to provide a more complete picture of the molecular structure.

Fourier Transform Infrared Spectroscopy (FTIR)

A measurement technique of obtaining the IR spectra, by measuring the Interferogram of the sample with the use of interferometer.

1. Source

2. Interferometer

3. Sample

4. Detector

5. Computer

Fourier Transform Infrared Spectroscopy (FTIR) parts (5)

Source

Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).

Interferometer

The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer.

Sample

The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed.

Detector

The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.

Computer

The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.

1. Chemical Analysis

2. Pharmaceutical Industry

3. Materials Science

4. Environmental Monitoring

5. Food and Beverage Analysis

6. Forensics

7. Art Conservation

Applications of FTIR (7)

ATR

The most common IR Sampling Technique

1. Dispersing the finely ground solid in liquid petrolatum

2. Incorporating the solid in KBr pellet

3. Dissolving the solid in a suitable solvent

What are the sample preparation (IR)? (3)

Mull technique

Dispersing the finely ground solid in liquid petrolatum is also known as?

1. A general sample handling technique useful with samples that undergo ion exchange.

2. Relatively inexpensive compared to KBr pellets because there is no need for accessories like dies and presses.

Advantages of Mull technique (2)

1. Nujol (refined mineral oil)

2. Fluorolube (perfluorinated hydrocarbon oil/ perfluorohydrocarbon

Mulling agents (2)

Incorporating the solid in KBr pellet

A technique that is very useful for solids and powders and for analysis of limited amount of samples (1-2mg).

1. KBr (most commonly used)

2. KCl

3. NaCl

4. CsI

Matrix Materials for Pellet-Making (4)

Christiansen effect

Occurs when a suspension of particles in a transparent medium is observed at a wavelength where the refractive indices of the particles and of the medium are equal, thus producing an optically homogeneous medium with optical bandpass or filter characteristics.

1. Good solubilizing property

2. Chemically inert

3. Solvent transparency

   • Chloroform, Carbon disulfide, Carbon tetrachloride, Benzene, Dimethylformamide, Tetrachloroethylene, Dioxane, Cyclohexane

Criteria for a Good Solvent (3)

1. Sealed Cells

2. Demountable Cells

Kinds of Cells in Sampling Handling (2)

Sealed Cells

Provides consistent pathlength, crucial for accurate quantitative measurements.

Sealed Cells

Suitable for analyzing volatile liquids as the sealed design prevents evaporation.

Sealed Cells

Use when you need precise quantitative measurements, are analyzing volatile liquids, and require consistent sample thickness.

Demountable Cells

Flexible for different samples and can be easily adapted to different sample types by changing spacers to adjust the pathlength.

Demountable Cells

Can be disassembled for thorough cleaning (Cleaning convenience).

Demountable Cells

Use when you need to analyze different sample types with varying thicknesses, require easy cleaning, or are performing qualitative analysis.