CHEM425, Instrumental Methods of Analysis

Infrared spectroscopy (FTIR)

Determination of mixtures of organic compounds

OBJECTIVE

To illustrate procedures and techniques of quantitative infrared spectroscopy, determination of film thickness of polymer films, measurement of path length of sealed cells and determination of molar absorptivity of organic compounds. Also, to observe the multiplex / Fellgett advantage of an FTIR spectrophotometer over a scanning infrared spectrophotometer.

MATERIALS AND INSTRUMENTS

Sealed cells (0.10 and 0.025 mm), Hypodermic syringe, isopropyl alcohol, toluene and methyl ethyl ketone . A Fourier transform infrared spectrophotometer and a scanning infrared spectrophotometer.

INTRODUCTION

Infrared spectroscopic techniques are most commonly used for qualitative identification of organic compounds. However, they can also be used for quantitative determinations. This is because the amount of sample present in the sample beam of an infrared spectrophotometer is directly related to the strength of an absorption band (or the absorbance at any frequency) in the infrared spectrum of the ample. This relationship is expressed mathematically by the Lambert-Beer’s law. This law may be represented in a number of ways depending on the units of concentration used:

(1)     The equations got lost in the HTML translation.

Where:

A = absorbance ; = molar absorptivity; c = concentration in grams per liter ; l = b = sample path length in cm (inside cell thickness) ; a = absorptivity, characteristic of the compound for that particular absorption (a = /mol. wt.) ; I_o = intensity of incident radiation ; I = intensity of transmitted radiation ; T = transmittance = (I/Io).

The basis for the measurement of film thickness of a polymer film or the path length of a sealed cell is the interference fringe pattern produced when the transmission of the film or empty cell is recorded over a range of frequencies. This pattern results from interaction between radiation which is reflected by the inner surfaces and then transmitted. The reflected radiation, as finally transmitted, may be exactly in phase with the radiation not reflected, it may be exactly out of phase or it may be somewhere in between. If in phase, reinforcement occurs and the cell or film transmission is maximum; if out of phase, destructive interference occurs and the transmission is minimum. Between the maxima and minima, the transmission changes gradually with frequency as radiation which is neither entirely in phase nor entirely out of phase interacts. As the spectrophotometer scans the cell or film transmission at one wavelength after another, a wavy interference fringe pattern emerges.

The thickness, d, of a cell or film can be calculated from data obtained from the fringe pattern by using Equations 2 and 3 below .

For cells:

(2)

For films:

(3)

where:

d = thickness, in cm ; ₁ = frequency at which first maximum (or minimum) occurs, in wavenumber (cm^-1) units ; ₂ = frequency at which the last maximum (or minimum) occurs, in wavenumber units. ; m = number of complete fringe maxima (or minima) in the interval from ₁ to _2, One complete fringe minimum is from point X to point Y; n = index of refraction for the film.

Note that the equations hold for situations where the incident radiation impinges on the sample at an angle of 90^o.

PROCEDURE

(A) Determination of film thickness of polystyrene film and path length of a sealed cell

Place an empty sealed cell containing a spacer (0.1 mm or 0.025 mm) in the sample beam and obtain the spectrum from 4000 to 400 cm^-1. Using as many as possible fringe maxima or minima calculate the thickness of the cell. Replace the cell with a polystyrene film and repeat the experiment but select fringes from either 4000 to 3200 cm^-1 region or 2800 to 1900 cm^-1 region. Calculate the thickness of the film.

(B) Determination of the molar absorptivity of toluene (at 695 cm^-1) and isopropyl alcohol (at 817 cm^-1).

Fill the sealed cell with isopropyl alcohol. Obtain the spectra. Using the absorbance of the peak at 815 cm^-1 , calculate the molar absorptivity. Repeat this experiment using toluene, and the toluene peak at 695 cm^-1 .

(C) Identification of constituent and determination of percent composition of an Unknown

Prepare accurately the following solutions (% by volume):

Obtain at least two spectra of each solution, determine A (from %T) for toluene (at 695 cm^-1 ) and isopropyl alcohol (at 817 cm^-1 ) in each solution and prepare plots of absorbance vs. Concentration for each solvent. Now obtain at least two spectra of the unknown solution. Determine the absorbance (from %T) of the toluene and isopropyl alcohol from the spectra of the unknown and using the calibration curve plotted from the standard solutions, determine the composition of the unknown solution.

(D) Observing the multiplex (Fellgett) advantage of FTIR

Fill the sealed cell with isopropyl alcohol. Obtain its spectra using the scanning Infrared spectrophotometer. Report the differences noted in the data acquisition method between the FTIR and the scanning IR. Give one more advantage of FTIR over the scanning IR.

This lab was adapted from "Basic Techniques and Experiments in Infrared and FTIR Spectroscopy" By L. Chia and S Ricketts. Perkin Elmer 1988