Quantitative Analysis From FTIR

Quantitative Analysis from FTIR spectrometer

M. Zafar Iqbal

Physical Properties of Polymers

Scheme• Introduction to quantitative analysis and sources• Sampling techniques for FTIR spectrometer• Base Line correction: Problems and their solutions• Curve fitting and its practical applications• Absorpitivity Ratios and Quantitation• References

Introduction to Quantitative Analysis• FTIR is commonly used for qualitative analysis• You must be aware of this type of analysis before going

towards quantitative analysis• The probe size of FTIR is ideal for the study of phase behavior

of polymers (i.e, Moleculer vibrations and size scale is in angstrom)

• It can characterize the nature and strength of strong interactions.

• It can determine the fraction of interacting groups in a certainly H-bonded system.

Cont’d• Consider the following blend system:

• s

Two copolymers are mixed on molecular level to form a blend.The fraction of carbonyl group H-bonded with phenolic group will tell the blending extent. For a single phase blend, there will be someEquilibrium conc. of H-bonded Carbonyl group present at a certainTemperature. The distribution of free and H-bonded C-group can be described by equilibrium constants from where free energy of Formation can be calculated.

C-stretching= 1650-1800 cm-1Careful while selecting yourrequired peaks. The breadth Strongly dictates the conc.of thatgroup.

Some important points• For quantitative measurements, Bee-Lambert law is strongly

recommended which isA = log10 (I0/I) = abc

Where A= Absorbance (dimensionless)I0 = Intensity of source radiation

I = Intensity of transmitted radiationa = absorpitivity co-efficientb = Thickness or path lengthc = Conc. Of the absorber

The spectrum should be in Beer-Lambert law absorbance units for quantitative measurements.

Cont’d

• Select the peaks (bands) well defined• Apply base line correction as recommended• Apply least square curve fitting• Determine the absorpitivities of free and H-bonded Carbonyl

groups in terms of ratio aR

• Determine the inter association fraction through certain formulae

The most important point is the judgment where your required groups lie and how much clear they are.

Experimental Necessities

Sample Preparation:The samples for IR analysis can be in different form:1- Solid 2- Liquid3- Gas

Neat sample: If sample is a thin film then mount it as it is.Solid: Mix with IR grade KBr in 1:100 ratioMull and salt window: fine powder of sample is mixed with a

hydrocarbon oil and it is placed between two salt plates.Typical salt windows: 1 inch diameter, 3-5 mm thickness

Cont’d

Dissolution and liquid cell: sample is dissolved in some solvent and place in the liquid cell. You must consider the absorption ranges of solvents to avoid from serious errors.

Conventional Liquid Cell

Gaskets help in making Non-leak seals and spacer.The sample is injected with The help of a syringe. AfterUsage, open the cell and rinse it.

Cont’d

Gas samples: A gas sample cell is required which is made of some standard salt like KBr or NaCl. The gas in injected with the help of a small piston system and kept at very low pressures. The main precaution in this system is the partial pressure of gas to be analyzed.

Films: less than 10 micron thickness, The absorbance should not be greater than 0.6.

Baseline and playing with spectra• Consider the following spectrum with sloping baseline.• 3 basic routes have been drawn here.

Why sloping baseline?• The most important factor for this sloping baseline is the un-

even film formation due to which IR light is scattered and 2nd is the coloured sample.

• Film cast method and KBr pellet method normally strike with this type of problem.

• In Film cast method, certain factors must be considered:1- Evaporation rate of the solvent2- Chemistry of the polymeric components3- Surface tension of the solution

Cont’d• If the sample is non-uniform, then IR radiation will be dispersed due to

difference in refractive indices at the curved surfaces and fraction of radiation will not reach the detector.

• Scattering is inversely proportional to the fourth power of wavelength. Therefore this effect is greatest in IR at higher frequencies (high wave numbers).

• A good judgment of the sample is that the absorbance should be with in 0.3-0.7 . Below 0.3, poor signals are observed which if interpreted would lead to serious errors. Modern FIR can go up to 0.05 absorbance units. But if more than 0.7, then bands are called over-absorbed and it goes out of Beer law range. This can never give good quantitative results.

• But if the following situation occurs:

The art of Curve Fitting

• Sample baseline correction: First Step

Understanding the Curve fitting mechanism• First of all you have to assume a specific shape of the band. This may be a

Gaussian type or Lorenzian type or mixed. Almost all the polymer sample bands resemble the Gaussian Shape but not necessarily.

• Let us do some sample exercises to understand what curve fitting is.

Consider five Gaussian curves named I-V, arbitrarily synthesized as:

Curve IIV=1700FWHH= 30A = 0.4

Curve IIIV= 1660FWHH = 15A = 0.3

Curve IVV= 1640FWHH= 15A 0.2

Curve VV=1620FWHH=15A=0.5

Curve IV= 1730 cm-1

FWHH = w1/2= 15 cm-1

A=0.15

Note: These all curves have Gaussian shape, Baseline at A=0 and are separated By frequency at distances greater than w1/2 of the individual bands.

Curve fitting: cont’d

Again consider V1=1730: V2=1700: V3= 1670: V4= 1655: V4= 1640

Curve fitting is the knowledge of the total no. of bandsThat comprise anEnvelop.

Second Derivative concept and its application• Sometimes, 2nd derivative of an IR-spectrum is helpful to

reveal unresolved bands like you have seen in last picture.

If bands are spaced too close, then..

Least square curve fitting methodology