Absorption

MOLECULAR ABSORPTION SPECTROSCOPY; THEORY,

INSTRUMENTATION AND APPLICATION

2.1 Terms employed in Absorption Spectroscopy

Term & Symbol Definition Alternative Name & Symbol

Incident radiant power, Po

Radiant power in watts in incident on sample

Incident intensity, Io

Transmitted radiant Power, P

Radiant power transmitted by sample

Transmitted intensity, I

Absorbance, A Log (Po/P) Optical density, D; extinction, E

Transmittance, T P/Po Transmission, T

Path length of sample, b

Length over which attenuation occurs

l,d

Absorptivity, a A/(bc) Extinction coefficient, k

Molar absorptivity, ε A/bc Molar extinction coefficient

2.1.1 Transmittance T

2.1.1 Transmittance T

2.1.2 Absorbance A

• The logarithm of the ratio between the initial power of a beam of radiation Po and its power after it has traversed an absorbing medium:

• When Absorbance of a solution increases, transmittance decreases

2.1.2 Absorbance A

2.1.3 Measuring Transmittance & Absorbance

• Losses in measuring: Reflection losses and scattering losses in solution

• To compensate these effects, the power of the beam transmitted through a cell containing the analyte solution is compared with one that traverses either in identical cell containing only the solvent/reagent blank

2.1.4 Sample Containers

2.2 Beer’s Law

2.2 Beer’s Law

Exercise:

A 7.25 x 10-5 M solution of potassium permanganate has a transmittance of 44.1% when measured in a 2.10 cm cell at a wavelength of 525 nm. Calculate (a) the absorbance of this solution; (b) the molar absorptivity of KMnO4

2.2.1 Application of Beer’s Law to Mixtures

• Beer’s Law also applies to solutions containing more than one kind of absorbance substance

• Provided that there is no interaction among the various species, the total absorbance of multicomponent system at a single wavelength is the sum of the individual absorbances:

2.2.2 Limitation to the applicability of Beer’s Law

• There are few exceptions to the linear relationship between absorbance and path length at a fixed concentration due to deviation:– Real deviation (fundamental)– Method:

• Instrumental• Chemical






2.2.3 Absorption Spectra

• Line spectra – Occur when the radiating species are individual atomic particles

that are well separated in gas– The individual particles in a gases medium behave

independently of one another, and spectrum consists of a series of sharp lines

• Band spectra– Are often produced in spectral source because of the presence

of gases radicals or small molecules– This spectra is not fully resolved by the instruments


• Continuum Spectra


2.3 Instruments for Optical Absorption Instruments

• Ultraviolet/Visible Spectrophotometers– Single beam instruments– Double beam instruments– Multichannel instruments

• Infrared Spectrophotometer– Dispersive Instruments– Fourier Transform Instruments

2.3.1.1 Single Beam Instruments

2.3.1.2 Double Beam Instruments

2.3.1.2 Double Beam Instruments

2.3.1.3 Multichannel Instruments

2.3.2.IR Spectrophotometers – Dispersive Instruments

2.3.2.IR Spectrophotometers – Fourier Transform

• Characteristics:– Great speed– High resolution– High sensitivity– Excellent wavelength precision and accuracy

• FTIR have been reduced to benchtop size which is reliable and easy to maintain, less price

• Contain no dispersive elements, all wavelengths are detected and measured simultaneously

• Interferometer is used to produce interference patterns that contain the IR spectral information

• Types of sources are the same as dispersive instruments

• Transducers: pyro-electric transducer, photoconductive transducer

• Most FTIR are of the single beam type

2.3.2.IR Spectrophotometers – Fourier Transform

• Advantages:– Better speed and sensivity– Simpler mechanical design– Better light-gathering power– More accurate wavelength calibration– Elimination of stray light and IR emission

Education

Absorption