Introduction to Spectroscopic Methods of Analysis (part 2)

1. Lecture 2 Introduction to Spectroscopic Methods of Analysis(part 2)

2. This lecture will cover: Molecular absorption spectroscopy Terms employed in absorption spectroscopy: Absorbance & Transmittance Beer’s Law

3. - is based on measurement of the transmittance, T or the absorbance, A of solutions contained in transparent cells Molecular Absorption Spectroscopy

4. Molecular absorption spectroscopy Absorption is a measure of the decrease in radiant power P0 P Absorbing solution

5. TERMS EMPLOYED IN ABSORPTION SPECTROSCOPY

6. T = P P0 A = -log T Transmittance- The fraction of incident radiation transmitted through the sample medium. b P0 P Power of incident radiation Power of transmitted radiation Absorbing solution of concentration, c Commonly expressed as a percentage: %T = P x 100 P0

7. Absorbance - A measurement of the amount of radiant power absorbed by the sample defined as the negative log of transmittance. b A = -log T A = log P0 P P0 P Power of incident radiation Power of transmitted radiation Absorbing solution of concentration, c •Absorbance has a linear relationship with sample concentration defined by Beer’s Law.

8. Questions: Convert the following percent transmittance data into absorbance: i) 33.6 ii) 92.1 iii) 1.75 Answers: i) 0.474 ii) 0.0357 iii) 1.76

9. Answer: i) %T = 33.6 T = 33.6 / 100 = 0.336 A = - log T = log 1/T = log 1/0.336 = 0.474

10. Questions: 2. Convert the following absorbance data into percent transmittance: i) 0.375 ii) 1.325 iii) 0.012 Answers: i) 42.2% ii) 4.73% iii) 97.3%

11. Beer’s Law A = bc - shows linear relationship between absorbance, concentration of the species measured, sample path length and the absorptivity of the species. ε = molar absorptivity, liter mol-1 cm-1 b = sample path length, cm c = concentration, mol per liter

12. A = abc Beer’s Law Concentration (g/liter) absorbance Absorptivity (Lg-1cm-1) Pathlength (cm) The term “a” is a proportionality constant called absorptivity. Absorptivity is a constant for a given chemical species at a specific wavelength.

13. Examples: E.g. 1 What is the concentration of an absorbing species if its molar absorptivity is 1500 L/mol cm and the measured absorbance in a 1.00 cm cuvette is 0.742? Answer: A = εbc c = A / εb c = 0.742 (1.00cm) (1500L/mol.cm) = 4.95 x 10-4 M

14. E.g. 2 The measured absorbance of a sample in a 1.00cm cuvette is 0.544. If the concentration is 1.40 x 10-3 M, what is the molar absorptivity for the species? Answer: A = εbc ε = A / b c ε = 0.544 (1.00cm)(1.40x10-3 mol/L) = 389 L/mol.cm

15. E.g. 3 A sample in a 1.0cm cell is determined with a spectrometer to transmit 80% light at a certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, what is the concentration of the substance? Answer: The percent transmittance is 80%. So, T = 0.80 A = abc log 1/T = 2.0 L/g.cm x 1 cm x c log 1/0.80 = 2.0 L/g x c c = 0.10 2.0 L/g = 0.050 g/L

16. A = abc Concentration Width of cuvette Inherent ability for the absorbing species to absorb light PARAMETERS THAT AFFECT ABSORBANCE

17. Width of cuvette Wider cuvette more absorbing species present in the path of the light, hence absorbance is greater. b b • Inherent ability for the absorbing species to absorb light - Chemical species vary with respect to this inherent ability since absorption depends on individual electronic and vibrational transitions available in a given species

18. Applying Beer’s Law to Mixtures - Beer’s law also applies to solutions containing more than one kind of absorbing substance, provided there is no interaction among the various species Total absorbance for a multicomponent system at a single wavelength is the sum of the individual absorbances. APPLICATION OF BEER’S LAW Atotal = A1 + A2 + ………+ An = ε1bc1 + ε2bc2 + ……… + εnbcn

19. LIMITATIONS TO THE APPLICABILITY OF BEER’S LAW - Deviations are frequently observed from the direct proportionality btw absorbance, A and concentration, c when pathlength, b is constant.

20. Deviations may be due to: 1. Fundamental 2. Instrumental 3. Chemical deviations

21. 1. Fundamental deviations • Real limitation to the law • At high concentration (0.01M) each particle affects the charge distribution of its neighbours. • Therefore, this interaction alter the ability of analyte species to absorb a given wavelength of radiation. Causing deviation from the linear relationship between absorbance and concentration.

22. 2. Instrumental deviations - due to polychromatic radiation • Beer’s Law strictly applies when measurements are made with monochromatic source radiation. • In practice, polychromatic sources that have a continuous distribution of wavelengths are being used. • Deviations occur if the radiation is polychromatic since the relationship btwnA and c is no longer linear when is differ.

23. 2. Instrumental deviations - due to presence of stray radiation • Due to instrument imperfections. • This stray radiation is the result of scattering and reflection off the surfaces of gratings, lenses or mirrors, filters and windows. • The wavelength of stray radiation differs greatly from the principal radiation & may not have passed thru’ the sample.

24. When measurements are made in the presence of stray radiation, A’ = log P0 + Ps P + Ps Ps – power of nonabsorbed stray radiation % stray radiation = Ps x 100 P0

25. 3. Chemical deviations • Occur when the analyte undergo dissociation, association or reaction with the solvent to give products that absorb differently than the analyte.