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U of M CHEM 4101 - Ultraviolet-Visible Absorption Spectrometry

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9/10/11 1 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Ultraviolet-Visible Absorption Spectrometry 1- Fundamental principles of molecular absorption 2- Beer’s Law 3- Factors that affect Beer’s Law -Sections 6A, 6B, 6C, 13A, 13B, 13D (Bonus 13C) Suggested exercises 6-5, 6-14, 6-15, 6-18, , 6-19, 13-4, 13-6, 13-12 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 I. Analyte = Chemical element - TABLE 1 II. Analyte = Small molecule (<1000 MW) – TABLE 2 III. Analyte = Large molecule (> 1000 MW) – TABLE 3 IV. Matrix = Solids or gels - TABLE 4 V. Matrix = Gas phase or liquid phase - TABLE 5 Analytical Problem - Topics 1. Sit at the Table that best matches the analytes (or matrix) of your analytical problem Done after slide 6…. 2. Discuss with your two nearest neighbors whether the analyte(s) or matrix have and absorption spectrum. Explain why. 3. Describe to the rest of the class your neighbor’s analyte(s) or matrix UV-VIS absorption properties (Random selection of one person per table). Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Electromagnetic Spectrum Figure 6-3, Section 6B9/10/11 2 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Spectroscopy Methods Section 6B Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Absorption of Radiation Figure 6-23 and 6-24, Section 6C Absorption Non-radiative relaxation Fluorescence Atoms - vapor Molecules - vapor Molecules - liquid Molecules - liquid Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Absorption Spectra are unique Figure 14-2, Section 14B Key Features: Molar absorptivity, ε (cm-1 M-1) Maximum absorption wavelength, λMax (nm)"Absorption range, , λ Range (νm)"Transmittance, T = I/Io (= P/Po), Eq. 6-30 % Transmittance, %T = 100% x I/Io Absorbance = -log10 T = log (Io/I) = εbC, Eqs: 6-32 and 6-349/10/11 3 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 UV-VIS Photometer or Spectrophotometer Figure 13-13(a), Section 13D Photometer: A or %T measurements at Preselected wavelengths Spectrophotomers: scans a wavelength range to measure A or %T Lecture 03 – Sep 12 Chem 4101 – Fall 2011 What happens at the cuvette? Figure 13-1, Section 13A Io I Use a blank to correct for light losses. Use a reference cell with only solvent Stray light Lecture 03 – Sep 12 Chem 4101 – Fall 2011 For quantitation, use a narrow wavelength selector Figure 13-4, Section 13B; Section 13C-3 Suggestions: Measure absorbance at the maximum. Use a wavelength selector with no more than 1/10 the width of the absorption band."9/10/11 4 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Practical aspects of Beer’s Law use Log (Io/I) = εbC = A Figure 13-4, Section 13B Assumptions: Diluted solutions (< 0.01M) Equilibrium"Stable molecules or species"Monochromatic radiation Effect of polychromatic radiation Understand derivation of these curves in pages 340-341 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Chemical deviations from Beer’s law (Work on your own: Example 13-1, titration indicators) At 430 nm, In-1 is the main absorber At 570 nm, HIn is the main absorber In mixtures at equilibrium, use equation for multi-component system: Atotal = A1 + A2 +… = ε1bC1 + ε2bC2… Figure 13-3, Section 14B; Example 13-1 € HIn → H++ In−ε430 ε570 HIn 6.30 × 102 7.12 × 103 In- 2.06 × 104 9.62 × 102 Ka = 1.42 × 10-5; 0.1 M Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Stray light effects on beer’s law Figure 13-6, Section 13B € A'= logPo+ PsP + PsPs: stray light On your own: Use Excel to replicate this graph9/10/11 5 Lecture 03 – Sep 12 Chem 4101 – Fall 2011 Advanced topics (Possible bonus points in exam 1) Figure 13-6, Section 13B Instrumental noise as a function of transmittance (Sections 13C-1,


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U of M CHEM 4101 - Ultraviolet-Visible Absorption Spectrometry

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