Essential Components of a UV-vis Spectrophotometer Monochromator Signal Processor Display Source Sample Transducer

Continuum Line Sources - Continuous Source - Line Source - Pulsed

Tungsten Xenon Arc Mercury Arc

Optical Components (lens and mirror) *Appropriate optics can be critical!

Monochromator - Filter - Monochromator: - Dispersion element (prism or grating) - Design Note the non-linear focal plane dispersion!

Czerny – Turner Design for Monochromator Note the linear focal plane dispersion!

Incident beam Reflected beam Normal to grating Normal to blaze i r   Monochromatic light source Polychromatic light source

Monochromator Figures of Merit Bandpass - The wavelength range that the monochromator transmits. Dispersion - The wavelength dispersing power, usually given as spectral range / slit width (nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order. For prismsFor gratings

Resolution - The minimum bandpass of the spectrometer, usually determined by the aberrations of the optical system. Acceptance angle - A measure of light collecting ability, focal length / mirror diameter Blaze wavelength - The wavelength of maximum intensity in first order.

Phototubes

Photomultiplier Tube (PMT)

Photodiode Array

Charge-coupled Device

Sample Compartment (Cuvettes)

Single Beam

Double Beam

Singlet state: All electrons in the molecule are spin-paired lifetime: – s Triplet state: One set of electron spins is unpaired lifetime < 10 s

Beers Law for a Multi-component Sample I I0I0 I I0I0

I I0I0 I I0I0

Concentration A Deviations from Beer’s Law Physical: a) Scattering b) Reflection f = [ (n 1 - n 2 ) / (n 1 + n 2 )] 2 = fraction reflected substance n = refractive index e.g. glass 1.5, air 1.0, water 1.3 c) Inhomogeneities d) Stray light 1. Scattering from grating (acting as a mirror causing scatter and reflection) 2. Overlapping orders 3. Diffraction at slits 4. Off-axis illumination 5. Scatter from interior 6. Dust

Deviations from Beer’s Law Chemical: a) Equilibria involving chromophore e.g.Cr 2 O H 2 O  2 H CrO 4 2- Absorptivities of Cr 2 O 7 2- and CrO 4 2- are quite different. b) Solute-Solvent interactions c) Solute-Solute interactions d) Fluorescence (gives positive deviation for %T negative deviation for Absorbance) Note: Non-zero intercept usually due to improper blank measurements or nonequivalent measurement conditions of blank and standards. Concentration A

Photometric Titrations  A > 0;  T =  P = 0 A + T P  T > 0;  A =  P = 0  P > 0;  T =  A = 0 Vol titrant Absorbance

 A >> 0;  T > 0  P = 0 A + T P Vol titrant Absorbance  T >> 0;  P > 0  A = 0  P >> 0;  A > 0  T = 0