Essential Components of a UV-vis Spectrophotometer

Monochromator

Signal Processor

Display

Source

SampleTransducer

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 gratingNormal to

blaze

ir

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.

d

dn

dn

d

d

d

cosd

m

d

d

xD

11 wDeff

For prisms For 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.

R

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-pairedlifetime: 10-5 – 10-8 s

Triplet state: One set of electron spins is unpairedlifetime < 10 s

Beers Law for a Multi-component Sample

nT AAAAA ......321

nnT bcbcbcbcA .........332211

II0 II0

Beers Law for a Multi-component Sample

nT AAAAA ......321

nnT bcbcbcbcA .........332211

II0 II0

Concentration

A

Deviations from Beer’s Law

Physical:

a) Scattering

b) Reflection

f = [ (n1 - n2) / (n1 + n2)] 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

S

Smeas II

IIT

0

Deviations from Beer’s Law

Chemical:

a) Equilibria involving chromophore

e.g. Cr2O72- + H2O 2 H+ + 2 CrO4

2-

Absorptivities of Cr2O72- and CrO4

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

Ab

sorb

ance

A >> 0; T > 0 P = 0

A + T P

Vol titrant

Ab

sorb

ance

T >> 0; P > 0 A = 0

P >> 0; A > 0 T = 0