Instrumental Analytics 4

Front 1

Components of Optical Instruments: General Design

Back 1

1. (Stable radiation) Source [Continuum/Line] ->
2. Wavelength selector [Dispersive/Non-dispersive] ->
3. Sample (Transpatent sample holder: cells/curvettes made of suitable material) [Liquids, gases, solids] ->
4. (Radiation) Detector [Single/multi-channel)->
5. Signal Processor and Readout

Front 2

General Design

Back 2

Optical spectroscopic methods based on 6 phenomena:
(1) absorption, (2) fluorescence, (3) phosphorescence, (4) scattering, (5) emission and (6) chemiluminescence

Front 3

Arrangement for Absorption measurement

Back 3

Source radiation of selected wavelength is sent through the sample and transmitted radiation is measured by detector-signalprocessing-readout unit.

(sometimes, reversed positions of sample and wavelength selector)

Front 4

Configuration for Fluorescence measurement

Back 4

Two wavelength selectors are needed to select excitation and emission wavelengths.

The selected source radiation is incident on the sample and the radiation emitted is measured (usually at right angles to avoid scattering).

Front 5

Configuration for Emission measurement

Back 5

Source of thermal energy (i.e. flame or plasma) produces an analyte vapor that emits radiation isolated by the wavelength selector and converted to an electrical signal by detector.

Front 6

Radiation Source

Back 6

Continuum Source: very broad range of wavelength.
e.g., Xe (169-850nm) arc lamp

Line Source: containing a few discrete lines.
e.g., Light amplification by Stimulated Emission of Radiation (LASER): ultimate line source

Front 7

Laser Source

Back 7

Critical component: lasing medium
- lasing medium is pumped by external energy to excited states, and a few photons are produced

- photons produced transmit back and forth between pair of mirror, trigging stimulated emission of photon of same energy -> ENORMOUS amplification

Front 8

Stimulated emission: Basis of Laser

Back 8

When the excited state is colliding with a photon whose energy matches the Ey-Ex, the excited electronic state will relax to ground state and simultaneously emit a photon of exactly the same energy and same direction and same phase angle

= Coherent radiation with incoming photon

Front 9

Stimulated Emission vs Absorption

Back 9

-

Front 10

Population inversion and amplification

Back 10

-

Front 11

Advantages of Laser

Back 11

• Spatial coherence:
- all photons in-phase
- high power density
- low beam divergence

• Spectral coherence: high monochromatic

• Pulsed (10^-15-10^-16s) or continuous

Front 12

Wavelength Selector: Ideal Output

Back 12

Ideal output for wavelength selector is to separate
electromagnetic into individual wavelength-component

Front 13

Wavelength Selector: Absorption Filters

Back 13

• Absorption filters are a colored glass or dye between two glass plates

• Properties: -wide bandwidth
-low transmittance at band peaks
-two filters can produce narrow band

Front 14

Wavelength Selector: Interference Filter

Back 14

• Two thin sheets of metal sandwiched between glass
plates, separated by transparent material.

• Interference for transmitted wave and the reflected wave from 2nd layer

Front 15

Wavelength Selector: Constructive Interference

Back 15

-

Front 16

Destructive Interference

Back 16

-

Front 17

Monochromator

Back 17

• entrance slit
• collimating lens or mirror
• grating
• focusing lens or mirror
• exit slit

Front 18

Grating

Back 18

A optically flat, polished surface with a large number of parallel and closed spaced grooves. 300-1400 grooves/mm for UV-VIS region, 10-200 grooves/mm for IR.

Front 19

Performance Characteristics of
Monochromators: Dispersion

Back 19

Ability to separate small wavelength
differences
Linear dispersion or reciprocal linear dispersion: variation in wavelength across the focal plane
SEE slide 26

Front 20

Resolution/Resolving Power

Back 20

The resolution if any spectral feature can be quantified by the peak separation and the valley depth.
Two peaks are commonly said to have resolved if the valley between them in 20%.

Resolving power is an instrument-specific measure: report wavelength/delta wavelength.