Spetrochemical Analysis: Instrument components

Instrument Components

© Glydenne Glaire P. Gayam

1. source of radiant energy

2. wavelength selector

3. sample container

4. detector

5. signal processor and readout


Two wavelength selectors are needed to select the excitationand the emission wavelengths. The selected source radiation isincident on the sample and the radiation emitted is measured,usually at right angles to avoid scattering.


A source of thermal energy, such as a flame, produces ananalyte vapor that emits radiation that is isolated by thewavelengths selector and converted to an electrical signalby the detector.


Cells

Windows

Lenses

Wavelength Dispersing Element


I. Optical Materials

Transmittance ranges for various optical materials.


I. Optical Materials

Silicate glass: Visible region

Fused silica or quartz: UV region (<380nm)

Halide salts: IR region© Glydenne Glaire P. Gayam

1. Continuum sources• emit radiation that changes in intensity only slowly as a

function of wavelength

• Widely use in absorption and fluorescence spectroscopyfor UV region

2. Line sources• emit a limited number of bands of radiation, each of

which spans a very limited range of wavelength

• Widely used in atomic absorption spectroscopy and inmolecular florescence spectroscopy

• Eg. Hg and Na vapors lamps

•Generate a beam of radiation with sufficient and stable power


The spectrum of a continuum source (a) is much broader than that of a line source (b).


Source Wavelength Region, nm Type of Spectroscopy

Xenon arc lamps 250-600 Molecular Fluorescence

H2 and D2 lamps 160-380 UV MolecularAbsorption

Tungsten/Halogen lamp 240-2500 UV/vis/near-IR molecularabsorption

Tungsten lamp 350-2200 Vis/near-IR molecular absorption

Nernst glower 400-20,000 IR molecular absorption

Nichrome wire 750-20,000 IR molecular absorption

Globar 1200-40,000 IR molecular absorption


provides radiation of all wavelength within a particular spectral region.


A tungsten lamp of thetype used in spectroscopyand its spectrum. Intensityof the tungsten source isusually quite low atwavelengths shorter thanabout 350 nm.


A cylindrical tube (contains deuterium at a low pressure) with a quartz window (the radiation exits)


A deuterium lamp of the type usedin spectrophotometers and itsspectrum.

Globar source: 1 - 40 μm (Globar heated to about 1500℃)

5- by 50-mm silicon carbide rod.

Nernst glower: a cylinder of zirconium and yttrium oxides.

Nichrome wire


1. Monochromators and Polychromators

2. Grating

3. Radiation Filters


- enhance both the selectivity and the sensitivity

1. Monochromators and Polychromators• Advantage: the output wavelength can be varied

continuously over a considerable spectral range. (the more common type)

2. Grating – disperse radiation into its component wavelengths• Qualitative analysis: narrow slits and minimum effective

bandwidths

• Quantitative analysis: wider slits permit operation at lower amplification (greater reproductibility


3. Radiation Filters• Advantage: simplicity, ruggedness and cheapness

• Interference filter: effective bandwidths of 5 to 20 nm• Dielectric material: CaF2 of MgF2

• Absorption filter: effective bandwidths of 50 to 250 nm


detector: indicates the existence of some physicalphenomenon.

ex: photographic film, pointer of a balance, mercury level ina thermometer, and human eye

transducer: converts signals, such as light intensity, pH,mass and temp. into electrical signals that can besubsequently amplified, manipulated and finallyconverted into numbers proportional to the magnitudeof the original signal.


Types of Transducers


1. Photon Detectors• Phototubes• Photomultiplier tubes• Silicon photodiodes• Photoconductive cells

2. Heat Detectors• Thermocouples• Bolometers• Pneumatic cells• Pyroelectric cells


Common Detectors for Absorption Spectroscopy

Type Wavelength Range, nm Type of Spectroscopy

Photon Detectors

Phototubes 150-1000 UV/visible and near-IR absorption

Photomultiplier tubes 150-1000 UV/visible and near-IR absorption, molecular fluorescence

Silicon photodiodes 350-1100 Visible and near-IR absorption

Photoconductive cells 1000-50,000 IR absorption



Common Detectors for Absorption Spectroscopy

Type Wavelength Range, nm Type of Spectroscopy

Heat Detectors

Thermocouples 600-20,000 IR absorption

Bolometers 600-20,000 IR absorption

Pneumatic cells 600-40,000 IR absorption

Pyroelectric cells 1000-20,000 IR absorption


Sample Containers


Typical examples of commercially available cells for the UV/visible region.

Cells or cuvettes: 0.1 to 1-cm path length

Spetrochemical Analysis: Instrument components

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