Introduction to AAS component parts

DISCUSSION WITH SHRI JKP SIR 

Introduction to AAS component parts

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Atomic absorption occurs when a ground state atom absorbs light of a specific wavelength.The amount of light absorbed is governed by Beer Lambert’s law and will increase as the number of atoms of the element in the light path increases. The component parts of Atomic Absorption Spectrometer are similar to a UV -Vis spectrophotometer as both operate on same principle with a basic difference that the sample cell of UV-Vis spectrophotometer is replaced by an atomization source (flame or graphite furnace)

AAS Schematic Diagram

 Light sources – Hollow Cathode Lamps

The light source commonly used is a hollow cathode lamp. A different element hollow cathode lamp is required for each element determination. Cathode is made of same metal that is to be estimated in the sample

Single element lamps are used commonly though multi-element lamps are also available

Lamps are made of glass with quartz windows and filled with an inert gas such as argon

Light sources – Electrodeless Discharge Lamps

Used for volatile elements such as As,Sb, Sn,Cd,Pb, etc

EDL’s have greater lamp life and high energy throughput

Burner system

The burner assembly comprises of nebulizer to reduce the liquid sample to a fine aerosol, a spray chamber and a burner head which is used to generate a flame to produce atoms of the same elements that are present in the sample.

Monochromator

A monochromator disperses the incident light beam and permits the selected wavelength to reach the detector.

Detector

Detector commonly used is a photomultiplier tube which produces a signal proportional to the amount of light received by it.

In this section you shall be introduced to two working configurations, namely single beam and double beam instruments

Single Beam Schematic Diagram

Light from the source is modulated electronically or chopped mechanically by rotating chopper. This helps isolate and remove sample cell emissions from light emitted by the source. The specific wavelength isolated by monochromator is led to the detector and the electrical signal generated is proportional to the elemental concentration in the sample

Double Beam Schematic

Double Beam AAS Schematic Diagram

 Light beam from source is split into two teams by the chopper. One beam passes directly through the flame and the other beam passes round the flame. Detector response represents the ratio of sample and reference beams. Fluctuations in light intensity are eliminated electronically to get greater reliability of results.

The sensitivity is lower than single beam instruments but the popular acceptance of double beam configuration is due to advantage of elimination of background changes in the atomizer.

Atomic Absorption Spectroscopy	Study of element specific light absorption by ground state atoms for estimation of concentration of the element in the sample solution.
Atomisation	Process of reduction of sample to ground state atoms by application of heat by means of a flame or a graphite furnace.
Atom	The smallest particle of an element or compound. It comprises of a central nucleus containing neutral particles called neutrons and positively charged protons. The electrons revolve the central nucleus in shells of different energy levels. The number of electrons equals the number of protons in the neutral atom.
Atomic Emission Spectroscopy	Qualitative identification and quantification of element by emission of characteristic wavelength of light on excitation of an element by means of a flame or plasma
Atomic Fluorescence Spectroscopy	Measurement of light emitted on decay of elements from excited states.Measurement is made at an angle to the optical beam path so that the detector sees only the fluorescence in the flame and not incidental light from the lamp.
Absorbance	The amount or fraction of incident light absorbed by the ground state atoms. It is directly proportional to the number of ground state atoms in the beam path and also on the optical path length of the flame in accordance to Beer Lambert law of light absorption
Absorbance unit	a ratio of intensity of transmitted flight to the intensity of incident light. It is a unit less quantity but is commonly expressed in absorbance units (EU)
Aspiration	losses of reduction of liquid sample stream into fine droplets for introduction into the flame
Acetylene	Commonly used gas as fuel to support combustion of the flame.Provides temperatures in the range 2150-23000C
Argon	Gas used commonly as a filling gas in hollow cathode lamps and as sample carrier in graphite furnace analysis
Air	Used as oxidant in combination with acetylene as fuel gas to support the flame
Air compressor	Device for delivery of air to the atomic absorption spectrometer. Oil less air compressor is preferred as contamination from oil is thereby avoided
Burner	A component of AAS system made of solid metal body having slit on the flat top surface to provide the flame required for atomisation of the sample
Blaze angle	It is the angle of cut of a mechanically ruled grating at which the angle of incidence is equal to the angle of reflection so that light intensity is greatest with minimal loss due to diffraction. For greater efficiency dual blazed ratings are used which provide greater light throughput over the wavelength range of the spectrometer
Background	any extraneous light other than the transmitted light that reaches the detector and affects the signal absorption
Background correction	Means applied to reduce the effects of background on the signal
Concentration	The amount of element present in a unit volume of solution.Usually expressed as ppm (mg/lit) or ppb (μg/lit)
Characteristic concentration	Concentration of an element expressed in mg/lit required to produce a 1% absorbance or 0.004 absorbance signal. Knowledge of characteristic concentration helps predict the concentration range required to produce optimum absorbance levels for analysis
Collimation	Condensation of beam of light as per size requirement
Cathode	An electrode inside the lamp made from the pure metal whose analysis is required in the sample solution
Chopper	A half transparent half opaque disc that rotates in the beam path to split the beam so as to alternately allow its passing through the sample or around it to give effective double beam performance
Cold vapour mercury analyser	Analyser fo mercury without using a heated sample cell as mercury is only element which exists as a liquid at room temperature
Deuterium sources	A broadband light source for providing background correction in flame analysis
Detector	A component of the system that records the intensity of the transmitted light. Photomultiplier tube is the commonly used detector in AAS
Double beam system	Optical arrangement which alternately permits the light beam to pass through the sample and round it as a reference beam.
Desolvation	Refrom sample droplets by heat inside the flame
Exhaust ventilation system	An assembly for removal of hot corrosive combustion gases and vapours arising from the flame
Electrode/discharge lamp	a lamp used for analysis of volatile elements.It is a high energy light source which has a longer life than corresponding hollow cathode lamps.
Excitation	Excitation of a ground state atom to higher energy states by means of electromagnetic radiation
End Cap	Removable cover of spray chamber that serves to introduce sample into spray chamber and also hold the nebuliser
Flow spoiler	A device inside spray chamber used for removal of large droplets of sample
Flame	Atomisation system which uses a flame. Commonly air – acetylene gas mixture or nitrous oxide – acetylene for higher temperature combustion
Flashback	Reverse movement of flame inside burner towards spray chamber due to greater proportion of oxidant or even pure oxygen in flame. It often results to a loud explosion and damage to spray chamber
Furnace	A graphite tube about a cm long with a hole on top for atomisation of sample using electrical heating of the tube
FIAS	Flow injection analysis system for automated analysis using hydride generation
Graphite furnace	same as furnace
Grating	A light dispersing device used in the monochromator
Hollow cathode lamp	A light source used for AAS analysis which is specific for metal to be analysed in the sample. For some elements multielement hollow cathode camps are also used
Hydride generation technique	Used for analysis of volatile hydride forming elements such as As, Bi, Ge, Pb, Sb, Se, Sn, Te.
Impact bead	A device inside spray chamber for removal of large sized droplets from sample stream
Interference	Effects resulting in variation of results due to spectral or non-spectral interferences
L’vov platform	small platform made from solid pyrolytically coated graphite which is placed at bottom inside graphite tube. Sample is put into a depression in the platform. Permits uniform heating and delays atomisation till stable temperature conditions are preached inside the furnace
Monochromator	A device used for dispersion of incident light using prism or grating, reflecting mirrors and a combination of entrance and exit slits for isolation of required wavelength and collimation of the light beam
Mirror's	Light reflecting component of monochromator with a aluminium or gold coated surface to reduce corrosion damage and provide high reflectivity
MHS	Mercury hydride system for analysis of volatile elements by hydride formation
Matrix interference	Interference arising due to differences in parameters such as viscosity, surface tension between sample and standards solutions
Microwave Reaction System	Automated digestion of samples in closed tubes using sonic waves. It offers advantages of speed of f digestion, cost and freedom from toxic vapours
Matrix modifier	Substance used for reduction of chemical interferences
Nebuliser	A device for producing an aerosol of sample inside spray chamber
Orifice	Small bore tube opening
Polychromatic	A light dispersion device using an array of detectors for simultaneous detection of elements in a sample
Photomultiplier tube detector	A detection device used in AAS which amplifies the current produced by impact of photons on a light sensitive surface
Prism	A light dispersing element
Quartz	A UV transparent material used for making hollow cathode lamp and graphite tube end windows
Slit width	Width of monochromator entry and exit slits expressed in millimeters
STPF	Stabilised temperature platform furnace is a combination of instrumental and analysis factors for providing high accuracy of results
Transverse heating	Heating of graphite furnace perpendicular to its axis to provide uniform heating of graphite tube along its length
UV range	Wavelength range 180 - 350 nm. Most elements have specific absorption bands in this region
Zeeman background correction	Advanced background correction used in graphite furnace analysis involving application of a magnetic field perpendicular to the graphite furnace. Effective for background correction of complex matrices.