atomic spectroscopy

intro

• uv-visible spectroscopy is based on molecular interactions

• measures electronic transition of atoms

• used for quantitative elemental analysis

• transition occurs in the visible region and are measured by absorption or emission

• samples must be atomised by heating

sample atomisation

• sample solution —> nebuliser (removal of solvent from aerosol) —> plasma, flame or electric heating (desolvation, liquefaction, vaporisation, atomisation)

after atomisation

• to persons AES a much higher temperature is required as atoms need to be in an excited state

• AES is very temperature dependant and instruments designed to protect flame from draughts

• elements with low ionisation potential are ideal for AES analysis

• range of elements can be increased by hotter flames

• flame temp can increase by using higher flow rates or different gas mixtures

plasma AES instrumentation

• flames have limited temperature —> limited elemental range

• plasma sources overcome this limitation

• plasma generated by energising argon gas

• plasma = ionised, electrically natural gas

• temperatures up to 10,000 degrees

• most common source: inductively couple plasma

ICP-AES advantages

• very high temp = all elements atomised

• multi element, simultaneous or sequential analysis

• requires less sample

• inert argon atmosphere —> chemical interferences

• stable signals and similar operating conditions for many elements

• low background emission —> low detection limits

• wide linear range (10³ - 10^6)

• excellent precision

• requires calibration with multi element standards

atomic absorption spectroscopy - instrumentation

• found state atoms absorb characteristic wavelength

• energy source - hollow cathode lamp

• each element requires its own lamp

• multiple lamps may be mounted but elements analysed individually

• instrumentation costly due to lamps

AAS - disadvantages

• large dilution of analyte —> low sensitivity

• high sample consumption

• flame conditions optimised for each element

minimising these problems

• use long path burners to increase absorption path length

• replace flames with graphite furnace —> EAAS

electrothermal atomic absorption

• uses electrically heated graphite tube

• sample retained longer and in smaller volume —> higher sensitivity

• very small samples size

EAAS - heating stages

• drying, charring, atomisation

• conditions must be optimised of specific analyte and sample matrix

• matrix. modifies added to prevent analyte loss during charring

atomic spectroscopy interferences

• common due to high temperatures

• produce reactive species and promote chemical reactions that can arise from the fuel and oxidant used

• less severe in plasma

• two main types - spectral interference, chemical interference

spectral interference - flam background

• broad absorption from molecular species

• reduced using background connection

spectral interference - overlapping lines

• emission from another element at similar wavelength

• minimise by changing wavenelgth or remove impurity

chemical interference - refractory compound formation

• anions in the sample can form stable, non volatile compounds with the analyte

• reduced AAS signal because atoms are no longer free to absorb radiation

• overcome by adding releasing agents

chemical interference - ionisation interference

• easily ionised elements released electrons that suppress analyte ionisation

• reduces signal intensity

• minimised by adding ionised element to both samples and standards to keep the effect constant

chemical interference - use of organic solvents

• water can increase background absorption and lower sensitivity

• organic solvents improve atomisation efficiency

• benefits include increased rate of aspiration, finer aerosols, faster evaporation, better combustion

AS sample preparation

• must be in solution form, solids aired to constant weight

• preparation varies by sample type

• plant materials - wet oxidation, extraction, titration

• biologic samples - protein preparation or ashing

• tissues - dry washing, acid extraction

• metals - acid digestion and filtration

quantitative analysis

• beer Lambert, measurements done in triplicate

• standards run first to check calibration

• AES is linear; ASS often quadratic