Lecture 2 M.Sc.

1. Lecture 2 M.Sc.

2. AA Spectrometer Components Lamp and Flame Detector Double-Click picture for VIDEO Nebulizer Fuel Oxidant

3. Nebulizer Components

4. Nebulizer Operation • Vacuum within chamber due to combustion process in the flame of the slot burner. • Vacuum results in aspiration (suction) of the analyte solution through the sampling tube. • Sample solution enters nebulizer at speed and strikes bead which breaks sample stream into tiny droplets. • Resulting mist/vapour is moved under low pressure through the nebulizer to pass a series of baffles. • Larger droplets strike the baffles and precipitate to the waste outlet. • Vapour consisting of the smallest droplets proceeds unimpeded to the flame.

5. Interferences • 1. Chemical Interference:Formation of stable analyte compounds. • Usually due to the presence of phosphate, silicate or aluminate • These cause the suppression of alkaline earth metal absorption signal in the flame, e.g., effect of PO43- on Ca. • To overcome this interference: - use hotter flame - add a releasing agent such as Sr, or La which preferentially react with phosphate. - add protective chelating agent such as EDTA, which will preferentially complex with the analyte.

6. Interferences • 2. Ionisation Interference:Most common for alkali and alkaline earth metals with low ionisation potentials • Reduces atom concentration • To reduce add ionisation suppressor or buffer. • Typically Caesium is used. • 3. Physical Interference:Due to differences between solvent and standards and sample. E.g. • Viscosity: viscous solvent aspirated with slower uptake rate, delivers less analyte per unit time to the flame. • Therefore, absorbance is lower than for an equivalent concentration in a less viscous solvent.

7. Interferences • 3. Physical Interference continued: • Surface Tension: Solvent with lower surface tension provides smaller average droplet size. • Less sample lost to drain and larger analyte concentrations reach flame per unit time, i.e., absorbance higher than for an equivalent concentration in a solvent with higher surface tension. • Overcome by trying to match physical characteristics of standard and sample.

8. Interferences • 4. Spectral Interference:Due to overlapping spectral lines. • Rare due to the narrow line emission of Hollow Cathode Lamp. • However, can occur if separation between two lines is around 0.01nm e.g. ANALYTE INTERFERENT Mg 285.02 Na 285.03 nm Al 308.215 V 308.211 Cu 324.753 Eu 324.754 • Main spectral interference due to molecular absorbance and scatter of source radiation.

9. Interferences • Molecular absorbance: commonly due to molecules such as NaCl and Ca(OH)2 • Spectral interferences are overcome by various background correction methods.

10. Interferences • 4. Spectral Interference continued: • Both reduce transmitted intensity and lead to positive analytical errors. • Scattering can be caused by Carbon particles in the flame or by unvapourised solid particles. • Usually caused by elements such as Ti, Zr and W, which form stable metal oxide particles. • 5. Occlusion: e.g., effect of Fe on Cr in steel analysis. • High concentration of Fe causes formation of particles where Cr is trapped and cannot evaporate or be reduced to atoms efficiently. • To overcome add EDTA or NH4Cl to standard and samples.

11. Interferences • 5. Occlusion continued: • EDTA complexes Fe and Cr and helps prevent formation of occlusion sites. • NH4Cl is highly volatile salt that explosively evaporates in the flame, resulting in a smaller, more easily evaporated particles.