Lecture 2 M.Sc.

AA Spectrometer Components Lamp and FlameDetector Fuel Oxidant Nebulizer Double-Click picture for VIDEO

Nebulizer Components

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.

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 PO 4 3- 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.

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.

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.

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. ANALYTEINTERFERENT Mg Na nm Al V Cu Eu Main spectral interference due to molecular absorbance and scatter of source radiation.

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

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 NH 4 Cl to standard and samples.

Interferences 5. Occlusion continued: EDTA complexes Fe and Cr and helps prevent formation of occlusion sites. NH 4 Cl is highly volatile salt that explosively evaporates in the flame, resulting in a smaller, more easily evaporated particles.