Question

In an experiment using flame atomic absorption spectroscopy, the goal is to examine how certain compounds that may be present in samples, can create interferences.

You start with with a 10ppm calcium standard, and measure the absorbance. (Concentration readout is 0.105 mg/L),

You add two drops of PO₄^3- (phosphate) to the standard, and measure the absorbance. (Concentration readout is now 8.400 mg/L)

Finally, you add 5 drops of La²⁺ (lanthanum) and measure absorbance. (Concentration readout is now 10.700 mg/L)

a) Explain the molecular mechanism by which phosphate interferes with calcium determination. What is the nature of the compound formed and what two actions can be performed to eliminate this type of interference?

b) What is the purpose of adding lanthanum? Explain the chemistry of when lanthanum is added.

Answer 1

(a) Phosphate interfare the determination of Calcium atom in Atomic Absorption Spectroscopy by combining with Calcium atom in condensed state and forming a less volatile compound. This compound decrease the concentration of free Calcium atom in the light beam of spectrometer. Hence results a less intense signal.

This problem can be solved by two ways-

(1) Adding Lanthanum to the mixture.

(2) Another way is, using Nitrous oxide/acetylene flame (3080⁰ C)

(b) Addition of lanthanum to the Calcium complex increase the intensity of the signal in Atomic absorption spectroscopy. This is because lanthanum formed a complex with phosphate and release free Calcium atom.