Study on stable coatings for determination of lead by flow-injection hydride generation and in situ concentration in graphite furnace atomic absorption spectrometry

Abstract

Lead hydride was generated by flow-injection from 0.05 M oxalic acid sample solution by using 0.1 M HCl carrier solution and the reaction with 1.5% sodium borohydride in the presence of 2% potassium hexacyanoferrate(III) as a mild oxidizing agent. Pb was determined by in situ concentration in graphite furnace AAS. The hydride generation by flow-injection and trapping in the graphite tube coated with a highly stable trapping reagent (e.g. tungsten) allows automatic Pb determination. In a systematic investigation, the in situ concentration of Pb was studied in the temperature range 50–600° on graphite tubes coated with noble metals (Ir, Ir/Mg, Pd/Ir), and with W or Zr. The highest response was found on the Ir coatings at trapping temperatures of 200–300°, followed by the W and Zr coatings. The radiotracer 210Pb was used to measure hydride generation (95%) and trapping efficiency (71%) on a W-coated tube. Signal stability and reproducibility was tested over 400 trapping and atomization cycles, and the better performance was found with the W and Zr coatings at a precision of 3%. Trapping temperatures above 450°C can lead to errors in absorbance values owing to an adsorptive “carry-over” effect. A characteristic mass of about 21 pg Pb for W-coated tube (283.3 nm) and a detection limit (3σ) of about 0.25 ng was obtained with a 0.5 ml sample loop. The problem with Pb hydride generation is the relatively high reagent blank (1.3 ng in 30 s trapping time) even using chemicals of the highest purity. The method has been tested by applying it to the determination of Pb in a sediment certified reference material.