Laboratory Exercise Demonstrating Systematic Error in Instrumental Analyses: Aberrant Atomic Absorption Spectroscopy of Cadmium

Commercial stock solutions are routinely used to prepare calibration standards for atomic absorption spectroscopy (AAS). Unfortunately, these solutions contain high concentrations of acid (e.g., 2-6 % HNO3) which may cause interferences in the AAS signal. Using calibration standards prepared from commercial stock solutions, 200 µM Cd (as Cd(NO3)2·4H20) test solutions were consistently quantified by air-acetylene flame AAS to contain ~135 µM Cd. Three sets of calibration standards, ranging from 0.5 to 1.5 ppm Cd, were prepared from two commercial 1000 ppm Cd solutions and a 1000 ppm Cd solution made from 3CdSO4·8H20. The 3CdSO4·8H20 standards produced a calibration curve with linear slope ~33% lower than the slopes of both sets of commercial standards. Using these results as a framework, a laboratory exercise appropriate for an Instrumental Analysis course is prepared. Students will design and perform experiments to identify and eliminate the systematic error in Cd AAS analyses.