Guided-Inquiry Experiment for Teaching the Calibration Method of Standard Addition in the Analysis of Lead with Graphite Furnace Atomic Absorption Spectroscopy

In this guided-inquiry lesson, students were tasked with determining the efficacy of an at-home drinking water filter at removing lead (Pb) from tap water using graphite furnace atomic absorption spectroscopy by determining the concentration of lead before and after filtration. The lesson was structured to allow students to choose and perform their own calibration methods. Although students believed the objective of the experiment was to determine the ability of the filter to remove Pb, the filtration process caused a change in matrix composition which prompted consideration of the associated advantages and disadvantages of the calibration methods of standard addition and calibration curves. The change in matrix composition of real-world samples can sometimes result in an observed increase in measured signal for Pb absorbance. If students selected external standardization (calibration curve), this change in matrix composition and subsequent measurements often suggested an increase in Pb concentration due to the filtration. Further reflection and critical analysis of their results, and guided support from their instructor, allowed students to discover that this scenario required standard addition calibration in lieu of using external standardization to compensate for the change in matrix composition. This lesson provided students with experience analyzing real-world samples and discovering the suitability of calibration methods to overcome authentic analytical challenges. The authors present this lesson in a manner that will allow adoption by others, including a student handout and instructor notes as supplemental resources. Possible extension activities are also suggested for characterizing the nature of the matrix and the matrix effects.