Her first step is to acidify the samples with a drop of high-purity concentrated nitric acid and shakes the bottles. She also prepares a reagent blank from the distilled water in her laboratory. She decides that sodium (589 nm), calcium (422 nm), magnesium (285 nm), copper (324 nm) and zinc (214 nm) will be determined using the flame AA in an air-acetylene flame. Lead (283 nm) and cadmium (228 nm) concentrations will (hopefully) be too low to see with the flame AA instrument and will thus require the use of the electrothermal AA. She prepares standards for calcium and begins the analysis. As shown in the recorder tracing above, her standards and reagent blanks appear normal, but when she measures the undiluted well water samples, she gets a negative signal! The vertical axis is absorbance and the horizontal axis is increasing time. What's going on?

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