Chemical Stability and Compatibility - Lead (Pb) instability in HNO3

While there are several possible causes for the results you are observing, it is unlikley that chemical instability is one of them; Pb at the diluted cconcentrations in 2% HNO₃ should be chemically stable indefinitely. The other possibilites fall into two broad categories with one related to preparation and the other related to measurement. If you are preforming your dilutions in glassware it is possible that Pb contamination is occurring, and what you're observing is inconsistent and variable Pb concentrations in your standards. If you are preparing a 2% HNO₃ blank in the same manner as your standards then this blank should indicate if this is the problem. If, however, you are observing highly variable Pb signal intensities for repeat measurements of the same Pb standard, then you coud be observing: 1) issues with the solution uptake, perhaps related to timing, 2) memory effects, and/or 3) some type of severe instrument drift. Unusually high RSDs for a single measurement could indicated a timing issue, and increasing or highly variable results for repeated measurements of the same solution could indicate memory or drift effects.