Hello. We prepare around 1-40 ppm Pb standards from 1000ppm Pb stock solution. Our choice of matrices are usually 2% nitric acid or 2%nitric:0.5-1% hydrochloric acid. These standards are usually prepared and stored in a corning plastic volumetric flask, with parafilmed caps. We like to ask you guys about the expiration or stability of these daughter standard solutions. Thanks in advance.