Expiration of Working (daughter) Standards

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trismetals
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Expiration of Working (daughter) Standards

 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.