Chemical Stability and Compatibility - Choosing best matrix for diluted standards

We would recommend diluting in the same matrix that is used to preserve the standard; in the case of QCP-QCS-1 that would be 5% HNO3. If that matrix is not compatible with the analytical method, then it is possible to use less acid, a different acid, or simply water, though each element in the standard may benefit differently from these approaches. Generally, 1-2% (v/v) of a strong mineral acid (HNO3 or HCl) is sufficient to stabilize most elements at concentrations <100 ppm. At concentrations less than 1 ppm, trace amounts of acid (0.1-0.2%) are usually all that is necessary to ensure chemical stability. For arsenic in particular, a relatively low pH is required to prevent the formation of insoluble metal arsenates, and we would recommend a pH of 2 or lower. If QCP-QCS-1 were diluted in only water to an As concentration of 30 ppm, the residual HNO3 content would be ~0.75% by volume, which would correspond to a pH of less than 1, and therefore we would expect no stability issues for As in this preparation.