LALman wrote:
Silver Chloride is slightly soluble in nitric acid. Each time you dilute you get a slight amount more (taking DF into account).
For 20+ years I've run ICP-AES and ICP-MS using a 2%HNO3-1%HCl final dilution matrix. I do my digestions with those acids in that ratio. This will keep at least up to 1 ppm Ag soluble at the instrument in standards and samples. That way chloride interference is not a problem. You also get better recoveries of As, Sb, and Tl in complex matrix with chloride present.
Silver is one of the worst elements to keep stable for certain. The EPA methods will suggest that anything over 100ppb can fall out of solution, personally I see it begin to fall out at 250ppb. The first day the calibration is good up to 1ppm, if I run those standards on consecutive days the 1ppm, 500ppb and 250ppb will begin to show lower recoveries, the 100ppb will usually be fairly stable. We just stop our calibration at 100ppb now. Higher HCl can help but that interferes with other elements so unless you are only looking for Silver it isn't worth it.
Also we learned not to use the older method of a single point calibration, especially for ICP-MS, they just are not linear over the wide range once thought. When I first began using ICP-AES everyone said you could calibrate using a 10ppm solution and it was linear down through the detection limit and many tried it at 1ppm for ICP-MS, but if you analyze a standard at 1-5ppb, it could calculate out either negative concentration or up to 5x the known concentration. If you are working in ppm concentrations this is ok, but if you are trying to monitor in low ppb, it just doesn't work. The newer CCD and CID detectors on the AES are also not as linear as the old PMT detectors nor do they have the dynamic range, but the methods are still written for the 30 year old instruments
If you are seeing that much drift, make sure to calibrate near to the expected concentration to help compensate for any day to day drift in sensitivity, and use an internal standard if you are not to help make it more reproducible.