ICP MS for silver amount change over time

[ym3142]

I have a solution, made by digesting polyglactin in strong acid by microwave assistance. The same solution was further diluted at three different days (i.e. the same, 2nd day and in 5 days) for 250 fold in 2% HNO3. The amount of silver found was 0.7 ppb, 2.5 ppb and 3.7 ppb.

what can be the cause?

[LALman]

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.

[James_Ball]

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.

[LALman]

I currently have a PE Elan 6000. My linear range check is 1000 ppb. I calibrate silver from 200 ppb in 5X dilution increments 200, 40, 8, 1.6, down to 0.32 ppb. For silver I usually drop the lowest standard and I use an MDL of 0.5 ppb.

Back when I had an HP-4500 ICP-MS my mdl for Ag was 0.04 ppb with an rdl/pql of 0.16 ppb. But the PE Elan 6000 is not as sensitive. The peltier cooled spray chamber and a bergner nebulizer is a potent combination on an HP-4500. The PE Elan 6000 just has the usual PE cross flow nebulizer and uncooled spray chamber.

So, silver and any ICP-MS analyte are log-linear if you use log of the concentration versus log of the (analyte/internal standard) intensity ratio. If the tygon tubing is old you won't be able to read the 1000 ppb to within 10% but the range from 200 down to 0.5 ppb will work every time.

[James_Ball]

Also something to remember is that silver is sensitive to UV light. I have had high concentration standards make a coating on a plastic tube if left for a few days under fluorescent lights or sunlight through a window. If you have a cover over the autosampler it helps with stability.