Lesley's point about matrix buildup is well taken. Silver is easily reduced even without a possible reducing matrix coating probe, etc., that will be removed by oxidizing medium. Remember the organic chemistry lab experiment for identification of aldehydes (easily oxidized) with Tollens' reagent (silver nitrate, silver ion is easily reduced, and forms a mirror coating on the test tube). But silver is even photocatalytically reduced by light, so if your standards or QCs are exposed to light constantly, and not prepared fresh frequently, that will explain buildup and weird calibrations.
In addition to storing away from light, I suggest one of two options: Increase your HCl to at least 2% or more to better coordinate the silver, slow the rate of reduction, OR use 0.01% to 0.1% (depending on your silver concentrations) high metals purity L-cysteine instead of HCl (Sigma has a low metals grade. I do not know who else does). The L-cysteine sulfur will be a stronger ligand for silver, is soluble in acid. Nitric acid will immediately oxidize L-cysteine thiol to cystine disulfide, and more gradually oxidize the disulfide, so decrease the nitric acid from 4% to 1% or less, even as low as 0.1% if you choose the L-cysteine option. You need to reprepare this solution at least monthly with 1% nitric acid. With 0.1% nitric acid, it may last a year.
R. Steven Pappas, Ph.D.
Team Lead, Tobacco Inorganics Group
Centers for Disease Control & Prevention
4770 Buford Highway, NE
M.S. F44, Building 110
Atlanta, GA 30341-3717, USA
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). <[log in to unmask]> On Behalf Of Lesley Swift
Sent: Wednesday, May 16, 2018 3:21 AM
To: [log in to unmask]
Subject: Re: Silver issue, Agilent 7700/7900
We found that matrix from samples progressively coats the inside of the probe and other tubing and this then acts as a chromatography column. This has a more significant effect on lower standards hence the quadratic nature of calibrations.
We found that cleaning probes with concentrated acids and changing tubing helped
Hope this is of some use
Senior Radiation Protection Scientist
Public Health England
155 Hardgate Road
Glasgow G51 4LS
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Michael Szoltysik
Sent: 16 May 2018 07:07
To: [log in to unmask]
Subject: Silver issue, Agilent 7700/7900
I have an issue with silver on some of my Agilent 7700 and 7900 instruments. My calibration curve and recoveries on my QCs gottten progressivly worse over time. Calibration curve (done on a custom multistandard, 4% HNO3, 1 %HCl) is more or less quadratic now, instead of linear before, recoveries on my QCs are ranging between 30-50 %. This problem has gotten progressivly worse over time, just on certain instruments (i couldn't find any correlation on what matrices are ran on those certain instruments). I've tested following: followed Agilent's advice to do a clean-up of the front end of the instrument (3 different solutions followed by a nitric acid wash), changed the entire front-end including sample intake, lenses even the octapole, tested with just a single element solution with different acid concentrations ranging from 1% HCl - 20 % HCl, different wash out solutions and timings. All solution/different acids testing has been done with and without autosampler, as well as with self aspiration. My conclusion was, based on real time monitor, that silver has much longer stabilisation and wash out time (regardless the acid conc) than the other elements on the instruments with this problem. Apart from that, the issue dissapears temporarly after changing vacuum pump (not oil only, but the entire vacuum pump). The same calibration solutions, QCs are working fine on other instruments (both Agilents and PE instruments) which rules out the solutions. Increasing timings is not a favourable thing, due to high sample volumes. I'm in a bit lost for ideas on what it might be and was hoping to get some insights, did anybody have a similar issue?
Thanks in advance!
Eurofins Environment Testing Sweden AB
Development Chemist & Team Leader
Phone: +46 10 490 8133