Good morning, Azam.
In addition to what Bill and Thomas said,
See Pappas RS. Sample Preparation Problem Solving for Inductively Coupled Plasma-Mass Spectrometry with Liquid Introduction Systems I. Solubility, Chelation, and Memory Effects. Spectroscopy 2012;27(5):20-31.
Since you are new to ICP-MS, it is apparent that you do not yet have experience with "sticky" elements that have "memory effects." Silver is one of those elements. Silver is easily reduced by reducing agents in the matrix. If you microwave digest the plants without HCl present to coordinate Ag+, it will get reduced by plant material as the material is oxidized, catalyzed by the acids. Ag+ is even photocatalytically reduced by light. When it is reduced to Ag(0), it is no longer water soluble, so it deposits everywhere in your introduction system and gradually rinses through. This is why you see a much higher concentration in your later samples. Read the article so that you have at least some understanding of Pearson hard/soft acid/base theory as applied to chelation/coordination. Compare silver to the discussion of mercury in the article. You should come away with an understanding of why you need at LEAST 1%HCl final concentration in your diluted samples (maybe more) and in your rinse solution, OR 0.1% high metals purity L-cysteine with MAXIMUM 1% nitric acid (nitric acid oxidizes L-cysteine over time). One of these approaches should at least aid with your silver problem, but you will probably need to replace your peristaltic pump tubing and run a 5% nitric acid + 5% HCl or similar solution through your system to clean out the deposited silver. If you have microwave digested without HCl, you need to perform a couple of dummy digestions (no sample), use something like 5 mL nitric acid, 5 mL HCl to clean the silver out of the vessels as well.
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 Azam Noori
Sent: Friday, June 1, 2018 10:12 AM
To: [log in to unmask]
Subject: Negative Data- ICP-MS
My name is Azam Noori. I am an assistant professor at Merrimack College. We have purchased an ICP-MS lately. It is NEXION 350X (Perkin Elmer). I am working with plants. I analyzed some samples yesterday. The calibration curve for most elements was good (Correlation factor: 0.998-0.999).
The concentration of elements in the first 5 samples was reasonable. Then I used the standard 10ppb as a sample to make sure the instrument is reporting the correct concentration. The concentration of most elements was much higher than 10 ppb in this sample. For example for silver (109), it was 623 ppb while when same sample was used as a standard the concentration was 9.88 ppb. I analyzed some samples after this, and the concentration of most elements (Ag, Mo, Cd, Sn, As, Ni, Fe, Cr, Pb, and V) in all samples were negative. It is not what I expected. Do you have any suggestions for troubleshooting?
Azam Noori, Ph.D.
Department of Biology
315 Turnpike Street
North Andover, MA 01845