It sounds like you are seeing the EIE effect. That's 200 mg/L of Mg that is stealing your plasma energy causing a suppression of your internal standard signal. Try lowering your Mg concentration by dilutions and/or make sure you have the same amounts of Mg in your calibration standards.
Trace Metals Task Lead
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From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). <[log in to unmask]> On Behalf Of Prasad Lavisetty
Sent: Tuesday, August 7, 2018 3:56 PM
To: [log in to unmask]
Subject: Intensities of all elements suppresing in presence of magnesium oxide
Currently I am working on a project that has 10% of magnesium oxide. It is pharmaceutical product. It dissolves freely in water. I am not digesting the sample. The acid matrix currently I am working is 2.5% nitric acid +0.5% hydrochloride acid+ 2% sulfuric acid. I am adding thiourea solution in dilution to get 0.1% concentration.
Calibration curve is good for all elements (cd, pb, as, hg, co, no, v), internal standards also the recovery is around 100%.
When it comes to samples and spiked samples the intensities are down to 75% (both analyte and internal standards). The intensities are same throughout the Samples.
I am thinking my sample containing magnesium oxide is causing suppression of intensities. The final concentration of magnesium oxide is going to ICPMS is 0.2mg/ mL. Anybody faced same issue ever before.
Thanks in advance