I have a problem with my standard for ICP-OES. It contains the following elements Cu, P, Al, Ca, Fe, K, Mg, Mn, Na and S in a solution of 7,8% HNO3 and H2O2. I cannot make it work for reading P and most worryingly the high concentration standard bottle expands and makes a high hiss when opened after a while indicating a reaction that makes a gas. Can anyone suggest what elements cross reacts? We have measured all elements in other combinations without problem.
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3$\begingroup$ To get any help, make readers feeling they know your scenario as well as you do. $\endgroup$– PoutnikCommented Sep 19, 2023 at 12:49
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4$\begingroup$ What is precisely the composition of your solution ? Apparently the solution is acidic and contains the cations $\ce{Cu^{2+}, Al^{3+}, Ca^{2+}, Fe^{3+}, K^+, Mg^{2+}, Mn^{2+}, Na^+}$ and the anions $\ce{NO3^-, PO4^{3-}}$. What is the concentration of $\ce{H2O2}$ in the solution ? Don't you think the "hiss" is not simply due to $\ce{O2}$ produced by the catalytic decomposition of $\ce{H2O2}$ ? It is not a cross reaction. It is well known that the ions of $\ce{Fe, Cu, Mn}$ act as catalyzers in the spontaneous $\ce{H2O2}$ decomposition. $\endgroup$– MauriceCommented Sep 19, 2023 at 15:43
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$\begingroup$ Are you following a standard protocol that has been shown to work and that you have tried before? What is the exact concentration? Are other elements giving the expected readout? $\endgroup$– Buck Thorn ♦Commented Sep 19, 2023 at 20:27
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$\begingroup$ @Buttonwood, P can easily have ppm or below ICP detection limits. $\endgroup$– ACRCommented Sep 20, 2023 at 0:37
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1 Answer
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The knowledge of classical chemistry remains important, irrespective of the sophistication of analytical techniques or the cost of the instrumentation. It is important to understand the incompatibility between iron and phosphorus, which form highly insoluble phosphates. Similarly, Mg, Cu, Al will all interact with dissolved phosphate with their insoluble phosphates. Consequently, the utility of a dissolved phosphate in the presence of these metallic ions to save analysis time goes to waste. This explains the poor limit of detection for phosphorus, which likely is resting at the bottom of the standard stock solution. A second concern is the catalytic decomposition of $\ce{H2O2}$. A repetition of the entire analysis is needed because the whole standard solution is messed up.
Until and unless these metallic ions were strongly chelated (perhaps not) by the standard solution manufacturer, the results of these ICP-OES analyses are not reliable at all.
