You are running an ICP-MS (Inductively coupled plasma mass spectrometry), with bracketing standards of 10 ppm(mg/L) K+ and 100 ppm K+ and sample of 500 ppm, prepared 5 mL diluted to 100 mL. Shouldn’t you adjust your upper limit bracketing standard, instead of just diluting your sample? Your original sample concentration of K+ (500 ppm) should not change adjusted for the dilution factor. What am I not understanding mathematically?

  • $\begingroup$ You have standards at 10 and 100 and your diluted sample will be about 25, ignoring units. That seems fine, so what is troubling you about it? $\endgroup$ – Ed V Aug 20 '19 at 12:43
  • $\begingroup$ Thanks for the feedback. I was trying to parse out the mathematics. I think I am confused as to how the software is reporting the data. I get that 5/100 would generate 500 ppm and 5/1000 would generate 50 ppm. However, I enter my dilutions into the software and I thought it is reporting the K+ in my original, undiluted sample. That, should not change. I should stick to calculating it myself instead of the software, ha. $\endgroup$ – Avocadonumber Aug 20 '19 at 17:32

As Ed said you are following the correct protocol by diluting the sample. The key reason is for doing so is to avoid a non-linear region in the calibration curve. This is why one would dilute the sample rather than make more concentrated standards. Several interesting effects happen with alkali metals at high concentrations in the flame or plasma. One of them is the concept of self-absorption.

Imagine you are monitoring the resonance line of K emission. Once you introduce lot of potassium atoms in the plasma (by using a concentrated standard) what may happen is that the ground state K atoms may absorb the light emitted by the excited potassium atoms? What is the result? You get less intensity than expected.

Try watching a sodium street lamp with a diffraction grating, or take a CD outside at night. You will see a prominent dark line in the emission spectrum of the lamp like this at 589 nm. This is the same self absorption phenomenon. There are so many Na atoms in the ground state that that they are self absorbing the light emitted by excited sodium atoms in the electrical discharge.

sodium lamp

  • $\begingroup$ This is very useful information. Thanks for taking the time. I did neglect the linearity of my standards and it make more sense to dilute the sample. I think I need a better understanding of how the software is reporting my data. $\endgroup$ – Avocadonumber Aug 20 '19 at 17:37
  • $\begingroup$ I always tell analytical chemists to take software output with a grain of salt. Modern softwares have become black-boxes. Check numbers manually in the good old Excel and see if they make sense. $\endgroup$ – M. Farooq Aug 20 '19 at 19:22

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