He is to eliminate the interferences. For example, to get rid of ArO when measuring 56Fe. Did you use Ar as the collision gas in 2019? That's very weird...

In general whether you can get the same results would depend on elements, concentration, storage, etc.

Initial counting was done on ICP-MS set on standard mode and standard uses Argon gas, in 2019. The nuclides of interest were/are U-238, Th-232 and K-40. Since the very same samples prepared in 2019 were analyzed on gamma spectroscopy, Uranium and Thorium results were in agreement. Then I noticed of very low concentration of potassium on ICP-MS, which I have decided to recount the samples but now with a different gas (helium) and the instrument uses this gas with kinetic energy distribution (KED) this year 2021. Now the results for all elements differ and I don't understand why. No new samples were digested, but the old digested samples stored for more than a year were counted on Helium (KED).

Dear Vaino Indongo ,

There should have been no reason to change the U, Th, and K from your original sample, as far as the hydrolyses were prevented along the time.

A year would be a reasonable time to have some slow cation hydrolyze, changing the relative amount. If the digested sample was conditioned in plastic containers due to gas permeation, it might lose some acidity.

At ICP-MS, only the soluble part is aspirated. The hydrolyses product could be small, not visible; nevertheless, the measurement method is significantly sensitive.

Before the new measurement, it is good to repeat the digestion procedure, now with the stocked solution. If some hydrolyses happen, the cations are going to be re-released to the solution.

Best regards,

WNM

Wagner Da Nova Mussel , my last resort is to re-digest the very samples and analyses again. The other one is to prepare potassium chloride solution and analyze again for quality assurance purpose to verify with the normal standard solution used in the lab.

You also mentioned about the digested samples being stored in plastic containers may lose some acidity. Yes they were kept/stored in plastic containers for all that duration. That could also affect, I suspect! I will revert back to you after. Thanks!

Vaino Indongo if I have interpreted your response correctly, you are using He as the KED gas, which collides with a fraction of the K ions and stops them reaching the detector. If you didn't use this in 2019, but have done so in 2021, then it is quite likely you are reducing the number of K ions reaching the detector, hence getting a lesser value for K. How are you measuring 40K in an argon-based plasma when >99% of this is 40Ar?

If your samples were stored appropriately, then you should get similar results to the initial analysis, however: evaporation, hydrolysis, calibration solutions, internal standards, sampler cones, RF generator & detector aging, pump tubing, argon supply etc have all had an effect on your samples and instrument, so I would not expect 100% agreement and be quite surprised if you did.

Dear Vaino:

Theoretically, for changes in measurement conditions and behaviour of the instrument, cones, use of KED mode, etc.. you can not expect to get the same "signal responses" on two different days. However, our fundamental assumption is that any such changes affect the samples and the calibration standards the same way, which is the main reason why we do a calibration for each measurement run. Then, regardless of the differences in "responses" or the absolute signal on different days, calculated concentrations based on corresponding calibrations should come out similar (within the statistical differences). So, do not make any quantitative comparisons of absolute intensities from the two days, other than the calculated concentrations.

- If your interference corrections (KED, etc.) affected samples and the standards differently on the two days for K, then the above assumption is not valid.

- If your measured values for U, Th are close to the Limit of Detection, there can be significant statistical differences that have to be tolerated.

- Usually, K is measured using K-39 (93.3% abundance), with possible interference from Ar38H1. Are you sure it was measured at mass 40, which is usually disallowed due to huge interference from Ar40?

Best wishes,

Nimal

The concentration of the samples may not give exactly what you got early due to evaporations of liquid. This is a sensitive instrument and even after one month samples may not give the exact concentrations.

it is always prepare fresh solutions.

It will all depend upon matrix of the samples and its storage. Always it should be freshly prepared