Trace Elements DRS clarification

dspence

I am slightly confused on how the Trace Element DRS uses external and internal standards to calculate trace element concentrations. For example, I have a sample with known Ni concentrations of ₃₀₀₀ ppm (using microprobe). I analyzed Ni60 on the laser, and when I run the DRS, I get Ni60_ppm concentrations of close to that value. Ni60 represents only 26.223% of naturally occurring Ni isotopes, so shouldn't this value be 26% of that? Unless that value is using the signal from Ni60 to calculate Ni concentrations independently of isotopic abundance. Would this make sense considering that most reference materials do not discriminate between isotopes?

Bence

Hi dspence

This is actually a pretty common question. Another form of this question that we are commonly asked is how do I get the right concentrations when using a very minor isotope of Ca such as Ca43, which only makes up 0.13% of calcium? You can insert whatever element you like, I just picked Ca because it's a fairly common internal standard.

The answer is that as long as you're measuring the same isotope in your reference materials (RMs) and your samples, and there is no large isotopic difference between the two, the isotopic abundance cancels out. That is, using Ca as an example again, if you're only measuring 0.13% of the calcium in your RM, and you're only measuring 0.13% of the calcium in your sample, it all works out.

If there is a large isotopic difference between your RM and your sample, you get situations as discussed here.

If I haven't answered your question, please just let me know.

Bence

dspence

Hi Bence,

I think that makes sense, though I am still confused about the output. In the Ca43 example, is the output being shown in Ca43 ppm or total Ca ppm? I guess it would make sense that it would be total Ca based on what you are saying, I am just trying to get it straight in my head.

Cheers.

Bence

I see what you're saying. Yes, that is a bit confusing, but instead of thinking of it being the concentration of Ca43, perhaps think of the output as the concentration of calcium, based on the Ca43 channel. That's why we include that bit in there. If you have measured two isotopes of the same element, you can check that they both give the same approx concentration because the output channels will be labelled according to the input channels they were calculated from. If the two are not approximately the same, there could be an interference on one of the masses etc.

dspence

Bence That totally makes sense, I was definitely thinking about that incorrectly. Thanks for the help!