Trace Element DRS and internal standard

sjaret

I have a few questions about the Trace element DRS
1) When selecting the reference materials for normalizing, how exactly does the program determine the intensity values? Is it just averaging between the first and last of what is selected? Or is it a more complicated than that?
2) Does the IS method also first normalize the external standards and then perform the internal standard normalization? (and similarly to my first question, does the internal standard average between the first and last part of the selected peak? I assume it is accounting for decreases in signal as you ablate down, but I'm not sure where within the peak it's choosing as the value to use).

Thanks,
Steven Jaret

Bence

Hi sjaret

We cover the trace element calculations in detail in our Basics and 3D Trace Elements DRS (the latter in particular goes into quite a bit of detail) but to answer your questions:

1/ When selecting the reference materials for normalizing, how exactly does the program determine the intensity values?

iolite calculates a concentration value for every data point within an experiment. It uses the Longerich et al equation. In that equation, it uses the spline for the baseline subtracted counts for the external reference material as the value for the counts for the RM in the Longerich et al equation. For example, when calculating a concentration for Sr88, it will use the spline of the RM for Sr88_CPS as the RM count rate. Depending on what you've chosen for your spline type, this may just be a single value, or it may be varying quite a bit.

2) Does the IS method also first normalize the external standards and then perform the internal standard normalization?

Similarly, the way iolite uses the IS is described by the Longerich et al equation (except if you're doing sum normalisation, where I would refer you to Liu et al., 2008, also discussed in the 3D Trace Elements webinar). The IS is basically a yield correction factor, and does not change within an ablation. When you set the IS value for a selection, it uses that IS value for all data points within the selection. Parts of the ablation that are outside the selection will have NO IS value, and a final ppm value will not be calculated for those data points.

I hope that helps. It's always good to get these finer details cleared up. If you (or anyone else) has any other questions about the DRS, please let us know.

-Bence

sjaret

Bence
Thanks, Bence. This helps. The background of why I'm asking this is that one of the grad students I work with and I are digging into the very nitty-gritty details surrounding reference materials and choices for best results. She has meteorite samples which are a whole range of phases and sizes, so we have lots of variables. We are also limited with what we have and in some cases the samples are very low concentrations. The question we stumbled into is whether or not we need to perfectly match spot sizes of standards and samples. Obviously, it's better to match, but we have so many phases that we're looking into whether or not we can come close (for example, can we use 40 micron standards for 35 or even 30 micron samples). We have done a whole series of tests on the USGS glasses looking at what happens if we do and don't use the same size for both samples and reference materials. It is interesting but seems like even using IS this difference can matter a lot (maybe even more than we would have anticipated). In any case, we want to make sure we really understand the calculations. I may have other questions later, but this helps a lot for now.

Thanks,
Steven