Those new to Iolite will notice that there are two version of the Trace elements DRS, called simply Trace_Elements and Trace_Elements_IS. Which one do I use?
Well, if you are analysing a whole bunch of samples with similar major element composition (e.g. speleothems or corals where you can reasonably expect stoichiometric behaviour), or you are simply doing a laser traverse or map you just need a fixed internal standard value, say 0.4004 for Ca (this is in fact the default setting ). For this you can use the Trace_Elements DRS and, if necessary, change the IS value in ‘edit settings’. Your final export data will be reported relative to this value.
However, let’s say that you have a whole bunch of pyroxenes or feldspars to analyse. You can still use the Trace_Elements DRS but it is likely that the Ca content will be slightly different for each mineral grain that you analyse. You could export and then play around for an hour or so in excel trying to make everything right but the far simpler solution is to use the Trace_Elements_IS routine.
Ideally you would set your samples up on the electron probe and determine the major element compositions and then transfer the digitised coordinates to your laser system to ablate the same spots. This should then give you a different (but correct) internal standard value for each laser spot analysis. Now…to processing.
Once you have selected the Trace_Elements_IS DRS, and selected baselines and standards, everything proceeds as normal but when you hit the ‘Crunch’ button you will be presented with a new window:
This Trace Element Control panel has a number of buttons. Pressing ‘Export Sample Name Table’ will generate a template in to which you can enter all the internal standard data (for multiple elements if required) for each sample – this is then re-saved as tab-delimited text.
You can load this file back in to Iolite using the ‘Reload Int Std Info’ button and choosing this file. Now results for each sample will be calculated according to the IS values contained in the table and using the element specified as the Internal Standard in the ‘Edit Settings’ dialog. The ‘Show Int Std data’ button can be used to view the IS data in current use – note all values in these tables to be in wt% element.
That’s pretty useful – we are now using different IS values for each grain BUT what if you have a whole bunch of different minerals and you need to use different elements as internal standard for each mineral? This is also straightforward.
After exporting data for some minerals (let’s say cpx and plag using Ca as internal std) we then simply recast the experiment with a different internal standard (let’s say Ti since we want to process some titano-magnetites) and using a different calibrant with higher Ti content than the NIST 612 that we used for the plag and cpx.
First, in the ‘Active Integration Type’ drop-down we can choose another calibration material – in this case ‘G_BCR2G’ (in the absence of an opaque oxide reference material this is a good choice of calibrate since it contains a reasonably high and well defined level of Ti). Then we simply add a few BCR2G integrations from the dataset. Now go in to ‘Edit Settings’ and change the Index Channel to ‘Ti49’, and Reference Standard to ‘G_BCR2G’ . Saving these settings should bring up the Trace Element Control Panel once more and you can now see that Ti is being used as the internal standard and the calculated results are now referenced to BCR2G. The oxide data should now be much more realistic than when you crunched the data with Ca as internal std and can be exported as usual.
So, it should be possible to process multiple minerals using multiple internal standard elements and multiple values for these in a relatively short period of time.