Below I've included references to lines in the python code that is available here (https://github.com/iolite-LA-ICP-MS/iolite4-python-examples/blob/master/drs/Sr_isotopes.py). These line references assume you haven't modified the code at all.
The DRS begins by subtracting the background from each channel (lines 77 - 81). It then calculates a preliminary fractionation factor
PFract from the observed intensities on m/z 88 and 86 (line 99 in the code). This fractionation factor is used to remove the interference of 46Ca40Ar and 48Ca40Ar on masses 86 and 88, respectively (lines 100 and 101). An updated fractionation factor
Fract is then determined from the interference corrected 86 and 88 masses (line 102). The Rb87 interference on mass 87 is then removed (lines 103-105), using Rb85 and the new fractionation factor. There is an option to adjust the Sr/Rb fractionation factor (that is, if you believe that Rb is not fractionated to the same extent as Sr, you can adjust it using the 'Rb Bias' variable).
The 42Ca40Ar inteference on Sr84 is then subtracted in the same manner (lines 106-108). Similarly, there is an option to adjust the CaAr fractionation factor from the observed Sr fractionation factor using the 'CaAr Bias' variable .
The 88 and 86 masses are interference corrected again (similar to lines 100 and 101) but now with the updated fractionation factor
Fract (lines 109-110).
With the interference corrections completed, the DRS then calculates several corrected and uncorrected ratios (lines 115-119). In this case, 'uncorrected' refers to ratios using the non-interference corrected channel values. For example, 'Sr8786_uncorr' is the ratio of mass 87 to mass 86 before CaAr interference correction, but mass fractionation corrected using the final
Fract factor. The '_corr' ratios use the interference corrected channels, with the final
Fract factor. Estimates of the Rb contribution to m/z 87 and CaAr to m/z 84 are also calculated (lines 120-121) as well as the total Sr signal (the sum of the correct Sr84, Sr86, Sr87 and Sr88 channels).
Finally, the DRS normalises the corrected 87Sr/86Sr ratio to the accepted value for the reference material (line 145) to create the channel
StdCorr_Sr87_86. This is done by multiplying the corrected 87Sr/86Sr values by the accepted value of the standard and dividing by the spline of 'Sr8786_corr'.
If you have any questions about the above, please just let me know by replying in this thread.
We hope to release a new DRS that combines the CaAr and REE interference corrections soon. Just waiting on a collaborator's go-ahead.