Hi @woodsmn
The ppm values in iolite are calculated according to the equations set out in Longerich et al (1996). Ca is commonly used as an internal standard to account for ablation differences between the reference material and the sample. The ablation yield factor is unitless and the amount of calcium in your sample is not included in the ppm value. However, because you provide the concentration of Ca in your sample, there is no calculated Ca ppm value (or channel). It would be circular reasoning to provide the value and then calculate it.
To convert to mmol/mol for Mg/Ca, you could use a stoichiometric value for Ca of 40.04 wt% (400,400 ppm) or whatever value you used when setting up the DRS, making sure you convert to ppm so that your units are the same. To convert your Mg ppm values, you would divide by the molecular weight ratio of Mg to Ca (i.e. 24.305 g/mol / 40.078 g/mol, = 0.606) and then multiply by 1000 to convert mol to mmol. In iolite's Calculator, this would look like:
Mg24_ppm / 400400.0 / 0.6064 * 1000.0
I hope that helps. Please let me know if you spot any errors in my calculations above.
Kind regards,
Bence