change from mass to molar units

Dear all, I have fitted a simple two-compartment model with linear elimination to some compound data (ADVAN4 TRANS4). Everything worked well. Because I want to extend this model to account for metabolite data as well, I now wanted to change all data to molar units, i.e. I changed the AMT and DV column in the same way. To my understanding, if I change both these columns, the obtained parameters should end up being the same. However, when I use mg and µg/mL the model runs just fine, whereas when I use µmol and nmol/L the parameter estimates change completely and always run into some boundary. Can anyone shed some light on why this happens? Is there any way to get around this? Could I keep the mass units and do some changes in the control stream when incorporating the metabolite data? Any help would be highly appreciated. Best regards Nele _________________________ Nele Plock Bayer Schering Pharma AG GPD/Pharmacokinetics Metabolism & Bioanalysis D- 13342 Berlin Phone : +49-30-468 15146 Fax: +49-30-468 95146 [EMAIL PROTECTED]