Hi, I have a problem with determination of distribution coefficient (LogD) using LC-MS system. I am not able to obtain, for strong lipophilic reference compounds, higher value of  LogD (pH 7.4) than 2.5. In public data for example distribition coefficient for ketoconazol determined in pH 7.4 is equal 3.64, for simvastatin logD (pH 7.4) = 4.64. I based on shaking-flask method with octanol:buffer ratio 1:1 (v/v) and spectrophotometric detection. I tried change ratio of these solvents in different way (1:2, 2:1) but it did not help. Therefore I confirmed my specrophotometer results using LC-MS system. Does anybody know what could I do better or in different way to obtain higher value of distribution coefficient than 2.5?