Azido impurities by APCI-LC-MS

Hello everyone.
I have been developing an analytical procedure of AZBT in sartan drugs by APCI-LC-MS and have found that when using high concentration of API (~ 20mg/ml), the recovery is unexpectedly high, about 150%, in different level. This phenomena correlated with the concentration of API in sample solution. The AZBT content in API <LOQ by the same procedure. Can anyone help me solve this problem? Thank you very much.