Diazepam has some solubility in water otherwise it couldn't act as a medicine. I'd expect ~0.3% (1g per 333 mL water). Depending on your method, cartridge, and amount of loading, you may see the compound in the wash. More information about the procedure would allow us to consider overload the cartridge, or the analyte being displaced by something else.

https://www.uspharmacist.com/article/diazepam-5-mgml-injection-32947#:~:text=Diazepam%20(Valium%2C%20C16H,C%20and%20135%C2%B0C.

If you check your analyte in solvent

through cartridge method you may get different when analyte with matrix it because of cartridge materials interaction with matrix and effect analyte recovery