i have a Dataframe with multiple columns like

df3=pd.DataFrame({"FACHEXPERTISE":["AQ01","AQ03","AQ05"],"ZUSATZBEZEICHNUNG":["ZF14","ZF45","ZF13"]})

and a second Dataframe which contains all possible combinations:

QUALI=pd.DataFrame({"FACHEXPERTISE":["AQ01","AQ01","AQ01"],"ZUSATZBEZEICHNUNG":["ZF15","ZF30","ZF40"]})

Now i want to check if all combinations in my df1 are valid and if not i want to delete the row.

The only way i could think of was something like this, but i always get an error that my str object does not have a .isin function:

df3.FACHEXPERTISE.apply(lambda x: x.isin(QUALI[QUALI.FACHEXPERTISE==x].ZUSATZBEZEICHNUNG))

could someone suggest another way?
Thanks in advanced