PH of dileunt and M.PH. for ciprofloxacin

[hossam kamal]

hi everybody ,
nowadays i am using a pharmacopoeial hplc method for analysis of ciprofloxacin HCl ,
mobile phase ammonium phosphate buffer (PH 3 ) : ACT 87:13
but sample dilutions are made by diluent ammonium phosphate buffer ( PH 2) : ACT 87:13
i can understand the reason for mobile phase ph ( 3 ) and its relation with ciprofloxacin dissociation constants knowing that ciprofloxacin molecule has -cooh gp and nitrogen on piperazinyl ring ( pKa = 6.09 (carboxylic acid group); pKa = 8.74 (nitrogen on piperazinyl ring)
what i cant understand is why the sample dilutions cant be in mobile phase ( ph 3) and what is the reason for the diluent ph ( 2 ) ,
we didnt use ph ( 2 ) as mobile phase as a protection of column packing form this PH ( hydrolysis of silanol gps, right ?
i hope if anyone can help me with any information
thanks to all

[aceto_81]

Don't know what Pharmacopea you use, but in the Ph. Eur. the diluent is equal to the mobile phase (pH 3.0)

Ace

[hossam kamal]

thanks alot for replying , it is according to USP 35