for the quantification of amino acid, I am using a C18 column. Mobile phase is 40mM phosphate puffer pH 7.8 (channel A) and acetonitrile/ methanol/ water 45:45:10 (channel B). . The method worked really well, but recently I have problems with too high pressure in the HPLC system (Agilent 1260). I have disconnected my column from the system, and the pressure remained high. I would assume that I have salt precipitation in the system. Therefore, I have used warm water to flush the system without the coulumn. The pressure is still very high.