I am running a Icpms 7900 for drinking waters. Lately I have been cleaning the cones once a week since I keep getting white residue on both the sample and skimmer cone. My oxide ratio also are getting up to 1.5% and my doublely charged can get up to 20%. After cleaning the numbers go down to less than 1% oxides and about 3 to 5% for doubly charged ratio. But then the numbers start to increase the very next day and within a week are to high again. I put in new cones last week and got the same result. Any ideas?