Increasing Pressure

[lehminh]

I have a situation in which the back pressure is increasing with each injection. I am using 60% (25mM KH2PO4 buffer, pH 6.3) w/ 40% ACN on a C18 50x4.6, 3.5µ. Can anyone give me some help on identifying what the issue is and how to correct?

Thanks

[Mark Tracy]

There is no reason that your mobile phase should have a solubility problem. Most likely you have particulate matter in your samples. Filter them; if you already did, use a smaller pore-size filter. You didn't say what the samples are, but soluble or colloidal proteins and carbohydrates can precipitate on contact with the mobile phase; check this by a test-tube experiment. Possibly your mobile phase needs filtering. Less commonly, a filter element inside the pump may be needing a change.

Once you find the source, it will be a lot easier to decide how to treat the column.

[Mattias]

Try to premix the mobile phase (if you havn't done that already)

Some instruments cannot do this mix without precipitation. See Mark's comments otherwise.