If you need to use both acid (e.g. formic) and ammonium salts (acetate or formate) with a Cogent Diamond Hydride™ HPLC column for your samples we recommend dedicating one column for use with mobile phases containing ammonium and another for use with mobile phases containing acids.

The rationale in this is that once you introduce ammonium acetate or formate to the column, it semi-permanently alters the chromatography for specific analytes. So a column that is used for a formic acid method should not be used for ammonium acetate methods and vice versa as a general rule of thumb.  One example is that of phenylalanine and phenylglycine, where selectivity between the two is known to decrease after introduction of ammonium acetate to the column.

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