I'm using RP-HPLC DAD and one of my samples is Diltiazem, I've used formicacid : ACN as mobile phase but it did not work and at the same time I want to avoid using phosphate buffer.
Try ammonium formate as a buffering agent, rather in higher pH range (8-10) to avoid amine ionisation. Be careful of low ammonia formate solubility in ACN.
I do not know in which matrix you want to analyze Diltiazem, but it should not be any problem on C18 or C phenyl bazed stacitonary phases with gradient acetonitrile in water (acidified with 0.1% of formic acid). I use Hypersil gold or HypersilGold aQ from Thermo Scientific, C18 Cogent Bidentate from MicroSolv Technology Corporation. For detailed chromatographic conditions see attached link.
http://www.sciencedirect.com/science/article/pii/S0039914012007370
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