Figure 6-10. Selectivity control by altering pH. Column: 50 mmx4.6 mm, 2.7 mm HPH Poroshell-C18; mobile phase A: 10 mM ammonium formate (pH 3), ammonium acetate (pH 4.8), or ammonium bicarbonate (pH 10) in water; mobile phase B: acetonitrile, gradient: 10%-90% B in 5min, hold 2 min at 90%; flow rate: 2 mL/min; detection UV absorbance at 254 nm; temperature 30^oC. Peaks: 1=procainamide, 2=caffeine, 3=acetyl salicylic acid, 4=hexanophenone degradant, 5=dipyrimadole, 6=diltiazem, 7=diflunisal, and 8=hexanophenone.

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