The separation and simultaneous determination, by high-performance liquid chromatography, of
acetaminophen (I),
guaifenesin (II),
pseudoephedrine hydrochloride (III), and
pholcodine (IV), together with a series of
parabens (methyl to butyl, V-VIII) in a
cough mixture, has been demonstrated using a chemically bonded
octadecylsilane stationary phase with a mobile phase of
methanol-water-
acetic acid (45:55:2) containing the ion-pairing agent octanesulfonic
acid. Retention volumes for the active ingredients were 3.8 ml, 5.4 ml, 9.4 ml, and 15.6 ml for compounds I-IV, respectively. Corrected retention volumes for the
parabens [5.4 ml for methyl (V), 9.6 ml for ethyl (VI), 18.5 ml for propyl (VII), and 37.9 ml for butyl (VIII)] showed an exponential relationship with chain length of the esterifying
alcohols.
Excipients did not interfere with the estimation of any of the compounds, hence pretreatment of the sample was unnecessary. Average recoveries of the active ingredients and of the
parabens from laboratory prepared samples were essentially 100% of theoretical with standard deviations of 1.7, 0.3, 1.5, 0.3, 0.3, 3.3, 0.7, and 2.7% for I-VIII, respectively.