Stability study of parabens in pharmaceutical formulations containing Paracetamol or Carbocisteine by high-performance liquid chromtatography

A new, simple, specific, sensitive, rapid, accurate and precise RP-HPLC method was developed for the proportioning of the methylparaben and the propylparaben in pharmaceutical formulations as well as the phenol, which is their main degradation product. These parabens are widely used as preservatives in pharmaceutical and cosmetic products. The separation was performed on a C18-column using a mobile phase elution gradient (Acetate buffer 50mM pH 5/Acetonitrile). The detection was performed at a fixed wavelength (l = 271nm), with a flow rate of 1.3 ml/min. The detector response was linear in concentrations ranging from 10 to 60 μg/ml for the methylparaben, from 3 to 15 µg/ml for the propylparaben and from 0.125 to 2.5 µg/ml for the phenol. The intra and inter day variation was found to be less than 2%. The limits of quantification and of detection of the phenol are of 0.1 µg/ml and 0.03 µg/ml, respectively. The recovery ratio was equal to 101.8 % for the methylparaben, 101.2 % for the propylparaben and 98.4 % for the phenol. The proposed method was used to monitor the degradation of the parabens and the production of the phenol in studied solutions

N′-(3-Methyl­quinoxalin-2-yl)-N′-phenyl­benzohydrazide

In the crystal structure of the title compound, C22H18N4O, the quinoxaline system makes dihedral angles of 86.59 (7) and 63.37 (9)° with the benzohydrazide and phenyl rings, respectively. The benzohydrazide ring makes a dihedral angle of 72.46 (10)° with the phenyl ring. The crystal structure is stabilized by inter­molecular N—H⋯O hydrogen bonds, C—H⋯O contacts and C—H⋯π inter­actions