Column temperature as an active variable in the isocratic, normal-phase high-performance liquid chromatography separation of lipophilic metabolites of nonylphenol ethoxylates

Normal-phase separation of technical grade nonylphenol (t-NP, about 90% 4-nonylphenol), 4-nonylphenol mono-ethoxylate (4-NPIEO) and 4-nonylphenol di-eihoxylate (4-NP2EO) was assessed, with the inclusion of column temperature as an active variable. The compound 2,4,6-trimethylphenol was evaluated for use as internal standard. Isocratic elution with 2-propanollhexanes mixtures from an amino-silica column and spectrometric UV detection at 277 mn were employed. Technical nonylphenol presented a significant contribution from unknown substances that eluted with retention times similar to that of 4-NPIEO. GC-MS analysis of the unknowns allowed to identify them as isomers of 2-NP. The response of the system to joint variations in flow rate, eluent composition and column temperature was investigated by means of Doehlert statistical experimental design. A model for retention of the analytes as a function of the experimental variables was proposed, and separation selectivity was studied. Selection of the optimal working zone was made through desirability function (D) calculations. Potential co-elution of 2-NP isomers with 4-NPIEO was considered when optimizing the separation. The occurrence of a restricted region of the experimental space where baseline resolution of analytes. associated impurities and internal standard results feasible (D:A 0) is apparent. (c) 2007 Elsevier B.V. All rights reserved.