A three-factor Doehlert matrix design in optimising the determination of octadecyltrimethylammonium bromide by cation-exchange chromatography with suppressed conductivity detection

A simple and effective chromatographic method with suppressed conductivity detection was developed and validated to determine dissolved samples of octadecyltrimethylammonium bromide (C18H37N+Me3Br−, ODTAB) for purity testing. A response surface methodology generated with a Doehlert matrix design was applied to optimize the chromatographic and detection conditions in ion-exchange chromatography (IEC) with conductivity detection in the chemical suppression mode. A three-factor Doehlert design was performed to fit a second-order model and jointly optimize the peak intensity and shorten analysis time through a global desirability function. Regenerant flow rate, volume fraction of acetonitrile in the acidic eluent and its flow rate were studied at seven, five and three levels, respectively. The optimized separation and detection conditions were accomplished by using a cation-exchange column eluted at 0.5 mL min−1 with an isocratic mobile phase composed of CH3CN and 25 mN H2SO4, 82/18 (v/v). Chemical suppression of ionic conductivity was performed by 100 mN tetrabutylammonium hydroxide (TBAOH) as a regenerant at a flow-rate of 4.0 mL min−1. Remarkably good agreement was found between predicted and experimental values of signal intensity and chromatographic retention. With the developed method, a linear calibration curve of ODTA+ as bromide salt from 5 to 1000 ppm was obtained using hexadecyltrimethylammonium bromide as internal standard. The estimated limit of detection was 0.3 ppm (S/N = 3). The effectiveness of electrochemically suppressed conductivity detection of ODTA+ was also demonstrated, thus making easier the whole detection operation and instrumental needs as well.