On-line emulsification of lubricating oils in a flow-injection system for chromium determination by electrothermal atomic absorption spectrometry

Abstract

A simple and quick procedure has been developed for the on-line preparation of a one phase emulsion in a flow injection (FI) system for chromium determination by electrothermal atomic absorption spectrometry (ETAAS) with Zeeman-effect background correction. The entire FI system was controlled by a computer, independent of the spectrometer. A 1-ml plug of sample solution was injected into the carrier stream of hexane and was subsequently mixed with streams of 3.8% (m/v) sodium chloride, 5.0% (v/v) sodium dodecylsulfate (SDS) and 5.0% (v/v) of sec-butanol. The flow rates of the carrier, sodium chloride, SDS and sec-butanol streams were 1.0, 0.5, 0.5 and 0.2 ml min−1, respectively. The sonication of the flowing solutions improved the stability of the emulsion and provided quantitative recoveries. An emulsified sub-sample of 200-µl was collected in a capillary of a sampling arm assembly. Thereafter, 20 µl of this sample solution were introduced by means of positive displacement with air through a time-based solenoid injector (TBSI) in the graphite tube atomizer. The furnace program included two drying steps (at 110 and 130 °C) to minimize the sputtering of the sample in the atomizer. In all instances, the pyrolysis, atomization and cleaning temperatures of 1600, 2400 and 2500 °C were used to obtain good peak profiles with low background signals without the addition of any chemical modifier. The method provides a linear range for chromium from 7.0 to 50 µg l−1 and a limit of detection of 4.0 µg l−1, which corresponds to 6.0 ng g−1. The results were precise (0.6–0.8% RSD) and the recovery values found in the analysis of spiked samples ranged from 99.2 to 102.2%. The agreement between the observed and certified values obtained from two NIST Standard Certified Materials was good. The method described is satisfactory for determining chromium in new and used lubricating oil samples.