Determination of Cu and Fe in fuel ethanol by ICP OES using direct sample introduction by an ultrasonic nebulizer and membrane desolvator

The aim of this work was the determination of copper and iron in fuel ethanol by inductively coupled plasma with optical emission spectroscopy (ICP OES) coupled to an ultrasonic nebulizer with a membrane for desolvation (USN-MD) without further sample pretreatments. The USN-MD-ICP OES methodology was optimized and the analytical performance parameters evaluated. Calibration curves were constructed using solutions in absolute ethanol, allowing the use of aqueous standard solutions. The different water contents of anhydrous and hydrated ethanol lead to different nebulization efficiencies, therefore the water content of the standards used to construct the calibration curves should be similar to the water content of the samples, in order to obtain accurate results. For example, a water content of about 7% (w/w) caused the loss of almost 30% of analyte signal intensities in ethanol solutions compared to samples prepared in absolute ethanol. Low detection limits between 0.1 and 0.5 µg kg−1 for Cu and Fe and good method precision (2–5%) were obtained. Finally, the methodology was validated by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) and aqueous Certified Reference Materials (CRM) analysis in ethanol.