Trace Metals Analysis in Water-Methanol-Oil Mixtures by HR ICP-OES Application Note · PlasmaQuant 9100 Elite

Introduction

Water-methanol-oil mixtures in varying compositions are used as additive in internal combustion engines. Here, water injection, also known as anti-detonant injection systems (ADI), is used to spray water or a water/methanol mixture into the fuel/air mixture in order to cool the introduction system avoiding premature ignition of the fuel. This leads to improved behavior in terms of engine knocking and compression ratio in engines of aircrafts and racing cars. Results are increased power and fuel efficiency, improved power output for short duration such as increase of thrust of jets during takeoff, and reduction of NOX and CO emissions by decreased soot formation during the combustion process.

In ADI fluids, methanol serves as an anti-freezing agent and as combustible additive. The small fraction of water-soluble oil mainly acts as corrosion inhibitor but also serves as lubrication agent. Quality control of ADI fluids in terms of trace element analysis is required to ensure the absence of compounds that compromise efficiency or trigger corrosion and eventually cause technical breakdown of the engine. Therefore, the lowest possible specification limits (e.g., limits of detection) are of high interest for QC laboratories. The complexity of the matrix of ADI fluids, however, demonstrates a demanding challenge for atomic spectrometry methods such as ICP-OES. A highly robust plasma for reliable excitation of the sample is a pre-requisite to achieve high precision and accuracy of the results. Additionally, exceptional optical resolution is required to resolve spectral interferences originating from matrix-based (methanol/oil) unspecific emission.

In this study, an aqueous ADI solution containing 7% of methanol and trace levels of oil was submitted to a limit control analysis of Al, As, Bi, Ca, Cd, Co, Cr, Cu, Fe, In, Mg, Mn, Ni, Sb, Sn, W, and Zn by the PlasmaQuant 9100 Elite. Aided by its argon counter gas technology, the highly transparent optical path of the PlasmaQuant 9100 Elite provides the lowest limits of detection in ICP-OES. In particular for arsenic and tin with their most sensitive emission lines located in the UV-range, the absence of oxygen, air, or nitrogen benefits their increased detectability and quality RSD values. Additionally, spectral interferences originating form unspecific emission lines of the organic matrix (methanol, oil) are removed successfully by the high-resolution optics (FWHM < 3 pm at 200 nm) and/or the CSI software tool. Furthermore, the high plasma robustness of the high-frequency generator and the sample introduction system with its centerpiece, the V-Shuttle torch, allow for the analysis of high matrix samples with high accuracy and precision.