Abstract
A prototype analyzer for the direct LIBS analysis of nebulized liquid samples was developed and tested, particularly for the analysis of petroleum, organic solvents and aqueous solutions. The LIBS analyzer requires about 750 μl of liquid sample, 1 l of N2 gas, and 10 s of the analysis time to record 100 spectra. The limits of detection in oil and solvent are as low as 0.01–0.04 ppm for Li, Mg, and Cu. They increase for the difficult elements, such as Pb and Hg (7–10 ppm), Cl (250 ppm), and S (~0.7%). The relative standard deviation of measuring 100 ppm vanadium in oil and solvent was 1.5%. The LIBS detection limits and repeatability are better than required by the standard method ASTM D5185 for the analysis of lubricating oils in ICP-OES. Several petroleum samples were analyzed by LIBS and the quantitative results for V, Ni, and Fe compared to the ICP-OES data. Light crude oils can be nebulized and analyzed directly. Medium crude oils may require minimal dilution at least 1:1, otherwise errors of determination may become large. Presumably, utilization of the internal standard and chemometrics can be useful to correct for the matrix effects. In addition to the trace element analysis, the LIBS prototype demonstrated ability to measure the hydrogen-to‑carbon ratio in organic liquid samples. Several unidentified features were observed in the carbon spectrum. Their possible origin is discussed.
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