Analysis of biodiesel by argon–oxygen mixed-gas inductively coupled plasma optical emission spectrometry

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Abstract
The determination of Ca, Cl, K, Mg, Na and P in biodiesel samples by ICP optical emission spectrometry was investigated using a new ICP spectrometer with CCD detectors and radial plasma viewing. Prior to analysis the samples were diluted 1∶4 (m/m) with kerosene. The effect of added oxygen to the intermediate gas, outer gas and nebulizer gas was studied. An overall increase of the signal to background ratios for Na I 588.995 nm, Na I 589.592 and K I 766.490 nm was observed due to reduced background emission of carbon and carbon compounds when oxygen was added. The addition of 0.35 l min−1 oxygen and 0.8 l min−1 argon to the nebulizer gas flow was found to be the most efficient in order to improve the detection limits for Na and K. The application of this cold argon–oxygen plasma enables Na and K determination at µg kg−1 concentrations in biodiesel. The limits of detection for Na I 588.995 nm, Na I 589.592 nm and K I 766.490 nm after the dilution of biodiesel with kerosene are 1.6, 1.4 and 7.1 µg kg−1, respectively, compared with 59, 74 and 220 µg kg−1, respectively, calculated for typical ICP operating conditions. The determination of environmentally relevant organic chlorine at low mg kg−1 levels is possible using the intense spectral lines in the vacuum ultraviolet spectral range at 134.724 or 135.166 nm. Limits of detection for these lines are 0.4 and 0.9 mg kg−1, respectively, after sample dilution as described above.

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