Abstract:
Microextraction sample preparation methods are miniaturized formats which contributes to green approaches in Analytical Chemistry. They are mainly characterized by the minimum use of organic solvents.
The aim of this research work was to develop environmentally friendly sample preparation methods followed by inductively coupled plasma optical emission spectroscopy for the determination of mercury in fossil fuels and their selected derivatives. Three different sample preparation methods were developed and applied for this purpose. These three different sample preparation methods were microwave-assisted hydrogen peroxide digestion, vortex assisted deep eutectic solvent based dispersive liquid-liquid microextraction and ultrasound-assisted magnetic dispersive solid phase microextraction.
Multivariate optimization tools were employed for the optimization of the most influential parameters for the three different sample preparation methods. For microwave-assisted hydrogen peroxide digestion, the optimized parameters were sample mass, digestion time, temperature, hydrogen peroxide concentration and methionine concentration. The vortex-assisted deep eutectic solvent based dispersive liquid-liquid microextraction optimized parameters were sample pH, extraction time, extractant volume, disperser solvent volume and centrifugation time. Finally for ultrasound-assisted magnetic dispersive solid phase microextraction, the optimized conditions were adsorbent mass, sonication time, pH, eluent concentration and elution time. The analyses were conducted using inductively coupled plasma-optical emission spectroscopy.
The three methods were validated by analyzing NIST SRM 2778 with certified mercury concentration levels of 38.98 μg/kg ±1.10 μg/kg. The recoveries obtained were 93-107 %, 99.9 % and 105 % for microwave-assisted hydrogen peroxide digestion, vortex-assisted deep eutectic solvent based dispersive liquid-liquid microextraction and ultrasound-assisted magnetic dispersive solid phase microextraction, respectively. The limits of detection and limits of quantification obtained were 0.25 μg/L and 0.80 μg/L for microwave-assisted hydrogen peroxide digestion respectively. For vortex-assisted deep
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eutectic solvent based dispersive liquid-liquid microextraction, they were 0.025 μg/L and 0.083 μg/L and for ultrasound-assisted magnetic dispersive solid phase microextraction were 0.035 μg/L and 0.119 μg/L, respectively. A good precision of less than 5 % was achieved for all the three different sample preparation methods.
The validated sample preparation methods were applied in real fossil fuels which were coal and crude oil. The methods were also applied in real crude oil derivatives which were gasoline, diesel oil and kerosene. Coal samples were obtained in triplicates from a coal mine in Mpumalanga, South Africa, while crude oil samples were obtained in triplicates from a petrochemical company in Johannesburg, South Africa. Crude oil derivatives were obtained from three local filling stations around Johannesburg, South Africa. The concentrations obtained were 0.876±0.023-0.975±0.025 μg/g in coal samples, 0.383±0.043-0.510±0.09 μg/g for crude oil samples, 0.306±0.010-0.390±0.035 μg/g for gasoline samples, 0.360±0.003-0.434±0.050 μg/g for diesel oil samples and 0.09±0.09-0.098±0.02 μg/g for kerosene samples.
The three different microextraction sample preparation methods were successfully developed and applied in fossil fuels and their selected derivatives prior to quantification of mercury using inductively coupled-plasma optical emission spectroscopy.
