Determination of five selenium compounds in urine by liquid chromatography with focused microwave assisted digestion and hydride generation–atomic absorption spectrometric detection
Abstract A method is proposed for the determination of trimethylselenonium ion, selenomethionine, selenocystine, selenite and selenate in urine, following clean-up with C18 cartridges. The on-line system developed consists of an anion-exchange chromatographic column for species separation, a focused microwave oven for the oxidation of organic Se species in the presence of 3% K2S2O8 and 3% NaOH solution and reduction of selenate to selenite in 10 mol lÿ1 HCl, a hydride generation system for the conversion of selenite to selenium hydride and ®nally an atomic absorption spectrometer detector. Optimum chromatographic conditions were obtained in a Hamilton PRP-X100 column, using 100 mmol lÿ1 phosphate buffer (pHˆ6.8) as the mobile phase. Detection limits were between 3 and 8 mg lÿ1. The relative standard deviation was <7% at 100 mg lÿ1 Se. The short-term stability test of the studied Se species in urine shows after 5 h of storage about 30% and 60% losses of selenite and SeCys, respectively. After taking pills containing Se about 60% of the Se ingested is excreted in the ®rst 12 h. Se(VI) ingested is partially excreted as Se(VI) and partially transformed and excreted as a species that behaves as SeCys. SeMet is metabolised and excreted as species that behave as Se(VI) and SeCys.
Focused microwave-assisted extraction of polycyclic aromatic hydrocarbons and alkanes from sediments and source rocks
Abstract Focused microwave-assisted extraction (FMW) of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes from sediments and rocks was compared to conventional extraction methods such as Soxhlet extraction. With the FMW technique, extraction time is reduced to 10 min and the solvent volume is small (30 ml). The extraction procedure and analysis was validated on a certi®ed marine sediment standard reference material, then applied to four source rocks of di€erent origins, compositions and analyte concentrations. Recoveries of individual compounds were calculated for both FMW and Soxhlet methods and the results suggested that there was no analyte selectivity. The moisture content of the sample matrix had a positive e€ect on the extraction recovery of some PAHs with a maximum increase of 30%, whereas n-alkane extraction seems less a€ected by this factor. The conventional biomarker parameters determined after FMW extraction were the same as those found after Soxhlet extraction. A study of matrix e€ects on the extraction recovery showed that the recovery of analytes was near 100% if the matrix was ground and above 74%, if the matrix was used without prior grinding
Modeling energy transfer in a focused microwave digestor
Abstract A physical model has been developed for studying microwave energy transfer and to predict temperature evolution of a liquid sample placed in an open microwave heating digestor. The thermal system was considered as involving three different media having different properties, i.e., the liquid sample, the glass flask and the microwave cavity. Energy balance equations were written taking into account conduction, convection and radiation heat transfer. The resolution of the differential equation set obtained gave the predicted temperatures. Experimental measurements of temperature were carried out with a thermocouple probe for several types of reagents classically used in wet ashing techniques. They were compared to predicted temperatures, and a Chi-squared parameter was used to estimate the fit of the model. The good results obtained demonstrated that this model can predict correctly the temperature of reagent heating in the presence of a microwave field. The addition of organic material did not significantly modify the results. It also produces abaci for predicting the time necessary to heat a reagent up to a given temperature. The model also gives a numerical estimation of the microwave energy loss during the transfer from the magnetron to the sample. It is then possible to explain some major differences observed between microwave ashing technique and conventional heating ashing technique, mainly the important reduction of time.
Optimisation of the extraction of arsenic species from mussels with low power focused microwaves by applying a Doehlert design
Abstract The extraction of arsenic species from mussel tissues is performed by using low power focused microwaves. The arsenic compounds are separated with anion-exchange liquid chromatography and detected by inductively coupled plasma mass spectrometry (ICP-MS). The extraction is optimised by applying a sequential Doehlert design, which allows us to quantify the in¯uence of the microwave power, the exposure time and the relative proportions of methanol/water in the extraction medium. Well correlated second-order models are ®tted to determine the extraction yields of arsenobetaine (AsBet), a probable arsenosugar (AsS), monomethylarsonate (MMA), dimethylarsinate (DMA) and As (V). By using focused microwaves the time of sample preparation is much shorter than in other methods. At powers close to 40 W with exposure times less than 4 min in methanol/water (55/45), the whole extraction yield reaches 85%. The proportion of species with respect to the total arsenic is 68% of AsBet, 14% of AsS‡MMA and 2% of DMA . Optimisation of the extraction of arsenic species from mussels with low power focused microwaves by applying a Doehlert design
Rapid determination of chemical oxygen demand using a focused microwave heating system featuring temperature control
Abstract This paper demonstrates the use of a microwave heating system, employed in the chemical digestion step, for the determination of chemical oxygen demand in wastewater. The results are first compared with those provided by standard methods using reference substances. The problems arising from abrupt heating of the sample and the potential thermal decomposition of potassium dichromate are examined. Two different approaches to sample digestion involving a gradually increasing irradiation time were tested. First, a constant power strategy is applied, and the second proposes a constant temperature approach by using a temperature control system. By optimising the operating conditions, the digestion time was reduced to 8–60 times with respect to the standard method. The reference digestion time is 5 min. In especially difficult digestions, the proposed approach provides a substantially improved degradation with respect to conventional procedures. The procedure was applied to wastewater from various industries and found to ensure thorough digestion of all samples and to provide favourable results in all cases tested.
Study of factors affecting on the extraction efficiency of polycyclic aromatic hydrocarbons from soils using open-vessel focused microwave-assisted extraction
Abstract As part of an evaluation of focused microwave-assisted extraction (FMAE) using an open-vessel system, the effects of matrix, moisture content, ageing, and solvent have been studied on the extraction efficiency of polycyclic aromatic hydrocarbons (PAHs) from spiked soils. PAHs were spiked onto three different uncontaminated air dried and originally wet soil matrices with 1- and 20-day ageing periods. Solvents used were hexane–acetone (1:1), cyclohexane–acetone (1:1) and dichloromethane. FMAE only required a small amount of solvent (20 ml) and short extraction time (10 min) in the open cell under 90 W of microwave power. The results revealed that the extraction efficiency strongly depends on the nature of soil matrix; moisture content may enhance the recoveries of PAHs for many cases; and, the influence of the type of solvent is not significant. A comparison between microwave extraction and 16-h Soxhlet extraction has been made on spiked soils. It evidenced that the microwave method under ambient pressure is a suitable alternative to Soxhlet method for the analysis of PAHs in soils. For the evaluation of the developed FMAE method, three reference materials were used. The PAHs recovered from three reference materials were in a good agreement with reference values.
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