On-line hydrogenation of biodiesel-Toward a reference method for the determination of glycerides and total glycerol.

A reliable method with ensured traceability of the measurement results for free and bound glycerol (as monoacylglycerides, diacylglycerides and triacylglyerides) in biodiesel was developed, giving results beyond the state of the art of the current standard methodologies. The proposed method is based on an on-line hydrogenation using gas chromatography coupled to flame ionization detection and hydrogen as carrier gas. After sample introduction the hydrogenation takes place on a fused silica pre-column coated with a palladium catalyst. This approach allows an immediate and reliable hydrogenation of vegetable oils and biodiesels from different feedstocks. All glycerides are converted into their saturated analogues, resulting in simplified chromatograms with structurally clearly defined analytes, and increased sensitivity for trace amounts of compounds. The method has been successfully in-house validated and combined uncertainty values have been assigned to the final results, which were less than 8% for free glycerol, the sum of monoacylglycerides, the sum of diacylglycerides and the sum of triacylglycerides.

In-capillary microextraction using monolithic polymers: application to preconcentration of carbamate pesticides prior to their separation by MEKC.

In the present work we report a novel procedure for in-capillary microextraction using a monolithic polymeric sorbent. In the proposed methodology, sample treatment takes place in the CE instrument but in a different capillary from that used for the electrophoretic separation. Polymers based on butyl methacrylate and divinylbenzene formed in situ inside a capillary column were assayed. The best results were found with the divinylbenzene-based polymers. The usefulness of the proposed procedure was checked for the determination of carbamate pesticides (Methomyl, Asulam, Carbendazim, Aldicarb, Carbetamide, Propoxur, Pirimicarb, Carbaryl, Carbofuran and Methiocarb) and three of their degradation compounds (Aldicarb sulphoxide, 2-isopropoxyphenol and alpha-naphthol) using MEKC. The optimization of the MEKC is reported, a good separation of the 13 analytes being obtained in less than 6 min. The analytical method using in-capillary microextraction was validated in terms of linearity, repeatability, precision (RSD< or =18% for 50 microg/L), and LODs (1-16 microg/L), and it revealed the usefulness of this in-capillary preconcentration procedure for the determination of analytes of intermediate polarity.

In-capillary preconcentration of pirimicarb and carbendazim with a monolithic polymeric sorbent prior to separation by CZE.

CZE was assayed for the separation of carbamate pesticides susceptible to protonation (Pirimicarb, Carbendazim). Different electrophoretic media with high organic contents were explored, adequate separation and resolution being achieved when a BGE based on ACN with acetic acid in the presence of SDS as an ionic additive was used. With a view to increasing the sensitivity of the method, an in-capillary SPE step prior to the electrophoretic separation was developed. We employed a monolithic polymer formed in situ within the capillary as a medium for analyte retention. The synthesized monolithic bed exhibited high porosity and allowed samples to be loaded at flow rates of about 65 microL/min by applying a pressure of 12 bar. A 5-cm length of monolithic sorbent was used to preconcentrate the target analytes from aqueous samples. The analytes retained were eluted from the polymeric phase directly in the separation capillary with the same electrophoretic medium used for their further separation by CZE. For a 15-min preconcentration time, the in-line SPE-CZE approach proposed here permitted the determination of these pesticides in drinking water at a concentration level of 0.1 microg/L, as demanded by current EU legislation.

Use of a polar-embedded stationary phase for the separation of tocopherols by CEC.

A polar-embedded stationary phase (ULTIMA C18) has been investigated for the separation of alpha-, beta-, gamma- and delta-tocopherols by CEC in comparison with commercially available C(18) and C(30) n-alkyl RPs. The behavior of this stationary phase was tested for different mobile phases based on methanol, ACN, or mixtures thereof and different separation parameters such as retention factors and resolution were evaluated. The main feature of this stationary phase is the improved selectivity for the separation of beta- and gamma-tocopherols (positional isomers) when compared with the pure n-alkyl C(18) material, which was unable to resolve these compounds. Additionally, it is possible to observe a reversal in the elution order of the beta- and gamma-tocopherol isomers with respect to that obtained on the C(30) column. The resulting data indicate that the enhanced selectivity obtained with the polar-embedded stationary phase, with respect to the conventional C(18) material, is due to the participation of both hydrophobic and polar interactions: these latter are of the hydrogen bridge type with the amide group of the polar-embedded stationary phase, which increases the retention of the tocopherols and facilitates the discrimination between the beta- and gamma-isomers. Adequate separation of the four tocopherols was obtained by CEC using the polar-embedded stationary phase and 95:5 v/v methanol/water (5 mM Tris, final concentration) as the mobile phase.

Behaviour of carbamate pesticides in gas chromatography and their determination with solid-phase extraction and solid-phase microextraction as preconcentration steps.

This work reports a study of the chromatographic behaviour of seven carbamate pesticides (aldicarb, carbetamide, propoxur, carbofuran, carbaryl, methiocarb, and pirimicarb) by gas chromatography-mass spectrometry (GC-MS). Variables such as injector temperature, solvent, injection mode, and the degree of ageing of the chromatographic column were studied. One of the aims of this work was to achieve a controlled decomposition of carbamates by a solid-phase microextraction (SPME) preconcentration step with a polyacrylate fibre in order to obtain reproducible chromatographic signals of the degradation products. Optimisation of the SPME process was accomplished by means of experimental design. Several methods using ultrapure water were developed with different preconcentration configurations: SPME-GC-MS, SPE followed by SPME-GC-MS, and SPE plus GC-MS. For all the pesticides studied, method detection limit (MDL) values below 0.1 microg L-1 were reached in at least one of the proposed configurations.