Quantification of oxygenated volatile organic compounds in seawater by membrane inlet-proton transfer reaction/mass spectrometry

The role of the ocean in the cycling of oxygenated volatile organic compounds (OVOCs) remains largely unanswered due to a paucity of datasets. We describe the method development of a membrane inlet-proton transfer reaction/mass spectrometer (MI-PTR/MS) as an efficient method of analysing methanol, acetaldehyde and acetone in seawater. Validation of the technique with water standards shows that the optimised responses are linear and reproducible. Limits of detection are 27 nM for methanol, 0.7 nM for acetaldehyde and 0.3 nM for acetone. Acetone and acetaldehyde concentrations generated by MI-PTR/MS are compared to a second, independent method based on purge and trap-gas chromatography/flame ionisation detection (P&T-GC/FID) and show excellent agreement. Chromatographic separation of isomeric species acetone and propanal permits correction to mass 59 signal generated by the PTR/MS and overcomes a known uncertainty in reporting acetone concentrations via mass spectrometry. A third bioassay technique using radiolabelled acetone further supported the result generated by this method. We present the development and optimisation of the MI-PTR/MS technique as a reliable and convenient tool for analysing seawater samples for these trace gases. We compare this method with other analytical techniques and discuss its potential use in improving the current understanding of the cycling of oceanic OVOCs.

An interlaboratory comparison for the quantification of aqueous dimethylsulfide

An interlaboratory comparison (ILC) was conducted to evaluate the proficiency of multiple laboratories to quantify dimethylsulfide (DMS) in aqueous solution. Ten participating laboratories were each supplied with blind duplicate test solutions containing dimethylsulfoniopropionate hydrochloride (DMSP HCl) dissolved in acidified artificial seawater. The test solutions were prepared by the coordinating laboratory from a DMSP HCl reference material that was synthesized and purity certified for this purpose. A concentration range was specified for the test solutions and the participating laboratories were requested to dilute them as required for their analytical procedure, together with the addition of excess alkali under gas-tight conditions to convert the DMSP to DMS. Twenty-two DMS concentrations and their estimated expanded measurement uncertainties (95% confidence level) were received from the laboratories. With two exceptions, the within-laboratory variability was 5% or less and the between-laboratory variability was ~ 25%. The magnitude of expanded measurement uncertainties reported from all participants ranged from 1% to 33% relative to the result. The information gained from this pilot ILC indicated the need for further test sample distribution studies of this type so that participating laboratories can identify systematic errors in their analysis procedures and realistically evaluate their measurement uncertainty. The outcome of ILC studies provides insights into the comparability of data in the global surface seawater DMS database.

Optimization of a total acid hydrolysis based protocol for the quantification of carbohydrate in macroalgae

Accurate quantification of carbohydrate content of biomass is crucial for many bio-refining applications. The standardised NREL two stage complete acid hydrolysis protocol was evaluated for its suitability towards seaweeds, as the protocol was originally developed for lignocellulosic feedstocks. The compositional differences between the major polysaccharides in seaweeds and terrestrial plants, and seaweed’s less recalcitrant nature, could suggest the NREL based protocol may be too extreme. Underestimations of carbohydrate content through the degradation of liberated sugars into furan compounds may yield erroneous data. An optimised analysis method for carbohydrate quantification in the brown seaweed L. digitata was thus developed and evaluated. Results from this study revealed stage 1 of the assay was crucial for optimisation however stage 2 proved to be less crucial. The newly optimised protocol for L. digitata yielded 210 mg of carbohydrate per g of biomass compared to a yield of only 166 mg/g from the original NREL protocol. Use of the new protocol on two other species of seaweed also gave consistent results; higher carbohydrate and significantly lower sugar degradation products generation than the original protocol. This study demonstrated the importance of specific individual optimisations of the protocol for accurate sugar quantification, particularly for different species of seaweed

A method for semi-automated objective quantification of linear bedforms from multi-scale Digital Elevation Models

The increasing availability of large, detailed digital representations of the Earth’s surface demands the application of objective and quantitative analyses. Given recent advances in the understanding of the mechanisms of formation of linear bedform features from a range of environments, objective measurement of their wavelength, orientation, crest and trough positions, height and asymmetry is highly desirable. These parameters are also of use when determining observation-based parameters for use in many applications such as numerical modelling, surface classification and sediment transport pathway analysis. Here, we (i) adapt and extend extant techniques to provide a suite of semi-automatic tools which calculate crest orientation, wavelength, height, asymmetry direction and asymmetry ratios of bedforms, and then (ii) undertake sensitivity tests on synthetic data, increasingly complex seabeds and a very large-scale (39 000km2) aeolian dune system. The automated results are compared with traditional, manually derived,measurements at each stage. This new approach successfully analyses different types of topographic data (from aeolian and marine environments) from a range of sources, with tens of millions of data points being processed in a semi-automated and objective manner within minutes rather than hours or days. The results from these analyses show there is significant variability in all measurable parameters in what might otherwise be considered uniform bedform fields. For example, the dunes of the Rub’ al Khali on the Arabian peninsula are shown to exhibit deviations in dimensions from global trends. Morphological and dune asymmetry analysis of the Rub’ al Khali suggests parts of the sand sea may be adjusting to a changed wind regime from that during their formation 100 to 10 ka BP.

Quantification of urinary excretion of ethosuximide enantiomers and their major metabolites in the rat.

A chiral gas chromatographic assay has been developed for quantitative analysis of ethosuximide and its major metabolites in rat urine. The extraction procedure was found to be precise and reproducible. Recovery was in the range of 94-98%, intraday CV(%) was 0.92% for (S)-ethosuximide (50 mug/ml) and 0.51% for (R)-ethosuximide (50 mug/ml). Interday CV(%) was 1.12% for (S)-ethosuximide and 0.72% for (R)-ethosuximide. The limit of detection was determined to be around 0.01 mug/ml for each enantiomer. Following administration of rac-ethosuximide by i.v., i.p. and oral routes, unchanged ethosuximide was detected in urine up to 72h after drug administration. The appearance of all detected metabolites occurred Within 24h of drug administration. Significantly more (S)-ethosuximide was excreted unchanged than (R)-ethosuximide with all three routes studied. A substantial amount of the drug was eliminated as the 2-(1-hydroxyethyl)-2-methylsuccinimide (2 pairs of diastereoisomers). Much less drug was eliminated as the 2-ethyl-3-hydroxy-2-methylsuccinimide with only one diastereoisomer observed. Examination of the one pair of diastereoisomers of 2-(1-hydroxyethyl)-2-methylsuccinimide that was resolved showed preferential excretion of one isomer. Comparison of both pairs of diastereoisomers showed that one pair was formed in preference to the other with a ratio of approximately 0.8:1. It is concluded that stereoselective metabolism of ethosuximide occurs. Copyright (C) 2001 John Wiley & Sons, Ltd. Author Keywords: chiral pharmacokinetics; ethosuximide enantiomers; metabolism; rat; urinary excretion; gas chromatography