Publications by authors named "Bert Wouters"

26 Publications

  • Page 1 of 1

Development of a fast, online three-phase electroextraction hyphenated to fast liquid chromatography-mass spectrometry for analysis of trace-level acid pharmaceuticals in plasma.

Anal Chim Acta 2022 Feb 11;1192:339364. Epub 2021 Dec 11.

Analytical Biosciences and Metabolomics, Systems Biomedicine and Pharmacology, Leiden Academic Centre for Drug Research, Leiden University, the Netherlands; Research Group Metabolomics, Leiden Center for Applied Bioscience, University of Applied Sciences Leiden, the Netherlands.

Sample preparation is a challenge for high-throughput analysis, especially for volume-limited samples with low-abundant analytes. Ideally, sample preparation enriches the analytes of interest while removing the interferents to reduce the matrix effect and improve both sensitivity and quantification. In this study, a three-phase electroextraction (EE) method hyphenated to fast online liquid chromatography-mass spectrometry (LC-MS) was developed. Four model acidic drugs of relevance for drug monitoring in plasma, i.e. naproxen, fenoprofen, flurbiprofen, and ibuprofen, were utilized for the optimization and evaluation of the method. A Design of Experiment approach (Box-Behnken design) was used to optimize the critical parameters of the method, i.e., the type of organic solvent, pH of the sample and acceptor phase, and the extraction voltage and time. Good fitness (P < 0.02, R > 0.95) was observed for the developed quadratic model. Extraction could be achieved in less than 2 min (115 s) with enrichment factors (EF) up to 190 and extraction recoveries (ER) up to 38% for academic samples. Additionally, the optimized three-phase EE method was successfully applied to spiked plasma samples with low-abundant (50 ng mL) analytes and a low sample volume of 15 μL plasma in 10-fold diluted samples. Finally, two crucial contributors to the matrix effect of three-phase EE application on plasma samples were determined. Specifically, the ion-suppression effect in the MS source was reduced by the fast LC separation, and the matrix effect during extraction was negligible for the diluted protein-precipitated plasma samples. The developed three-phase EE method is easy to operate and provides fast and online extraction of trace-level acidic analytes from volume-limited biological samples. Therefore, this method can provide a potential solution for sample-preparation bottlenecks in high-throughput bioanalysis workflows.
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http://dx.doi.org/10.1016/j.aca.2021.339364DOI Listing
February 2022

A high-throughput, ultrafast, and online three-phase electro-extraction method for analysis of trace level pharmaceuticals.

Anal Chim Acta 2021 Mar 7;1149:338204. Epub 2021 Jan 7.

Analytical Biosciences and Metabolomics, Systems Biomedicine and Pharmacology, Leiden Academic Centre for Drug Research, Leiden University, the Netherlands; Research Group Metabolomics, Leiden Center for Applied Bioscience, University of Applied Sciences Leiden, the Netherlands. Electronic address:

Sample preparation is often reported as the main bottleneck of analytical processes. To meet the requirements of both high-throughput and high sensitivity, improved sample-preparation methods capable of fast analyte preconcentration are urgently needed. To this end, a new three-phase electroextraction (EE) method is presented that allows for ultrafast electroextraction hyphenated to flow-injection analysis mass spectrometry (FIA-MS). Four model compounds, i.e., propranolol, amitriptyline, bupivacaine, and oxeladin, were used to optimize and evaluate the method. Within only 30 s extraction time, enrichment factors (EF) of 105-569 and extraction recoveries (ER) of 10.2%-55.7% were achieved for these analytes, with limits of detection (LODs) ranging from 0.36 to 3.21 ng mL, good linear response function (R > 0.99), low relative standard deviation (0.6%-17.8%) and acceptable accuracy (73-112%). Finally, the optimized three-phase EE method was successfully applied to human urine and plasma samples. Our three-phase electroextraction method is simple to construct and offers ultrafast, online extraction of trace amounts of analytes from biological samples, and therefore has great potential for high-throughput analysis.
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http://dx.doi.org/10.1016/j.aca.2021.338204DOI Listing
March 2021

Damage accelerates ice shelf instability and mass loss in Amundsen Sea Embayment.

Proc Natl Acad Sci U S A 2020 10 14;117(40):24735-24741. Epub 2020 Sep 14.

ENVEO IT GmbH, 6020 Innsbruck, Austria.

Pine Island Glacier and Thwaites Glacier in the Amundsen Sea Embayment are among the fastest changing outlet glaciers in West Antarctica with large consequences for global sea level. Yet, assessing how much and how fast both glaciers will weaken if these changes continue remains a major uncertainty as many of the processes that control their ice shelf weakening and grounding line retreat are not well understood. Here, we combine multisource satellite imagery with modeling to uncover the rapid development of damage areas in the shear zones of Pine Island and Thwaites ice shelves. These damage areas consist of highly crevassed areas and open fractures and are first signs that the shear zones of both ice shelves have structurally weakened over the past decade. Idealized model results reveal moreover that the damage initiates a feedback process where initial ice shelf weakening triggers the development of damage in their shear zones, which results in further speedup, shearing, and weakening, hence promoting additional damage development. This damage feedback potentially preconditions these ice shelves for disintegration and enhances grounding line retreat. The results of this study suggest that damage feedback processes are key to future ice shelf stability, grounding line retreat, and sea level contributions from Antarctica. Moreover, they underline the need for incorporating these feedback processes, which are currently not accounted for in most ice sheet models, to improve sea level rise projections.
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http://dx.doi.org/10.1073/pnas.1912890117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7547219PMC
October 2020

Observing and Modeling Ice Sheet Surface Mass Balance.

Rev Geophys 2019 Jun 13;57(2):376-420. Epub 2019 Jun 13.

Institute for Marine and Atmospheric Research Utrecht University Utrecht The Netherlands.

Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large-scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state-of-the-art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5-30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller-scale SMB processes.
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http://dx.doi.org/10.1029/2018RG000622DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6774314PMC
June 2019

Experimental and numerical study of band-broadening effects associated with analyte transfer in microfluidic devices for spatial two-dimensional liquid chromatography created by additive manufacturing.

J Chromatogr A 2019 Aug 22;1598:77-84. Epub 2019 Mar 22.

Universiteit van Amsterdam, Van' t Hoff Institute for Molecular Sciences, Science Park 904, 1098 XH, Amsterdam, the Netherlands.

Conventional one-dimensional column-based liquid chromatographic (LC) systems do not offer sufficient separation power for the analysis of complex mixtures. Column-based comprehensive two-dimensional liquid chromatography offers a higher separation power, yet suffers from instrumental complexity and long analysis times. Spatial two-dimensional liquid chromatography can be considered as an alternative to column-based approaches. The peak capacity of the system is ideally the product of the peak capacities of the two dimensions, yet the analysis time remains relatively short due to parallel second-dimension separations. Aspects affecting the separation efficiency of this type of systems include flow distribution to homogeneously distribute the mobile phase for the second-dimension (D) separation, flow confinement during the first-dimension (D) separation, and band-broadening effects during analyte transfer from the D separation channel to the D separation area. In this study, the synergy between computational fluid dynamics (CFD) simulations and rapid prototyping was exploited to address band broadening during the D development and analyte transfer from D to D. Microfluidic devices for spatial two-dimensional liquid chromatography were designed, simulated, 3D-printed and tested. The effects of presence and thickness of spacers in the D separation area were addressed and leaving these out proved to be the most efficient solution regarding band broadening reduction. The presence of a stationary-phase material in the D channel had a great effect on the analyte transfer from the D to the D and the resulting band broadening. Finally, pressure limit of the fabricated devices and printability are discussed.
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http://dx.doi.org/10.1016/j.chroma.2019.03.041DOI Listing
August 2019

On-line microfluidic immobilized-enzyme reactors: A new tool for characterizing synthetic polymers.

Anal Chim Acta 2019 Apr 12;1053:62-69. Epub 2018 Dec 12.

Universiteit van Amsterdam, Van 't Hoff Institute for Molecular Sciences, Science Park 904, 1098 XH, Amsterdam, the Netherlands.

Biodegradable polymeric materials may eventually replace biostable materials for medical applications, including therapeutic devices, scaffolds for tissue engineering, and drug-delivery vehicles. To further develop such materials, a more fundamental understanding is necessary to correlate parameters including chemical-composition distribution within a macromolecular structure with the final properties of the material, including particle-size. A wide variety of analytical techniques have been applied for the characterization of polymer materials, including hyphenated techniques such as comprehensive two-dimensional liquid chromatography (LC × LC). In this context, we have investigated enzymatic degradation of polyester-based nanoparticles, both in-solution and by the use of an immobilized-enzyme reactor (IMER). We have demonstrated for the first time the implementation of such an IMER in a size-exclusion chromatography system for on-line degradation and subsequent analysis of the polymer degradation products. The effect of residence times ranging from 12 s to 4 min on polymer degradation was assessed. IMER-assisted degradation is much faster compared to in-solution degradation, which requires several hours to days, and opens the possibility to use such reactors in LC × LC modulation interfaces.
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http://dx.doi.org/10.1016/j.aca.2018.12.002DOI Listing
April 2019

Seasonal to decadal variability in ice discharge from the Greenland Ice Sheet.

Cryosphere 2018 3;12(12):3813-3825. Epub 2018 Dec 3.

Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, Netherlands.

Rapid changes in thickness and velocity have been observed at many marine-terminating glaciers in Greenland, impacting the volume of ice they export, or discharge, from the ice sheet. While annual estimates of ice-sheet wide discharge have been previously derived, higher-resolution records are required to fully constrain the temporal response of these glaciers to various climatic and mechanical drivers that vary in sub-annual scales. Here we sample outlet glaciers wider than 1 km (N = 230) to derive the first continuous, ice-sheet wide record of total ice sheet discharge for the 2000-2016 period, resolving a seasonal variability of 6 %. The amplitude of seasonality varies spatially across the ice sheet from 5 % in the southeastern region to 9 % in the northwest region. We analyze seasonal to annual variability in the discharge time series with respect to both modelled meltwater runoff, obtained from RACMO2.3p2, and glacier front position changes over the same period. We find that year-to-year changes in total ice sheet discharge are related to annual front changes ( = 0.59, p = 10) and that the annual magnitude of discharge is closely related to cumulative front position changes ( = 0.79), which show a net retreat of > 400 km, or an average retreat of > 2 km at each surveyed glacier. Neither maximum seasonal runoff or annual runoff totals are correlated to annual discharge, which suggests that larger annual quantities of runoff do not relate to increased annual discharge. Discharge and runoff, however, follow similar patterns of seasonal variability with near-coincident periods of acceleration and seasonal maxima. These results suggest that changes in glacier front position drive secular trends in discharge, whereas the impact of runoff is likely limited to the summer months when observed seasonal variations are substantially controlled by the timing of meltwater input.
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http://dx.doi.org/10.5194/tc-12-3813-2018DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6582977PMC
December 2018

Nanoparticle Analysis by Online Comprehensive Two-Dimensional Liquid Chromatography combining Hydrodynamic Chromatography and Size-Exclusion Chromatography with Intermediate Sample Transformation.

Anal Chem 2017 09 8;89(17):9167-9174. Epub 2017 Aug 8.

Analytical-Chemistry Group, University of Amsterdam, van't Hoff Institute for Molecular Sciences , Science Park 904, 1098 XH Amsterdam, The Netherlands.

Polymeric nanoparticles have become indispensable in modern society with a wide array of applications ranging from waterborne coatings to drug-carrier-delivery systems. While a large range of techniques exist to determine a multitude of properties of these particles, relating physicochemical properties of the particle to the chemical structure of the intrinsic polymers is still challenging. A novel, highly orthogonal separation system based on comprehensive two-dimensional liquid chromatography (LC × LC) has been developed. The system combines hydrodynamic chromatography (HDC) in the first-dimension to separate the particles based on their size, with ultrahigh-performance size-exclusion chromatography (SEC) in the second dimension to separate the constituting polymer molecules according to their hydrodynamic radius for each of 80 to 100 separated fractions. A chip-based mixer is incorporated to transform the sample by dissolving the separated nanoparticles from the first-dimension online in tetrahydrofuran. The polymer bands are then focused using stationary-phase-assisted modulation to enhance sensitivity, and the water from the first-dimension eluent is largely eliminated to allow interaction-free SEC. Using the developed system, the combined two-dimensional distribution of the particle-size and the molecular-size of a mixture of various polystyrene (PS) and polyacrylate (PACR) nanoparticles has been obtained within 60 min.
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http://dx.doi.org/10.1021/acs.analchem.7b01906DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5588091PMC
September 2017

Prototyping of thermoplastic microfluidic chips and their application in high-performance liquid chromatography separations of small molecules.

J Chromatogr A 2017 Nov 1;1523:224-233. Epub 2017 Jun 1.

Vrije Universiteit Brussel (VUB), Department of Chemical Engineering, Brussels, Belgium. Electronic address:

The present paper discusses practical aspects of prototyping of microfluidic chips using cyclic olefin copolymer as substrate and the application in high-performance liquid chromatography. The developed chips feature a 60mm long straight separation channel with circular cross section (500μm i.d.) that was created using a micromilling robot. To irreversibly seal the top and bottom chip substrates, a solvent-vapor-assisted bonding approach was optimized, allowing to approximate the ideal circular channel geometry. Four different approaches to establish the micro-to-macro interface were pursued. The average burst pressure of the microfluidic chips in combination with an encasing holder was established at 38MPa and the maximum burst pressure was 47MPa, which is believed to be the highest ever report for these polymer-based microfluidic chips. Porous polymer monolithic frits were synthesized in-situ via UV-initiated polymerization and their locations were spatially controlled by the application of a photomask. Next, high-pressure slurry packing was performed to introduce 3μm silica reversed-phase particles as the stationary phase in the separation channel. Finally, the application of the chip technology is demonstrated for the separation of alkyl phenones in gradient mode yielding baseline peak widths of 6s by applying a steep gradient of 1.8min at a flow rate of 10μL/min.
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http://dx.doi.org/10.1016/j.chroma.2017.05.063DOI Listing
November 2017

A cyclic-olefin-copolymer microfluidic immobilized-enzyme reactor for rapid digestion of proteins from dried blood spots.

J Chromatogr A 2017 Mar 28;1491:36-42. Epub 2017 Jan 28.

University of Amsterdam, van't Hoff Institute for Molecular Sciences, Analytical-Chemistry Group, Science Park 904, 1098 XH Amsterdam, The Netherlands.

A critical step in the bottom-up characterization of proteomes is the conversion of proteins to peptides, by means of endoprotease digestion. Nowadays this method typically uses overnight digestion and as such represents a considerable bottleneck for high-throughput analysis. This report describes protein digestion using an immobilized-enzyme reactor (IMER), which enables accelerated digestion times that are completed within seconds to minutes. For rapid digestion to occur, a cyclic-olefin-copolymer microfluidic reactor was constructed containing trypsin immobilized on a polymer monolithic material through a 2-vinyl-4,4-dimethylazlactone linker. The IMER was applied for the rapid offline digestion of both singular protein standards and a complex protein mixture prior to liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) analysis. The effects of protein concentration and residence time in the IMER were assessed for protein standards of varying molecular weight between 11 and 240kDa. Compared to traditional in-solution digestion, IMER-facilitated protein digestion at room temperature for 5min yielded similar results in terms of sequence coverage and number of identified peptides. Good repeatability was demonstrated with a relative standard deviation of 6% for protein-sequence coverage. The potential of the IMER was also demonstrated for a complex protein mixture in the analysis of dried blood spots. Compared to a traditional workflow a similar number of proteins could be identified, while reducing the total analysis time from 22.5h to 4h and importantly omitting the sample-pre-treatment steps (denaturation, reduction, and alkylation). The identified proteins from two workflows showed similar distributions in terms of molecular weight and hydrophobic character.
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http://dx.doi.org/10.1016/j.chroma.2017.01.078DOI Listing
March 2017

Geodetic measurements reveal similarities between post-Last Glacial Maximum and present-day mass loss from the Greenland ice sheet.

Sci Adv 2016 Sep 21;2(9):e1600931. Epub 2016 Sep 21.

Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, Netherlands.

Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5031466PMC
http://dx.doi.org/10.1126/sciadv.1600931DOI Listing
September 2016

Spatial and temporal Antarctic Ice Sheet mass trends, glacio-isostatic adjustment, and surface processes from a joint inversion of satellite altimeter, gravity, and GPS data.

J Geophys Res Earth Surf 2016 02 3;121(2):182-200. Epub 2016 Feb 3.

School of Geographical Sciences University of Bristol Bristol UK.

We present spatiotemporal mass balance trends for the Antarctic Ice Sheet from a statistical inversion of satellite altimetry, gravimetry, and elastic-corrected GPS data for the period 2003-2013. Our method simultaneously determines annual trends in ice dynamics, surface mass balance anomalies, and a time-invariant solution for glacio-isostatic adjustment while remaining largely independent of forward models. We establish that over the period 2003-2013, Antarctica has been losing mass at a rate of -84 ± 22 Gt yr, with a sustained negative mean trend of dynamic imbalance of -111 ± 13 Gt yr. West Antarctica is the largest contributor with -112 ± 10 Gt yr, mainly triggered by high thinning rates of glaciers draining into the Amundsen Sea Embayment. The Antarctic Peninsula has experienced a dramatic increase in mass loss in the last decade, with a mean rate of -28 ± 7 Gt yr and significantly higher values for the most recent years following the destabilization of the Southern Antarctic Peninsula around 2010. The total mass loss is partly compensated by a significant mass gain of 56 ± 18 Gt yr in East Antarctica due to a positive trend of surface mass balance anomalies.
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http://dx.doi.org/10.1002/2015JF003550DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4845667PMC
February 2016

Towards ultra-high peak capacities and peak-production rates using spatial three-dimensional liquid chromatography.

Lab Chip 2015 Dec;15(23):4415-22

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium.

In order to successfully tackle the truly complex separation problems arising from areas such as proteomics research, the development of ultra-efficient and fast separation technology is required. In spatial three-dimensional chromatography, components are separated in the space domain with each peak being characterized by its coordinates in a three-dimensional separation body. Spatial three-dimensional (3D-)LC has the potential to offer unprecedented resolving power when orthogonal retention mechanisms are applied, since the total peak capacity is the product of the three individual peak capacities. Due to parallel developments during the second- and third-dimension separations, the analysis time is greatly reduced compared to a coupled-column multi-dimensional LC approach. This communication discusses the different design aspects to create a microfluidic chip for spatial 3D-LC. The use of physical barriers to confine the flow between the individual developments, and flow control by the use of (2)D and (3)D flow distributors is discussed. Furthermore, the in situ synthesis of monolithic stationary phases is demonstrated. Finally, the potential performance of a spatial 3D-LC systems is compared with the performance obtained with state-of-the-art 1D-LC and (coupled-column) 2D-LC approaches via a Pareto-optimization approach. The proposed microfluidic device for 3D-LC featuring 16 (2)D channels and 256 (3)D channels can potentially yield a peak capacity of 8000 in a total analysis time of 10 minutes.
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http://dx.doi.org/10.1039/c5lc01169hDOI Listing
December 2015

Design of a microfluidic device for comprehensive spatial two-dimensional liquid chromatography.

J Sep Sci 2015 Apr 9;38(7):1123-9. Epub 2015 Mar 9.

Department of Chemical Engineering, Vrije Universiteit Brussel, Brussels, Belgium.

This study discusses the design aspects for the construction of a microfluidic device for comprehensive spatial two-dimensional liquid chromatography. In spatial two-dimensional liquid chromatography each peak is characterized by its coordinates in the plane. After completing the first-dimension separation all fractions are analyzed in parallel second-dimension separations. Hence, spatial two-dimensional liquid chromatography potentially provides much higher peak-production rates than a coupled column multi-dimensional liquid chromatography approach in which the second-dimension analyses are performed sequentially. A chip for spatial two-dimensional liquid chromatography has been manufactured from cyclic olefin copolymer and features a first-dimension separation channel and 21 parallel second-dimension separation channels oriented perpendicularly to the former. Compartmentalization of first- and second-dimension developments by physical barriers allowed for a preferential flow path with a minimal dispersion into the second-dimension separation channels. To generate a homogenous flow across all the parallel second-dimension channels, a radially interconnected flow distributor containing two zones of diamond-shaped pillars was integrated on-chip. A methacrylate ester based monolithic stationary phase with optimized macroporous structure was created in situ in the confines of the microfluidic chip. In addition, the use of a photomask was explored to localize monolith formation in the parallel second-dimension channels. Finally, to connect the spatial chip to the liquid chromatography instrument, connector ports were integrated allowing the use of Viper fittings. As an alternative, a chip holder with adjustable clasp locks was designed that allows the clamping force to be adjusted.
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http://dx.doi.org/10.1002/jssc.201401192DOI Listing
April 2015

Using contemporary liquid chromatography theory and technology to improve capillary gradient ion-exchange separations.

J Chromatogr A 2014 Nov 23;1370:63-9. Epub 2014 Oct 23.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium. Electronic address:

The gradient-performance limits of capillary ion chromatography have been assessed at maximum system pressure (34.5 MPa) using capillary columns packed with 4.1 μm macroporous anion-exchange particles coated with 65 nm positively-charged nanobeads. In analogy to the van-Deemter curve, the gradient performance was assessed applying different flow rates, while decreasing the gradient time inversely proportional to the increase in flow rate in order to maintain the same retention properties. The gradient kinetic-performance limits were determined at maximum system pressure, applying tG/t0=5, 10, and 20. In addition, the effect of retention on peak width was assessed in gradient mode for mono-, di-, and trivalent inorganic anions. The peak width of late-eluting ions can be significantly reduced by using concave gradient, resulting in better detection sensitivity. A signal enhancement factor of 8 was measured for a late-eluting ion when applying a concave instead of a linear gradient. For the analysis of a complex anion mixture, a coupled column with a total length of 1.05 m was operated at the kinetic-performance limit applying a linear 250 min gradient (tG/t0=10). The peak capacity varied between 200 and 380 depending on analyte retention, and hence on charge and size of the ion.
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http://dx.doi.org/10.1016/j.chroma.2014.10.028DOI Listing
November 2014

Design and performance evaluation of a microfluidic ion-suppression module for anion-exchange chromatography.

J Chromatogr A 2014 Aug 12;1355:253-60. Epub 2014 Jun 12.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium. Electronic address:

A microfluidic membrane suppressor has been constructed to suppress ions of alkaline mobile-phases via an acid-base reaction across a sulfonated poly(tetrafluoroethylene)-based membrane and was evaluated for anion-exchange separations using conductivity detection. The membrane was clamped between two chip substrates, accommodating rectangular microchannels for the eluent and regenerant flow, respectively. Additionally, a clamp-on chip holder has been constructed which allows the alignment and stacking of different chip modules. The response and efficacy of the microfluidic chip suppressor was assessed for a wide range of eluent (KOH) concentrations, using 127 and 183μm thick membranes, while optimizing the flow rate and concentration of the regenerant solution (H2SO4). The optimal operating eluent flow rate was determined at 5μL/min, corresponding to the optimal van-Deemter flow velocity of commercially-available column technology, i.e. a 0.4mm i.d.×250mm long column packed with 7.5μm anion-exchange particles. When equilibrated at 10mM KOH, a 99% decrease in conductivity signal could be obtained within 5min when applying 10mM H2SO4 regenerant at 75μL/min. A background signal as low as 1.2μS/cm was obtained, which equals the performance of a commercially-available electrolytic hollow-fiber suppressor. When increasing the temperature of the membrane suppressor from 15 to 20°C, ion suppression was significantly improved allowing the application of 75mM KOH. The applicability of the chip suppressor has been demonstrated with an isocratic baseline separation of a mixture of seven inorganic ions, yielding plate numbers between 5300 and 10,600 and with a gradient separation of a complex ion mixture.
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http://dx.doi.org/10.1016/j.chroma.2014.06.025DOI Listing
August 2014

Monitoring the morphology development of polymer-monolithic stationary phases by thermal analysis.

J Sep Sci 2014 Jan 22;37(1-2):179-86. Epub 2013 Nov 22.

Department of Chemical Engineering, Vrije Universiteit Brussel, Brussels, Belgium.

Thermal analysis and SEM were employed to gain insights in the different stages of morphology development and the thermal properties of polymer-monolithic stationary phases. The studied system was a thermally initiated free-radical copolymerization reaction at 70°C of styrene and divinylbenzene in the presence of tetrahydrofuran and 1-decanol. The key events in the early stages of morphology development are initiation, chain growth, branching, and cyclization, leading to microgel particles. Interparticle reactions through pendant vinyl groups lead to the formation of microgel clusters. The rapid increase in molecular weight and cross-link density of the microgel clusters causes a reaction-induced phase separation, and the formation of a macroscopic network of interconnected globules was observed (macrogelation) at around 45 min. After 3 h or 65% conversion, a space-filling macroporous monolithic network was observed. Afterwards, mainly growth of existing globules takes place, reducing the macropore size. The porogen ratio affects the timing of the reaction-induced phase separation, strongly influencing the morphology of the polymer material. The use of a mixture of divinylbenzene isomers yielded a monolithic material that is less cross-linked at the surface compared to the central part of the polymer backbone due to copolymerization-composition drift. The less cross-linked outer layer starts devitrifying at 100°C.
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http://dx.doi.org/10.1002/jssc.201301104DOI Listing
January 2014

A reconciled estimate of glacier contributions to sea level rise: 2003 to 2009.

Science 2013 May;340(6134):852-7

Graduate School of Geography, Clark University, Worcester, MA 01610, USA.

Glaciers distinct from the Greenland and Antarctic Ice Sheets are losing large amounts of water to the world's oceans. However, estimates of their contribution to sea level rise disagree. We provide a consensus estimate by standardizing existing, and creating new, mass-budget estimates from satellite gravimetry and altimetry and from local glaciological records. In many regions, local measurements are more negative than satellite-based estimates. All regions lost mass during 2003-2009, with the largest losses from Arctic Canada, Alaska, coastal Greenland, the southern Andes, and high-mountain Asia, but there was little loss from glaciers in Antarctica. Over this period, the global mass budget was -259 ± 28 gigatons per year, equivalent to the combined loss from both ice sheets and accounting for 29 ± 13% of the observed sea level rise.
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http://dx.doi.org/10.1126/science.1234532DOI Listing
May 2013

Visualization procedures for proteins and peptides on flat-bed monoliths and their effects on matrix-assisted laser-desorption/ionization time-of-flight mass spectrometric detection.

J Chromatogr A 2013 Apr 27;1286:222-8. Epub 2013 Feb 27.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium.

The present study concerns the application of visualization methods, i.e. coomassie-brilliant-blue-R staining (CBB-R), silver-nitrate staining, and fluorescamine labeling, and subsequent MALDI-MS analysis of intact proteins and peptides on the surface of flat-bed monoliths, intended for spatial two-dimensional chromatographic separations. The use of 100-μm thick macroporous poly(butyl methacrylate-co-ethylene dimethacrylate) flat-bed monoliths renders a fixation step obsolete, so that CBB-R and silver-nitrate staining and destaining could be achieved in 10-15 min as opposed to up to 24h, as is typical on 2D-PAGE gels. The detection limits remained comparable. The compatibility of the monolithic layer with subsequent MALDI-MS analysis of individual proteins and peptide spots was investigated with regards to mass accuracy, mass precision, resolution, and signal intensity. When comparing results from MALDI-MS analysis of proteins and peptides on a flat-bed monolith to results obtained directly on stainless-steel target plates, significant losses in mass precision, signal intensity, and an increased variation in resolution were observed. In addition, a loss in signal intensity up to two orders of magnitude was observed when using monolithic layers. After CCB-R and silver-nitrate staining and destaining to disrupt the protein-dye complexes no MALDI spectra with significant S/N ratios could be achieved. After fluorescamine labeling heterogeneous signals were observed, which resulted from a distribution in the number of fluorescence-labeled lysine groups and from the presence of labeled derivatives that had undergone condensation reactions.
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http://dx.doi.org/10.1016/j.chroma.2013.02.064DOI Listing
April 2013

Capillary ion chromatography at high pressure and temperature.

Anal Chem 2012 Aug 2;84(16):7212-7. Epub 2012 Aug 2.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium.

The application of high pressure and temperature in ion chromatography (IC) can significantly improve the efficiency and reduce the analysis time. In this work, the kinetic-performance limits of capillary IC columns with inner diameters of 400 μm packed with 4 and 7 μm macroporous anion-exchange particles were investigated employing a capillary ion-exchange instrument allowing column pressures up to 34 MPa and column temperatures up to 80 °C. Plate heights below 10 μm could be realized using capillary columns packed with 4 μm particles. Compared to conventional IC using 7 μm particles and pressures up to 21 MPa, a 40% improvement in plate number could be achieved when working at the kinetic performance limits at 34 MPa and using columns packed with 4 μm particles. Using coupled columns with a total length of 400 mm, a mixture of seven anions was separated within 7.5 min while yielding 20,000 plates. Increasing the temperature improved the performance limits when operating in the C-term region (for fast IC separation using columns <75 cm). Temperature also affected the retention properties and hence the selectivity. At higher temperature, retention for monovalent ions was mainly governed by ion diameter. An increase in retention with temperature was observed for small ions, and there was a decrease for ions having a larger diameter. The retention factor for divalent and trivalent anions increased with temperature.
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http://dx.doi.org/10.1021/ac301598jDOI Listing
August 2012

High-resolution separations of protein isoforms with liquid chromatography time-of-flight mass spectrometry using polymer monolithic capillary columns.

J Chromatogr A 2011 Aug 21;1218(32):5504-11. Epub 2011 Jun 21.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium.

The separation of intact proteins, including protein isoforms arising from various amino-acid modifications, employing a poly(styrene-co-divinylbenzene) monolithic capillary column in high-performance liquid chromatography coupled on-line to a time-of-flight mass spectrometer (MS) is described. Using a 250 mm × 0.2 mm monolithic capillary column high-sensitivity separations yielding peak capacities of >600 were achieved with a 2h linear gradient and formic acid added in the mobile phase as ion-pairing agent. The combination of high-resolution chromatography with high-accuracy MS allowed to distinguish protein isoforms that differ only in their oxidation and biotinylation state allowing the separation between structural isoforms. Finally, the potential to separate proteins isoforms due to glycosylation is discussed.
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http://dx.doi.org/10.1016/j.chroma.2011.06.049DOI Listing
August 2011

Sharply increased mass loss from glaciers and ice caps in the Canadian Arctic Archipelago.

Nature 2011 May 20;473(7347):357-60. Epub 2011 Apr 20.

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada.

Mountain glaciers and ice caps are contributing significantly to present rates of sea level rise and will continue to do so over the next century and beyond. The Canadian Arctic Archipelago, located off the northwestern shore of Greenland, contains one-third of the global volume of land ice outside the ice sheets, but its contribution to sea-level change remains largely unknown. Here we show that the Canadian Arctic Archipelago has recently lost 61 ± 7 gigatonnes per year (Gt yr(-1)) of ice, contributing 0.17 ± 0.02 mm yr(-1) to sea-level rise. Our estimates are of regional mass changes for the ice caps and glaciers of the Canadian Arctic Archipelago referring to the years 2004 to 2009 and are based on three independent approaches: surface mass-budget modelling plus an estimate of ice discharge (SMB+D), repeat satellite laser altimetry (ICESat) and repeat satellite gravimetry (GRACE). All three approaches show consistent and large mass-loss estimates. Between the periods 2004-2006 and 2007-2009, the rate of mass loss sharply increased from 31 ± 8 Gt yr(-1) to 92 ± 12 Gt yr(-1) in direct response to warmer summer temperatures, to which rates of ice loss are highly sensitive (64 ± 14 Gt yr(-1) per 1 K increase). The duration of the study is too short to establish a long-term trend, but for 2007-2009, the increase in the rate of mass loss makes the Canadian Arctic Archipelago the single largest contributor to eustatic sea-level rise outside Greenland and Antarctica.
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http://dx.doi.org/10.1038/nature10089DOI Listing
May 2011

Parameters affecting the separation of intact proteins in gradient-elution reversed-phase chromatography using poly(styrene-co-divinylbenzene) monolithic capillary columns.

J Chromatogr A 2010 Apr 6;1217(18):3085-90. Epub 2010 Mar 6.

Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, 1050 Brussels, Belgium.

An experimental study was performed to investigate the effects of column parameters and gradient conditions on the separation of intact proteins using styrene-based monolithic columns. The effect of flow rate on peak width was investigated at constant gradient steepness by normalizing the gradient time for the column hold-up time. When operating the column at a temperature of 60 degrees C a small C-term effect was observed in a flow rate range of 1-4 microL/min. However, the C-term effect on peak width is not as strong as the decrease in peak width due to increasing flow rate. The peak capacity increased according to the square root of the column length. Decreasing the macropore size of the polymer monolith while maintaining the column length constant, resulted in an increase in peak capacity. A trade-off between peak capacity and total analysis time was made for 50, 100, and 250 mm long monolithic columns and a microparticulate column packed with 5 microm porous silica particles while operating at a flow rate of 2 microL/min. The peak capacity per unit time of the 50mm long monolithic column with small pore size was superior when the total analysis time is below 120 min, yielding a maximum peak capacity of 380. For more demanding separations the 250 mm long monolith provided the highest peak capacity in the shortest possible time frame.
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http://dx.doi.org/10.1016/j.chroma.2010.03.002DOI Listing
April 2010

Partitioning recent Greenland mass loss.

Science 2009 Nov;326(5955):984-6

Institute for Marine and Atmospheric Research, Utrecht University, Netherlands.

Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000-2008 mass loss of approximately 1500 gigatons, equivalent to 0.46 millimeters per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 millimeters per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.
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http://dx.doi.org/10.1126/science.1178176DOI Listing
November 2009

Cholesterol-lowering effect of a theaflavin-enriched green tea extract: a randomized controlled trial.

Arch Intern Med 2003 Jun;163(12):1448-53

Division of Cardiovascular Medicine, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA.

Background: Tea consumption has been associated with decreased cardiovascular risk, but potential mechanisms of benefit are ill-defined. While epidemiologic studies suggest that drinking multiple cups of tea per day lowers low-density lipoprotein cholesterol (LDL-C), previous trials of tea drinking and administration of green tea extract have failed to show any impact on lipids and lipoproteins in humans. Our objective was to study the impact of a theaflavin-enriched green tea extract on the lipids and lipoproteins of subjects with mild to moderate hypercholesterolemia.

Methods: Double-blind, randomized, placebo-controlled, parallel-group trial set in outpatient clinics in 6 urban hospitals in China. A total of 240 men and women 18 years or older on a low-fat diet with mild to moderate hypercholesterolemia were randomly assigned to receive a daily capsule containing theaflavin-enriched green tea extract (375 mg) or placebo for 12 weeks. Main outcome measures were mean percentage changes in total cholesterol, LDL-C, high-density lipoprotein cholesterol (HDL-C), and triglyceride levels compared with baseline.

Results: After 12 weeks, the mean +/- SEM changes from baseline in total cholesterol, LDL-C, HDL-C, and triglyceride levels were -11.3% +/- 0.9% (P =.01), -16.4% +/- 1.1% (P =.01), 2.3% +/- 2.1% (P =.27), and 2.6% +/- 3.5% (P =.47), respectively, in the tea extract group. The mean levels of total cholesterol, LDL-C, HDL-C, and triglycerides did not change significantly in the placebo group. No significant adverse events were observed.

Conclusion: The theaflavin-enriched green tea extract we studied is an effective adjunct to a low-saturated-fat diet to reduce LDL-C in hypercholesterolemic adults and is well tolerated.
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http://dx.doi.org/10.1001/archinte.163.12.1448DOI Listing
June 2003

Documentation of delirium in elderly patients with hip fracture.

J Gerontol Nurs 2002 Nov;28(11):23-9

Katholieke Universiteit Leuven, Faculty of Medicine, Center for Health Services and Nursing Research, Kapucijnenvoer 35, 4th Floor, 3000 Leuven, Belgium.

This study determined the accuracy of diagnosis and documentation of delirium in the medical and nursing records of 55 elderly patients with hip fracture (mean age = 78.4, SD = 8.4). These records were reviewed retrospectively on a patient's discharge for diagnosis of delirium, and for description of clinical indicators or symptoms of delirium. Additionally, all patients were monitored by one of the research members on days 1, 3, 5, 8, and 12 postoperatively for signs of delirium, as measured by the Confusion Assessment Method (CAM). Clinicians were blinded to the purpose of the study. According to the CAM criteria, the incidence of delirium was 14.5% on postoperative Day 1; 9.1% on postoperative Day 3; 10.9% on postoperative Day 5; 7.7% on postoperative Day 8; and 5.6% on postoperative Day 12. For those same days, no formal diagnosis of delirium or a description of clinical indicators was found in the medical records. In the nursing records, a false-positive documentation of 8.5%, 4%, 4.1%, 4.2%, and 5.9%, respectively was noted. False-negative documentation was found in 87.5%, 80%, 66.7%, 75%, and 50% of the cases on the respective days. Documentation of essential symptoms--namely onset and course of the syndrome--and disturbances in consciousness, attention, and cognition, were seldom or never found in the nursing records. However, behaviors of the hyperactive variant of delirium and which are known to interfere with nursing care were documented more often (e.g., 13.4% restless, 10.3% fidget with materials, 7.2% annoying behavior). Both medical and nursing records showed poor documentation and under-diagnosis of delirium. However, a correct diagnosis and early recognition of delirium may enhance the management of this syndrome.
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http://dx.doi.org/10.3928/0098-9134-20021101-07DOI Listing
November 2002
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