An integrated solid-phase extraction system for sub-picomolar detection

A microchip structure etched on a glass substrate for packed column solid-phase extraction (SPE) and capillary electrochromatography (CEC) is described. A 200 microm long, octadecylsilane (ODS) packed column was secured using two different approaches: solvent lock or polymer entrapment. The former method was utilized for SPE while the latter approach was applied for CEC. In SPE, the ODS packed chamber gave a detection limit of 70 fM for a nonpolar BODIPY (493/503) dye when concentrated for 3 min at an electroosmotic flow rate of 4.14 nL/min, compared to 30 pM for this detector without the SPE step. SPE beds showed reproducible, linear calibration curves (R(2) = 0.9989) between 1 and 100 pM BODIPY at fixed preconcentration times. Breakthrough curves for the 330 pL (ODS-packed) bed indicated a capacity for BODIPY dye of 8.1 x 10(-14) mmol, or 0.25 mmol dye per liter of bed. The ODS-chamber could also be used to analyze dilute amino acid and peptide solutions. In the CEC format, two neutral dyes (BODIPY and acridine orange) were baseline-separated in an isocratic run with a theoretical plate count of 84 (420 000 plates/m) and a reduced plate height of about 1. A labeled peptide was also analyzed by CEC, using the acidic eluent (84% acetonitrile, and 26% aqueous trifluoroacetic acid (0.05%)) preferred for peptide separations on ODS-coated silica particles.     

Miniaturization of separation techniques using planar chip technology

Miniaturization of separation columns implies equally reduced vol- umes of injectors, detectors, and the connecting channels. Planar chip technology provides a powerful means for the fabrication of micron-sized structures such as channels. This is demonstrated by two examples. An optical absorbance detector chip exhibits the expected behavior of a 1 mm optical path length cell despite its volume of 1 nL. A capillary electrophoresis device allows integrated injections of 100 pL samples, efficiencies of 70,000 to 160,000 theoretical plates in 10 to 20 seconds, and external laser-induced fluorescence detection at any capillary length of choice between 5 and 50 mm.     

Tandem GC/MS: A useful tool for studying end-capping reactions of oligo(styryl)lithium anions

Anionic living polymerization methods, using organometallic initiators such as butyllithium reagents, have proven useful for, inter alia, styrene polymerization and are amenable to subsequent functionalization of poly(styryl)lithium in the termination step. In this study, general methods for the selective preparation of small styrene oligomers and termination of the intermediate oligo(styryl)lithium anions were investigated. The crude reaction mixtures were analyzed directly by tandem gas chromatography/mass spectrometry (GC/MS). Of the carbon- and silicon-based electrophiles tested, chloro(chloroalkyl)silanes, Cl-SiR₂(CH₂)_nCl in particular, were shown by GC/MS to be regioselective end-capping reagents, thus allowing subsequent transformation to the primary amine. The combined GC/MS data allow not only an estimate of the degree of functionalization, but also the identification of by-products, thus providing insight into the end-capping process that otherwise might be difficult to access. © 1995 John Wiley & Sons, Inc.     

CFD evaluation of drop retraction methods for the measurement of interfacial tension of surfactant-laden drops

Drop retraction methods are popular means of measuring the interfacial tension between immiscible polymers. Experiments show that two different drop retraction methods, imbedded fiber retraction (IFR) and deformed drop retraction (DDR), give inconsistent results when a surfactant is present on the surface of the drop. These inconsistencies are deemed to be due to dilution of the surfactant and due to gradients in interfacial concentration of surfactant along the drop surface. This physical picture is quantified for the simple case of a Newtonian drop in a Newtonian matrix, with an insoluble, nondiffusive surfactant at the interface. The drop is deformed in computational fluid dynamics simulations by shearing the matrix, and then allowed to retract. Dilution and interfacial tension gradients effects are found to be especially large at the early stages of retraction, making IFR unsuitable for measuring the interfacial tension of surfactant-laden interfaces. The effects of surfactant dilution and gradients are found to persist even at late stages of retraction, causing the DDR method to underestimate the equilibrium interfacial tension significantly. The largest underestimates occur when the drop viscosity is lower than the matrix viscosity.     

Misting of non-Newtonian liquids in forward roll coating

Misting of liquids in forward roll coating is a problem under certain conditions. The relaxation time is known to influence misting but the fundamental mechanisms are not clear. A new mechanism for misting of dilute non-Newtonian liquids was proposed based on visualizations with a high-speed camera. With these liquids, filaments were created which sometimes transformed into beads-on-string structures and the beads were ejected as mist droplets when the structures broke. Misting was quantified by measuring sizes of the generated droplets, their count and mass concentration. The measurements were related to elasticity of the solutions through their relaxation times. Small levels of elasticity reduce the amount of misting, but higher levels lead to an increase.     

A comparison of boundary element and finite element methods for modeling axisymmetric polymeric drop deformation

A modified boundary element method (BEM) and the DEVSS‐G finite element method (FEM) are applied to model the deformation of a polymeric drop suspended in another fluid subjected to start‐up uniaxial extensional flow. The effects of viscoelasticity, via the Oldroyd‐B differential model, are considered for the drop phase using both FEM and BEM and for both the drop and matrix phases using FEM. Where possible, results are compared with the linear deformation theory. Consistent predictions are obtained among the BEM, FEM, and linear theory for purely Newtonian systems and between FEM and linear theory for fully viscoelastic systems. FEM and BEM predictions for viscoelastic drops in a Newtonian matrix agree very well at short times but differ at longer times, with worst agreement occurring as critical flow strength is approached. This suggests that the dominant computational advantages held by the BEM over the FEM for this and similar problems may diminish or even disappear when the issue of accuracy is appropriately considered. Fully viscoelastic problems, which are only feasible using the FEM formulation, shed new insight on the role of viscoelasticity of the matrix fluid in drop deformation. Copyright © 2001 John Wiley & Sons, Ltd.