Open sessile droplet viscometer with low sample consumption.

This paper reports a portable viscometer that requires less than 10 µL of sample for a measurement. Using a two-droplet Laplace-induced pumping system on an open microfluidic substrate, the device measures the viscosity of a liquid by determining the time required for one droplet to completely pump into a second droplet. The pumping behaviour follows the Hagen-Poiseuille and Laplace relations where the flow rate, Q, is proportional to the liquid's kinematic viscosity, µ. The progress of pumping is measured by tracking the change in curvature of one of the droplets using a laser that is positioned perpendicular to the microfluidic chip and directed at the "tail" of the shrinking droplet. The angle of incidence and degree of refraction changes depending on the size of the droplet, which is tracked by a linear diode array placed beneath the microfluidic chip. Droplet reservoirs and connecting channels were defined by precise patterning of a glass substrate coated with a commercially available omniphobic coating (Ultra Ever Dry®) using laser micromachining. A 500 µm wide and 20 mm long channel with circular reservoirs (d = 1.5 mm) enabled the measurement of dynamic viscosities in the range of η = 1.0-2.87 mPa s. The materials cost for the entire viscometer (fluidics and electronics, etc.) is <15 USD.

Polymer micronozzle array for multiple electrosprays produced by templated synthesis and etching of microstructured fibers.

Microstructured fibers (MSFs) having raised polymer nozzles in each channel are custom designed, fabricated, and tested for use as multiple electrospray (MES) emitters for mass spectrometry (MS). There is strong motivation to develop electrospray emitters that operate at practical flow rates but give the much greater ionization efficiency associated with lower (nano) flow rates. This can be accomplished by splitting the flow into many lower-volume electrosprays, an approach known as MES. To couple with most modern mass spectrometers, the MES emitter must have a small diameter to allow efficient ion collection into the MS. In this work, a MSF, defined as a fiber having many empty channels running along its length, was designed to have 9 channels, 9 µm each, >100 µm apart arranged in a radial pattern, all in a fiber having a compatible diameter with both front-end LC equipment and typical MS inlets. This design seeks to promote independent electrospray from each channel while maintaining electric field homogeneity. While the MSFs themselves do not support MES, the formation of polymer nozzles protruding from each channel at the tip face enables independent electrospray from each nozzle. Microscope imaging, electrospray current measurement, and ESI-MS detection of a model analyte all confirm the MES behavior of the 9-nozzle emitter, showing significant signal enhancement relative to a single-nozzle emitter at the same total flow rate. LC/MS data from a protein digest obtained at an independent laboratory demonstrates the applicability and robustness of the emitter for real scientific challenges using modern LC/MS equipment.

Development of fluorous porous polymer monolith (FPPM) for the capillary electrochromatographic separation of fluorous analytes based on fluorous-fluorous interaction.

This is the first report on the CEC separation of fluorous analytes on a fluorous porous polymer monolith (FPPM) stationary phase based on fluorous-fluorous interaction. Monolithic columns do not require retaining frits and can be conveniently photo-patterned within a capillary. Two groups of fluorous compounds, a N-f-Cbz-4-nitro-benzylamine (N) series and a N-f-Cbz-4-phenyl-benzylamine (P) series, each series having compounds differing only by the length of their perfluorinated tag, were employed to evaluate the ability of the fluorinated column to separate fluorous analytes using a variety of mobile phase compositions and separation conditions. Fluorous monoliths showed enhanced separation performance by providing better selectivity, higher resolution and shorter analysis time compared to a similar non-fluorous (reversed phase) monolithic column. Under optimal conditions, column efficiency as high as 234,000 plates per metre was achieved, and all four compounds of the N series were fully resolved in <5 minutes. Perfluoromethylene selectivity was used to quantitatively evaluate the interaction between the perfluorinated chain on the analytes and both the FPPM and non-FPPM columns. It was found that the non-FPPM column resolves fluorous analytes mainly based on reversed phase interaction while the FPPM column resolves them mainly based on fluorous-fluorous interaction. Results are compared to fluorous monolith columns used in a nano-liquid-chromatographic (nano-LC) separation with gradient elution. The FPPM column required less than one fifth the analysis time in CEC mode than was required in nanoLC mode, with superior separation efficiency and resolution. FPPM stationary phases provide an attractive option for the analysis of perfluorinated analytes, which is expected to be useful in areas such as proteomics for the separation of fluorously tagged proteins, and in environmental analysis where fluorinated species are of increasing concern.

Multiple electrosprays generated from a single polycarbonate microstructured fibre.

Electrospray ionization (ESI) has been invaluable to the mass spectrometric detection of biomolecules, due largely to the sensitivity afforded by the ionization technique. Lower flow rates, e.g. in the nanoelectrospray regime, result in smaller initial electrosprayed droplets, leading to higher ionization efficiency and greater signal. One approach to improving sensitivity without lowering flow rate is to generate multiple electrosprays (MESs) from the same sample, essentially splitting one larger flow into smaller flows in the nanoESI regime. Presented here is a series of novel MES emitters in the form of polycarbonate fibres. Based on microstructured fibre (MSF) technology whereby a set of homogeneous parallel channels are formed in a heat-drawn fibre intended to conduct light, a custom design was fabricated in which 3, 6, 9 and 12 holes were arranged in a radial pattern to prevent inhomogeneities in the electric field. The MSFs have dimensions that are compatible with current standards in nanoESI equipment, and the tip is more compatible with standard MS orifices than other larger multielectrospray emitters. By measuring the spray current provided by the various emitters under the same solvent/voltage/total flow rate conditions, a plot was obtained clearly demonstrating the expected dependence on the square root of the number of holes, i.e. the number of independent electrosprays. With this firm proof of principle using this design/format, further effort is justified in developing similar emitters in alternative materials that better prevent surface wetting and allow greater hole density, ultimately leading to greater signal enhancement.

Bundled capillary electrophoresis using microstructured fibres.

Joule heating, arising from the electric current passing through the capillary, causes many undesired effects in CE that ultimately result in band broadening. The use of narrow-bore capillaries helps to solve this problem as smaller cross-sectional area results in decreased Joule heating and the rate of heat dissipation is increased by the larger surface-to-volume ratio. Issues arising from such small capillaries, such as poor detection sensitivity, low loading capacity and high flow-induced backpressure (complicating capillary loading) can be avoided by using a bundle of small capillaries operating simultaneously that share buffer reservoirs. Microstructured fibres, originally designed as waveguides in the telecommunication industry, are essentially a bundle of parallel ∼5 µm id channels that extend the length of a fibre having otherwise similar dimensions to conventional CE capillaries. This work presents the use of microstructured fibres for CZE, taking advantage of their relatively high surface-to-volume ratio and the small individual size of each channel to effect highly efficient separations, particularly for dye-labelled peptides.

Fast preparation of photopolymerized poly(benzyl methacrylate-co-bisphenol A dimethacrylate) monoliths for capillary electrochromatography.

A novel porous polymer monolith was prepared in situ in a fused-silica capillary using photoinitiated polymerization. Bisphenol A dimethacrylate (BPADMA) was selected as a crosslinker, copolymerized with benzyl methacrylate (BMA) in the presence of a binary porogenic solvent consisting of cyclohexanol and 1-decanol in < or =10 min. The resulting poly(BMA-co-BPADMA) monoliths exhibited good permeability and mechanical stability. Mixtures of alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs) or phenolic compounds were successfully separated by CEC. A similar monolith was also prepared with ethylene dimethacrylate (EDMA) as the crosslinker instead of BPADMA to compare the separation ability of the resulting monoliths. The results indicated that poly(BMA-co-BPADMA) monoliths have better selectivity for aromatic analytes and greater chromatographic stability in higher aqueous mobile phase.

Reversed phase capillary HPLC using polymer-entrapped C18 particles.

Novel capillary columns containing polymer-entrapped octadecylsilyl-modified silica microspheres were evaluated for chromatographic performance. The polymer forms only at the particle surface and in as little as 10 s strongly immobilizes them, obviating the need for a frit. The photoinitiated polymerization is patternable, and various entrapment styles were compared, all of which withstood pressure drops of at least 5000 psi (345 bar). The fritless nature of the columns allows a unique mass production capability whereby long packed columns are entrapped and simply cut afterward. Since the material has previously been shown to be a proficient nanoelectrospray emitter, the columns have enormous potential for use in capillary LC-MS. The evaluation suggests that for the designs that have minimal entrapped regions, the column performance is equivalent to commercial columns containing similar particles.

Microstructured photonic fibers as multichannel electrospray emitters.

Novel multichannel electrospray emitters are presented that use silica-based microstructured fibers (MSFs) to split the flow allowing efficient desolvation during electrospray. The MSFs investigated in this study possess 30-168 individual fluidic channels (each channel being 5 microm in diameter) that form a 2D emitting array. Multiple flow paths afford stable electrospray at flow rates ranging from the microspray (e.g., 1000 nL/min) to the nanoelectrospray (e.g., 10 nL/min) regime with moderate to negligible flow-induced backpressures. The electrospray stability of highly aqueous solutions (up to 99.9% water with 0.1% acetic acid) is enhanced through modification of the emitting surface with a hydrophobic silylation reagent (chlorotrimethylsilane). Furthermore, by successfully spraying highly concentrated salt solutions, this study demonstrates that multichannel MSF emitters provide enhanced robustness to clogging, leading to increased operational throughput.

Nanoelectrospray emitters: trends and perspective.

The benefits of electrospray ionization are many, including sensitivity, robustness, simplicity and the ability to couple continuous flow methods with mass spectrometry. The technique has seen further improvement by lowering flow rates to the nanoelectrospray regime (<1,000 nL/min), where sample consumption is minimized and sensitivity increases. The move to nanoelectrospray has required a shift in the design of the electrospray source which has mostly involved the emitter itself. The emitter has seen an evolution in architecture as the shape and geometry of the device have proved pivotal in the formation of sufficiently small droplets for sensitive MS detection at these flow rates. There is a clear movement toward the development of emitters that produce multiple Taylor cones. Such multielectrospray emitters have been shown to provide enhanced sensitivity and sample utilization. This article reviews the development of nanoelectrospray emitters, including factors such as geometry and the manner of applying voltage. Designs for emitters that take advantage of multielectrospray are emphasized.

Entrapment of functionalized silica microspheres with photo-initiated acrylate-based polymers.

The entrapment of silica-based microspheres, commonly used as stationary phases in chromatography, with an organic porous polymer based on poly(butyl acrylate-co-1,3-butanediol diacrylate) was explored. The spheres were immobilized by photopolymerization leading to entrapped beds within 75 microm i.d. fused silica capillaries, and were mechanically stable, resisting pressure drops of over 5600 psi (38.6 MPa) for only 1 cm of material. The morphology of the polymer formation around the spheres was investigated by SEM and corroborated with back pressure measurements, which indicated that the spheres were held together by encapsulating polymer. The entrapped material was extruded from the capillary in some cases to facilitate imaging. The entrapment conditions were explored, varying the polarity of the sphere surface, the solvent, and the monomers, revealing that polymer formation is based on partitioning of the monomers between the surface and solvent. The resulting polymer morphology is discussed with respect to the effects of confinement, supported by experiments with varying microsphere diameters. The columns described here have favourable properties for use in capillary chromatography and supported catalysis among other applications, and is suitable for lab-on-a-chip devices.

Potentiometric titration of metal ions in ethanol.

The potentiometric titrations of Zn2+, Cu2+ and 12 Ln3+ metal ions were obtained in ethanol to determine the titration constants (defined as the at which the [-OEt]/[Mx+]t ratios are 0.5, 1.5, and 2.5) and in two cases (La3+ and Zn2+) a complete speciation diagram. Several simple monobasic acids and aminium ions were also titrated to test the validity of experimental titration measurements and to establish new constants in this medium that will be useful for the preparation of buffers and standard solutions. The dependence of the titration constants on the concentration and type of metal ion and specific counterion effects is discussed. In selected cases, the titration profiles were analyzed using a commercially available fitting program to obtain information about the species present in solution, including La3+ for which a dimer model is proposed. The fitting provides the microscopic values for deprotonation of one to four metal-bound ethanol molecules. Kinetics for the La3+-catalyzed ethanolysis of paraoxon as a function of are presented and analyzed in terms of La3+ speciation as determined by the analysis of potentiometric titration curves. The stability constants for the formation of Zn2+ and Cu2+ complexes with 1,5,9-triazacyclododecane as determined by potentiometric titration are presented.

La3+-catalyzed methanolysis of N-aryl-beta-lactams and nitrocefin.

The kinetics of the La(3+)-catalyzed methanolysis of N-phenyl-beta-lactam (2) and N-p-nitrophenyl-beta-lactam (3) as well as that of nitrocefin (1) were studied at 25 degrees C under buffered conditions. In the case of 2 and 3, the observed second-order rate constants (k(2)(obs)) for catalysis plateau at pH 7.5-7.8, reaching values of 1 x 10(-)(2) and 35 x 10(-)(2) M(-)(1) s(-)(1) respectively. Potentiometric titrations of solutions of 2 x 10(-)(3) M La(OTf)(3) were analyzed in terms of a dimer model (La(3+)(2)((-)OCH(3))(n)()), where the number of methoxides varies from 1 to 5. The species responsible for catalysis in the pH range investigated contain 1-3 methoxides, the one having the highest catalytic activity being La(3+)(2)((-)OCH(3))(2), which comprises 80% of the total La(3+) forms present at its pH maximum of 8.9. The catalysis afforded by the La(3+) dimers at a neutral pH is impressive relative to the methoxide reactions: at pH 8.4 a 1 mM solution of catalyst (generated from 2 mM La(OTf)(3)) accelerated the methanolysis of 2 by approximately 2 x 10(7)-fold and 3 by approximately 5 x 10(5)-fold. As a function of metal ion concentration, the La(3+)-catalyzed methanolysis of 1 proceeds by pathways involving first one bound metal ion and then a second La(3+) leading to a plateau in the k(obs) vs [La(3+)](total) plots at all pH values. The k(max)(obs) pseudo-first-order rate constants at the plateaus, representing the spontaneous methanolysis of La(3+)(2)(1(-)) forms, has a linear dependence on [(-)OCH(3)] (slope = 0.84 +/- 0.05 if all pH values are used and 1.02 +/- 0.03 if all but the two highest pH values are used). The speciation of bound 1 at a La(3+) concentrations corresponding to that of the onset of the kinetic plateau region was approximated through potentiometric titration of the nonreactive 3,5-dinitrobenzoic acid in the presence of 2 equiv of La(OTf)(3). A total speciation diagram for all bound forms of La(3+)(2)(1(-))((-)OCH(3))(n)(), where n = 0-5, was constructed and used to determine their kinetic contributions to the overall pH vs k(max)(obs) plot under kinetic conditions. Two kinetically equivalent mechanisms were analyzed: methoxide attack on La(3+)(2)(1(-))((-)OCH(3))(n)(), n = 0-2; unimolecular decomposition of the forms La(3+)(2)(1(-))((-)OCH(3))(n)(), n = 1-3.