The performance of some liquid phase flow-through detectors

Miniaturization of flow-through analytical systems such as flow injection analysis and liquid chromatography is necessary to achieve higher rates of sample throughput and smaller absolute detection limits. During this miniaturization, however, it is essential to preserve the concentration detection limits of the devices used. Some principles for liquid phase flow-through detection, u.v.-visible absorption photometry, fluorimetry and electrochemical detection, are discussed from this point of view. It appears that, although technical difficulties will be large, there are good prospects of detecting concentrations of about 1 ng ml^?1, while keeping the peak broaadening down to values of about 1 nl.     

Spectroelectrochemical detector for flow-injection systems and liquid chromatography

Flow-through spectroelectrochemical detectors for flow-injection systems and liquid chromatography are described. The detectors have a rectangular flow channel with a reticulated vitreous carbon working electrode followed by an open optical window. The dead volumes of the cells are 27 μl (liquid chromatography) and 80 μl (flow injection). In situ spectral monitoring of reaction products and intermediates for compounds that are both weakly and highly absorbing is demonstrated by using o-tolidine and N,N,N′,N′-tetramethyl-p-phenylenediamine. As a detector for flow-injection systems, components in two-component mixtures can be quantified. As a detector for liquid chromatography, simultaneous absorbance and electrochemical chromatograms allow more eluting compounds to be identified and quantified. Mixtures of nitro- and chloro-phenols are used to illustrate the simultaneous profiling of spectral and redox properties.     

Characterization of different reversed phase systems in liquid adsorption chromatography of polymer homologous series

The interaction parameter of a given repeat unit in liquid adsorption chromatography (LAC) can be determined from the slope in a plot of the elution volumes versus the difference in elution volumes of subsequent monomers. In such a plot, the intercept represents the void volume, and from the slope the adsorption interaction parameter can be calculated. This parameter is independent of column dimensions and pore diameter and can thus be used as a measure of the interaction of a given repeat unit with the surface of a stationary phase in a given mobile phase composition. The interaction parameter can also be obtained from the slope in a plot of the logarithmic retention factors k versus the number of repeat units n or from the slope in a plot of the logarithmic difference in elution volumes of subsequent monomers versus n. The values obtained by the three different approaches are in good agreement. In a given mobile phase, the interaction parameter of a given repeat unit was found to be almost the same for various alkyl bonded columns from different producers. The retention of a given oligomer depends as well on the interaction parameter as on the pore dimensions (and thus the internal pore surface) of the stationary phase. The pore surface can be determined from the intercept in a plot of the logarithmic difference in elution volumes of (subsequent) nonfunctional monomers as a function of n. As the interaction parameter of a given repeat unit in a given mobile phase is the same for stationary phases with the same chemical nature, retention can be adjusted by selection of the pore surface. On a given stationary phase, there is a linear dependence between interaction parameter and mobile phase composition.     

Electrochemical flow-through detector for the determination of cystine and related compounds

An electrochemical double cell for the detection of cystine in aqueous liquid streams is described. A column electrode of amalgamated silver powder is used to reduce cystine quantitatively to cysteine, which is detected amperometrically at a mercury electrode. The double cell is applied to the detection of cystine, cysteine and penicillamine in high-pressure liquid chromatography and to the selective determination of cystine and cysteine by flow-injection analysis.     

On-line coupling of packed column supercritical fluid chromatography with multiple wavelength detector and radioactivity flow-through detector

Summary The coupling of packed column supercritical fluid chromatography with a commercial Radioactivity Flow-Through Detector for the sensitive and highly selective detection of radioactive nuclides (e.g.¹⁴C is described. The radioactivity flow-through detector showed no baseline shift when pressure or modifier (methanol) gradients were applied. The detector cell was pressure resistant even at high flow rates, over a period of approx. 100 hours and showed no leakage problems even at 40°C, 340 bar, 4.0 ml/min and a modifier content up to 20%.     

Lyotropic liquid crystalline phase behaviour in amphiphile-protic ionic liquid systems

Approximate partial phase diagrams for nine amphiphile-protic ionic liquid (PIL) systems have been determined by synchrotron source small angle X-ray scattering, differential scanning calorimetry and cross polarised optical microscopy. The binary phase diagrams of some common cationic (hexadecyltrimethyl ammonium chloride, CTAC, and hexadecylpyridinium bromide, HDPB) and nonionic (polyoxyethylene (10) oleyl ether, Brij 97, and Pluronic block copolymer, P123) amphiphiles with the PILs, ethylammonium nitrate (EAN), ethanolammonium nitrate (EOAN) and diethanolammonium formate (DEOAF), have been studied. The phase diagrams were constructed for concentrations from 10 wt% to 80 wt% amphiphile, in the temperature range 25 °C to >100 °C. Lyotropic liquid crystalline phases (hexagonal, cubic and lamellar) were formed at high surfactant concentrations (typically >50 wt%), whereas at <40 wt%, only micelles or polydisperse crystals were present. With the exception of Brij 97, the thermal stability of the phases formed by these surfactants persisted to temperatures above 100 °C. The phase behaviour of amphiphile-PIL systems was interpreted by considering the PIL cohesive energy, liquid nanoscale order, polarity and ionicity. For comparison the phase behaviour of the four amphiphiles was also studied in water.     

Characterization of retentivity of reversed phase liquid chromatography columns

There are dozens of commercially available reversed phase columns, most marketed as C-8 or C-18 materials, but with no useful way of classifying their retentivity. A useful way of ranking these columns in terms of column "strength" or retentivity is presented. The method utilizes a value for ln k'(w), the estimated retention of a solute from a mobile phase of 100% water, and the slope of the plot of ln k' vsE(T)(30), the solvent polarity. The method is validated with 26 solutes varying in ln k'(w) from about 2 to over 20, on 14 different reversed phase columns. In agreement with previous work, it is found that the phase volume ratio of the column is the most important parameter in determining retentivity. It is strongly suggested that manufacturers adopt a uniform method of calculating this value and that it be made available in advertising, rather than the uninterpretable "% carbon".     

A flow-through detector for simultaneous determination of glucose and urea in serum samples

Glucose and urea electrodes are prepared by immobilizing glucose oxidase and urease on nylon net and fixing the nets on oxygen and ammonia gas sensors. Both enzyme electrodes are fixed in a single flow cell (40 μl volume). Serum is diluted tenfold with 0.1 M Tris buffer (pH 8.3) to fit the calibration graphs for both sensors. Samples are pumped for 1 min, with wash periods of 2 min for recovery to the baseline. Results on serum samples are in good agreement with the results obtained by conventional spectrophotometry.     

Cure reaction and phase behavior of liquid crystalline epoxides-anhydride systems

A series of novel liquid crystalline epoxides with lateral substituents were cured with anhydrides and the cure kinetics was investigated by non-isothermal DSC technique. The results showed that the lengths of lateral substituents have great effect on the value of Ea. The curing reaction became less active, when the liquid crystalline epoxides have long lateral substituents and were controlled by diffusion at the late stage of cure. A nematic structure was observed by POM and XRD.     

Interdependence of liquid crystal phase transitions in mixed systems

From calorimetric measurements, the phase transition enthalpies and entropies in mixed systems of each of two cholesteryl esters with a nematogenic compound PCPB have been determined as a function of composition. The calorimetric data for the LC-I transition, as well as the slope (dp/dT)_t of the phase boundary line LC-I, show a conspicuous discontinuity at the critical mole fraction of PCPB where the blue phases disappear. The enthalpy of the SmA-N* transition becomes zero at the same critical mole fraction. Possible reasons for the observed transition interdependence are discussed.     

Characteristics of an amperometric flow-through detector with a renewable stationary mercury electrode

The construction of the detector is described and its performance is assessed for 1,4-benzoquinone by flow-injection analysis. The detection limit is 0.5 ng and the relative standard deviation of the peak current is 0.4% for a 102-ng sample. The peak current is highly sensitive to temperature changes of the mercury electrode system. The relative temperature coefficient is 15% K^-1, whereas the corresponding value with respect to the mobile phase is 0.6% K^-1. The dependence of the peak current on the volume flow rate is described and explained by appropriate models.     

Interdependence of liquid crystal phase transitions in mixed systems

From calorimetric measurements, the phase transition enthalpies and entropies in mixed systems of each of two cholesteryl esters with a nematogenic compound PCPB have been determined as a function of composition. The calorimetric data for the LC-I transition, as well as the slope (dp/dT)_t of the phase boundary line LC-I, show a conspicuous discontinuity at the critical mole fraction of PCPB where the blue phases disappear. The enthalpy of the SmA-N* transition becomes zero at the same critical mole fraction. Possible reasons for the observed transition interdependence are discussed.     

Characterization of multiple carbon fibre microelectrodes in a flow-through cell and amperometric detection with dilute electrolyte for liquid chromatography

A simple, convenient and sensitive flow-through cell incorporating multiple carbon fibres in a polyvinyl chloride tube was constructed for high performance liquid chromatography. The voltammetric behaviour, electrode treatment, stability and hydrodynamic voltammograms of such electrodes are discussed. An in-situ electrochemical pretreatment is proposed for activation and recovery of the activity of the carbon fibre electrode in flow-stream detection. The desirable detector was found to be nearly flow-rate independent. Catecholamines can be detected at concentrations as low as 2 × 10⁻⁹ M. The cell could be used in the mobile phase with little electrolyte. It was found that the solution resistance in the flow pathway was the major source of distortion of the shape of hydrodynamic voltammograms and of high noise at dilute electrolyte. Parallel detection using dual working electrodes is demonstrated for reversed-phase chromatographic separation of catecholamines.     

Modified cylindrical cavity-type phase separator for liquid/liquid extraction in flow-injection systems

The dimensions of a cylindrical cavity-type phase separator for liquid/liquid extraction in flow-injection analysis were examined in order to achieve smooth, long-term and efficient phase separations with good reproducibility and high sensitivity. Several kinds of filter (PTFE membrane filters and a phase-separating filter) for the organic phase were also compared. A phase separator with a cylindrical cavity (4.2-mm diameter, 2.3-mm deep) and a conical cavity (5-mm diameter, 1.7-mm deep) was the most desirable. A less expensive phase-separating filter can be used instead of a PTFE membrane filter.     

Application of pulse techniques in a polarographic flow-through detector with potential-controlled drop synchronization

The use of pulse techniques and current sampling available at the dropping mercury electrode (DME) is described for polarographic flow-through detectors. The necessary synchronization between the drop lifetime and the desired measuring technique is achieved by means of a potential pulse supplied to a horizontal DME with a partially-conical glass capillary.     

Ultra rapid prototyping of microfluidic systems using liquid phase photopolymerization

We present a method for the ultra rapid prototyping of microfluidic systems using liquid phase photopolymerization, requiring less than 5 min from design to prototype. Microfluidic device fabrication is demonstrated in a universal plastic or glass cartridge. The method consists of the following steps: introduction of liquid prepolymer into the cartridge, UV exposure through a mask to define the channel geometry, removal of unpolymerized prepolymer, and a final rinse. Rapidly fabricated masters for polydimethylsiloxane micromolding are also demonstrated. The master making process is compared to SU-8 50 photoresist processes. Press-on connectors are developed and demonstrated. All materials used are commercially available and low cost. An extension of these methods (mix and match) is presented that allows for maximal design flexibility and integration with a variety of existing fluidic geometries, components, and processes.     

3-D electrode designs for flow-through dielectrophoretic systems

Traditional methods of dielectrophoretic separation using planar microelectrodes have a common problem: the dielectrophoretic force, which is proportional to nabla|E|2, rapidly decays as the distance from the electrodes increases. Recent advances in carbon microelectromechanical systems have allowed researchers to create carbon 3-D structures with relative ease. These developments have opened up new possibilities in the fabrication of complex 3-D shapes. In this paper, the use of 3-D electrode designs for high-throughput dielectrophoretic separation/concentration/filtration systems is investigated. 3-D electrode designs are beneficial because (i) they provide a method of extending the electric field within the fluid. (ii) The 3-D electrodes can be designed so that the velocity field coincides with the electric field distribution. (iii) Novel electrode designs, not based on planar electrodes designs, can be developed and used. The electric field distribution and velocity fields of 3-D electrode designs that are simple extensions of 2-D designs are presented, and two novel electrode designs that are not based on 2-D electrode designs are introduced. Finally, a proof-of-concept experimental device for extraction of nanofibrous carbon from canola oil is demonstrated.     

Miscibility and phase behavior of water-dicarboxylic acid type ionic liquid mixed systems

Tetra-n-butylphosphonium type ionic liquids with fumarate anion and maleate anion exhibit different physico-chemical properties and different solubility to water in their cis and trans conformations; fumarate showed the usual upper critical solution temperature (UCST) behavior, whereas maleate had highly unusual lower critical solution temperature (LCST) behavior after mixing with water.