Effect Of Dead Volume Research Articles

Enhanced machining of micron-scale features in microchip molding masters by CNC milling

An improved approach for the generation of aluminum masters used in the replication of polymer-based microfluidic devices is proposed. It is commonly assumed that conventional computer numerical controlled (CNC)-milling yields insufficient specifications for use in micro-milling. As molding masters suitable for microfluidic device replication require features whose dimensions are on the order of tens to hundreds of microns, the previous limitations of CNC-milling include machining tolerances that exceed 25% of the machined feature and inability to mill intersecting features. Here, we present several novel and advanced machining techniques to overcome existing limitations of a CNC-milling based approach. To achieve microfeatures with tolerances up to a factor of ten better than expected from the CNC-mill used, external calibration and measurement of the milling process was employed. To reduce the unwanted volume occurring from two intersecting features, 102 μm dia. high-density micrograin carbon endmills were used with all milling steps externally calibrated and measured. To generate masters with abutting features of different widths, keyway slots were milled into the master that could accommodate various size molding master keys. In addition to adding flexibility to the downstream embossing step, the use of keys eliminated dead-volume effects. A variety of molding masters have been machined and characterized in this work. Taken together, these approaches illustrate that conventional CNC-milling using equipment commonly found in university machine shops is a viable alternative to other more expensive master generation processes. Molding masters can be machined to yield high aspect ratio microfeatures with a tolerance of ±4 μm, and these masters can then be used in conjunction with casting or hot embossing to produce polymer microchip platforms suitable for analytical use.

Atmospheric molded poly(methylmethacrylate) microchip emitters for sheathless electrospray.

Disposable poly(methylmethacrylate) (PMMA) sheathless electrospray microchip emitters were prepared for the first time using the atmospheric molding fabrication protocol. A sheathless electrospray from uncoated channel outlets, machined to cone-shaped three-dimensional tips, is demonstrated utilizing a simple cross design with an on-chip liquid junction to obviate the need for external unions to voltage electrodes, thus reducing the dead volume effects as well as the complexity of fabrication. The fast replication of microchip emitters was performed by molding prepolymeric methylmethacrylate solutions into silicon-master/aluminum-spacer/glass-plate molds followed by UV-initiated free radical polymerization. The performance of the new microchip emitters was demonstrated for mass spectral measurements of methionine enkephalin, adrenocorticotropic hormone and insulin peptide/protein mixtures. The samples were infused through capillary connections using hydrodynamic pumping. The polymeric emitters prepared by this flexible fabrication route offer an easy way of operation and high stability, without a need for attachment of external voltage unions or metallizing the emitter tips. The new approach should provide a useful low-cost tool for widespread coupling of mass spectrometry to chip systems.

Residence time model for assessing liquid-liquid extraction systems

A model based on the residence time of solvent in the extraction system may be used to describe the dynamic operation of a continuous liquid-liquid extractor. It is proposed that optimum performance occurs when the extract exiting from the contactor is near equilibrium with the solution being extracted. This approach was tested on two commercially available continuous extraction systems. The difference in their performance was found to be related to the level of agitation of the two contactor vessels: the stirred system was approximately six times more efficient than the simple column continuous extractor. The stirred system achieved a near equilibrium analyte distribution between the solvent and sample and could be described accurately in terms of residence time theory. This was in marked contrast to its unmixed counterpart where the analyte distribution between solvent and sample reached 14% of its equilibrium value during its residence in the contactor. The effect of dead volume of solvent within the extraction assembly on the extraction rate was also apparent; its main effect was to delay the extraction process.

Tandem magnetic refrigerator for 1.8 K

A tandem magnetic refrigerator has been developed for continuous operation between 4.2 and 1.8 K. This paper presents the design, computer simulation and construction of the prototype machine. The regenerative concept is employed in the magnetic refrigerator to obtain a cascaded magnetic Carnot cycle effect along the temperature axis from the cold end to the warm end. Entropy pumping action from the cold heat reservoir to the warm heat reservoir occurs in the active magnetic regenerator. The inherent sources of irreversibility in regenerative magnetic refrigerators, i.e. heat capacity imbalance between magnetic refrigerant and heat transport medium, helium entrainment and dead volume effect, have been minimized by design optimization of the system components. The magnetic system of the tandem refrigerator has two virtually identical units, each consisting of a gadolinium gallium garnet (GGG; Gd 3Ga 5O 12) magnetic core, a superconducting magnet, a warm end heat exchanger and a cold end heat exchanger. These components are united by a cryogenic displacer which shuttles the heat transport medium, subatmospheric 3He gas, between the two units. The prototype of the designed magnetic refrigerator has operated continuously, producing a net refrigeration rate of 12 mW per magnetic core at 1.8 K.

Accounting for the effect of turbulent flow in large-bore fluidized bed pressure probes

Pneumatic pressure probes have been used to collect information such as bubble frequency, size and presence from fluidized beds. In a common set-up, a probe is inserted into the bed and connected to a pressure transducer via small-diameter tubing. The resistance of the tubing to friction and the effects of dead volume in the probe and transducer can cause the time-varying pressure signals at the transducer to be unrepresentative of the pressure in the bed at the tip of the probe. In these systems, it is assumed by previous models that the flow in the probe system is laminar. The model in this work takes into consideration the effects of turbulent flow in the probe transmission line. A numerical model was constructed and data show that oscillating measurements are possible. Experimental evidence was also collected from a slugging fluidized bed to examine system response.

A simple procedure for hydrodynamic injection in flow injection analysis applied to the atomic absorption spectrometry of chromium in steels

The proposed technique for hydrodynamic injection is based on commutation in a simple flow-injection system with only one peristaltic pump. Precise results (usually r.s.d. < 1%) are attained even when injected volumes are small (1–10 μl). Dead-volume effects are associated with the mechanical flexibility of the system and with diffusion; differences between effective sample size and the dimensions of the injection duct are emphasized, particularly for small volumes. The proposed technique is checked for a spectrophotometric model system and confirmed by the atomic absorption spectrometry of chromium in steels. With a sampling rate of 120 h −1, the precision is better than 99% for chromium contents in the 0.0–30.0% (w/w) range. The results agree well with those obtained by i.c.p./a/e/s. and conventional a.a.s.

The determination of zeolite crystal diffusivity by gas chromatography—I. Theoretical

A mathematical model is developed which describes the pulse response of a chromatographic column packed with crystal zeolite powder, thus allowing the crystal diffusivity to be determined independently of the pellet diffusivity which is present in more conventional beds. This continuum model allows for the effects of both pressure drop and dead volume in the apparatus.The first and second moments of the pulse response are derived and criteria are presented for the neglect of the dead volume terms in these moments, baThe model equations are solved numerically and simulations are presented to illustrate the pressure drop and dead volume effects.

Reliability and limitations of pulse chromatography in evaluating properties of flow systems II. end effects

AbstractDead volume cannot be avoided in measurements of kinetic and transport parameters using pulse gas chromatography. A criterion is derived for the bed volume necessary to make these dead volume effects negligible. Application of the criterion and assumptions made in its development are discussed.

Effect of dead volume on efficiency of a gas chromatographic system

Effect of dead volume on efficiency of a gas chromatographic system

Terpenoids : LXXXV. Evaluation of polyesters as stationary phases in GLC columns

Polyesters from diferent 1,ω-glycols, ranging from ethylene glycol to 1,15-pentadecanediol, with adipic acid were prepared and evaluated as substrates for gas-liquid chromatography. Several terpenic and a few non-terpenic compounds were analysed. It was observed that the retention time is a characteristic of the number of methylene groups in the glycolic fragment of the polyester and, as before, follows a linear relationship, which starts deviating at 1,10-decanediol. The effect on retention times by using ethylene glycol and changing the acid fragment was also studied. Similarly, the effect on retention times by using 1,5-pentanediol (which has the same number of polyvalent atoms in the chain unit as diethylene glycol) and changing the acidic fragment was also examined. From the above mentioned observations, it was clear that the retention times of hydrocarbon-type compounds may be enormously increased by using a polyester derived from very-long-chain dicarboxylic acid and long-chian, 1,ω-glycol. However, the retention times of hydrocarbons reach a certain saturation point above which their solubilities do not increase appreciably. Finally, the effect of dead volume of the column was discussed with respect to the case of long-chain as well as short-chain polyesters. Some of the stationary phases described in the paper are likely to be very useful for capillary columns and for preparative gas-liquid chromatography.

Estimation and Correction of Post-Column Dead Volume Effect in Chromatography.

Estimation and Correction of Post-Column Dead Volume Effect in Chromatography.