A robust multi-syringe system for process flow analysis. Part 3. Time based injection applied to the spectrophotometric determination of nickel(II) and iron speciation.

A new software-controlled time-based system for sample or reagent introduction in process flow injection analysis was developed. By using a multi-syringe burette coupled with one multi-port selection valve, the time-based injection of precise known volumes was accomplished. Characteristics and performance of the injection system were studied by injecting an indicator in a buffered carrier. Two multi-syringe time-based injection (MS-TBI) systems were implemented: first, the injection of a sample in a multiple-channel manifold where the sample would sequentially merge and react with different reagents, and second, the sequential injection of several solutions (sample and reagents) into a particular flowing stream. The first system was applied to the spectrophotometric determination of nickel(II) in diluted samples from the acidic nickel ore leaching process, by using ammonium citrate as carrier, a saturated solution of iodine as oxidizing agent and alkaline dimethylglyoxime as chromogenic reagent. The sampling frequency attained was 57 h-1. Determinations on process samples compared well at the 95% confidence level with the reference values obtained by ICP-OES. The second time-based injection system was applied to the speciation of iron. Total iron and iron(II) concentrations were separately and sequentially determined using 1,10-phenanthroline in acetic buffer medium as reagent. The developed manifold allowed the optional use of two different carrier solutions, containing or not containing ascorbic acid, for performing the separate determinations. Also, in the sequential procedure, plugs of reducing carrier were alternatively intercalated before the sample injections used for total iron determinations. Sampling frequencies of 68 injections per hour were routinely used. Accuracy was assessed by analyzing synthetic known mixtures of Fe(III) and Fe(II) standard solutions. Recoveries of 98-100.5% with a maximum relative standard deviation of 3.6% were found. Results obtained for various samples of fertilizers agreed well with those attained by the standard batch procedure.     

Further improvement of hydrostatic pressure sample injection for microchip electrophoresis

Hydrostatic pressure sample injection method is able to minimize the number of electrodes needed for a microchip electrophoresis process; however, it neither can be applied for electrophoretic DNA sizing, nor can be implemented on the widely used single-cross microchip. This paper presents an injector design that makes the hydrostatic pressure sample injection method suitable for DNA sizing. By introducing an assistant channel into the normal double-cross injector, a rugged DNA sample plug suitable for sizing can be successfully formed within the cross area during the sample loading. This paper also demonstrates that the hydrostatic pressure sample injection can be performed in the single-cross microchip by controlling the radial position of the detection point in the separation channel. Rhodamine 123 and its derivative as model sample were successfully separated.     

High performance flow flame atomic absorption spectrometry for interference-free trace determination

In High Performance Flow Atomic Spectrometry (HPF-AS), the aerosol is generated by injecting the sample solution into the gas mixing chamber with the aid of an HPLC pump (Hydraulic High Pressure Nebulization, HHPN, working pressure about 200 bar). Using an additional analytical HPLC column, between sample injection valve and nebulization nozzle, the elements under investigation can be separated from interfering elemental traces or major components (High Performance/Pressure Flow System, HPF). The elements to be determined and the interfering substances reach the flame at different times. This means, for example, that traces of Mg in solutions containing aluminum can be determined without interference. Compared to pneumatic nebulization, there is a 20 fold increase in the detection power when determining calcium in a saturated NaCl solution. HPF-AS is a rapid and sensitive method which allows separation of the calcium from the matrix with subsequent determination in less than 90 s.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Control-free air vent system for ultra-low volume sample injection on a microfabricated device.

An improved method of sample injection was demonstrated for introducing ultra-low volume liquid on a microfabricated device. In our previous study, a pressure-driven injection method has been introduced and was applied to on-chip electrophoresis. In this study, the need for control of the air vent, which was indispensable for sample injection in the previous study, was completely eliminated, facilitating sample injection with great simplicity and high accuracy. This was realized by altering the topology of the air vent channel, which is connected to a hydrophobic and narrow channel (called a passive valve). Several types of air vent channels were designed and their injection performances were tested. In addition, by modifying the shape and the position of air vent channel and passive valve, the residual liquid volume inside the passive valve after sample injection was decreased to approximately 0.5% of the injected volume, a value which showed high reproducibility.     

Hydrodynamic injection with pneumatic valving for microchip electrophoresis with total analyte utilization

A novel hydrodynamic injector that is directly controlled by a pneumatic valve has been developed for reproducible microchip CE separations. The PDMS devices used for the evaluation comprise a separation channel, a side channel for sample introduction, and a pneumatic valve aligned at the intersection of the channels. A low pressure (≤ 3?psi) applied to the sample reservoir is sufficient to drive sample into the separation channel. The rapidly actuated pneumatic valve enables injection of discrete sample plugs as small as ~ 100?pL for CE separation. The injection volume can be easily controlled by adjusting the intersection geometry, the solution back pressure, and the valve actuation time. Sample injection could be reliably operated at different frequencies (< 0.1?Hz to > 2?Hz) with good reproducibility (peak height relative standard deviation ≤ 3.6%) and no sampling biases associated with the conventional electrokinetic injections. The separation channel was dynamically coated with a cationic polymer, and FITC-labeled amino acids were employed to evaluate the CE separation. Highly efficient (≥ 7.0 × 103 theoretical plates for the ~2.4-cm-long channel) and reproducible CE separations were obtained. The demonstrated method has numerous advantages compared with the conventional techniques, including repeatable and unbiased injections, little sample waste, high duty cycle, controllable injected sample volume, and fewer electrodes with no need for voltage switching. The prospects of implementing this injection method for coupling multidimensional separations for multiplexing CE separations and for sample-limited bioanalyses are discussed.     

Optimization of injection technique in capillary ion electrophoresis for the determination of trace level anions in environmental samples

Capillary ion electrophoresis (Waters' trade name: Capillary Ion Analysis) offers several advantages compared to ion chromatography for the analysis of ionic solutes, primarily simplicity, matrix independence, and a different separation selectivity. The use of electromigration sample introduction leads to on-capillary enrichment of ionic analytes at the sample—buffer interface, permitting the determination of low ng/ml levels of anions in environmental samples of moderate ionic strength. This injection method allows improved sensitivity compared to hydrostatic injection; and is significantly more rapid than precolumn concentration ion chromatography. The analyte enrichment rate, hence peak response, is strongly dependent upon ionic strength and appropriate measures, such as standard addition or internal standard techniques, must be used to account for differences in standard and sample conductance.     

Effects of sample matrix and injection plug on dsDNA migration in capillary gel electrophoresis

Reproducible DNA migration times are required for accurate basepair assignment in restriction fragment mapping and polymerase chain reaction product identification. Our data shows DNA migration time shifts with changes in sample ionic strength. Secondly, loss of resolution with replaceable polyacrylamide gels was observed when increasing the length of the sample plug with pressure injection. An easy way to correct for the migration time shifts is to incorporate an internal DNA standard directly into the separation process by consecutively injecting the DNA sample and the DNA standard. This allows for compensation of any possible migration time variation caused by high ionic strength sample matrices. Also high-resolution separations can be maintained with large injection volumes (long injection plug) by using consecutive injections of 0.1 M Tris-acetate buffer and the DNA sample.     

Injection and flow control system for microchannels

In spite of considerable efforts, flow control in micro-channels remains a challenge owing to the very small ratio of channel/supply-system volumes, as well as the induction of spurious flows by extremely small pressure or geometry changes. We present here an inexpensive and robust system for flow control in a microchannel system, based on a dynamic control of reservoir pressures at the end of each channel. This system allows flow equilibration with a time constant smaller than one second, and is also able to maintain stable flux from stopped flow to many microl min(-1) range over several hours. It is robust to changes in ambient pressure and temperature. This system further includes a feature for sub-microliter sample injection during the experiment. We quantify flow control in elastomer and thermoplastic channels, and demonstrate the impact on one application of the system, namely the reproducible, automated separation of large DNA by electrophoresis in a self-organized magnetic bead matrix in a microchannel.     

“Groove-injection” as a novel method for sub-microliter (nanoliter) injections in liquid chromatography using a conventional multi-port valve

In liquid chromatography with “low-dispersion methods”, there is an increasing need to reproducibly inject nanoliter sample volumes. Low-dispersion methods produce very narrow peaks because of short column length, narrow column bore, small particle packing, low particle surface area, open tubular configuration, or combinations of these parameters. This paper describes a new injector method, the “groove-injector” which involves simple plumbing changes for use of a conventional multi-port valve (8-ports or more) to inject sample volumes approximating a single groove in such a valve (e.g 30 nanoliters with aceptable reproducibility, ca. 8% RSD). In addition, by changing a resistor, volumes between 30, and over 2,000 nanoliters (nearly two orders of magnitude) can be injected with reproducibilities generally below 2% RSD. Different samples can be injected by using an autosampler. Compared to commonly used 4-port valves for nanoliter injections, multi-port valves have a number of advantages. Multiport valves generally are more commonly available and they are a better financial investment because of their versatility for column switching, sample enrichment, or variable volume injections. Previously, submicroliter (nanoliter) injections have not been possible with multi-port valves in as direct and simple a manner as described here.     

Parallel separation of multiple samples with negative pressure sample injection on a 3-D microfluidic array chip

A simple and powerful microfluidic array chip-based electrophoresis system, which is composed of a 3-D microfluidic array chip, a microvacuum pump-based negative pressure sampling device, a high-voltage supply and an LIF detector, was developed. The 3-D microfluidic array chip was fabricated with three glass plates, in which a common sample waste bus (SW(bus)) was etched in the bottom layer plate to avoid intersecting with the separation channel array. The negative pressure sampling device consists of a microvacuum air pump, a buffer vessel, a 3-way electromagnet valve, and a vacuum gauge. In the sample loading step, all the six samples and buffer solutions were drawn from their reservoirs across the injection intersections through the SW(bus) toward the common sample waste reservoir (SW(T)) by negative pressure. Only 0.5 s was required to obtain six pinched sample plugs at the channel crossings. By switching the three-way electromagnetic valve to release the vacuum in the reservoir SW(T), six sample plugs were simultaneously injected into the separation channels by EOF and electrophoretic separation was activated. Parallel separations of different analytes are presented on the 3-D array chip by using the newly developed sampling device.     

A flow injection-capillary electrophoresis system with high-voltage contactless conductivity detection for automated dual opposite end injection

The system comprises two flow injection-capillary electrophoresis interfaces into which the opposite ends of the separation capillary are inserted. The electrolyte solution flows through both interfaces by use of hydrostatic pressure. The injection of the samples into the electrolyte flow is accomplished by a rotary-type chromatographic valve at the grounded side and by a pinch-valve injector at the high-voltage side that provides sufficient isolation from the high electric field. The system allows a fully automated dual-injection sequence of samples from both capillary ends and simultaneous electrophoretic separation of anions and cations in the samples. The analytes are detected by a high-voltage contactless conductometric detector positioned approximately in the middle of the separation capillary. The parameters of the system were evaluated. The repeatability of the flow injection-capillary electrophoresis system for the simultaneous determination of anions and cations was evaluated for ten consecutive injections and relative standard deviation (RSD) values for peak areas were better than 1.0%. The sample throughput for total ionic analysis was estimated to be 25 samples per hour. The system was used for automated simultaneous analysis of anions and cations in various real samples. Using a short separation capillary, rapid total ionic analysis in less then 1 min is demonstrated.     

Acupuncture injection for field amplified sample stacking and glass microchip‐based capillary gel electrophoresis

Acupuncture sample injection is a simple method to deliver well‐defined nanoliter‐scale sample plugs in PDMS microfluidic channels. This acupuncture injection method in microchip CE has several advantages, including minimization of sample consumption, the capability of serial injections of different sample solutions into the same microchannel, and the capability of injecting sample plugs into any desired position of a microchannel. Herein, we demonstrate that the simple and cost‐effective acupuncture sample injection method can be used for PDMS microchip‐based field amplified sample stacking in the most simplified straight channel by applying a single potential. We achieved the increase in electropherogram signals for the case of sample stacking. Furthermore, we present that microchip CGE of ΦX174 DNA‐HaeⅢ digest can be performed with the acupuncture injection method on a glass microchip while minimizing sample loss and voltage control hardware.     

Multiplexed detection of nitrate and nitrite for capillary electrophoresis with an automated device for high injection efficiency

A simple automated nanoliter scale injection device which allows for reproducible 5 nL sample injections from samples with a volume of <1 microL is successfully used for conventional capillary electrophoresis (CE) and Hadamard transform (HT) CE detection. Two standard fused silica capillaries are assembled axially through the device to function as an injection and a separation capillary. Sample solution is supplied to the injection capillary using pressure controlled with a solenoid valve. Buffer solution flows gravimetrically by the junction of the injection and separation capillaries and is also gated with a solenoid valve. Plugs of sample are pushed into the space between the injection and separation capillaries for electrokinectic injection. To evaluate the performance of the injection device, several optimizations are performed including the influence of flow rates, the injected sample volume and the control of the buffer transverse flow on the overall sensitivity. The system was then applied to HT-CE-UV detection for the signal-to-noise ratio (S/N) improvement of the nitric oxide (NO) metabolites, nitrite and nitrate. In addition, signal averaging was performed to explore the possibility of greater sensitivity enhancements compared to single injections.     

Automated stopped-in-dual-loop flow analysis system for catalytic determination of vanadium in drinking water

An automated stopped-in-dual-loop flow analysis (SIDL-FA) system is proposed for the determination of vanadium in drinking water. The chemistry is based on the vanadium-catalyzed oxidation reaction of p-anisidine by bromate in the presence of Tiron as an activator to produce a dye (λ_max = 510 nm). A SIDL-FA system basically consists of a selection valve, three pumps (one is for delivering of standard/sample, and others are for reagents), two six-way injection valves, a spectrophotometric detector and a data acquisition device. A 100-μL coiled loop around a heated device is fitted onto each six-way injection valve. A well-mixed solution containing reagents and standard/sample is loaded into the first loop on a six-way valve, and then the same solution is loaded into the second loop on another six-way valve. The solutions are isolated by switching these two six-way valves, so that the catalytic reaction can be promoted. The net waste can be zero in this stage, because all pumps are turned off. Then each resulting solution is dispensed to the detector with suitable time lag. A touchscreen controller is developed to automatically carry out the original SIDL-FA protocol. The proposed SIDL-FA method allows vanadium to be quantified in the range of 0.1-2 μg L⁻¹ and is applied to the determination of vanadium in drinking water samples.     

Microfluidic sequential injection analysis system based on polydimethylsiloxane (PDMS) chip with integrated pneumatic-actuated valves

A sequential injection analysis (SIA) system based on polydimethylsiloxane (PDMS) chip with integrated pneumatic-actuated valves was developed. A novel SIA operation mode using multiphase laminar flow effect and pneumatic microvalve control was proposed. The sample and reagent solutions were synchronously loaded and injected in the chip-based sample injection module instead of multi-step sequential injection by a multiposition valve and a reciprocating pump as in conventional SIA system. The sample and reagent injection volumes were reduced to ca. 1.1 nL. The present system has the advantages of simple structure, fast and convenient operation, low sample and reagent consumption, and high degree of integration and automation. The system operation conditions were optimized using fluorescein as model sample. Its feasibility in biological analysis was preliminarily demonstrated in enzyme inhibition assay.     

Sample preconcentration by field amplification stacking for microchip-based capillary electrophoresis

A microchip structure for field amplification stacking (FAS) was developed, which allowed the formation of comparatively long, volumetrically defined sample plugs with a minimal electrophoretic bias. Up to 20-fold signal gains were achieved by injection and separation of 400 microm long plugs in a 7.5 cm long channel. We studied fluidic effects arising when solutions with mismatched ionic strengths are electrokinetically handled on microchips. In particular, the generation of pressure-driven Poiseuille flow effects in the capillary system due to different electroosmotic flow velocities in adjacent solution zones could clearly be observed by video imaging. The formation of a sample plug, stacking of the analyte and subsequent release into the separation column showed that careful control of electric fields in the side channels of the injection element is essential. To further improve the signal gain, a new chip layout was developed for full-column stacking with subsequent sample matrix removal by polarity switching. The design features a coupled-column structure with separate stacking and capillary electrophoresis (CE) channels, showing signal enhancements of up to 65-fold for a 69 mm long stacking channel.     

Rapid and efficient isotachophoretic preconcentration in free solution coupled with gel electrophoresis separation on a microchip using a negative pressure sampling technique

We present a novel isotachophoresis–gel electrophoresis (ITP–GE) microchip system designed for rapid and efficient isotachophoretic preconcentration coupled with gel electrophoresis separation by using a negative pressure sampling technique. The overall ITP–GE procedure involves only three steps: sample loading, ITP preconcentration and GE separation and was controlled by a simple and compact negative pressure sampling device, which is composed of a vacuum vessel, a three-way electromagnetic valve and a single high voltage power supply. During the sample loading stage, a negative pressure was applied via a three-way electromagnetic valve in headspace of the two sealed sample waste reservoirs (SWs). A sandwiched sample zone between a leading and a terminating electrolyte zone was formed in the channel intersection in less than 1 s. Once the three-way electromagnetic valve was switched to connect SWs to ambient atmosphere to release vacuum in SWs, ITP preconcentration in free solution and GE separation in the 4% hydroxyethylcellulose (HEC) sieving material were consequently activated under the electric potentials applied. The performance of present approach was evaluated by using DNA fragments as model analytes. Compared to conventional cross microchip GE using electrokinetic pinched injection, an average signal enhancement of 185-fold was obtained with satisfactory resolution. The results demonstrated the ITP–GE approach possessing an exciting potential of high sensitivity and short sampling time with significant simplification in operation and instrumentation.     

Development a robust ionic liquid-based dispersive liquid-liquid microextraction against high concentration of salt combined with flame atomic absorption spectrometry using microsample introduction system for preconcentration and determination of cobalt in water and saline samples

A robust ionic liquid-based method for dispersive liquid-liquid microextraction was developed for the determination of cobalt in saline samples. Its robustness was increased by introducing a common ion of the IL into the sample solution. The concentration of cobalt in the settled phase of the ionic liquid was determined by the FAAS using a homemade microsample injection valve. Under the optimized conditions, the calibration graph was linear in the concentration range from 0.4 to 120 ??g L^?1 with the detection limit of 0.1 ??g L^?1. Accuracy was checked against the ISO standard method. The method was successfully applied to the determination of cobalt in water and saline samples.     

New approach to large-volume injection in reversed-phase high performance liquid chromatography: determination of atrazine and hydroxyatrazine in soil samples

A well established method of direct injection of larger than conventional sample volumes ranging from 0.1 mL to 10 mL in HPLC is the injection valve method in which a loop of tubing is totally or partially filled with sample. Recent HPLC pumps have a flow-rate setting accuracy of +/- 1-2% over a flow-rate range from 0.1 mL/min to 10 mL/min and the flow stability is 0.2% or less. Quarternary low pressure gradient pumps are widely available and used, but all their hydraulic lines are seldom utilised. The idea of using one line of a common commercial HPLC quaternary low-pressure pump for direct on-column injection (pumping) of large sample volumes ranging from 1 mL to 100 mL was tested. This approach was evaluated during practical work on the development of an RP-HPLC method of determination of residual atrazine and hydroxyatrazine. In lysimetric environmental experiments hydroxyatrazine was formed in situ in a soil column by hydrolysis of atrazine. The results proved the applicability of this approach not only in experiments with model mixtures of analytes at microg/L levels in solutions. Analysis of 20 mL of soil leachates and extracts of soil samples containing atrazine and hydroxyatrazine at the 10 microg/kg level (in dry soil) revealed that good figures-of-merit were preserved, even in the presence of a large excess of humic substances.     

On-line introduction of high-pressure gas-liquid sample for capillary gas chromatographic analysis

An on-line sample introduction technique in capillary gas chromatograph (CGC) for the analysis of high-pressure gas-liquid mixtures has been designed and evaluated. A sample loop of 0.05 microL and a washing solvent loop of 0.5 microL are mounted on a 10-port switching valve, which serves as the injection valve. A capillary resistor was connected to the vent of sample loop in order to maintain the pressure of the sample. Both the sample and the washing solvent are transferred into the split-injection port through a narrow bore fused silica capillary inserted into the injection liner through a septum. The volume of the liner is used both as the pressure-release damper and evaporation chamber of the sample. On-line analysis of both reactants and resultants in ethylene olimer reaction mixture at 5 MPa was carried out, which demonstrated the applicability of the technique.