Programmable Millifluidic Platform Integrating Automatic Electromembrane Extraction Cleanup and In-Line Electrochemical Detection: A Proof of Concept

A fully automatic millifluidic sensing platform coupling in-line nonsupported microelectromembrane extraction (μ-EME) with electrochemical detection (ECD) is herein proposed for the first time. Exploiting the features of the second generation of flow analysis, termed sequential injection (SI), the smart integration of SI and μ-EME-ECD enables (i) the repeatable formation of microvolumes of phases for the extraction step in a membrane-less (nonsupported) arrangement, (ii) diverting the acceptor plug to the ECD sensing device, (iii) in-line pH adjustment before the detection step, and (iv) washing of the platform for efficient removal of remnants of wetting film solvent, all entirely unsupervised. The real-life applicability of the miniaturized sensing system is studied for in-line sample cleanup and ECD of diclofenac as a model analyte after μ-EME of urine as a complex biological sample. A comprehensive study of the merits and the limitations of μ-EME solvents on ECD is presented. Under the optimal experimental conditions using 14 μL of unprocessed urine as the donor, 14 μL of 1-nonanol as the organic phase, and 14 μL of 25 mM NaOH as the acceptor in a 2.4 mm ID PTFE tubing, an extraction voltage of 250 V, and an extraction time of 10 min, an absolute (mass) extraction recovery of 48% of diclofenac in urine is obtained. The proposed flow-through system is proven to efficiently remove the interfering effect of predominantly occurring organic species in human urine on ECD with RSD% less than 8.6%.

A Robust Flow-Based System for the Spectrophotometric Determination of Cr(VI) in Recreational Waters

A flow-based method for the spectrophotometric determination of chromium (VI) in recreational waters with different salinities was developed. Chromium can occur in the environment in different oxidation states with different related physiological properties. With regard to chromium, the speciation is particularly important, as the hexavalent chromium is considered to be carcinogenic. To achieve that purpose, the use of the diphenylcarbazide (DPC) selective colored reaction with the hexavalent chromium was the chosen strategy. The main objective was to develop a direct and simple spectrophotometric method that could cope with the analysis of different types of environmental waters, within different salinity ranges (fresh to marine waters). The potential interference of metal ions, that can usually be present in environmental waters, was assessed and no significant interferences were observed (<10%). For a complete Cr(VI) determination (three replicas) cycle, the corresponding reagents consumption was 75 µg of DPC, 9 mg of ethanol and 54 mg of sulfuric acid. Each cycle takes about 5 min, including the system clean-up. The limit of detection was 6.9 and 12.2 µg L−1 for waters with low and high salt content, respectively. The method was applied for the quantification of chromium (VI) in both fresh and marine water, and the results were in agreement with the reference procedure.

Sequential Injection Analysis for Automation and Evaluation of Drug Liberation Profiles: Clotrimazole Liberation Monitoring

A fully automated sequential injection system was tested in terms of its application in liberation testing, and capabilities and limitations were discussed for clotrimazole liberation from three semisolid formulations. An evaluation based on kinetic profiles obtained in short and longer sampling intervals and steady-state flux values were applied as traditional methods. The obtained clotrimazole liberation profile was faster in the case of Delcore and slower for Clotrimazol AL and Canesten cream commercial formulations. The steady-state flux values for the tested formulations were 52 µg cm^-2 h^-1 for Canesten, 35 µg cm^-2 h^-1 for Clotrimazol AL, and 7.2 µg cm^-2 h^-1 for Delcore measured in 4 min sampling intervals. A simplified approach for the evaluation of the initial rate based on the gradient between the second and third sampling points was used for the first time and was found to correspond well with the results of the conventional methods. A comparison based on the ratio of the steady-state flux and the initial rate values for Canesten and Clotrimazol AL proved the similarity of the obtained results. The proposed alternative was successfully implemented for the comparison of short-term kinetic profiles. Consequently, a faster and simpler approach for dissolution/liberation testing can be used.

Flow Injection Techniques for Tetracycline Quantification: A Review

The multiple therapeutic potentials of tetracycline and its worldwide usage have encouraged researchers to develop various methods for its assay in various matrices and for different purposes. In this regard, different analytical techniques have been exploited. Among those techniques is flow injection (FI), which is an extended family of three generations and five versions. The current manuscript reviews the utilization of FI techniques for developing assay methods for tetracycline. The review covers more than forty methods, since the inception of FI techniques and up to date. The review highlights the advantages of the application of FI techniques for quantification of tetracycline in terms of reagent consumption, sample frequency, accuracy, and practitioner safety, besides instrumentation simplicity and cost-effectiveness. The review also addresses applications to several matrices ranging from simple matrices such as standard solutions and pharmaceutical formulations to complex matrices such as biological fluids and food. Prior to the review, a brief background on the principles and developments of FI techniques is illustrated.

Use of a Polymer Inclusion Membrane and a Chelating Resin for the Flow-Based Sequential Determination of Copper(II) and Zinc(II) in Natural Waters and Soil Leachates

A bi-parametric sequential injection method for the determination of copper(II) and zinc(II) when present together in aqueous samples was developed. This was achieved by using a non-specific colorimetric reagent (4-(2-pyridylazo)resorcinol, PAR) together with two ion-exchange polymeric materials to discriminate between the two metal ions. A polymer inclusion membrane (PIM) and a chelating resin (Chelex 100) were the chosen materials to retain zinc(II) and copper(II), respectively. The influence of the flow system parameters, such as composition of the reagent solutions, flow rates and standard/sample volume, on the method sensitivity were studied. The interference of several common metal ions was assessed, and no significant interferences were observed (<10% signal deviation). The limits of detection were 3.1 and 5.6 µg L⁻¹ for copper(II) and zinc(II), respectively; the dynamic working range was from 10 to 40 µg L⁻¹ for both analytes. The newly developed sequential injection analysis (SIA) system was applied to natural waters and soil leachates, and the results were in agreement with those obtained with the reference procedure.

Development of an In Situ Analyzer Based on Sequential Injection Analysis and Liquid Waveguide Capillary Flow Cell for the Determination of Dissolved Reactive Phosphorus in Natural Waters

This study presents an innovative technique for the in situ analysis of aquatic biochemical elements detected through wet chemical processes. A new compact in situ phosphate analyzer based on sequential injection analysis, liquid waveguide capillary flow cell and spectrophotometry was developed, and a safe and modular electronics-chemical separation mechanical structure was designed. The sequential injection system of this analyzer was optimized, and the major functions of this analyzer were studied and estimated. With a 10 cm liquid waveguide capillary flow cell and a 6.3 min time cost of detection, the analyzer reaches a detection limit of 1.4 μg·L⁻¹ (≈14.7 nM, [PO₄³⁻]) and a consumption of 23 μL at most for each reagent. This analyzer was operated in situ and online during two scientific research cruises in the Pearl River Estuary and northern South China Sea. The advantages of this analyzer include its simple versatile manifold, full automation, low chemical consumption and electronics-chemical separate safe structure. Long-term in situ performance of this analyzer will be validated in the future.

Novel Approach to Sample Preconcentration by Solvent Evaporation in Flow Analysis

A preconcentration module operated in flow mode and integrated with a sequential injection system with spectrophotometric detection was developed. Using the system, preconcentration was performed in continuous mode and was based on a membraneless evaporation process under diminished pressure. The parameters of the proposed system were optimized and the system was tested on the example of the spectrophotometric determination of Cr(III). The preconcentration effectiveness was determined using the signal enhancement factor. In the optimized conditions for Cr(III), it was possible to obtain the signal enhancement factors of around 10 (SD: 0.9, n = 4) and determine Cr(III) with precision and intermediate precision of 8.4 and 5.1% (CV), respectively. Depending on the initial sample volume, signal enhancement factor values of about 20 were achieved. Applicability of the developed preconcentration system was verified in combination with the capillary electrophoresis method with spectrophotometric detection on the example of determination of Zn in certified reference materials of drinking water and wastewater. Taking into account the enhancement factor of 10, a detection limit of 0.025 mg L⁻¹ was obtained for Zn determination. Zn was determined with precision less than 6% (CV) and the results were consistent with the certified values.

Automatic On-Line Purge-and-Trap Sequential Injection Analysis for Trace Ammonium Determination in Untreated Estuarine and Seawater Samples

An innovative automatic purge-and-trap (P&T) system coupled with fluorimetric sequential injection (SI), for the on-line separation and preconcentration of volatile compounds, is presented. The truth of concept is demonstrated for the ammonium fluorimetric determination in environmental water samples with complex matrices without any pretreatment. The P&T flow system comprises a thermostated purge-vessel where ammonium is converted into gaseous ammonia and a trap-vessel for ammonia collection. This configuration results in matrix removal as well as analyte preconcentration, avoiding membrane-associated problems. All the main parameters affecting the efficiency of a P&T system were studied and optimized. The proposed method is characterized by a working range of 2.7–150.0 μg L⁻¹ of NH₄⁺, with a detection and quantification limit of 0.80 and 2.66 μg L⁻¹, respectively, for a 10-mL sample consumption. The accuracy of the method was assessed by recovery assays in seawater, estuarine, and lake water samples as well as by the analysis of standard reference material.

Development of a Sequential Injection Analysis System for the Determination of Saccharin

Saccharin is a powerfully sweet nonnutritive sweetener that has been approved for food-processing applications within the range of 100-1200 mg/kg. A simple, rapid, and cost-effective sequential injection analysis (SIA) technique was developed to determine the saccharin level. This method is based on the reaction of saccharin with p-chloranil in an ethanol medium with a hydrogen peroxide (H₂O₂) acceleration, and the resultant violet-red compound was detected using a UV-Vis spectrophotometer at λ_max = 420 nm. To ascertain the optimal conditions for the SIA system, several parameters were investigated, including buffer flow rate and volume, p-chloranil concentration, and reactant volumes (saccharin, p-chloranil, and H₂O₂). The optimum setup of the SIA system was achieved with a buffer flow rate, buffer volume, and draw-up time of 1.2 mL/min, 2900 µL, and ~145 s, respectively. The optimal p-chloranil concentration is 30 mM, and the best reactant volumes, presented in an ordered sequence, are as follows: 30 µL of H₂O₂, 450 µL of saccharin, and 150 µL of p-chloranil. The optimized SIA configuration produced a good linear calibration curve with a correlation coefficient (R² = 0.9812) in the concentration range of 20-140 mg/L and with a detection limit of 19.69 mg/L. Analytical applications in different food categories also showed acceptable recovery values in the range of 93.1-111.5%. This simple and rapid SIA system offers great feasibility for the saccharin quality control in food-product processing.

Development of an Evaluation Method for Hydroxyl Radical Scavenging Activities Using Sequential Injection Analysis with Chemiluminescence Detection

A method for evaluating hydroxyl radical (·OH) scavenging activities using sequential injection analysis (SIA) with chemiluminescence (CL) detection was developed. In this system, CL was produced by the reaction of luminol with ·OH generated from the Fenton reaction. The scavenging activity was expressed as a diminution rate of the CL due to the scavenging of ·OH by a sample. The SIA system allows the automation of a series of experimental procedures including Fenton's reaction, scavenging of ·OH, and luminol CL reaction. The evaluation of scavenging activities in one sample (n = 3) was completed within 3.0 min. Relative standard deviations (n = 3) of scavenging activity with 700 μM L-ascorbic acid were 2.6% (intraday) and 3.7% (interday). The SIA-CL system was applied to measure ·OH scavenging activities of several antioxidants and pharmaceuticals.

Monitoring lipase/esterase activity by stopped flow in a sequential injection analysis system using p-nitrophenyl butyrate

Lipases and esterases are biocatalysts used at the laboratory and industrial level. To obtain the maximum yield in a bioprocess, it is important to measure key variables, such as enzymatic activity. The conventional method for monitoring hydrolytic activity is to take out a sample from the bioreactor to be analyzed off-line at the laboratory. The disadvantage of this approach is the long time required to recover the information from the process, hindering the possibility to develop control systems. New strategies to monitor lipase/esterase activity are necessary. In this context and in the first approach, we proposed a lab-made sequential injection analysis system to analyze off-line samples from shake flasks. Lipase/esterase activity was determined using p-nitrophenyl butyrate as the substrate. The sequential injection analysis allowed us to measure the hydrolytic activity from a sample without dilution in a linear range from 0.05-1.60 U/mL, with the capability to reach sample dilutions up to 1000 times, a sampling frequency of five samples/h, with a kinetic reaction of 5 min and a relative standard deviation of 8.75%. The results are promising to monitor lipase/esterase activity in real time, in which optimization and control strategies can be designed.

Automated evaluation of pharmaceutically active ionic liquids' (eco)toxicity through the inhibition of human carboxylesterase and Vibrio fischeri

The toxicity of 16 pharmaceutical active ionic liquids (IL-APIs) was evaluated by automated approaches based on sequential injection analysis (SIA). The implemented bioassays were centered on the inhibition of human carboxylesterase 2 and Vibrio fischeri, in the presence of the tested compounds. The inhibitory effects were quantified by calculating the inhibitor concentration required to cause 50% of inhibition (EC50). The EC50 values demonstrated that the cetylpyridinium group was one of the most toxic cations and that the imidazolium group was the less toxic. The obtained results provide important information about the safety of the studied IL-APIs and their possible use as pharmaceutical drugs. The developed automated SIA methodologies are robust screening bioassays, and can be used as a generic tools to identify the (eco)toxicity of the structural elements of ILs, contributing to a sustainable development of drugs.

Amperometric biosensor for oxalate determination in urine using sequential injection analysis

An amperometric flow biosensor for oxalate determination in urine samples after enzymatic reaction with oxalate oxidase immobilized on a modified magnetic solid is described. The solid was magnetically retained on the electrode surface of an electrode modified with Fe (III)-tris-(2-thiopyridone) borate placed into a sequential injection system preceding the amperometric detector. The variables involved in the system such as flow rate, aspired volumes (modified magnetic suspension and sample) and reaction coil length were evaluated using a Taguchi parameter design. Under optimal conditions, the calibration curve of oxalate was linear between 3.0-50.0 mg·L⁻¹, with a limit of detection of 1.0 mg·L⁻¹. The repeatability for a 30.0 mg·L⁻¹ oxalate solution was 0.7%. The method was validated by comparing the obtained results to those provided by the spectrophotometric method; no significant differences were observed.