Hyphenation of flow injection/sequential injection with chemical hydride/vapor generation atomic fluorescence spectrometry

The dominant role played by flow injection/sequential injection (FI/SI, including lab-on-valve, LOV) in automatic on-line sample pretreatments coupling to various detection techniques is amply demonstrated by the large number of publications it has given rise to. Among these, its hyphenation with hydride/vapor generation atomic fluorescence spectrometry (HG/VG-AFS) has become one of the most attractive sub-branches during the last years, attributed not only to the high sensitivity of this technique, but also to the superb separation capability of hydride/vapor forming elements from complex sample matrices. In addition, it also provides potentials for the speciation of the elements of interest.It is worth mentioning that quite a few novel developments of sample pretreatment have emerged recently, which attracted extensive attentions from the related fields of research. The aim of this mini-review is thus to illustrate the state-of-the-art progress of implementing flow injection/sequential injection and miniaturized lab-on-valve systems for on-line hydride/vapor generation separation and preconcentration of vapor forming elements followed with detection by atomic fluorescence spectrometry, within the period from 2004 up to now. Future perspectives in this field are also discussed.     

Development of an Amperometric Biosensor for β‐N‐Oxalyl‐L‐α,β‐Diaminopropionic Acid (β‐ODAP)

An amperometric method for the determination of the neurotoxic amino acid β‐N‐oxalyl‐L ‐α,β‐diaminopropionic acid (β‐ODAP) using a screen printed carbon electrode (SPCE) is reported. The electrode material was bulk‐modified with manganese dioxide and used as a detector in flow injection analysis (FIA). The enzyme glutamate oxidase (GlOx) was immobilized in a Nafion‐film on the electrode surface. The performance of the biosensor was optimized using glutamate as an analyte. Optimum parameters were found as: operational potential 440 mV (vs. Ag/AgCl), flow rate 0.2 mL min^?1, and carrier composition 0.1 mol L^?1 phosphate buffer (pH 7.75). The same conditions were used for the determination of β‐ODAP. The signal was linear within the concentration range 53–855 μmol L^?1 glutamate and 195–1950 μmol L^?1 β‐ODAP. Detection limits (as 3σ value) for both analytes were 9.12 and 111.0 μmol L^?1, respectively, with corresponding relative standard deviations of 3.3 and 4.5%. The biosensor retained more than 73% of its activity after 40 days of on‐line use.     

Evaluation of different mediator-modified screen-printed electrodes used in a flow system as amperometric sensors for NADH

This work presents a comparative study between two different methods for the preparation of mediator-modified screen-printed electrodes, to be used as detectors in a reliable flow injection system for the determination of the nicotinamide adenine dinucleotide (NADH) coenzyme. The best strategy was selected for the final development of compact biosensors based on dehydrogenase enzymes. For the first immobilisation strategy, different redox mediators were electropolymerised onto the SPE surface. The second immobilisation strategy was carried out using polysulfone–graphite composites, which were deposited by screen-printing technology onto the screen-printed electrode (SPE) surface. Both methods achieved an effective and reliable incorporation of redox mediators to the SPE configuration. Finally, a flow system for ammonium determination was developed using a glutamate dehydrogenase (GlDH)-Meldola's Blue (MB)-polysulfone-composite film-based biosensor.

The stability of the redox mediators inside the composite films as well as the negligible fouling effect observed on the electrode surface improve the repeatability and reproducibility of the sensors, important features for continuous analysis in flow systems. Furthermore, the optimised bio/sensors, incorporated in a flow injection system, showed good sensitivities and short response times. Such a good analytical performance together with the simple and fast sensor construction are interesting characteristics to consider the polysulfone-composite films as attractive electrochemical transducer materials for the development of new dehydrogenase-based SPEs.     

Flow analysis-hydride generation-Fourier transform infrared spectrometric determination of antimony in pharmaceuticals

In this work, a flow analysis system with hydride generation and Fourier transform infrared (FTIR) spectrometric detection has been developed for the determination of antimony in pharmaceuticals. The method is based on the on-line mineralization/oxidation of the organic antimonials present in the sample and pre-reduction of Sb(V) to Sb(III) with K₂S₂O₈ and KI, respectively; prior to the stibine generation. The gaseous SbH₃ is separated from the solution in a gas phase separator, and transported by means of a nitrogen carrier into a short pathway (10 cm) IR gas cell, where the corresponding FTIR spectrum is acquired by accumulating 3 scans in a continuous mode. The 1893 cm⁻¹ band was used for the quantification of the antimony. The procedure is carried out in a closed system, which reduces sample handling and makes possible the complete automation of the antimony determination. The figures of merit of the proposed method (linear range: 0-600 mg l⁻¹, limit of detection (3σ)=0.9 mg l⁻¹, limit of quantification (10σ)=3 mg Sb l⁻¹, precision (R.S.D.) less than 1% and sample frequency=28 h⁻¹), are appropriate for the designed application. Furthermore, precise and accurate results were found for the analysis of different antimonial pharmaceutical samples, indicating that the methodology developed represents a valid alternative for the determination of antimony in pharmaceuticals, which could be suitable for the routine control analysis.     

Small-volume radial flow cell for all-solid-state ion-selective electrodes

A flow cell with a radial distribution of four all-solid-state ion selective electrodes (ISEs), or alternatively three ISEs and one reference electrode, was designed and optimized for mass production. The radial distribution of the electrodes reduces the cell volume and is expected to minimize cross-contamination between different electrodes. Two different cell prototypes were developed and tested for all-solid-state K⁺-ISEs based on a solvent polymeric ion-selective membrane (ISM) and a conducting polymer, poly(3,4-ethylenedioxythiophene), as solid internal contact. In the first prototype, PEDOT was electropolymerized from an aqueous solution of the monomer and the doping ion salt, sodium polystyrenesulfonate (NaPSS). The second prototype employed an aqueous dispersion of PEDOT(PSS) that is commercially available (Baytron P, Bayer AG). Compared to electrochemical synthesis, solution casting of the polymer dispersion was found to be a more advantageous method to deposit the conducting polymer layer aiming at mass production. The resulting prototypes of the flow cell had a small volume (ca. 17-37 μl), which makes them suitable for application in clinical analysis.     

On-column polymer-imbedded graphite inlet electrode for capillary electrophoresis coupled on-line with flow injection analysis in a poly(dimethylsiloxane) interface

A method for coupling an electrophoretic driven separation to a liquid flow, using conventional fused-silica capillaries and a soft polymeric interface is presented. A novel design of the electrode providing high voltage to the electrophoretic separation was also developed. The electrode consisted of a conductive polyimide/graphite imbedded coating immobilized onto the capillary electrophoresis (CE) column inlet. This integrated electrode gave the same separation performance as a commonly used platinum electrode. The on-column electrode also showed good electrochemical stability in chronoamperometric experiments. In addition, with this electrode design, the electrode position relative to the inlet end of the CE column will always be constant and well defined. The on-line flow injection analysis (FIA)-CE system was used with electrospray ionization (ESI)-time of flight (TOF)-mass spectrometry detection. The preparation of the PDMS (poly(dimethylsiloxane)) interface for FIA-CE is described in detail and used for initial tests of the on-column polymer-imbedded graphite inlet electrode. In this interface, a pressure-driven liquid flow, a make up CE electrolyte and a CE column inlet meet in a two-level cross (95 microm ID) in the PDMS structure, enabling independent flow characterization.     

Self-assembled monolayer gold electrode for surfactant analysis

A gold electrode coated with a self-assembled monolayer of octane-thiol (SAM/Au) has been used as an amperometric detector for the determination of surfactants. This detector operated in the presence of a high percentage of organic solvent and was adapted to an HPLC System. At the SAM/Au, the electrochemical response of an electroactive tracer (potassium ferricyanide) was completely inhibited, but, in the presence of a cationic surfactant, the electrochemical reduction was progressively restored. In flow injection analysis, using the SAM/Au in an amperometric flow-through detector polarised at 0.0 V vs Ag/AgCl, a linear response (i=f{[surfactant]}) was observed for cationic surfactants e.g. cetylpyridinium chloride in the concentration range 2 × 10⁻⁶–1 × 10⁻³ M. The electrochemical data along with the determination of the ion pair stoichiometry between the redox tracer and the surfactant suggest an electrochemical response related to ion pair formation and governed by electron transfer by tunneling effect. Received: 28 January 1997 / Accepted: 7 March 1997     

Simultaneous Determination of Ascorbic Acid and Uric Acid by Using a PVC/TTF‐TCNQ Composite Electrode as Detector in a FIA System

A PVC/TTF‐TCNQ composite electrode has been employed as detector in a flow injection system. The proposed method allows the simultaneous detection of ascorbic acid (AA) and uric acid (UA) in mixtures by using a FIA system in a simple manner, without pre‐treatment or modified electrode. This method is based on the amperometric determination of (a) ascorbic acid at 0.15 V and (b) both analytes at 0.35 V, being the response linear in the range 1×10^?2–4×10^?4 M for both analytes with detection limits (S/N=3) of 1.2×10^?4 M and 8.1×10^?5 M for AA and UA, respectively.     

A photometric flow measurements method for characterisation of HPLC pumps

Summary The measurement of flow constancy and pulsation amplitudes of HPLC pump based on the photoconversion of malachite green leucocyanide is described. The irradiation time and hence the degree of conversion of the leucocyanide is correlated to flow fluctuations of piston driven HPLC pumps. It is possible to measure the amplitude of pulsations and determine the constancy of the flow rate. Most of the pumps show a flow stability better than ±1% measured as relative standard deviation of the flow rate under HPLC conditions (pressure drop 100 bar at 1 ml/min flow rate). The most expensive pump of those tested showed less than ±0.5% flow instability, however, this result was achieved by installation of a large-volume pulsation damper. The method described also allows flow rate measurements under FIA conditions where there is little or no pressure drop at the pump outlet.     

双光束流动注射光度法测定生活用水中氯离子

将流动注射分析技术应用到双光束分光光度法中，对其理论和实验技术进行了研究，采用自行组装的流动注射系统进行了生活用水中氯离子的测定，获得满意结果。