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.     

On-line renewable solid-phase extraction hyphenated to liquid chromatography for the determination of UV filters using bead injection and multisyringe-lab-on-valve approach

For the first time, an automatic sample pre-treatment/detection method is proposed for the multiclass determination of UV filters (namely, benzophenone-3, ethylhexylmetoxycinnamate, butylmethoxydibenzoylmethane and homosalate) in environmental samples. The new methodology comprises in-line solid-phase extraction (SPE) of the target analytes by exploiting the bead injection (BI) concept in a mesofluidic lab-on-valve (LOV) format, with subsequent determination by liquid chromatography (LC). The proposed microanalytical system, using a multisyringe burette as propulsion unit, automatically performed the overall SPE steps, including the renewal of the sorbent in each analytical cycle to prevent sample cross-contamination and the post-extraction adjustment of the eluate composition to prevent chromatographic band broadening effects. In order to expedite the LC separation, a C₁₈ monolithic column was applied and an accelerated isocratic elution was carried out by using a cationic surfactant as mobile phase additive. The LOV-BI-LC method was proven reliable for handling and analysis of complex matrices, e.g., spiked swimming pool water and seawater, with limits of detection ranging between 0.45 and 3.2 μg L⁻¹ for 9 mL sample volume. Linear calibration was attained up to 160 μg L⁻¹ for homosalate and up to 35 μg L⁻¹ for the other target analytes, with good reproducibility (RSD < 13%, for 5 different SPE columns). The hyphenated scheme is able to process a given sample simultaneously and within the same time frame than the chromatographic separation/determination of the formerly pre-treated sample, providing concentration values every 9 min. Hence, the sample throughput was enhanced up to 33 times when compared with previously reported off-line SPE methods. A drastic reduction in reagent consumption and effluent production was also attained, contributing to the development of an environment-friendly analyzer.     

Multicommutation flow techniques in the hydride generation-atomic fluorescence determination of arsenic

This review outlines automated methodologies developed for measuring arsenic in environmental samples. We report the state of the art of the most significant methods exploiting multicommutation flow techniques coupled to hydride generation-atomic fluorescence determination. We review analytical methods used and present a comparative evaluation of them. We also discuss the on-line pre-concentration procedure as being of particular interest in the development of fully automated methods.     

A review on sequential injection methods for water analysis

The development of fast, automatic and less expensive methods of analysis has always been the main aim of flow methodologies. The search for new procedures that still maintain the reliability and accuracy of the reference procedures is an ever growing challenge. New requirements are continually added to analytical methodologies, such as lower consumption of samples and reagents, miniaturisation and portability of the equipment, computer interfaces for full decision systems and so on. Therefore, the development of flow methodologies meeting the extra requirements of water analysis is a challenging work.Sequential injection analysis (SIA) presents a set of characteristics that make it highly suitable for water analysis. With sequential injection analysis, most routine determinations in waters can be performed more quickly with much lower reagent consumption when compared to reference procedures. Additionally, SIA can be a valuable tool for analyte speciation and multiparametric analysis. This paper critically reviews the overall work in this area.     

On-line coupling of sequential injection lab-on-valve to differential pulse anodic stripping voltammetry for determination of Pb in water samples

Sequential injection lab-on-valve (LOV) was first proposed for analyzing ultra-trace amounts of Pb using differential pulse anodic stripping voltammetry (DPASV) with a miniaturized electrochemical flow cell fabricated in the LOV unit. Deposition and stripping processes took place between the renewable mercury film carbon paste electrode and sample solution, the peak current was employed as the basis of quantification. The mercury film displayed a long-term stability and reproducibility for at least 50 cycles before next renewal, the properties of integrated miniature LOV unit not only enhanced the automation of the analysis procedure but also declined sample/reagent consumption. Potential factors that affect the present procedure were investigated in detail, i.e., deposition potential, deposition time, electrode renewable procedure and the volume of sample solution. The practical applicability of the present procedure was demonstrated by determination of Pb in environmental water samples.     

Determination of cadmium with a sequential injection lab-on-valve by anodic stripping voltammetry using a nafion coated bismuth film electrode

This work exploited a sequential injection lab-on-valve (LOV) system for the determination of cadmium by anodic stripping voltammetry (ASV). A miniaturized electrochemical flow cell (EFC) was fabricated in LOV, in which a nafion coated bismuth film electrode was used as working electrode. The cadmium was electrodeposited on the electrode surface in bismuth solution, and measured with the subsequential stripping scan. Under optimal conditions, the proposed system responded linearly to cadmium concentrations in a range 2.0-100.0 μg L⁻¹. The detection limit of this method was found to be 0.88 μg L⁻¹. By loading a sample volume of 800 μL, a sampling frequency of 22 determinations h⁻¹ was achieved. The repeatability expressed as relative standard derivation (R.S.D.) was 3.65% for 20 μg L⁻¹ cadmium (n = 11). The established method was applied to analysis of trace cadmium in environmental water samples and the spiked recoveries were satisfactory.     

Field speciation of chromium with a sequential injection lab-on-valve incorporating a bismuthate immobilized micro-column

A fully automated and portable analyzer for field speciation of inorganic chromium in wastewater was developed. The instrument consists of a micro-sequential injection lab-on-valve (LOV) system and a miniature USB2000 spectrophotometer. A multi-purpose flow cell was incorporated on one side of the main body of the LOV, which offers vast potentials and versatilities in its compatibility with various detection modes. On-line oxidation of trivalent chromium was performed on a bismuthate immobilized silica micro-column reactor integrated in the LOV. When determining Cr(VI), its chromogenic reaction with 1,5-diphenylcarbazide (DPC) was facilitated in the flow cell and the absorbance was monitored in situ at 548 nm via optical fibers. While for the quantification of total chromium, Cr(III) was oxidized on-line by aspirating sample solution through the oxidizing column reactor, followed by chromogenic reaction with DPC and the absorbance was monitored in the flow cell. With a sampling volume of 200 μl, the detection limits of 5.6 μg l⁻¹ for Cr(VI) and 6.8 μg l⁻¹ for total chromium were achieved along with a sampling frequency of 60 h⁻¹. A R.S.D. value of 2.0% was recorded at 32 μg l⁻¹ of Cr(VI). The practical applicability of the speciation analyzer was validated by analyzing Cr(VI) and total chromium contents in two certified reference materials. The feasibility of performing rapid field speciation of chromium in wastewater samples was also demonstrated.     

Lab-on-valve (LOV) system coupled to irreversible biamperometric detection for the on-line monitoring of catechol

The analytical performance of lab-on-valve (LOV) system using irreversible biamperometry for the determination of catechol was evaluated. By integrating miniaturized electrochemical flow cell (EFC) designed and processed which is furnished with two identical polarized platinum electrodes, into the LOV unit, the lab-on-valve system combines sampling with analysis, realizing automated on-line analysis for catechol in a closed system. The biamperometric detection system was established to record the relationship between oxidation current and time by coupling the irreversible oxidation of catechol at one pretreated platinum electrode with the irreversible reduction of platinum oxide at the other pretreated platinum electrode. Factors influencing the analytical performance were optimized, including the potential difference (ΔE), buffer solution and pH, and flow variables in the LOV. A linear calibration curve was obtained within the range of 1.0 × 10⁻⁶-5.0 × 10⁻⁴ mol L⁻¹ of catechol with the detection limit (3σ) of 5.09 × 10⁻⁷ mol L⁻¹. The relative standard deviation (R.S.D.) was 2.39% for 11 successive determinations of 1 × 10⁻⁵ mol L⁻¹ catechol and the sample throughput was 35 h⁻¹. Moreover, this proposed method was applied to the analysis of catechol in beer sample, which was testified by high-performance liquid chromatography (HPLC).     

DNA purification on a lab-on-valve system incorporating a renewable microcolumn with in situ monitoring by laser-induced fluorescence

Bead injection in a lab-on-valve (LOV) system was adopted for DNA purification via micro solid-phase extraction (SPE) with a renewable silica microcolumn packed in a channel of the LOV unit. The complex matrix components in human whole blood, including proteins, were well eliminated by choosing properly the sample loading and elution media. The DNA purification process was monitored on-line by using laser-induced fluorescence in a demountable side part of the LOV unit incorporating optical fibers. The practical applicability of the entire system was demonstrated by separation/purification of λ-DNA in a simulated matrix and human blood genetic DNA by performing SPE, in situ monitoring of the purified products, and postcolumn PCR amplification. When DNAs in a simulated matrix (10.0 ng μl⁻¹ λ-DNA, 50 ng μl⁻¹ bovine serum albumin, 1.0% Triton X-100) were processed in the present system and laser-induced fluorescence was monitored at 610 nm, an overall extraction/collection efficiency of 70% was achieved by employing identical sample loading and an elution flow rate of 0.5 μl s⁻¹, along with a precision of 3.8% relative standard deviation. DNA separation and purification from human whole-blood samples were performed under similar conditions. Figure Lab-on-valve mesofluidic system employed for DNA separation and purification integrating a demountable fluorescence flow cell for in-situ laser induced fluorescence detection     

Universal approach for mesofluidic handling of bead suspensions in lab-on-valve format

In the present report, new protocols are introduced for automatic mesofluidic handling of irregularly shaped and non-uniformly sized bead materials for renewable micro-solid phase extraction (μSPE) under the lab-on-valve (LOV) format. To this end, two alternative strategies were studied comprising either (i) the direct aspiration of bead suspension placed at a container attached to LOV device or (ii) the aspiration of beads after a resuspension step, allowing the formation of a fluidized bed inside the beads’ container. Suspensions with homogeneously dispersed beads were also tested in the first strategy above, as prepared by increasing the viscosity of the suspension milieu with 75:15:10 glycerol/MeOH/H₂O (in wt). The bead injection protocols were applied to four commercial reversed-phase sorbent materials with different sorptive surfaces: Oasis HLB, SupelMIP β-receptors, Lichrolut EN and Discovery DSC-MCAX, and the mass of sorbent packed in each microcolumn was assessed. Direct aspiration of methanolic suspensions gave rise to bead stacking and clogging of the LOV microconduits, resulting in a source of results with poor precision (RSD: 3.8-67.6%). The use of glycerolic suspensions was merely effective for repeatable sampling and packing of Oasis HLB and SupelMIP β-receptor beads without sorbent settlement along time. The resuspension strategy was able to handle all the materials tested with acceptable precision (RSD: 1.6-13.8%). Enhanced precision was attained (RSD <4.1%) when the sorbent bed was physically restricted to the volume of the LOV microchannel cavity. Different volumes of suspension aiming at a target mass of sorbent of 10 mg were successfully handled (RSD: 3.1-13.8%), showing the reliability of the bead resuspension approach for varied nominal bead sampling volumes. The proposed bead handling protocols were applied to μSPE of propranolol taken as a model of β-blocker from aqueous solutions by SupelMIP β-receptors and Discovery DSC-MCAX beads with high repeatability (RSD <6%) and absolute recoveries between 69 and 74% in a bead-injection mode.