On-line sample processing involving microextraction techniques as a front-end to atomic spectrometric detection for trace metal assays: a review.

Within the last decade, liquid-phase microextraction (LPME) and micro-solid phase extraction (µSPE) approaches have emerged as substitutes for conventional sample processing procedures for trace metal assays within the framework of green chemistry. This review surveys the progress of the state of the art in simplification and automation of microextraction approaches by harnessing to the various generations of flow injection (FI) as a front end to atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS) or inductively coupled plasma atomic emission spectrometry or mass spectrometry (ICP-AES/MS). It highlights the evolution of flow injection analysis and related techniques as vehicles for appropriate sample presentation to the detector and expedient on-line matrix separation and pre-concentration of trace levels of metals in troublesome matrices. Rather than being comprehensive this review is aimed at outlining the pros and cons via representative examples of recent attempts in automating green sample preparation procedures in an FI or sequential injection (SI) mode capitalizing on single-drop microextraction, dispersive liquid-phase microextraction and advanced sorptive materials including carbon and metal oxide nanoparticles, ion imprinted polymers, superparamagnetic nanomaterials and biological/biomass sorbents. Current challenges in the field are identified and the synergetic combination of flow analysis, nanotechnology and metal-tagged biomolecule detection is envisaged.

Recent advances and future prospects of mesofluidic lab-on-a-valve platforms in analytical sciences--a critical review.

Miniaturization and automation in analytical sciences have evolved tremendously over the past decade within the framework of green analytical chemistry. This manuscript outlines the unrivalled merits of advanced flow methodology capitalizing on mesofluidic platforms for the simplification and acceleration of the overall analytical process. Introduced back in 2000, the Lab-on-a-Valve concept (LOV), allied to sequential injection analysis, has emerged as an appealing downscaled analytical tool for pressure-driven sampling at the low µL level. Not the least, for advanced on-chip sample processing involving renewable micro-solid phase extraction (so-called bead injection analysis), non-chromatographic speciation or chemical vapor generation using programmable flow, for optical and electrochemical detection on-chip including optosensing approaches, or as a front end to modern detection equipment or column separation systems. It is the intention of this work to offer the authors' own critical vision as to where the field of LOV is being directed on the basis of the survey of the current state-of-the art of mesofluidic systems and identify what are the major cutting-edge challenges to be yet undertaken and what opportunities are offered by LOV for real-world applications that might not be at present tackled by lab-on-a-chip microfluidic approaches.

Evaluation of the readsorption of plutonium and americium in dynamic fractionations of environmental solid samples.

A dynamic extraction system exploiting sequential injection (SI) for sequential extractions incorporating a specially designed extraction column is developed to fractionate radionuclides in environmental solid samples such as soils and sediments. The extraction column can contain up to 5 g of a soil sample, and under optimal operational conditions it does not give rise to creation of back pressure. Attention has been placed on studies of the readsorption problems during sequential extraction using a modified Standards, Measurements and Testing (SM&T) scheme with four-step sequential extractions. The degree of readsorption in dynamic and conventional batch extraction systems is compared and evaluated by using a double-spiking technique. A high degree of readsorption of plutonium and americium (>75%) was observed in both systems, and they also exhibited similar distribution patterns of the two radionuclides. However, the dynamic system is fully automated, eliminates manual separations, significantly reduces the operational time required, and offers detailed kinetic information.

Miniaturization of environmental chemical assays in flowing systems: the lab-on-a-valve approach vis-à-vis lab-on-a-chip microfluidic devices.

The analytical capabilities of the microminiaturized lab-on-a-valve (LOV) module integrated into a microsequential injection (muSI) fluidic system in terms of analytical chemical performance, microfluidic handling and on-line sample processing are compared to those of the micro total analysis systems (muTAS), also termed lab-on-a-chip (LOC). This paper illustrates, via selected representative examples, the potentials of the LOV scheme vis-à-vis LOC microdevices for environmental assays. By means of user-friendly programmable flow and the exploitation of the interplay between the thermodynamics and the kinetics of the chemical reactions at will, LOV allows accommodation of reactions which, at least at the present stage, are not feasible by application of microfluidic LOC systems. Thus, in LOV one may take full advantage of kinetic discriminations schemes, where even subtle differences in reactions are utilized for analytical purposes. Furthermore, it is also feasible to handle multi-step sequential reactions of divergent kinetics; to conduct multi-parametric determinations without manifold reconfiguration by utilization of the inherent open-architecture of the micromachined unit for implementation of peripheral modules and automated handling of a variety of reagents; and most importantly, it offers itself as a versatile front end to a plethora of detection schemes. Not the least, LOV is regarded as an emerging downscaled tool to overcome the dilemma of LOC microsystems to admit real-life samples. This is nurtured via its intrinsic flexibility for accommodation of sample pre-treatment schemes aimed at the on-line manipulation of complex samples. Thus, LOV is playing a prominent role in the environmental field, whenever the monitoring of trace level concentration of pollutants is pursued, because both matrix isolation and preconcentration of target analytes is most often imperative, or in fact necessary, prior to sample presentation to the detector.

A multisyringe flow-through sequential extraction system for on-line monitoring of orthophosphate in soils and sediments.

A fully automated flow-through microcolumn fractionation system with on-line post-extraction derivatization is proposed for monitoring of orthophosphate in solid samples of environmental relevance. The system integrates dynamic sequential extraction using 1.0moll(-1) NH(4)Cl, 0.1moll(-1) NaOH and 0.5moll(-1) HCl as extractants according to the Hieltjes-Lijklema (HL) scheme for fractionation of phosphorus associated with different geological phases, and on-line processing of the extracts via the Molybdenum Blue (MB) reaction by exploiting multisyringe flow injection as the interface between the solid containing microcolumn and the flow-through detector. The proposed flow assembly, capitalizing on the features of the multicommutation concept, implies several advantages as compared to fractionation analysis in the batch mode in terms of saving of extractants and MB reagents, shortening of the operational times from days to hours, highly temporal resolution of the leaching process and the capability for immediate decision for stopping or proceeding with the ongoing extraction. Very importantly, accurate determination of the various orthophosphate pools is ensured by minimization of the hydrolysis of extracted organic phosphorus and condensed inorganic phosphates within the time frame of the assay. The potential of the novel system for accommodation of the harmonized protocol from the Standards, Measurement and Testing (SMT) Program of the Commission of the European Communities for inorganic phosphorus fractionation was also addressed. Under the optimized conditions, the lowest detectable concentration at the 3sigma level was

Hyphenating multisyringe flow injection lab-on-valve analysis with atomic fluorescence spectrometry for on-line bead injection preconcentration and determination of trace levels of hydride-forming elements in environmental samples.

In this work the third generation of flow injection analysis, that is, the so-called micro-lab-on-valve (microLOV) approach, is proposed for the first time for the separation, preconcentration, and monitoring of metalloids as hyphenated with atomic fluorescence spectrometry (AFS). This was made feasible by interfacing the micromachined LOV-module with AFS by a multisyringe flowing stream network for on-line postcolumn derivatization of the eluate aimed at generation of hydride species. The potential of this new hyphenated technique for environmental assays was ascertained via determination of ultratrace level concentrations of total inorganic arsenic in freshwater. Employing quantitative preoxidation of As(III) to As(V) in the samples by means of permanganate, the method involves preconcentration of arsenate at pH 10 on a renewable anion exchanger, namely, Q-Sepharose, packed in a LOV microcolumn. The analyte species is afterward stripped out and concurrently prereduced by a 300 microL eluent plug containing 6 mol L(-)1 HCl and 10% KI. The eluate is downstream merged with a metered volume of sodium tetrahydroborate (0.3% w/v) for generation of arsine, which is subsequently quantified by AFS. The flow system facilitates on-column reduction of the retained arsenic with no need for application of programmable stopped flow. Yet, the high concentration of reductant and extreme pH conditions for elution hinder the sorbent to be reused due to gradual deactivation of the functional moieties, so that maximum benefit can be taken from the application of the bead renewable strategy. The proposed procedure is characterized by a high tolerance to metal species and interfering hydride-forming elements. In fact, ratios of Se(IV) to As < or = 5000 and Sb(V) to As < or = 500 are tolerated at the 10% interference level. Under the optimized experimental conditions, a detection limit (3sigma) of 0.02 ng mL(-1) As, a dynamic linear range of 0.05-2.0 ng mL(-1) As (by tailoring the AFS gain), an enrichment factor of 8.8 for arsenate, and a precision better than 6.0% at the 0.1 ng mL-1 level were obtained for the bead-injection mode whenever the loading sample volume was affixed at 3.0 mL. The reliability and accuracy of the automated procedure was ascertained by determining total inorganic arsenic in both spiked environmental waters and certified reference materials of variable matrix complexity (TMDA-54.3 and ERM-CA010) at the low ng mL(-1) level. No significant differences were found between the experimental results and the certified values at a significance level of 0.05.

Highly selective micro-sequential injection lab-on-valve (muSI-LOV) method for the determination of ultra-trace concentrations of nickel in saline matrices using detection by electrothermal atomic absorption spectrometry.

A highly selective procedure is proposed for the determination of ultra-trace level concentrations of nickel in saline aqueous matrices exploiting a micro-sequential injection Lab-On-Valve (muSI-LOV) sample pretreatment protocol comprising bead injection separation/pre-concentration and detection by electrothermal atomic absorption spectrometry (ETAAS). Based on the dimethylglyoxime (DMG) reaction used for nickel analysis, the sample, as contained in a pH 9.0 buffer, is, after on-line merging with the chelating reagent, transported to a reaction coil attached to one of the external ports of the LOV to assure sufficient reaction time for the formation of Ni(DMG)(2) chelate. The non-ionic coordination compound is then collected in a renewable micro-column packed with a reversed-phase copolymeric sorbent [namely, poly(divinylbenzene-co-N-vinylpyrrolidone)] containing a balanced ratio of hydrophilic and lipophilic monomers. Following elution by a 50-muL methanol plug in an air-segmented modality, the nickel is finally quantified by ETAAS. Under the optimized conditions and for a sample volume of 1.8 mL, a retention efficiency of 70 % and an enrichment factor of 25 were obtained. The proposed methodology showed a high tolerance to the commonly encountered alkaline earth matrix elements in environmental waters, that is, calcium and magnesium, and was successfully applied for the determination of nickel in an NIST standard reference material (NIST 1640-Trace elements in natural water), household tap water of high hardness and local seawater. Satisfying recoveries were achieved for all spiked environmental water samples with maximum deviations of 6 %. The experimental results for the standard reference material were not statistically different to the certified value at a significance level of 0.05.

On-line dynamic extraction and automated determination of readily bioavailable hexavalent chromium in solid substrates using micro-sequential injection bead-injection lab-on-valve hyphenated with electrothermal atomic absorption spectrometry.

A novel and miniaturized micro-sequential injection bead-injection lab-on-valve (microSI-BI-LOV) fractionation system was developed for on-line microcolumn soil extraction under simulated environmental scenarios and accurate monitoring of the content of easily mobilisable hexavalent chromium in soil environments at the sub-low parts-per-million level. The flow system integrates dynamic leaching of hexavalent chromium using deionized water as recommended by the German Standard DIN 38414-S4 method; on-line pH adjustment of the extract by a 0.01 mol L(-1) Tris-HNO(3) buffer solution; isolation of the chromate leached from the matrix constituents onto a Q Sepharose strong anion-exchanger freshly packed into the microconduits of the microSI-assembly; air-segmented elution of the sorbed species by a 40 microL plug of 0.5 mol L(-1) NH(4)NO(3) (pH 8) eluent; and detection by electrothermal atomic absorption spectrometry (ETAAS). The effect of simulated acidic rain on the accessibility of chromate forms for plant uptake was also investigated. The proposed approach offers several advantages over conventional speciation/fractionation protocols in the batch mode, including immediate separation with concomitant preconcentration of the released chromate, minimization of Cr(vi) to Cr(iii) interconversion risks, enhanced accuracy, and non-existence of re-adsorption/re-distribution problems along with a detailed pattern of the kinetics of the leaching process. The reliability of the proposed method was evaluated via spiking of a moderately polluted agricultural soil material (San Joaquin Soil-Baseline Trace Element Concentrations) with water-soluble Cr(vi) salts at different concentration levels. The potential of the microSI-BI-LOV set-up with renewable surfaces for flame-AAS determination of high levels of readily bioavailable chromate in contaminated soils is also addressed.

On-line dynamic fractionation and automatic determination of inorganic phosphorus in environmental solid substrates exploiting sequential injection microcolumn extraction and flow injection analysis.

Sequential injection microcolumn extraction (SI-MCE) based on the implementation of a soil-containing microcartridge as external reactor in a sequential injection network is, for the first time, proposed for dynamic fractionation of macronutrients in environmental solids, as exemplified by the partitioning of inorganic phosphorus in agricultural soils. The on-line fractionation method capitalises on the accurate metering and sequential exposure of the various extractants to the solid sample by application of programmable flow as precisely coordinated by a syringe pump. Three different soil phase associations for phosphorus, that is, exchangeable, Al- and Fe-bound, and Ca-bound fractions, were elucidated by accommodation in the flow manifold of the three steps of the Hieltjes-Lijklema (HL) scheme involving the use of 1.0M NH4Cl, 0.1M NaOH and 0.5M HCl, respectively, as sequential leaching reagents. The precise timing and versatility of SI for tailoring various operational extraction modes were utilized for investigating the extractability and the extent of phosphorus re-distribution for variable partitioning times. Automatic spectrophotometric determination of soluble reactive phosphorus in soil extracts was performed by a flow injection (FI) analyser based on the Molybdenum Blue (MB) chemistry. The 3sigma detection limit was 0.02 mg P L(-1) while the linear dynamic range extended up to 20 mg P L(-1) regardless of the extracting media. Despite the variable chemical composition of the HL extracts, a single FI set-up was assembled with no need for either manifold re-configuration or modification of chemical composition of reagents. The mobilization of trace elements, such as Cd, often present in grazed pastures as a result of the application of phosphate fertilizers, was also explored in the HL fractions by electrothermal atomic absorption spectrometry.

Universal approach for selective trace metal determinations via sequential injection-bead injection-lab-on-valve using renewable hydrophobic bead surfaces as reagent carriers.

A new concept is presented for selective and sensitive determination of trace metals via electrothermal atomic absorption spectrometry based on the principle of bead injection (BI) with renewable reversed-phase surfaces in a sequential injection-lab-on-valve (SI-LOV) mode. The methodology involves the use of poly(styrene-divinylbenzene) beads containing pendant octadecyl moieties (C18-PS/DVB), which are preimpregnated with a selective organic metal chelating agent prior to the automatic manipulation of the beads in the microbore conduits of the LOV unit. By adapting this approach, the immobilization of the most suitable chelating agent can be effected irrespective of the kinetics involved, optimal reaction conditions can be used for implementing the chelating reaction of the target metal analyte with the immobilized re-agent, and an added degree of freedom is offered in selecting the most favorable elution mode in order to attain the highest sensitivity. The potential of the SI-BI-LOV scheme is demonstrated by taking Cr(VI) as a model analyte, using a 1,5-diphenylcarbazide (DPC)-loaded bead column as the active microzone. As this reaction requires the use of high acidity, it is also shown that the bead material exhibits excellent chemical stability at low pH values. On-line pH sample adjustment prevents alteration of the original distribution of chromium species while ensuring fast rates for the DPC-Cr(VI) reaction. The proposed procedure was successfully applied to the determination of trace levels of Cr(VI) in natural waters containing high levels of dissolved salts (such as seawater and hard tap water) without requiring any dilution step. Method validation was performed by determination of total chromium in an NIST standard reference material (NIST 1640, natural water) after Cr(III) oxidation, and the results were in good agreement with the certified value.

Use of flow injection and sequential injection analysis schemes for the determination of trace-level concentrations of metals in complex matrices by ETAAS and ICPMS.

Despite their excellent analytical chemical capacities for determination of low levels of metal species, electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry often require suitable pretreatment (separation and preconcentration) of the sample material to facilitate the desired sensitivity and selectivity of measurement. Such pretreatment schemes are advantageously performed in flow injection (FI) or sequential injection (SI) manifolds, where all unit operations can be effected on-line and under enclosed and strictly controlled conditions. Various separation/preconcentration procedures are feasible, such as liquid-liquid extraction (possibly including back extraction), (co)precipitation with collection in knotted coils, adsorption on hydrophilic or hydrophobic reactors, hydride generation, or the use of ion-exchange/chelating packed columns. After describing the particulars and characteristics of FI and SI, we present, via selected examples, various separation/preconcentration FI/SI schemes for the determination of trace levels of metals, with particular emphasis on the use of the novel extension of FI/SI, that is, the so-called lab-on-valve concept.

Automated sequential injection-microcolumn approach with on-line flame atomic absorption spectrometric detection for implementing metal fractionation schemes of homogeneous and nonhomogeneous solid samples of environmental interest.

An automated sequential injection (SI) system incorporating a dual-conical microcolumn is proposed as a versatile approach for the accommodation of both single and sequential extraction schemes for metal fractionation of solid samples of environmental concern. Coupled to flame atomic absorption spectrometric detection and used for the determination of Cu as a model analyte, the potentials of this novel hyphenated approach are demonstrated by the ability of handling up to a 300 mg sample of a nonhomogeneous sewage amended soil (viz., CRM 483). The three steps of the endorsed Standards, Measurements, and Testing sequential extraction method have been also performed in a dynamic fashion and critically compared with the conventional batchwise protocols. The ecotoxicological relevance of the data provided by both methods with different operationally defined conditions is thoroughly discussed. As compared to traditional batch systems, the developed SI assembly offers minimal risks of sample contamination, the absence of metal re-distribution/readsorption, and dramatic saving of operational times (from 16 h to 40-80 min per partitioning step). It readily facilitates the accurate manipulation of the extracting reagents into the flow network and the minute, well-defined injection of the desired leachate volume into the detector. Moreover, highly time-resolved information on the ongoing extraction is given, which is particularly relevant for monitoring fast leaching kinetics, such as those involving strong chelating agents. On-line and off-line (for Cu, Pb, and Zn) single extraction schemes are also proven to constitute attractive alternatives for fast screening of metal pollution in solid samples and for predicting the current, rather than the potential, element bioavailability by the assessment of the readily mobilizable metal forms.

Determination of trace metal ions via on-line separation and preconcentration by means of chelating Sepharose beads in a sequential injection lab-on-valve (SI-LOV) system coupled to electrothermal atomic absorption spectrometric detection.

The analytical performance of an on-line sequential injection lab-on-valve (SI-LOV) system using chelating Sepharose beads as sorbent material for the determination of ultra-trace levels of Cd(II), Pb(II) and Ni(II) by electrothermal atomic absorption spectrometry (ETAAS) is described and discussed. The samples are adjusted to pH 5.0 on-line in the system for optimum operation. The target ions are adsorbed by chelation on the surface of the beads, contained in a 20mul microcolumn within the LOV, and following elution by 50mul 2M nitric acid, the eluate is, as sandwiched by air segments, introduced into the ETAAS. Based on the consumption of 1.8ml sample solution, retention efficiencies of 95, 75 and 90%, enrichment factors of 34, 27 and 32, and determination limits of 0.001, 0.07 and 0.02mugl(-1) were obtained for Cd(II), Pb(II) and Ni(II), respectively. The beads can be used repeatedly for at least 20 times without decrease of performance, yet can be replaced at will if the circumstances should so dictate. The optimized procedural parameters showed that 12 samples per hour could be prepared and successfully analyzed. The results obtained for three standard reference materials agreed very well with the certified values.

The impact of flow injection on modern chemical analysis: has it fulfilled our expectations? And where are we going?

Presenting a condensation of the opening lecture of the 12th ICFIA conference, this communication presents a view of the impact that flow injection analysis (FIA) has had on modern analytical chemistry, evaluated both within the academic community and outside it, i.e. in "industry". The ensuing developments of FIA, encompassing sequential injection analysis (SIA) and bead injection lab-on-valve (BI-LOV), are described and their individual features discussed. Finally, some recent results of the activities from the author's own research group are briefly mentioned.