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.     

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.     

Trends of flow injection sample pretreatment approaching the new millennium

The important role of flow injection (FI) techniques for the automation, acceleration and miniaturization of solution handling in sample pretreatment as well as some recent trends in the development of the field are discussed, illustrated mainly by recent achievements in the author’s laboratory, including: (a) sample pretreatment for vapor generation and electrothermal AAS based on sequential injection (SI) techniques, with low reagent consumption and enhanced ruggedness; (b) combination of FI and SI sample pretreatment (filtration, dialysis, gas diffusion, column sorption) with capillary electrophoresis (CE) giving enhanced reproducibility and efficiency; (c) application of on-line microdialysis in in vivo monitoring of blood glucose in test animals and (d) application of on-line microdialysis and solvent extraction in continuous monitoring of drug dissolution processes with high resolution of process events. Future perspectives of FI sample pretreatment are discussed, emphasizing the improvement in ruggedness of the equipment and methods, the combination and synchronization of different means for liquid propulsion, and the development of miniaturized systems.     

Exploiting flow injection and sequential injection anodic stripping voltammetric systems for simultaneous determination of some metals

Flow injection (FI) and sequential injection (SI) systems with anodic stripping voltammetric detection have been exploited for simultaneous determination of some metals. A pre-plated mercury film on a glassy carbon disc electrode was used as a working electrode in both systems. The same film can be repeatedly applied for at least 50 analysis cycles, thus reducing the mercury consumption and waste. A single line FI voltammetric system using an acetate buffer as a carrier and an electrolyte solution was employed. An injected standard/sample zone was mixed with the buffer in a mixing coil before entering a flow cell. Metal ions were deposited on the working electrode by applying a potential of −1.1 V vs Ag/AgCl reference electrode. The stripping was performed by anodically scanning potential of working electrode to +0.25 V, resulting a voltammogram. Effects of acetate buffer concentration, flow rate and sample volume were investigated. Under the selected condition, detection limits of 1 μg l⁻¹ for Cd(II), 18 μg l⁻¹ for Cu(II), 2 μg l⁻¹ for Pb(II) and 17 μg l⁻¹ for Zn(II) with precisions of 2–5% (n=11) were obtained. The SI voltammetric system was similar to the FI system and using an acetate buffer as a carrier solution. The SI system was operated by a PC via in-house written software and employing an autotitrator as a syringe pump. Standard/sample was aspirated and the zone was then sent to a flow cell for measurement. Detection limits for Cd(II), Cu(II), Pb(II) and Zn(II) were 6, 3, 10 and 470 μg l⁻¹, respectively. Applications to water samples were demonstrated. A homemade UV-digester was used for removing organic matters in the wastewater samples prior to analysis by the proposed voltammetric systems.     

Application of flowing stream techniques to water analysis Part III. Metal ions: alkaline and alkaline-earth metals, elemental and harmful transition metals, and multielemental analysis

In the earlier parts of this series of reviews [1] and [2], the most relevant flowing stream techniques (namely, segmented flow analysis, continuous flow analysis, flow injection (FI) analysis, sequential injection (SI) analysis, multicommuted flow injection analysis and multisyringe flow injection analysis) applied to the determination of several core inorganic parameters for water quality assessment, such as nutrients and anionic species including nitrogen, sulfur and halogen compounds, were described.In the present paper, flow techniques are presented as powerful analytical tools for the environmental monitoring of metal ions (alkaline and alkaline-earth metals, and elemental and harmful transition metals) as well as to perform both multielemental and speciation analysis in water samples. The potentials of flow techniques for automated sample treatment involving on-line analyte separation and/or pre-concentration are also discussed in the body of the text, and demonstrated for each individual ion with a variety of strategies successfully applied to trace analysis. In this context, the coupling of flow methodologies with atomic spectrometric techniques such as flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma mass spectrometry (ICPMS) or hydride-generation (HG)/cold-vapor (CV) approaches, launching the so-called hyphenated techniques, is specially worth mentioning.     

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 1.0 M NH₄Cl, 0.1 M NaOH and 0.5 M 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 3σ 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.     

Evaluation of different sample introduction approaches for the determination of boron in unalloyed steels by inductively coupled plasma mass spectrometry

An extended study of different sampling introduction approaches using inductively coupled plasma mass spectrometry (ICP-MS) is presented for the determination of boron in steel samples. The following systems for sample introduction were applied: direct sample solution nebulization by continuous nebulization (CN) using a cross-flow nebulizer and with flow injection (FI), applied to 0.1% (m/v) and 0.5% (m/v) sample solutions, respectively; FI after iron matrix extraction, using acetylacetone–chloroform, and isotopic dilution (ID) analysis as the calibration method; FI with on-line electrolytic matrix separation; and spark ablation (SA) and laser ablation (LA) as solid sampling techniques. External calibration with matrix-matching samples was used with CN, SA, and LA, and only acid solutions (without matrix matching) with FI methods. When FI was directly applied to a sample solution, the detection limit was of 0.15 μg g⁻¹, improving by a factor of 4 that was obtained from the CN measurements. Isotopic dilution analysis, after matrix removal by solvent extraction, made it possible to analyse boron with a detection limit of 0.02 μg g⁻¹ and, with the on-line electrolytic process, the detection limit was of 0.05 μg g⁻¹. The precision for concentrations above 10 times the detection limit was better than 2% for CN, as well as for FI methods. Spark and laser ablation sampling systems, avoiding digestion and sample preparation procedures, provided detection limits at the μg g⁻¹ levels, with RSD values better than 6% in both cases. Certified Reference Materials with B contents in the range 0.5–118 μg g⁻¹ were used for validation, finding a good agreement between certified and calculated values.     

Flow injection on-line solid phase extraction for ultra-trace lead screening with hydride generation atomic fluorescence spectrometry

A flow injection (FI) on-line solid phase extraction (SPE) procedure for ultra-trace lead separation and preconcentration was developed, followed by hydride generation and atomic fluorescence spectrometric (AFS) detection. Lead is retained on an iminodiacetate chelating resin packed microcolumn, and is afterward eluted with 2.5% (v/v) hydrochloric acid to facilitate the hydride generation by reaction with alkaline tetrahydroborate solution with 1% (m/v) potassium ferricyanide as an oxidizing (or sensitizing) reagent. The hydride was separated from the reaction medium in the gas-liquid separator and swept into the atomizer for quantification. The chemical variables and the FI flow parameters were carefully optimized. With a sample loading volume of 4.8 ml, quantitative retention of lead was obtained, along with an enrichment factor of 11.3 and a sampling frequency of 50 h(-1). A detection limit of 4 ng l(-1), defined as 3 times the blank standard deviation (3 sigma), was achieved along with a RSD value of 1.6% at the 0.4 microg l(-1) level. The procedure was validated by determining lead contents in two certified reference materials, and its practical applicability was further demonstrated by analysing a variety of biological and environmental samples.     

Development of on-line single-drop micro-extraction sequential injection system for electrothermal atomic absorption spectrometric determination of trace metals

A novel automatic sequential injection (SI) single-drop micro-extraction (SDME) system is proposed as versatile approach for on-line metal preconcentration and/or separation. Coupled to electrothermal atomic absorption spectrometry (ETAAS) the potentials of this SI scheme are demonstrated for trace cadmium determination in water samples. A non-charged complex of cadmium with ammonium diethyldithiophosphate (DDPA) was produced and extracted on-line into a 60 μL micro-drop of di-isobutyl ketone (DIBK). The extraction procedure was performed into a newly designed flow-through extraction cell coupled on a sequential injection manifold. As the complex Cd(II)-DDPA flowed continuously around the micro-droplet, the analyte was extracting into the solvent micro-drop. All the critical parameters were optimized and offered good performance characteristics and high preconcentration ratios. For 600 s micro-extraction time, the enhancement factor was 10 and the sampling frequency was 6 h⁻¹. The detection limit was 0.01 μg L⁻¹ and the precision (RSD at 0.1 μg L⁻¹ of cadmium) was 3.9%. The proposed method was evaluated by analyzing certified reference material.     

Comparison of traditional cloud-point extraction and on-line flow-injection cloud-point extraction with a chemiluminescence method using benzo[a]pyrene as a marker

In this work, using benzo(a)pyrene (BaP) as marker, the analytical merits of on-line flow-injection cloud-point extraction (FI CPE), including preconcentration factor, extraction efficiency, sample throughput, and analysis time were evaluated by use of peroxyoxalate chemiluminescence (CL) detection. Moreover, by detailed discussion of several preconcentration conditions for traditional and on-line FI CPE the advantages of on-line FI CPE became conspicuously apparent. When coupled with separation techniques such as high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE), on-line FI CPE–CL has much potential for analysis of low concentrations of polycyclic aromatic hydrocarbons (PAH) in environmental samples.     

Flow injection on-line displacement/solid phase extraction system coupled with flame atomic absorption spectrometry for selective trace silver determination in water samples

A novel flow injection (FI) on-line displacement solid phase extraction preconcentration and/or separation method coupled with FAAS in order to minimize interference from other metals was developed for trace silver determination. The proposed method involved the on-line formation and subsequently pre-sorption of lead diethyldithiocarbamate (Pb-DDTC) into a column packed with PTFE-turnings. The preconcentration and/or separation of the Ag(I) took place through a displacement reaction between Ag(I) and Pb(II) of the pre-sorbed Pb-DDTC. Finally, the retained analyte was eluted with isobutyl methyl ketone (IBMK) and delivered directly to nebulizer for measuring. Interference from co-existing ions with lower DDTC complex stability in comparison with Pb-DDTC, was eliminated without need for any masking reagent. With 120 s of preconcentration time at a sample flow rate of 7.6 mL min−1, an enhancement factor of 110 and a detection limit (3 s) of 0.2 μg L−1 were obtained. The precision (RSD, n = 10) was 3.1% at the 10 μg L−1 level. The developed method was successfully applied to trace silver determination in a variety of environmental water samples and certified reference material.     

Determination of essential metals in complete diet feed by flow injection and flame atomic absorption spectrometry

A prior study of different sample pre-treatments for the determination of metallic elements in complete diet feeds was performed in order to choose the most suitable for these samples. The studied pre-treatment were: acid extraction (lixiviation), wet digestion (on microwave oven) and dry ashing mineralization (calcination). Lixiviation (acid extraction) with hydrochloric acid was selected due to its accuracy, fast and simple pre-treatment procedure. Due of the different levels of concentration of the metallic elements in the samples, the same manifold was used but with small variations. Copper (with on-line pre-concentration by chelating Chelex-100 resin), calcium (with on-line dilution) and iron determination gave suitable accuracy and precision and required a little time for analysis. Five different samples were analyzed by flow injection and the results were contrasted with dry ashing mineralization in batch procedure and with the labeled contents.     

毛细管电泳结合压力辅助电动进样测定水样中4种酚类雌激素

在毛细管电泳的胶束电动色谱（MEKC）模式下,采用压力辅助电动进样（PAEKI）的进样方式在线富集4种酚类雌激素（PEs）。对影响PAEKI的进样电压、进样时间等进行考察,并与传统的压力进样比较。结果表明,在最优的PAEKI条件下（-9 kV,0.3 psi（约2.1 kPa）,0.4 min）,4种PEs在7 min内基线分离,线性关系良好,相关系数（r）大于0.9936,己烷雌酚和双烯雌酚的线性范围为0.05~5 mg/L、双酚A和己烯雌酚的线性范围为0.1~10 mg/L;检出限（S/N=3）为0.0071~0.017 mg/L,富集倍数为11~15。使用该MEKC-PAEKI法对自来水和湖水水样进行测定,得到定量限（S/N=10）分别为0.029~0.064 mg/L和0.033~0.079 mg/L;加标回收率为75.6%~110.1%,相对标准偏差（n=5）为4.6%~11.8%。PAEKI不需要使用其他试剂,只需对电泳仪的参数进行适当调整即可实现对分析物的在线富集,简单、快速、自动化程度高。     

Flow injection-electrothermal atomic absorption spectrometry configurations: recent developments and trends

This review aims to provide an account of the state-of-the-art, of recently introduced techniques and of future possibilities offered by flow analysis in order to automate sample pretreatment for electrothermal atomic absorption spectrometry (ETAAS). Topics such as on-line solid-liquid extraction, precipitation, coprecipitation, on-wall retention, solvent extraction and chemical vapor generation processes are used to illustrate the versatility of coupling flow injection (FI) for analyte separation and preconcentration in the flow system or in the atomizer. The use of FI to make the introduction of slurries and highly viscous samples easier is considered as well as on-line microwave sample digestion.     

Determination of tellurium at ultra-trace levels in drinking water by on-line solid phase extraction coupled to graphite furnace atomic absorption spectrometer

In this paper, two time-based flow injection (FI) separation pre-concentration systems coupled to graphite furnace atomic absorption spectrometry (GFAAS) for tellurium determination are studied and compared. The first alternative involves the pre-concentration of the analyte onto Dowex 1X8 employed as packaging material of a micro-column inserted in the flow system. The second set-up is based on the co-precipitation of tellurium with La(OH)₃ followed by retention onto XAD resins. Both systems are compared in terms of limit of detection, linear range, RSD%, sample throughput, micro-columns lifetime and aptitude for fully automatic operation.     

Simultaneous on-line preconcentration and determination of trace metals in environmental samples by flow injection combined with inductively coupled plasma mass spectrometry using a nanometer-sized alumina packed micro-column

A flow injection (FI) on-line preconcentration procedure by using a nanometer-sized alumina packed micro-column coupled to inductively coupled plasma mass spectrometry (ICP-MS) was described for simultaneous determination of trace metals (V, Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb) in the environmental samples. The effects of pH value, sample flow rate, preconcentration time, and interfering ions on the preconcentration of analytes have been investigated. Under the optimized operating conditions, the adsorption capacity of the nanometer-sized alumina for V, Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb were found to be 11.7, 13.6, 15.7, 9.5, 12.2, 13.3, 17.1, 17.7 and 17.5 mg g⁻¹, respectively. With 60 s preconcentration time and 60 s elution time, an enrichment factor of 5 and the sampling frequency of 15 h⁻¹ were obtained. The proposed method has been applied to the determination of trace metals in environmental certified reference materials and natural water samples with satisfactory results.     

On-line flow injection solvent extraction for electrothermal atomic absorption spectrometry: Determination of nickel in biological samples

A flow injection (FI) on-line solvent extraction system for electrothermal atomic absorption spectrometry (ETAAS) was developed with nickel as a model trace element. The nickel pyrrolidine-dithiocarbamate chelate was extracted on line into isobutyl methyl ketone, which was delivered into the FI system by a peristaltic pump equipped with poly(tetrafluoroethylene) tubing. The organic phase was separated from the aqueous phase by a novel gravity phase separator with a small conical cavity, and stored in a collector tube, from which 50 μl organic concentrate was introduced into the graphite tube by an air flow. ETAAS determination of the analyte was performed in parallel with the extraction process. An enrichment factor of 25 was obtained in comparison with 50 μl direct introduction while achieving a detection limit of 4 ng l⁻¹ (3σ), and a precision of 1.5% relative standard deviation for 1.0 μg l⁻¹ nickel (n = 11). The proposed method was successfully applied to the determination of nickel in body fluids and other biological samples.     

A hydride generation atomic fluorescence spectrometric procedure for selenium determination after flow injection on-line co-precipitate preconcentration

A flow injection (FI) on-line preconcentration procedure for ultra-trace inorganic selenium was developed with detection by atomic fluorescence spectrometry. Selenium (IV) is co-precipitated with lanthanum hydroxide and collected on a PTFE beads packed column, the precipitate is afterwards dissolved with hydrochloric acid followed by hydride generation with reduction by tetrahydroborate. A thorough scrutiny was made for chemical variables and FI parameters. With a sampling volume of 3.4 ml, quantitative retention of selenium (IV) was obtained, along with an enrichment factor of 11 and a sampling frequency of 38 h^− 1. The detection limit, defined as 3 times the blank standard deviation (3σ), was 5 ng l^− 1. The precision was characterized by a RSD value of 1.2% (at the 0.5 μg l^− 1 level, n = 11). The enrichment factor was further enhanced to 20 along with a detection limit of 3 ng l^− 1, with a sample loading volume of 6.8 ml. The procedure was validated with certified reference materials and biological samples. It was also applied to the speciation of inorganic selenium in surface waters.     

New developments in flow injection separation and preconcentration techniques for electrothermal atomic absorption spectrometry

Flow injection (FI) on-line separation and preconcentration systems for electrothermal atomic absorption spectrometry (ETAAS) are reviewed, highlighting the main developments in the field since 1990 and work achieved in the authors' laboratory. These include developments in on-line preconcentration systems based on column techniques, solvent extraction, coprecipitation and hydride sequestration. Advantages, limitations and potentials of the FI-ETAAS combination are discussed.     

New developments in flow injection separation and preconcentration techniques for electrothermal atomic absorption spectrometry

Flow injection (FI) on-line separation and preconcentration systems for electrothermal atomic absorption spectrometry (ETAAS) are reviewed, highlighting the main developments in the field since 1990 and work achieved in the authors laboratory. These include developments in on-line preconcentration systems based on column techniques, solvent extraction, coprecipitation and hydride sequestration. Advantages, limitations and potentials of the FI-ETAAS combination are discussed.