Chapter I.11
On-line sample processing methods in flow analysis
Manuel Miró1*and Elo Harald Hansen2
1Department of Chemistry, Faculty of Sciences, University of the Balearic Islands, Carretera de Valldemossa, km. 7.5, E-07122-Palma de Mallorca, Illes Balears, Spain.E-mail:
2Department of Chemistry, TechnicalUniversity of Denmark, Kemitorvet, Building 207, DK-2800 Kgs. Lyngby, Denmark
Corresponding author
Abstract
In this chapter, the state of the art of flow injection and related approaches thereof for automation and miniaturization of sample processing regardless of the aggregate state of the sample medium is overviewed. The potential of the various generation of flow injection for implementation of in-line dilution, derivatization, separation and preconcentration methods encompassing solid reactors, solvent extraction, sorbent extraction, precipitation/coprecipitation, hydride/vapor generation and digestion/leaching protocols as hyphenated to a plethora of detection devices is discussed in detail and relevant examples published in the literature up to April 2007 are pinpointed.
1.Introduction
The low concentration levels of target measurands and the complexity of the matrices of the real samples commonly encountered in any field of analytical chemistry, encompassing environmental, biological, industrial and biotechnological applications including food analyses, process monitoring and quality control testing,frequently impede the direct determination of core parameters by exploiting even modern analytical instrumentation. This is a consequence of the influence of the concomitant matrix components on the analytical signal, and the fact that the concentration of the target species is often below the dynamic linear range of the detection device. Thus, there is a need for the development of rugged and effective sample pretreatment procedures prior to the quantification step aimed at removing interfering matrix constituents and at the same time improving measurand detectability by preconcentration. When performed in a manual fashion, these preliminary operations are labour-intensive and time consuming, difficult to control systematically, and a source of major bias and accidental errors (for example, sample contamination) that might have a decisive impact on the accuracy and precision of the analytical results. Therefore, they are viewed as the major bottleneck of the overall analytical process as regards to the reliability of the analytical data [[1]].
The development of on-line sample pretreatment procedures exploiting the various generations of flow injection, that is, flow injection analysis (FIA), sequential injectionanalysis (SIA), multicommutation-based approaches and hyphenated techniques,has opened new avenues regarding automation and miniaturization of sample handling with the added advantage of saving of sample and reagent consumption and reducing waste generation [[2][3]-,[4],[5]].
This chapter presents and discusses with selected examples the progress of the state-of-the-art within the last decade in implementing flow injection systems for on-line matrix separation and/or preconcentration (and possibly dilution) as interfaced to a variety of detection instruments. These schemes comprise the use of solvent extraction, including salient variants such as micelle-mediated extraction, flow-batch extraction andwetting-film extraction; solid-phase extractions embracing sorbent optosensing and bead injection, on-wall molecular sorption and precipitate/(co)precipitate retention using knotted reactors; hydride/vapour generation; membrane-based separations involving (micro)dialysis, gas-diffusion, pervaporation and supported-liquid membranes; and digestion protocols, such as microwave-, ultrasound- and UV-irradiation assisted extractions. It should be noted that several of the methods herein reported hold equally well for the earlier methodology of air-segmented flow analysis though the FIA/SIA counterparts offer unique facilities. Special attention is also paid to the potential of flow based approaches for accommodation of sample processing steps regardless of the aggregate state of the sample medium. To this end, new trends for on-line handling of solid samples, particle and colloidal suspensions as well as air samples are thoroughly described. The chapter concludes with the discussion on further downscaling of these preliminary operations by tailoring solid-phase microextraction and solvent microextraction protocols into the flow system, thus rendering the so-called greenchemistry approaches where the use of hazardous chemicals and/or organic solvents iseliminated or kept toa minimum.
2. On-line sample pre-treatment protocols for aqueous and air samples
2.1. On-line dilution
Sample dilution is the simplest unit operation in flow systems. It is inherent to the concept of controlled dispersion and reproducible timing in flow analysis. The idea behind it is to perform single standard calibration using gradient dilution [[6]], to minimize matrix interfering effects, or to bring the measurand concentration within the dynamic linear range of the detection device. Moderate dilution factors might be readily accomplished via microfluidic manipulation within the flow network using flow-splitting, zone sampling or merging zone procedures as demonstrated with the atomic absorption/emission determination of metal elements in saline matrices [[7]]. Another elegant alternative is to exploit flow-reversal approaches in SIAaimed at dispersing the sample segment into the carrier stream to different extents [[8],[9]]. The degree of dilution might be tailored to the requirement of the assays by selecting the number of forward and backward movements and the lengths of the sample displacements [[10]]. Yet, sample dilutions higher than 20-fold are difficult to achieve.
The use of a small mixing chamber with an inner volume of a few milliliters and clustered to a peripheral port of the selection valve of anSIA manifold might be regardedas the most reproducible and reliable way of performing on-line controlled sample dilution within a wide concentration range [10,[11]]. Dilution factors of a few hundreds might be attained in a fully automatic fashion by appropriate selection of that part of the diluted zone to be entrapped within the holding coil [11].Particular interesting possibilities for dilution of polar species arise from usingsingle or serial arrangements of flow-through parallel-plate dialyzers furnished with regenerated cellulose (Cuprophan) membranes [[12]].
2.2. Derivatization reactions
Since the early developments of flow analysis, many analytical applications are based on its role as a mechanized system for homogeneous chemical derivatization. FIA and related approaches thereof have proven to be ideal vehicles for the conversion of non-detectable species into detectable ones through kinetically controlled chemical reactions.Both the highly reproducible mixing of streams and controllable timing enable novel applications never thought of before. The monitoring of unstable intermediate reaction products clearly illustrates the powerful capabilities of flow analysis. For example, the classical spectrophotometric determination of cyanide is based on the halogenation of the target species with chloramine-T, whereafter it reacts with a mixture of pyrazolone or barbituric acid to form a violet polymethine dye. Yet, the determination of the metastable red-coloured intermediate product allows a significant sensitivity improvement since the colour development is more intense.
The use of solid-phase reactors for heterogeneous chemical derivatization should be however considered as one of the most rapidly developing and challenging areas in flow analysis research [[13]]. The implementation of packed-bed microcolumns in the flow network for on-line reduction or oxidation of targeted compounds has proven an excellent avenue for speciation studies [2]. A worthwhile virtue of FIA as opposed to batch procedures is the likelihood of using unstable reagents in solution via entrapment into suitable matrices, thus obtaining highly stable reagent sources, or in-line generation of the active species at a solid-phase redox reactor [[14]]. Displacement reactions based on the detection of the delivered species following passage of the sample through a packed column containing an insoluble salt have attracted the interest of a plethora of researchers [[15],[16]] owing to the feasibility of reagent reutilisation, precipitate collection in the reactor as well as on-line hyphenation with various detection instruments such as spectrophotometers or atomic absorption/emission spectrometers.
The coupling of catalytic heterogeneous reactions using enzyme packed-bed or open-tubular reactors with numerous detection techniques has been presented as an appealing approach for determination of substrates of enzymatic reactions and activity of enzymes [[17]].The most relevant benefits gained by the exploitation of flow analysis are the strict repeatability of time-dependent operations and the limited consumption of the often costly biocomponents via immobilisation procedures, advantage being taken by the increased enzyme stability after physical or chemical binding to appropriate supports[[18]. An increased rigidity of the structure of the immobilised enzyme results in preserving the native configuration of the protein and making unfold less probable.The advantage of immobilisation of the enzyme in flow-through column reactors via Schiff base formation in comparison to integrated biosensors is the ability of entrapping much larger amount of catalyst. The ensuing high activity facilitates an extensive and rapid conversion of substrate into the detectable product, thus yielding lower detection limits [[19] .
2.3. Solvent Extraction
Liquid-liquid extraction was the first sample processing approach for isolation and preconcentration of target species adapted to automatic flow systems. The foremost asset was to overcome the main shortcomings that had arisen from their batch counterparts, namely, loss of measurand by manipulation, handling of large volumes of hazardous reagents, low sampling frequency, contamination of the laboratory atmosphere by organic vapors, and generation of large amounts of residual solvents. The essential steps of a batch liquid-liquid extraction procedure, that is, introduction of defined volumes of the aqueous and organic phases, transfer of derivatized species by bringing the immiscible solutions into intimate contact, and physical separation of the two phases, are conventionally carried out in continuous flow systems using a phase segmentor, an extraction coil and a phase separator, as schematically shown in Figure1A.
Despite of the initial progress for the separation and preconcentration of transition metals from a variety of matrices [[20]], along with the establishment of official standard flow-based methods for phenol index [[21]] and anionic surfactant [[22]] determinations, its further development has not enjoyed the attractions it deserves. In fact, the repeatability, sensitivity and accuracy of the methods are drastically limited by the existence of the above-mentioned components of the flow-through extractor. The phase separator should be actually viewed as the Achilles’ Heel of the flow analyzer due to fouling and carry-over effects and low efficiency in phase separation [[23],[24]].Although the overall performance of solvent extraction in flow-through systems can be improved by exploiting either novel separation designs for quantitative recovery of both the low-density and high-density phases [[25],[26]]or back-extraction schemes for enhanced selectivity and versatility as to the coupling to detection devices[[27]], recent trends are directed towards the development of novel concepts and strategies for improving the ruggedness of phase separation with no need for the classical extractor components.
The utilization of a microporous, hydrophobic material to hold the organic extractant by capillary forces (hence called liquid membrane) offers an appealing alternative to segmented flow injection systems for the construction of efficient and automatic sample pre-treatment set-ups [[28]]. The most accepted methodologies are the so-called supported liquid membrane extraction (SLME) [[29],[30]], or hollow-fiber assisted liquid-liquid microextraction[[31]] based on the extraction of targeted species from the donor stream into another aqueous, but frequently temporally stagnant, phase through the immobilized organic phase. For SLME, closed-loop type arrangements for the acceptor volume are frequently exploited as depicted in Figure 1B. It should be emphasized that the applicability of SLME is not solely restricted to the preconcentration of ionisable organic compounds, because permanently charged compounds (for example., metal ions) can be equally well determined by carrier mediated extraction. Readers are referred to the following review articles [28,[32]] for an exhaustive description of the fundamentals of on-line liquid membrane extraction and application for clinical and environmental research.
Single-phase liquid-liquid continuous extraction is as an optimal approach for expediting pre-extraction derivatization reactions as the sample is processed in a homogeneous medium. In this context, in-line micelle-mediated extraction techniques [[33]], such as flow-injection cloud-point extraction (CPE), have recently attracted considerable attention for the isolation and enrichment of hydrophobic organic species or metal ions, which are either in their native form or in non-charged covalent chelates or ion-pairs [[34],[35]]. In CPE, following the generation of derivatives of the target compounds, the increase of temperature causes the single phase system to be broken down into two distinct phases; one of them, the so-called surfactant-enriched phase, entraps the target species from the bulk medium into organized entities by hydrophobic or electrostatic interactions [[36]].This mixture is commonly delivered on-line to a microcolumn packed with a suitable filtering material, namely, glass wool, cotton or nylon fibers, for retention of the large-size measurand-entrapped surfactant aggregates, as illustrated in Figure1C.However, it should beborne in mind that the adaptation of CPE to FIAspectrophotometric/spectrofluorimetric assemblies might be hindered by the vapor bubbles generated within the flow network when raising the temperature. This shortcoming can be overcome by using salting-out agents for inducing on-line phase separations [[37]]. Other organized assemblies encompassing vesicles, admicelles and microemulsions have been also utilized in flow systems for tuning the sensitivity and selectivity of analytical methods by concentration or solubilization of measurands, respectively [36].
A noteworthy advance in terms of enhanced sensitivity and avoidance of carryover between samples has been achieved with the advent of the novel SIA-wetting film methodology [[38]]. It is based on coating of the inner walls of a polytetrafluoroethylene tubular reactor with a thin layer of organic phase, as facilitatedby the hydrophobic interactions between the solvent and the reactor material, which will cause a delay of the organic phase in respect to that of the aqueous solutions. The enhanced aqueous/organic phase volume ratio as compared with classical FIA solvent extraction renders high concentration factors. This film is also responsible for high extraction efficiencies, avoiding the axial dispersion of the measurand occurring in segmented FIA extraction. Wetting-film extraction has proven suitable for the preconcentration and speciation of metal traces in hyphenation with atomic spectrometric detection, for the simultaneous determination of various phenolic isomers and for monitoring of radionuclides at low activity levels [[39]]. The most serious shortcoming of this approach is the pseudo-stationary nature of the organic phase [[40]], which requires careful optimization of the hydrodynamic variables for each specific analytical application. Besides, the capacity of the film is rather limited.
The so-called extraction chromatography (EC) has also exploited the virtues of SIA for sample handling. It is utilized for the isolation of target radioisotopes from inactive matrix ingredients and interfering radioactive fission products by using inert polymeric columns impregnated with selective chelating agents or macrocyclic ionophores. Thus, Grate and co-workers [[41],[42]] have highlighted the potential of SIA-EC as a front-end to ICP-MS for actinide isotopic measurements and to non-selective liquid scintillation spectrometers for on-line radiometric detection, as demonstrated via separation, identification and/or quantification of 90Sr, 99Tc, and actinide isotopes, viz., Am, Cm, Pu, Th, Np and U, in nuclear waste waters. Not the least, enhanced chromatographic resolution of actinide species may be accomplished by application of multiple elution schemes entailing the on-line modification of the chemical composition of the mobile phase [42].
The implementation of microextraction chambers in a flow manifold furnished with a set of commutators and optical sensors constitutes the basis of a fully automatic solvent extraction approach supplementary to continuous FIA extraction [[43]]. The resulting microscale liquid-phase analysers operating under discontinuous-flow mode encompass a suite of advantages, including minimization of solvent consumption, gravitational phase separation under stagnant in lieu of dynamic regime, and reliable transportation of the enriched organic phase to the flow-through detector [[44]]. Identical assets are to be gained by clustering a conventional separation module to one of the peripheral ports of the multiposition valve in SIA systems [24] (see Figure1D).
The analytical capabilities of other innovative extraction modules and flow-through arrangements for on-line solvent extraction, involving iterative flow reversal extraction, on-tube detection, chromatomembrane extraction and extractive optrodes are described in detail in arecent fundamental review article [39]. The influence of external energy sources, such as ultrasounds, on mass transfer in LLE systems has been also explored, and a significant effect on those systems involving chemical derivatization has been reported [[45]].
2.4. Sorbent extraction
On-line solid-phase extraction (SPE), also called sorbent extraction, is the predominant sample processing method that has been growing rapidly as a consequence of its straightforward operation and high separation and preconcentration capabilities. Most often it is employed by using packed-bed or disk-phase based microcolumns, which are filled with appropriate sorptive materials and placed within the flow network prior to the detection device (see figure 2for a schematic illustration of an SIAsystem furnished with a packed column for SPE purposes). The ultimate goal is to improve the sensitivity of the analytical procedure and/or overcome the inherent low tolerance of the detection system to sample constituents by preconcentrating the measurand and/or removing interfering components from complex biological, industrial or environmental matrices. Whenever trace analysis is pursued, matrix separation occurs concomitantly with measurand enrichment.
Sorbent extraction plays an outstanding role in combination with atomic spectrometric detection, as has been demonstrated in several recent review articles [2,25,[46],[47]]. The discontinuous operation ofSIA makes this approach well suited for hyphenation of SPE with ETAAS in a column-in-tip or traditional column-in-manifold mode as a consequence of the discrete, non-continuous nature of the detector [47,[48]] (see figure 2). It should be stressed that inorganic metal speciation analyses are compatible with FIA/SIA-SPE by employing selective sorption or chemical elution protocols as demonstratedwith the determination of, for example, Cr(III)/Cr(VI), As(III)/As(V), Fe(II)/ Fe(III), Se(IV)/Se(VI) and Sb(III)/ Sb(V) [[49]-[50]-,[51],[52]].