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sequential injection on-line sample-pretreatment schemes coupled to ETAAS
Trends in Analytical Chemistry, Vol. 24, No. 1, 2005
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Trends and perspectives of flow injection/sequential injection on-line sample-pretreatment schemes coupled to ETAAS Jianhua Wang, Elo Harald Hansen
Flow injection (FI) analysis, the first generation of this technique, became in the 1990s supplemented by its second generation, sequential injection (SI), and most recently by the third generation (i.e., Lab-on-Valve). The dominant role played by FI in automatic, on-line, sample pretreatments in recent decades is amply demonstrated by the large number of publications to which it has given rise. Among these, its hyphenation with electrothermal atomic absorption spectrometry (ETAAS) is one of the most attractive sub-branches because of the high sensitivity of ETAAS instruments for metal species. After a decade of development, it is apparent from the literature that coupling FI sample pretreatment with ETAAS remains most dynamic in the new millenium; meanwhile, it is exciting to note that some novel trends associated with this subject have also emerged. The aim of this mini-review is thus to illustrate the state-of-the-art progress in implementing miniaturized FI/SI systems for on-line separation and preconcentration of trace metals with detection by ETAAS since 2000. We also discuss future perspectives in this field. ª 2004 Elsevier Ltd. All rights reserved. Keywords: ETAAS; Flow injection; Lab-on-Valve; On-line sample pretreatment; Sequential injection
1. Introduction Jianhua Wang Research Center for Analytical Sciences, Northeastern University, Chemistry Building, Box 332, Shenyang 110006, China Elo Harald Hansen* Department of Chemistry, Technical University of Denmark, Building 207, Kemitorvet, DK-2800, Kgs-Lyngby, Denmark *Corresponding author. Tel.: +45 4525-2346; Fax: +45 4588-3136; E-mail: [email protected]
ETAAS is one of the most attractive instrumental techniques for the determination of ultratrace levels of metal species, because of its high sensitivity [1,2]. However, ETAAS signals are inherently very susceptible to the composition of the sample matrix and notably to variations in it [3,4]. The most effective approach to completely eliminate the matrix effects is therefore simply to separate the analytes from the matrix components prior to ETAAS detection. Various separation and preconcentration schemes have been devel-
0165-9936/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.08.011
oped for this purpose. When effected manually in the batch mode, they all share the characteristics of being time consuming and labor intensive, plus tending to consume excessive amounts of reagents, which in itself is even more troublesome when large amounts of organic solvents are involved. But, most critically, the high risk of sample contamination in regular laboratory environments constitutes an important source of error, especially when dealing with ultratrace levels of metals in complex matrices, considering the extremely low limits of detection (LODs) of ETAAS instruments. Special care should therefore always be taken in order to minimize sample contamination during the sample-pretreatment processes. In this respect, it is particularly noteworthy that the three generations of FI can be applied in the aforementioned circumstances, whereby the drawbacks of the batch procedures can be avoided, to a large extent, in automated, miniaturized enclosed FI systems, and, at the same time, improved precision and LODs can be ensured [5,6]. It is obvious from the literature that, ever since the first report of interfacing FI on-line sample pre-treatment system with ETAAS successfully was implemented [7], elegantly solving the apparent incompatibility of integrating the continuously operating FI system with the discrete features of the ETAAS, this field has attracted much attention. And, in fact, the topic 1
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005
remains hot in the new millenium after a decade of developments. This review presents an update on progress in implementing the three generations of FI in facilitating on-line matrix removal/elimination as well as the preconcentration of trace levels of metal species with detection by ETAAS in the period since 2000. In this context, we discuss the frequently employed on-line sample-pretreatment schemes, including solid phase extraction (SPE), onwall molecular sorption and precipitate/coprecipitate retention via use of PTFE knotted reactors, liquid-liquid solvent extraction and back extraction, and hydride/vapour generation (HG/VG). We also address some novel protocols for on-line sample-pretreatment interfaced to ETAAS (e.g., an Lab-on-Valve (LOV)-bead injection (BI) scheme by means of a renewable microcolumn) and new approaches with PTFE surface sorption.
2. On-line sample pretreatment coupled to ETAAS Extensive studies in recent decades have indicated that FI/SI/SI-LOV, on-line, sample-pretreatment systems are well suited for interfacing with any flow-through spectrometric detectors. The various separation and preconcentration techniques coupled to atomic spectrometry are illustrated in Table 1. The on-line hyphenation of some of these protocols with ETAAS and pertinent details are discussed in the following sections. 2.1. Solvent extraction and extraction/back extraction Liquid-liquid solvent extraction is one of the most effective sample-pretreatment techniques for the elimination/separation of interfering matrix constituents and
for concurrent preconcentration of the analytes, such as trace metals. However, in the last decade attempts dedicated to the interfacing of on-line solvent-extraction systems with ETAAS have scarcely been reported. This has been attributed to the lack of robustness of conventional FI systems and to the difficulty in manipulating organic solvents, as well as the trouble associated with most of the on-line phase separators. However, in this respect, the simplicity, robustness and versatility of an SI system make it most suitable for executing on-line, solvent-extraction operations. Furthermore, the hyphenation of SI and FI facilitates the manipulation of more complicated manifolds, and the conical gravitational phase separator has been shown to be very effective for on-line separation of two immiscible phases [8]. With this background, some novel exploitations via solvent extraction have recently been reported. For example, an FI/SI on-line solvent-extraction procedure for the preconcentration of ultratrace levels of Cr(VI) via the formation of an ammonium pyrrolidinedithiocarbamate (APDC)–Cr(VI) complex was established by using a conical gravitational phase separator [9]. A modified version of the gravitational phase separator has also been used for FI on-line solvent extraction of cadmium [10]. Both procedures are suitable for trace-metal species in high-salinity matrices. In order to improve the preconcentration efficiency, the aqueous/organic phase ratio in the stream should be maintained as large as possible. When using gravitational phase separators, the practical phase ratio is, in our experience, best controlled by not exceeding a factor of 30 to ensure an effective separation of the two phases. Otherwise, at too high an aqueous/organic phase ratio, aqueous droplets tend to be entrapped within the organic phase. As a consequence, further
Table 1. Various FI separation and preconcentration schemes for metals as interfaced to atomic spectrometry On-line pretreatment protocols
Means for collection of analytes
Collected form of metals
Solvent extraction
Extraction (from aqueous to organic phase) Extraction/back extraction (from organic to aqueous phase)
Metal chelates in organic phase Free cations in aqueous phase
SPE
Column-based ion exchange – permanent and renewable column approaches. Column-based adsorption – permanent and renewable column approaches
Free cations in aqueous phase Metal chelates in aqueous or organic phase Metal chelates in aqueous or organic phase Free cations
Adsorption on the PTFE surface, including PTFE knotted reactor, microcolumn packed with PTFE beads, turnings and fibres Molecular recognition in molecularly imprinted polymers
2
Precipitate/ coprecipitate
On-wall collection on the interior surface of a PTFE knotted reactor, or on-line collection by means of filter
Precipitates/coprecipitates
Hydride/vapour generation
In-atomizer trapping of the metal species
Hydrides/vapors of hydride/ vapour-forming metals
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005
improvement in preconcentration capability is difficult to attain. Considering that the chelates of the heavy metal are generally non-polar rather than polar in nature, and thus are more soluble in organic solvents than in water, their distribution ratios are usually quite large. As a result, most of the reported on-line solvent-extraction procedures make use of a single extraction, whereby the majority of the analyte metal is transferred into the organic phase [8,9]. And, for the same reason, the gain by performing on-line multi-extraction on the same aliquot of sample solution will be very limited. However, employment of a fixed amount of organic phase to extract different portions of fresh sample solutions should prove to be an effective approach for improving extraction efficiency, provided, of course, that the organic phase does not become saturated with the metal of interest. Thus, a recently developed dual-stage FI/SI hybrid on-line solvent-extraction procedure was shown to be very effective in improving the extraction efficiency [11]. The working principle is illustrated in Fig. 1. In this case, a small, fixed amount of organic phase is used to extract a first portion of sample solution. Afterwards, the organic phase is separated in a gravitational phase separator and successively allowed to extract another fresh portion of sample solution, separation being effected in a second phase separator. For the preconcentration of bismuth, roughly twice as high an enrichment factor was obtained compared to that in a single-stage extraction procedure under identical experimental conditions. In most cases, the organic phase is separated from the mixture and directed into the atomizer for quantification. Studies revealed that organic solvents tend to distribute along the length of the graphite tube because of their lower surface tension and good wetting ability, thus resulting in loss of sensitivity and causing deterioration of precision in the detection step [12,13]. To take advantage of the high surface tension of water, it is Sample Analyte (Org) (O ETAAS
Org Sample PS1
PS2
Waste Waste Figure 1. Schematic representation of the separation processes in an FI/SI, on-line, dual-stage, solvent-extraction system. PS1 and PS2, phase separators.
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therefore beneficial to back extract the analytes from the organic phase into an aqueous phase prior to ETAAS quantification. For this purpose, an FI/SI hybrid on-line solventextraction–back-extraction procedure coupled to ETAAS by using two newly designed PEEK dual-conical gravitational phase separators has been described [14]. The new phase separator is reported to be effective for the separation of both organic and aqueous phases. Illustrated for the determination of cadmium via complexation with APDC, the Cd–APDC chelate was first extracted into a small amount of IBMK, which subsequently was successfully back extracted into an aqueous phase containing dilute nitric acid with Hg(II) as a stripping agent to accelerate the slow back-extraction process by replacing Cd from the Cd–APDC complex. The aqueous phase was afterwards introduced into the ETAAS for quantification. In this case, analyte enrichments were obtained in both stages of the extraction process. The flow manifold is illustrated in Fig. 2. 2.2. SPE and the renewable surface approach Extensive studies have shown that column-based SPE is one of the most efficient on-line sample-pretreatment approaches. Conventionally, a variety of sorbent materials has been employed for on-line column separation/ preconcentration coupled to ETAAS [15,16]. The broad range of choice for sorbent materials along with various chelating reagents and eluents makes this technique very attractive for on-line sample pretreatment. In the time span covered by this review, most of the efforts for on-line sample pretreatments were still devoted to column-based SPE. As stated elsewhere [17], the dispersion of the pretreated sample zone in an FI/SI on-line system should be kept at a minimum in order to achieve the highest enrichment efficiency, which equally well holds true in microcolumn systems. Thus, it is most beneficial to employ time-based sample loading. At the same time, shortening the distance for eluate transportation helps to minimize the loss of eluate. Several procedures have been proposed for using this mode. In one of these, the preconcentration of platinum was effected on a column, packed with silica gel functionalized with 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide (DPTH gel), inserted into the autosampler arm of the ETAAS [18]. The modification of the tubing circuitry of the autosampler allowed either sample to flow through the column or operation of the autosampler in the normal mode (i.e., nitric acid was directed through the column to elute the platinum and the eluate was directly deposited in the graphite tube). Platinum(IV) has also been determined in microcolumns packed with different chelating cellulose exchangers (i.e., oxime, sulphoxine and 2,20 -diaminodiethylamine (DEN) were used for preconcentration of http://www.elsevier.com/locate/trac
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005
(a)
Load
ETAAS
SV
PP/on APDC
W
EC
Sample
PS
EC
W
W
SL PS
IBMK BEx
HC
(b)
IV
SP2
i.d./0.4 mm
SP3
45 mm
SP1
~140µ ~140 l
ETAAS
Inject
SV
PP/off APDC
SL
EC
25 mm
Sample
PS
EC
W
W PS
IBMK
(c)
Phase separator
BEx
HC
SP1 SP2
IV SP3
Figure 2. Manifold for an SI on-line solvent-extraction–back-extraction system coupled with ETAAS: (a) load position, (b) inject position, (c) dual-conical gravitational phase separator (not scaled). SP1 , SP2 and SP3 , Syringe pumps; PP, Peristaltic pump; EC, Extraction coils; PS, Phase separators; SV, 6-port selection valve; HC, Holding coil; SL, Sample loop; IV, 2-position injection valve; and, W, Waste. (For details, see text.)
platinum after reduction by iodide or sulphite ions). The loaded platinum(II) was afterwards eluted with a mixture of thiourea–HCl [19]. Chromium speciation has been performed with another on-line system, where two microcolumns were inserted in the sample tip of the autosampler arm [20], as shown in Fig. 3. Cr(VI) was adsorbed when the column was packed with Amberlite IRA-910, while both chromium species were retained when the packing sorbent was silica gel functionalized with 1,5-bis(di-2pyridyl)methylene thiocarbohydrazide (DPTH gel). A similar procedure was established for simultaneous determination of bismuth, cadmium and lead in urine by employing a multi-element SIMMA 6000 ETAAS instrument [21]. The metal ions were retained on a miniature Muromac A-1 resin (iminodiacetate type) column, which was inserted at the tip of the autosampler arm, and 20% (v/v) nitric acid was used for elution. An interesting procedure for the separation and determination of tributyltin (TBT) in mineral and tap water has been described [22]. The procedure was based on the selective retention of TBT by a chelating resin, Amberlite XAD-2 impregnated with tropolone. The addition of 0.8% sulphuric acid to the water sample led to the retention of TBT by the resin, while monobutyltin (MBT), dibutyltin (DBT) and inorganic tin remained in solution. TBT was afterwards eluted with methyl isobutyl ketone (MIBK). 4
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Figure 3. Flow manifold for on-line, SPE coupled to ETAAS for chromium speciation by means of two minicolumns: P, External peristaltic pump; PAAS , Autosampler pump; Vs, Switching valve; A, Autosampler arm; S, Switches for the automatic control of the peristaltic pump and switching valve; W1 , W2 , Wastes (from [20], courtesy The Royal Society of Chemistry).
A modified cloud point extraction (i.e., a micellemediated preconcentration procedure) has been established for the analysis of lead [23]. The lead-entrapped surfactant micelles were adsorbed on a microcolumn packed with silica gel, and were then eluted with
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acetonitrile with ensuing ETAAS determination. By this approach, phase separation of the lead-entrapped surfactant micelles from the aqueous solution was avoided. In most of the on-line column-based operations, the sorbent is conventionally treated as a stationary component, being used repetitively and renewed only after numerous analytical runs. In this format, deterioration in analytical performance, associated with creation of flow resistance/back pressure because of the progressively tighter packing of the column material, or malfunctions of the reactive surface, caused by contamination and/or deactivation of the surface, or even the loss of functional groups, is eventually unavoidable. This effect is particularly critical when analyzing biological samples [24]. An interesting alternative was recently presented by adopting the renewable microcolumn approach with the SI-BI format in the so-called LOV system. In this mode, the microcolumn is renewed for each analytical run, which effectively eliminates the flow resistance and any potential changes or malfunctions of the surface properties encountered in traditional approaches. This approach does open the possibility of two different protocols for treating the loaded sorbent beads, as schematically shown in Fig. 4. Thus, in the first, an elution procedure is incorporated after the microcolumn has been loaded with analytes [25,26], while, in the other, the loaded beads are transported directly into the graphite tube of the ETAAS instrument [27,28], advantage being taken of the fact that the beads primarily comprise organic
Bead suspension spension
Waste Column packing Analyte Sample
Waste Matrix
Analyte loading
Eluent
Carrier
Beads/Analyte
Eluate Elution Carrier
Bead transporting Be ransporting ETAAS
ETAAS
Waste Bead discarding iscarding
(a) Bead ead elution
(b) Bead ead transportation ransportation
Figure 4. Illustration of the two schemes for dealing with the analyte-loaded beads in the renewable microcolumn approach. (a) The beads are eluted and the eluate is exploited for quantification; (b) the beads are transported directly into the graphite tube, where the analyte is quantified, following pyrolysis of the beads.
material, which, after pyrolysis, allows the analyte to become quantified. 2.3. Separations and preconcentrations on PTFE surfaces PTFE knotted reactors (KRs) have been widely recognized as a trouble-free, easy-to-control sorption medium for SPE and precipitate/coprecipitate collection [29]. The creation of increased secondary, radial flow in the KR, and thus the generation of strong centrifugal forces in the stream, facilitates the adsorption of neutral analyte compounds or precipitate/coprecipitate onto the inner wall of the KR through molecular sorption. The adsorbed analyte is subsequently eluted by an appropriate eluent and is then, via air segmentation, transported into the graphite furnace for quantification. A precipitation-dissolution procedure with microwave assisted sample digestion has been developed by Burguera et al. [30] for trace molybdenum(VI) determination in human-blood serum and whole blood. The on-line generation of the precipitate of molybdenum was achieved by merging the sample zone with a plug of potassium hexacyanoferrate(II). The reddish-brown precipitate of molybdenyl hexacyanoferrate was collected on the walls of the PTFE KR, and the precipitate was afterwards dissolved by introduction of a 3.0 mol/l sodium hydroxide solution. The same group reported another procedure for the determination of ‘‘total’’ and ‘‘soluble’’ silicon in water samples [31]. In order to allow the determination of total silicon within the 300–1000 lg/l range, the sample solution was first on-line diluted, whereupon a subsample portion was collected in the capillary of the sampling arm. For the determination of the ‘‘soluble’’ fraction, the silicon was precipitated with ammonium chloride and collected on the walls of the KR. The precipitate was afterwards dissolved with a slightly acidified ammonium molybdate solution, and the resulting yellow solution of the heteropolyacid of ammonium molybdosilicate was collected in the sampling arm assembly, followed by transportation of it into the graphite tube to facilitate the determination of ‘‘soluble’’ silicon within the 280–850 lg/l range. Another approach for KR adsorption is to pre-coat the complexing reagent directly onto the interior surface of the KR, followed by sample loading and executing the separation/preconcentration process. Thus, a variety of hydroxyquinoline (HQ) derivatives (i.e., 8-HQ, 2-methyl8-HQ, 5,7-dichloro-2-methyl-8-HQ, 5,7-dibromo-8-HQ, 5-sulfo-7-iodo-8-HQ and 5-sulfo-8-HQ) have been compared as chelating reagents for on-line sorption preconcentration of cobalt in a KR pre-coated with these reagents [32]. The results showed that, among the various variables affecting the performance of the precoated PTFE KR, the most important parameters are, as expected, the hydrophobicity of the reagents and the http://www.elsevier.com/locate/trac
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stability of the chelating complexes formed with the analyte. However, although the open-ended nature of the KR entails the clear advantage of low flow resistance/ hydrodynamic impedance and allows high sampleloading flow rates for obtaining better enrichment factors, the low retention efficiency for most of the analyte complexes restricts further improvement of its preconcentration capability. The reported values of retention efficiency are generally within the range of 30–60%, while some are even lower than 20% [33]. This drawback is eliminated by performing the sorption on PTFE surfaces using packed columns. Typically, a microcolumn is packed with PTFE beads, nominally of 100 lm. A good example is the sorption and determination of cadmium and chromium(VI) by using diethyldithiophosphate (DDPA) and pyrrolidinedithiocarbamate (APDC), respectively, as chelating reagents [34,35]. The results showed that both the retention efficiencies and the enrichment factors obtained by a PTFE bead-packed column are improved substantially compared to those when using a KR of a similar internal active surface area. PTFE turnings have similarly been employed to pack a microcolumn for on-line preconcentration of cobalt via the sorption of a Co–APDC complex [36], and results were obtained similar to those of PTFE bead-packed columns. Besides their remarkable improvement in retention efficiency, both PTFE bead- and turning-packed microcolumns additionally entail low hydrodynamic resistance of the flow system (i.e., they possess the figures of merit of long life time and low flow resistance characteristic of the PTFE KR). 2.4. On-line HG/VG coupled to ETAAS The interfacing of an FI/SI on-line HG/VG system to ETAAS is not straightforward. The incompatibility between the flow system and the discrete feature of the ETAAS requires a vapour-sequestration procedure before the atomization process. This can be effected by in-atomizer trapping of the analytes via successive sequestration of the hydride/vapor in a pre-coated graphite tube, allowing appropriate preconcentration and, therefore, substantial improvement of the sensitivity. The determination of lead in media of high calcium carbonate content via the HG approach is quite problematic because of the matrix effect encountered. However, an FI-HG procedure was successfully developed for this purpose [37]. Lead hydride was generated from an acid solution – with potassium hexacyanoferrate(III) added as oxidizing agent (to convert Pb(II) to Pb(IV)) – by reaction with alkaline tetrahydroborate solution. The hydride was successively sequestrated on the interior surface of an iridium pretreated transversely heated graphite atomizer at 400C for 40 s, followed by 6
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005
atomization and quantification. The results obtained were in full agreement with validated methods. It has been demonstrated that the VG approach is equally effective for the determination of some metals that cannot form hydrides as such, provided that a proper metal vapour can be obtained. Thus, a volatile species of gold in acidified medium was generated at room temperature by using an FI-VG system [38]. The gold vapour, the form of which has not yet been specified, was generated in acidified aqueous medium with NaBH4 as reducing reagent in the presence of microamounts of sodium diethyldithiocarbamate (DDTC). The vapour was afterwards sequestrated in the graphite furnace by in-atomizer trapping and followed by atomization. However, an LOD of 0.8 lg/l for the procedure is not satisfactory, when compared with conventional VG approaches coupled with ETAAS. In this context, it should be mentioned that although the in-atomizer sequestration procedure might appear attractive, it is, from an operational point of view, much preferable to effect all the manipulations on-line in a single cycle, yet separate the preconcentration part of the procedure from that of the HG part. Being readily feasible in an FI system, it renders an extra degree of freedom because it allows not only selection of the preconcentration chemistry completely independently of the HG scheme used, but also obtains much higher enrichment factors in considerably shorter sampling times. 2.5. Miscellaneous on-line sample-pretreatment approaches In addition to the aforementioned on-line sample-pretreatment protocols, some other on-line procedures have also been reported. Dialysis is one of the most effective approaches for the separation/collection and analysis of trace species from complex matrices, such as whole blood samples, although on-line microdialysis schemes coupled to ETAAS detection have rarely been reported. However, an on-line microdialysis procedure with detection by ETAAS was developed for in vivo monitoring of extracellular diffusive manganese in brains of living rats [39], the manifold used being illustrated in Fig. 5. The microdialysates, perfused through implanted microdialysis probes, were collected in a sample loop, the content of which was afterwards introduced directly into the atomizer of the ETAAS instrument for quantification. An automatic separation/preconcentration procedure for the determination of total arsenic at the ng/g level in wheat flour has been proposed [40]. Total arsenic, in the form of arsenate anions, was selectively precipitated with silver(I) as Ag3 AsO4 . The precipitate was collected with a Scientific System column furnished with a removable screen-type stainless-steel filter, which was originally designed as a cleaning device for high-performance liquid chromatography (HPLC). The precipitate was
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Figure 5. Flow manifold for on-line microdialysis coupled with ETAAS for in vivo monitoring of extracellular diffusive manganese in brains of living rats: P1, Microinjection syringe pump; MP, microdialysis probe; T, Micro-Tee; V, On-line injection valve; A, Air; P2, Peristaltic pump; W, waste; Valve position: (a) loading; (b) injection (adapted from [39], by permission of The Royal Society of Chemistry).
subsequently dissolved with ammonia and the resulting solution collected in an autosampler cup; afterwards, it was directed into the graphite tube of the ETAAS instrument, followed by atomization and quantification. When precipitating As(V) from 10 ml of digested sample solution by using a weakly acidified Ag(I) solution, this approach allowed the sensitivity to be increased by a factor of 20. Another on-line precipitation–dissolution procedure was developed for ETAAS determination of ultratrace amounts of beryllium in water samples [41]. Beryllium was precipitated quantitatively with NH4 OH–NH4 Cl and the resulting Be(OH)2 collected in a hydrophilic Tygon KR. The precipitate was dissolved with nitric acid and a sub-sample was collected in the capillary of the sampling arm assembly, followed by introduction into the graphite tube.
3. Perspectives On-line separation and preconcentration procedures can effectively eliminate the interfering effects from matrix components and offer pertinent improvements in sensitivity. Although the coupling of on-line samplepretreatment protocols with ETAAS has been subjected to extensive investigations, there are still some vacant areas that need to be further exploited. For instance, microcolumns packed with PTFE beads, turnings and fibres could not only be employed for improving the retention efficiency of metal chelates, but also potentially be used for precipitate/coprecipitate collection, and further combined with HG. SPE with the online renewable microcolumn approach is merely the
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start of employing the LOV format in sample pretreatment. In fact, practically, most of, or all, the conventional sample-pretreatment schemes should potentially be feasible to implement in the LOV system, such as in-valve solvent-extraction, precipitate/coprecipitate collection, and in-valve precipitation/coprecipitation/SPE followed by HG. So far, on-line sample-pretreatment schemes coupled to ETAAS have predominately been focused on the determination of total metal contents, while speciation studies have rarely been touched. Thus, more efforts should be devoted to the speciation of metals via the combination of on-line sample-pretreatment protocols with ETAAS. Another scheme attracting considerable attention is sample pretreatment by molecular recognition using molecularly imprinted polymers (MIPs). Thus, Say et al. [42] recently demonstrated excellent selectivity for the target, Cu(II), when executing SPE with a MIP-packed microcolumn reactor. However, the interface of this approach with ETAAS has not yet been reported. Thus, this format could not only offer highly selective separation/preconcentration procedures, but also open a promising avenue for metal speciation by coupling on-line molecular recognition with ETAAS detection.
Acknowledgement One of us (J.-H.W.) is indebted to The National Natural Science Foundation of China (NSFC-20375007) and to The China Postdoctoral Science Foundation for financial support. References [1] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 424 (2000) 223. [2] E.V. Alonso, A.G. Torres, J.M.C. Pavon, Talanta 55 (2001) 219. [3] M. Grotti, R. Leardi, C. Gnecco, R. Frache, Spectrochim. Acta, Part B 54 (1999) 845. [4] K. B€ackstr€ om, L.G. Danielsson, Anal. Chim. Acta 232 (1990) 301. [5] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 225. [6] E.H. Hansen, J.-H. Wang, Anal. Chim. Acta 467 (2002) 3. [7] Z. Fang, M. Sperling, B. Welz, J. Anal. At. Spectrom. 5 (1990) 639. [8] G. Tao, Z. Fang, Spectrochim. Acta, Part B 50 (1995) 1747. [9] S.C. Nielsen, E.H. Hansen, Anal. Chim. Acta 422 (2000) 47. [10] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, J. Anal. At. Spectrom. 18 (2003) 1400. [11] J.-H. Wang, E.H. Hansen, Anal. Lett. 33 (2000) 2747. [12] K. B€ackstr€ om, L.G. Danielsson, Anal. Chim. Acta 232 (1990) 301. [13] A.B. Volynsky, B.Y. Spivakov, Y.A. Zolotov, Talanta 31 (1984) 449. [14] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 456 (2002) 283. [15] E.V. Alonso, A.G. de Torres, J.M.C. Pavon, Talanta 55 (2001) 219.
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Trends [16] J.L. Burguera, M. Burguera, Spectrochim. Acta, Part B 56 (2001) 1801. [17] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 836. [18] C.B. Ojeda, F.S. Rojas, J.M.C. Pavon, A.G. de Torres, Anal. Chim. Acta 494 (2003) 97. Kelk [19] A. L asztity, A. o-Levai, K. Zih-Perenyi, I. Varga, Talanta 59 (2003) 393. [20] M.T.S. Cordero, E.I.V. Alonso, A.G. de Torres, J.M.C. Pavon, J. Anal. At. Spectrom. 19 (2004) 398. [21] Y.-H. Sung, S.-D. Huang, Anal. Chim. Acta 495 (2003) 165. [22] P. Bermejo-Barrera, R.M. Anllo-Sendia, M.J. Cantelar-Barbazan, A. Bermejo-Barrera, Anal. Bioanal. Chem. 372 (2002) 837. [23] J. Nan, Y. Jiang, X.-P. Yan, J. Anal. At. Spectrom. 18 (2003) 946. [24] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 225. [25] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 435 (2001) 331. [26] J.-H. Wang, E.H. Hansen, Atom. Spectrosc. 22 (2001) 312. [27] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 424 (2000) 223. [28] P. Ampan, J. Ruzicka, R. Atallah, G.D. Christian, J. Jakmunee, K. Grudpan, Anal. Chim. Acta 499 (2003) 167. [29] Z. Fang, S. Xu, L. Dong, W. Li, Talanta 41 (1994) 2165. [30] J.L. Burguera, M. Burguera, C. Rondon, Talanta 58 (2002) 1167. [31] J.L. Burguera, M. Burguera, P. Carrero, C. Rondon, Talanta 58 (2002) 1157. [32] S. Tsakovski1, K. Benkhedda, E. Ivanova, F.C. Adams, Anal. Chim. Acta 453 (2002) 143. [33] E. Ivanova, K. Benkhedda, F. Adams, J. Anal. At. Spectrom. 13 (1998) 527. [34] J.-H. Wang, E.H. Hansen, J. Anal. At. Spectrom. 17 (2002) 248. [35] W. Som-aum, S. Liawruangrath, E.H. Hansen, Anal. Chim. Acta 463 (2002) 99.
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005 [36] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, J. Anal. At. Spectrom. 17 (2002) 1330. [37] J.F. Tyson, R.I. Ellis, G. Carnrick, F. Fernandez, Talanta 52 (2000) 403. [38] H. Ma, X. Fan, H. Zhou, S. Xu, Spectrochim. Acta, Part B 58 (2003) 33. [39] W.-C. Tseng, Y.-C. Sun, M.-H. Yang, T.-P. Chen, T.-H. Lin, Y.-L. Huang, J. Anal. At. Spectrom. 18 (2003) 38. [40] M.M. Gonzalez, M. Gallego, M. Valcarcel, Talanta 55 (2001) 135. [41] J.L. Burguera, M. Burguera, C. Rondon, P. Carrero, M.R. Brunetto, Y.P. de Pena, Talanta 52 (2000) 27. [42] R. Say, E. Birlik, A. Ers€ oz, F. Y ylmaz, T. Gedikbey, A. Denizli, Anal. Chim. Acta 480 (2003) 251. Elo Harald Hansen, who is one of the two inventors of FI, is Professor of Analytical Chemistry at the Department of Chemistry, Technical University of Denmark. His research interest is focused on instrumental and automated analysis with special emphasis on the development and application of procedures based on FI/SI. In recent years, he has placed particular emphasis on developing FI/SI/LOV-schemes for the determination of trace levels of metals in complex matrices by hyphenation with atomic spectrometric techniques and ICP-MS. Jianhua Wang is Professor and Director of the Research Center for Analytical Sciences, Northeastern University, China. He received his B.Sc. and M.Sc. degrees at Nankai University and Jilin University, respectively. He was awarded a Ph.D. degree at the Technical University of Denmark. After nine years’ employment at Yantai Normal University, he spent one year at the University of Delaware conducting research on analytical applications of tandem mass spectrometry. He is currently focusing his research interests on SI/LOV and related techniques.
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Trends and perspectives of flow injection/sequential injection on-line sample-pretreatment schemes coupled to ETAAS Jianhua Wang, Elo Harald Hansen
Flow injection (FI) analysis, the first generation of this technique, became in the 1990s supplemented by its second generation, sequential injection (SI), and most recently by the third generation (i.e., Lab-on-Valve). The dominant role played by FI in automatic, on-line, sample pretreatments in recent decades is amply demonstrated by the large number of publications to which it has given rise. Among these, its hyphenation with electrothermal atomic absorption spectrometry (ETAAS) is one of the most attractive sub-branches because of the high sensitivity of ETAAS instruments for metal species. After a decade of development, it is apparent from the literature that coupling FI sample pretreatment with ETAAS remains most dynamic in the new millenium; meanwhile, it is exciting to note that some novel trends associated with this subject have also emerged. The aim of this mini-review is thus to illustrate the state-of-the-art progress in implementing miniaturized FI/SI systems for on-line separation and preconcentration of trace metals with detection by ETAAS since 2000. We also discuss future perspectives in this field. ª 2004 Elsevier Ltd. All rights reserved. Keywords: ETAAS; Flow injection; Lab-on-Valve; On-line sample pretreatment; Sequential injection
1. Introduction Jianhua Wang Research Center for Analytical Sciences, Northeastern University, Chemistry Building, Box 332, Shenyang 110006, China Elo Harald Hansen* Department of Chemistry, Technical University of Denmark, Building 207, Kemitorvet, DK-2800, Kgs-Lyngby, Denmark *Corresponding author. Tel.: +45 4525-2346; Fax: +45 4588-3136; E-mail: [email protected]
ETAAS is one of the most attractive instrumental techniques for the determination of ultratrace levels of metal species, because of its high sensitivity [1,2]. However, ETAAS signals are inherently very susceptible to the composition of the sample matrix and notably to variations in it [3,4]. The most effective approach to completely eliminate the matrix effects is therefore simply to separate the analytes from the matrix components prior to ETAAS detection. Various separation and preconcentration schemes have been devel-
0165-9936/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.08.011
oped for this purpose. When effected manually in the batch mode, they all share the characteristics of being time consuming and labor intensive, plus tending to consume excessive amounts of reagents, which in itself is even more troublesome when large amounts of organic solvents are involved. But, most critically, the high risk of sample contamination in regular laboratory environments constitutes an important source of error, especially when dealing with ultratrace levels of metals in complex matrices, considering the extremely low limits of detection (LODs) of ETAAS instruments. Special care should therefore always be taken in order to minimize sample contamination during the sample-pretreatment processes. In this respect, it is particularly noteworthy that the three generations of FI can be applied in the aforementioned circumstances, whereby the drawbacks of the batch procedures can be avoided, to a large extent, in automated, miniaturized enclosed FI systems, and, at the same time, improved precision and LODs can be ensured [5,6]. It is obvious from the literature that, ever since the first report of interfacing FI on-line sample pre-treatment system with ETAAS successfully was implemented [7], elegantly solving the apparent incompatibility of integrating the continuously operating FI system with the discrete features of the ETAAS, this field has attracted much attention. And, in fact, the topic 1
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remains hot in the new millenium after a decade of developments. This review presents an update on progress in implementing the three generations of FI in facilitating on-line matrix removal/elimination as well as the preconcentration of trace levels of metal species with detection by ETAAS in the period since 2000. In this context, we discuss the frequently employed on-line sample-pretreatment schemes, including solid phase extraction (SPE), onwall molecular sorption and precipitate/coprecipitate retention via use of PTFE knotted reactors, liquid-liquid solvent extraction and back extraction, and hydride/vapour generation (HG/VG). We also address some novel protocols for on-line sample-pretreatment interfaced to ETAAS (e.g., an Lab-on-Valve (LOV)-bead injection (BI) scheme by means of a renewable microcolumn) and new approaches with PTFE surface sorption.
2. On-line sample pretreatment coupled to ETAAS Extensive studies in recent decades have indicated that FI/SI/SI-LOV, on-line, sample-pretreatment systems are well suited for interfacing with any flow-through spectrometric detectors. The various separation and preconcentration techniques coupled to atomic spectrometry are illustrated in Table 1. The on-line hyphenation of some of these protocols with ETAAS and pertinent details are discussed in the following sections. 2.1. Solvent extraction and extraction/back extraction Liquid-liquid solvent extraction is one of the most effective sample-pretreatment techniques for the elimination/separation of interfering matrix constituents and
for concurrent preconcentration of the analytes, such as trace metals. However, in the last decade attempts dedicated to the interfacing of on-line solvent-extraction systems with ETAAS have scarcely been reported. This has been attributed to the lack of robustness of conventional FI systems and to the difficulty in manipulating organic solvents, as well as the trouble associated with most of the on-line phase separators. However, in this respect, the simplicity, robustness and versatility of an SI system make it most suitable for executing on-line, solvent-extraction operations. Furthermore, the hyphenation of SI and FI facilitates the manipulation of more complicated manifolds, and the conical gravitational phase separator has been shown to be very effective for on-line separation of two immiscible phases [8]. With this background, some novel exploitations via solvent extraction have recently been reported. For example, an FI/SI on-line solvent-extraction procedure for the preconcentration of ultratrace levels of Cr(VI) via the formation of an ammonium pyrrolidinedithiocarbamate (APDC)–Cr(VI) complex was established by using a conical gravitational phase separator [9]. A modified version of the gravitational phase separator has also been used for FI on-line solvent extraction of cadmium [10]. Both procedures are suitable for trace-metal species in high-salinity matrices. In order to improve the preconcentration efficiency, the aqueous/organic phase ratio in the stream should be maintained as large as possible. When using gravitational phase separators, the practical phase ratio is, in our experience, best controlled by not exceeding a factor of 30 to ensure an effective separation of the two phases. Otherwise, at too high an aqueous/organic phase ratio, aqueous droplets tend to be entrapped within the organic phase. As a consequence, further
Table 1. Various FI separation and preconcentration schemes for metals as interfaced to atomic spectrometry On-line pretreatment protocols
Means for collection of analytes
Collected form of metals
Solvent extraction
Extraction (from aqueous to organic phase) Extraction/back extraction (from organic to aqueous phase)
Metal chelates in organic phase Free cations in aqueous phase
SPE
Column-based ion exchange – permanent and renewable column approaches. Column-based adsorption – permanent and renewable column approaches
Free cations in aqueous phase Metal chelates in aqueous or organic phase Metal chelates in aqueous or organic phase Free cations
Adsorption on the PTFE surface, including PTFE knotted reactor, microcolumn packed with PTFE beads, turnings and fibres Molecular recognition in molecularly imprinted polymers
2
Precipitate/ coprecipitate
On-wall collection on the interior surface of a PTFE knotted reactor, or on-line collection by means of filter
Precipitates/coprecipitates
Hydride/vapour generation
In-atomizer trapping of the metal species
Hydrides/vapors of hydride/ vapour-forming metals
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improvement in preconcentration capability is difficult to attain. Considering that the chelates of the heavy metal are generally non-polar rather than polar in nature, and thus are more soluble in organic solvents than in water, their distribution ratios are usually quite large. As a result, most of the reported on-line solvent-extraction procedures make use of a single extraction, whereby the majority of the analyte metal is transferred into the organic phase [8,9]. And, for the same reason, the gain by performing on-line multi-extraction on the same aliquot of sample solution will be very limited. However, employment of a fixed amount of organic phase to extract different portions of fresh sample solutions should prove to be an effective approach for improving extraction efficiency, provided, of course, that the organic phase does not become saturated with the metal of interest. Thus, a recently developed dual-stage FI/SI hybrid on-line solvent-extraction procedure was shown to be very effective in improving the extraction efficiency [11]. The working principle is illustrated in Fig. 1. In this case, a small, fixed amount of organic phase is used to extract a first portion of sample solution. Afterwards, the organic phase is separated in a gravitational phase separator and successively allowed to extract another fresh portion of sample solution, separation being effected in a second phase separator. For the preconcentration of bismuth, roughly twice as high an enrichment factor was obtained compared to that in a single-stage extraction procedure under identical experimental conditions. In most cases, the organic phase is separated from the mixture and directed into the atomizer for quantification. Studies revealed that organic solvents tend to distribute along the length of the graphite tube because of their lower surface tension and good wetting ability, thus resulting in loss of sensitivity and causing deterioration of precision in the detection step [12,13]. To take advantage of the high surface tension of water, it is Sample Analyte (Org) (O ETAAS
Org Sample PS1
PS2
Waste Waste Figure 1. Schematic representation of the separation processes in an FI/SI, on-line, dual-stage, solvent-extraction system. PS1 and PS2, phase separators.
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therefore beneficial to back extract the analytes from the organic phase into an aqueous phase prior to ETAAS quantification. For this purpose, an FI/SI hybrid on-line solventextraction–back-extraction procedure coupled to ETAAS by using two newly designed PEEK dual-conical gravitational phase separators has been described [14]. The new phase separator is reported to be effective for the separation of both organic and aqueous phases. Illustrated for the determination of cadmium via complexation with APDC, the Cd–APDC chelate was first extracted into a small amount of IBMK, which subsequently was successfully back extracted into an aqueous phase containing dilute nitric acid with Hg(II) as a stripping agent to accelerate the slow back-extraction process by replacing Cd from the Cd–APDC complex. The aqueous phase was afterwards introduced into the ETAAS for quantification. In this case, analyte enrichments were obtained in both stages of the extraction process. The flow manifold is illustrated in Fig. 2. 2.2. SPE and the renewable surface approach Extensive studies have shown that column-based SPE is one of the most efficient on-line sample-pretreatment approaches. Conventionally, a variety of sorbent materials has been employed for on-line column separation/ preconcentration coupled to ETAAS [15,16]. The broad range of choice for sorbent materials along with various chelating reagents and eluents makes this technique very attractive for on-line sample pretreatment. In the time span covered by this review, most of the efforts for on-line sample pretreatments were still devoted to column-based SPE. As stated elsewhere [17], the dispersion of the pretreated sample zone in an FI/SI on-line system should be kept at a minimum in order to achieve the highest enrichment efficiency, which equally well holds true in microcolumn systems. Thus, it is most beneficial to employ time-based sample loading. At the same time, shortening the distance for eluate transportation helps to minimize the loss of eluate. Several procedures have been proposed for using this mode. In one of these, the preconcentration of platinum was effected on a column, packed with silica gel functionalized with 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide (DPTH gel), inserted into the autosampler arm of the ETAAS [18]. The modification of the tubing circuitry of the autosampler allowed either sample to flow through the column or operation of the autosampler in the normal mode (i.e., nitric acid was directed through the column to elute the platinum and the eluate was directly deposited in the graphite tube). Platinum(IV) has also been determined in microcolumns packed with different chelating cellulose exchangers (i.e., oxime, sulphoxine and 2,20 -diaminodiethylamine (DEN) were used for preconcentration of http://www.elsevier.com/locate/trac
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(a)
Load
ETAAS
SV
PP/on APDC
W
EC
Sample
PS
EC
W
W
SL PS
IBMK BEx
HC
(b)
IV
SP2
i.d./0.4 mm
SP3
45 mm
SP1
~140µ ~140 l
ETAAS
Inject
SV
PP/off APDC
SL
EC
25 mm
Sample
PS
EC
W
W PS
IBMK
(c)
Phase separator
BEx
HC
SP1 SP2
IV SP3
Figure 2. Manifold for an SI on-line solvent-extraction–back-extraction system coupled with ETAAS: (a) load position, (b) inject position, (c) dual-conical gravitational phase separator (not scaled). SP1 , SP2 and SP3 , Syringe pumps; PP, Peristaltic pump; EC, Extraction coils; PS, Phase separators; SV, 6-port selection valve; HC, Holding coil; SL, Sample loop; IV, 2-position injection valve; and, W, Waste. (For details, see text.)
platinum after reduction by iodide or sulphite ions). The loaded platinum(II) was afterwards eluted with a mixture of thiourea–HCl [19]. Chromium speciation has been performed with another on-line system, where two microcolumns were inserted in the sample tip of the autosampler arm [20], as shown in Fig. 3. Cr(VI) was adsorbed when the column was packed with Amberlite IRA-910, while both chromium species were retained when the packing sorbent was silica gel functionalized with 1,5-bis(di-2pyridyl)methylene thiocarbohydrazide (DPTH gel). A similar procedure was established for simultaneous determination of bismuth, cadmium and lead in urine by employing a multi-element SIMMA 6000 ETAAS instrument [21]. The metal ions were retained on a miniature Muromac A-1 resin (iminodiacetate type) column, which was inserted at the tip of the autosampler arm, and 20% (v/v) nitric acid was used for elution. An interesting procedure for the separation and determination of tributyltin (TBT) in mineral and tap water has been described [22]. The procedure was based on the selective retention of TBT by a chelating resin, Amberlite XAD-2 impregnated with tropolone. The addition of 0.8% sulphuric acid to the water sample led to the retention of TBT by the resin, while monobutyltin (MBT), dibutyltin (DBT) and inorganic tin remained in solution. TBT was afterwards eluted with methyl isobutyl ketone (MIBK). 4
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Figure 3. Flow manifold for on-line, SPE coupled to ETAAS for chromium speciation by means of two minicolumns: P, External peristaltic pump; PAAS , Autosampler pump; Vs, Switching valve; A, Autosampler arm; S, Switches for the automatic control of the peristaltic pump and switching valve; W1 , W2 , Wastes (from [20], courtesy The Royal Society of Chemistry).
A modified cloud point extraction (i.e., a micellemediated preconcentration procedure) has been established for the analysis of lead [23]. The lead-entrapped surfactant micelles were adsorbed on a microcolumn packed with silica gel, and were then eluted with
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acetonitrile with ensuing ETAAS determination. By this approach, phase separation of the lead-entrapped surfactant micelles from the aqueous solution was avoided. In most of the on-line column-based operations, the sorbent is conventionally treated as a stationary component, being used repetitively and renewed only after numerous analytical runs. In this format, deterioration in analytical performance, associated with creation of flow resistance/back pressure because of the progressively tighter packing of the column material, or malfunctions of the reactive surface, caused by contamination and/or deactivation of the surface, or even the loss of functional groups, is eventually unavoidable. This effect is particularly critical when analyzing biological samples [24]. An interesting alternative was recently presented by adopting the renewable microcolumn approach with the SI-BI format in the so-called LOV system. In this mode, the microcolumn is renewed for each analytical run, which effectively eliminates the flow resistance and any potential changes or malfunctions of the surface properties encountered in traditional approaches. This approach does open the possibility of two different protocols for treating the loaded sorbent beads, as schematically shown in Fig. 4. Thus, in the first, an elution procedure is incorporated after the microcolumn has been loaded with analytes [25,26], while, in the other, the loaded beads are transported directly into the graphite tube of the ETAAS instrument [27,28], advantage being taken of the fact that the beads primarily comprise organic
Bead suspension spension
Waste Column packing Analyte Sample
Waste Matrix
Analyte loading
Eluent
Carrier
Beads/Analyte
Eluate Elution Carrier
Bead transporting Be ransporting ETAAS
ETAAS
Waste Bead discarding iscarding
(a) Bead ead elution
(b) Bead ead transportation ransportation
Figure 4. Illustration of the two schemes for dealing with the analyte-loaded beads in the renewable microcolumn approach. (a) The beads are eluted and the eluate is exploited for quantification; (b) the beads are transported directly into the graphite tube, where the analyte is quantified, following pyrolysis of the beads.
material, which, after pyrolysis, allows the analyte to become quantified. 2.3. Separations and preconcentrations on PTFE surfaces PTFE knotted reactors (KRs) have been widely recognized as a trouble-free, easy-to-control sorption medium for SPE and precipitate/coprecipitate collection [29]. The creation of increased secondary, radial flow in the KR, and thus the generation of strong centrifugal forces in the stream, facilitates the adsorption of neutral analyte compounds or precipitate/coprecipitate onto the inner wall of the KR through molecular sorption. The adsorbed analyte is subsequently eluted by an appropriate eluent and is then, via air segmentation, transported into the graphite furnace for quantification. A precipitation-dissolution procedure with microwave assisted sample digestion has been developed by Burguera et al. [30] for trace molybdenum(VI) determination in human-blood serum and whole blood. The on-line generation of the precipitate of molybdenum was achieved by merging the sample zone with a plug of potassium hexacyanoferrate(II). The reddish-brown precipitate of molybdenyl hexacyanoferrate was collected on the walls of the PTFE KR, and the precipitate was afterwards dissolved by introduction of a 3.0 mol/l sodium hydroxide solution. The same group reported another procedure for the determination of ‘‘total’’ and ‘‘soluble’’ silicon in water samples [31]. In order to allow the determination of total silicon within the 300–1000 lg/l range, the sample solution was first on-line diluted, whereupon a subsample portion was collected in the capillary of the sampling arm. For the determination of the ‘‘soluble’’ fraction, the silicon was precipitated with ammonium chloride and collected on the walls of the KR. The precipitate was afterwards dissolved with a slightly acidified ammonium molybdate solution, and the resulting yellow solution of the heteropolyacid of ammonium molybdosilicate was collected in the sampling arm assembly, followed by transportation of it into the graphite tube to facilitate the determination of ‘‘soluble’’ silicon within the 280–850 lg/l range. Another approach for KR adsorption is to pre-coat the complexing reagent directly onto the interior surface of the KR, followed by sample loading and executing the separation/preconcentration process. Thus, a variety of hydroxyquinoline (HQ) derivatives (i.e., 8-HQ, 2-methyl8-HQ, 5,7-dichloro-2-methyl-8-HQ, 5,7-dibromo-8-HQ, 5-sulfo-7-iodo-8-HQ and 5-sulfo-8-HQ) have been compared as chelating reagents for on-line sorption preconcentration of cobalt in a KR pre-coated with these reagents [32]. The results showed that, among the various variables affecting the performance of the precoated PTFE KR, the most important parameters are, as expected, the hydrophobicity of the reagents and the http://www.elsevier.com/locate/trac
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stability of the chelating complexes formed with the analyte. However, although the open-ended nature of the KR entails the clear advantage of low flow resistance/ hydrodynamic impedance and allows high sampleloading flow rates for obtaining better enrichment factors, the low retention efficiency for most of the analyte complexes restricts further improvement of its preconcentration capability. The reported values of retention efficiency are generally within the range of 30–60%, while some are even lower than 20% [33]. This drawback is eliminated by performing the sorption on PTFE surfaces using packed columns. Typically, a microcolumn is packed with PTFE beads, nominally of 100 lm. A good example is the sorption and determination of cadmium and chromium(VI) by using diethyldithiophosphate (DDPA) and pyrrolidinedithiocarbamate (APDC), respectively, as chelating reagents [34,35]. The results showed that both the retention efficiencies and the enrichment factors obtained by a PTFE bead-packed column are improved substantially compared to those when using a KR of a similar internal active surface area. PTFE turnings have similarly been employed to pack a microcolumn for on-line preconcentration of cobalt via the sorption of a Co–APDC complex [36], and results were obtained similar to those of PTFE bead-packed columns. Besides their remarkable improvement in retention efficiency, both PTFE bead- and turning-packed microcolumns additionally entail low hydrodynamic resistance of the flow system (i.e., they possess the figures of merit of long life time and low flow resistance characteristic of the PTFE KR). 2.4. On-line HG/VG coupled to ETAAS The interfacing of an FI/SI on-line HG/VG system to ETAAS is not straightforward. The incompatibility between the flow system and the discrete feature of the ETAAS requires a vapour-sequestration procedure before the atomization process. This can be effected by in-atomizer trapping of the analytes via successive sequestration of the hydride/vapor in a pre-coated graphite tube, allowing appropriate preconcentration and, therefore, substantial improvement of the sensitivity. The determination of lead in media of high calcium carbonate content via the HG approach is quite problematic because of the matrix effect encountered. However, an FI-HG procedure was successfully developed for this purpose [37]. Lead hydride was generated from an acid solution – with potassium hexacyanoferrate(III) added as oxidizing agent (to convert Pb(II) to Pb(IV)) – by reaction with alkaline tetrahydroborate solution. The hydride was successively sequestrated on the interior surface of an iridium pretreated transversely heated graphite atomizer at 400C for 40 s, followed by 6
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atomization and quantification. The results obtained were in full agreement with validated methods. It has been demonstrated that the VG approach is equally effective for the determination of some metals that cannot form hydrides as such, provided that a proper metal vapour can be obtained. Thus, a volatile species of gold in acidified medium was generated at room temperature by using an FI-VG system [38]. The gold vapour, the form of which has not yet been specified, was generated in acidified aqueous medium with NaBH4 as reducing reagent in the presence of microamounts of sodium diethyldithiocarbamate (DDTC). The vapour was afterwards sequestrated in the graphite furnace by in-atomizer trapping and followed by atomization. However, an LOD of 0.8 lg/l for the procedure is not satisfactory, when compared with conventional VG approaches coupled with ETAAS. In this context, it should be mentioned that although the in-atomizer sequestration procedure might appear attractive, it is, from an operational point of view, much preferable to effect all the manipulations on-line in a single cycle, yet separate the preconcentration part of the procedure from that of the HG part. Being readily feasible in an FI system, it renders an extra degree of freedom because it allows not only selection of the preconcentration chemistry completely independently of the HG scheme used, but also obtains much higher enrichment factors in considerably shorter sampling times. 2.5. Miscellaneous on-line sample-pretreatment approaches In addition to the aforementioned on-line sample-pretreatment protocols, some other on-line procedures have also been reported. Dialysis is one of the most effective approaches for the separation/collection and analysis of trace species from complex matrices, such as whole blood samples, although on-line microdialysis schemes coupled to ETAAS detection have rarely been reported. However, an on-line microdialysis procedure with detection by ETAAS was developed for in vivo monitoring of extracellular diffusive manganese in brains of living rats [39], the manifold used being illustrated in Fig. 5. The microdialysates, perfused through implanted microdialysis probes, were collected in a sample loop, the content of which was afterwards introduced directly into the atomizer of the ETAAS instrument for quantification. An automatic separation/preconcentration procedure for the determination of total arsenic at the ng/g level in wheat flour has been proposed [40]. Total arsenic, in the form of arsenate anions, was selectively precipitated with silver(I) as Ag3 AsO4 . The precipitate was collected with a Scientific System column furnished with a removable screen-type stainless-steel filter, which was originally designed as a cleaning device for high-performance liquid chromatography (HPLC). The precipitate was
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Figure 5. Flow manifold for on-line microdialysis coupled with ETAAS for in vivo monitoring of extracellular diffusive manganese in brains of living rats: P1, Microinjection syringe pump; MP, microdialysis probe; T, Micro-Tee; V, On-line injection valve; A, Air; P2, Peristaltic pump; W, waste; Valve position: (a) loading; (b) injection (adapted from [39], by permission of The Royal Society of Chemistry).
subsequently dissolved with ammonia and the resulting solution collected in an autosampler cup; afterwards, it was directed into the graphite tube of the ETAAS instrument, followed by atomization and quantification. When precipitating As(V) from 10 ml of digested sample solution by using a weakly acidified Ag(I) solution, this approach allowed the sensitivity to be increased by a factor of 20. Another on-line precipitation–dissolution procedure was developed for ETAAS determination of ultratrace amounts of beryllium in water samples [41]. Beryllium was precipitated quantitatively with NH4 OH–NH4 Cl and the resulting Be(OH)2 collected in a hydrophilic Tygon KR. The precipitate was dissolved with nitric acid and a sub-sample was collected in the capillary of the sampling arm assembly, followed by introduction into the graphite tube.
3. Perspectives On-line separation and preconcentration procedures can effectively eliminate the interfering effects from matrix components and offer pertinent improvements in sensitivity. Although the coupling of on-line samplepretreatment protocols with ETAAS has been subjected to extensive investigations, there are still some vacant areas that need to be further exploited. For instance, microcolumns packed with PTFE beads, turnings and fibres could not only be employed for improving the retention efficiency of metal chelates, but also potentially be used for precipitate/coprecipitate collection, and further combined with HG. SPE with the online renewable microcolumn approach is merely the
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start of employing the LOV format in sample pretreatment. In fact, practically, most of, or all, the conventional sample-pretreatment schemes should potentially be feasible to implement in the LOV system, such as in-valve solvent-extraction, precipitate/coprecipitate collection, and in-valve precipitation/coprecipitation/SPE followed by HG. So far, on-line sample-pretreatment schemes coupled to ETAAS have predominately been focused on the determination of total metal contents, while speciation studies have rarely been touched. Thus, more efforts should be devoted to the speciation of metals via the combination of on-line sample-pretreatment protocols with ETAAS. Another scheme attracting considerable attention is sample pretreatment by molecular recognition using molecularly imprinted polymers (MIPs). Thus, Say et al. [42] recently demonstrated excellent selectivity for the target, Cu(II), when executing SPE with a MIP-packed microcolumn reactor. However, the interface of this approach with ETAAS has not yet been reported. Thus, this format could not only offer highly selective separation/preconcentration procedures, but also open a promising avenue for metal speciation by coupling on-line molecular recognition with ETAAS detection.
Acknowledgement One of us (J.-H.W.) is indebted to The National Natural Science Foundation of China (NSFC-20375007) and to The China Postdoctoral Science Foundation for financial support. References [1] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 424 (2000) 223. [2] E.V. Alonso, A.G. Torres, J.M.C. Pavon, Talanta 55 (2001) 219. [3] M. Grotti, R. Leardi, C. Gnecco, R. Frache, Spectrochim. Acta, Part B 54 (1999) 845. [4] K. B€ackstr€ om, L.G. Danielsson, Anal. Chim. Acta 232 (1990) 301. [5] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 225. [6] E.H. Hansen, J.-H. Wang, Anal. Chim. Acta 467 (2002) 3. [7] Z. Fang, M. Sperling, B. Welz, J. Anal. At. Spectrom. 5 (1990) 639. [8] G. Tao, Z. Fang, Spectrochim. Acta, Part B 50 (1995) 1747. [9] S.C. Nielsen, E.H. Hansen, Anal. Chim. Acta 422 (2000) 47. [10] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, J. Anal. At. Spectrom. 18 (2003) 1400. [11] J.-H. Wang, E.H. Hansen, Anal. Lett. 33 (2000) 2747. [12] K. B€ackstr€ om, L.G. Danielsson, Anal. Chim. Acta 232 (1990) 301. [13] A.B. Volynsky, B.Y. Spivakov, Y.A. Zolotov, Talanta 31 (1984) 449. [14] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 456 (2002) 283. [15] E.V. Alonso, A.G. de Torres, J.M.C. Pavon, Talanta 55 (2001) 219.
http://www.elsevier.com/locate/trac
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Trends [16] J.L. Burguera, M. Burguera, Spectrochim. Acta, Part B 56 (2001) 1801. [17] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 836. [18] C.B. Ojeda, F.S. Rojas, J.M.C. Pavon, A.G. de Torres, Anal. Chim. Acta 494 (2003) 97. Kelk [19] A. L asztity, A. o-Levai, K. Zih-Perenyi, I. Varga, Talanta 59 (2003) 393. [20] M.T.S. Cordero, E.I.V. Alonso, A.G. de Torres, J.M.C. Pavon, J. Anal. At. Spectrom. 19 (2004) 398. [21] Y.-H. Sung, S.-D. Huang, Anal. Chim. Acta 495 (2003) 165. [22] P. Bermejo-Barrera, R.M. Anllo-Sendia, M.J. Cantelar-Barbazan, A. Bermejo-Barrera, Anal. Bioanal. Chem. 372 (2002) 837. [23] J. Nan, Y. Jiang, X.-P. Yan, J. Anal. At. Spectrom. 18 (2003) 946. [24] J.-H. Wang, E.H. Hansen, Trends Anal. Chem. 22 (2003) 225. [25] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 435 (2001) 331. [26] J.-H. Wang, E.H. Hansen, Atom. Spectrosc. 22 (2001) 312. [27] J.-H. Wang, E.H. Hansen, Anal. Chim. Acta 424 (2000) 223. [28] P. Ampan, J. Ruzicka, R. Atallah, G.D. Christian, J. Jakmunee, K. Grudpan, Anal. Chim. Acta 499 (2003) 167. [29] Z. Fang, S. Xu, L. Dong, W. Li, Talanta 41 (1994) 2165. [30] J.L. Burguera, M. Burguera, C. Rondon, Talanta 58 (2002) 1167. [31] J.L. Burguera, M. Burguera, P. Carrero, C. Rondon, Talanta 58 (2002) 1157. [32] S. Tsakovski1, K. Benkhedda, E. Ivanova, F.C. Adams, Anal. Chim. Acta 453 (2002) 143. [33] E. Ivanova, K. Benkhedda, F. Adams, J. Anal. At. Spectrom. 13 (1998) 527. [34] J.-H. Wang, E.H. Hansen, J. Anal. At. Spectrom. 17 (2002) 248. [35] W. Som-aum, S. Liawruangrath, E.H. Hansen, Anal. Chim. Acta 463 (2002) 99.
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Trends in Analytical Chemistry, Vol. 24, No. 1, 2005 [36] A.N. Anthemidis, G.A. Zachariadis, J.A. Stratis, J. Anal. At. Spectrom. 17 (2002) 1330. [37] J.F. Tyson, R.I. Ellis, G. Carnrick, F. Fernandez, Talanta 52 (2000) 403. [38] H. Ma, X. Fan, H. Zhou, S. Xu, Spectrochim. Acta, Part B 58 (2003) 33. [39] W.-C. Tseng, Y.-C. Sun, M.-H. Yang, T.-P. Chen, T.-H. Lin, Y.-L. Huang, J. Anal. At. Spectrom. 18 (2003) 38. [40] M.M. Gonzalez, M. Gallego, M. Valcarcel, Talanta 55 (2001) 135. [41] J.L. Burguera, M. Burguera, C. Rondon, P. Carrero, M.R. Brunetto, Y.P. de Pena, Talanta 52 (2000) 27. [42] R. Say, E. Birlik, A. Ers€ oz, F. Y ylmaz, T. Gedikbey, A. Denizli, Anal. Chim. Acta 480 (2003) 251. Elo Harald Hansen, who is one of the two inventors of FI, is Professor of Analytical Chemistry at the Department of Chemistry, Technical University of Denmark. His research interest is focused on instrumental and automated analysis with special emphasis on the development and application of procedures based on FI/SI. In recent years, he has placed particular emphasis on developing FI/SI/LOV-schemes for the determination of trace levels of metals in complex matrices by hyphenation with atomic spectrometric techniques and ICP-MS. Jianhua Wang is Professor and Director of the Research Center for Analytical Sciences, Northeastern University, China. He received his B.Sc. and M.Sc. degrees at Nankai University and Jilin University, respectively. He was awarded a Ph.D. degree at the Technical University of Denmark. After nine years’ employment at Yantai Normal University, he spent one year at the University of Delaware conducting research on analytical applications of tandem mass spectrometry. He is currently focusing his research interests on SI/LOV and related techniques.