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Direct Solid-Phase Optical Measurements in
Flow Systems: A Review ANALYTICAL LETTERS, v.44, n.1/Mar, p.528-559, 2011http://producao.usp.br/handle/BDPI/16997
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Direct Solid-Phase Optical Measurementsin Flow Systems: A ReviewFábio R. P. Rocha a , Ivo M. Raimundo Jr. b & Leonardo S. G. Teixeirac
a Centro de Energia Nuclear na Agricultura, Universidade de SãoPaulo, São Paulo, Brazilb Instituto de Química, Universidade Estadual de Campinas, SãoPaulo, Brazilc Instituto de Química, Universidade Federal da Bahia, CampusUniversitário de Ondina, Salvador, Brazil
Version of record first published: 18 Feb 2011.
To cite this article: Fábio R. P. Rocha, Ivo M. Raimundo Jr. & Leonardo S. G. Teixeira (2011): DirectSolid-Phase Optical Measurements in Flow Systems: A Review, Analytical Letters, 44:1-3, 528-559
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Flow and Sequential Injection—General Approaches
DIRECT SOLID-PHASE OPTICAL MEASUREMENTSIN FLOW SYSTEMS: A REVIEW
Fabio R. P. Rocha,1 Ivo M. Raimundo, Jr.,2 andLeonardo S. G. Teixeira31Centro de Energia Nuclear na Agricultura, Universidade de Sao Paulo,Sao Paulo, Brazil2Instituto de Quımica, Universidade Estadual de Campinas, Sao Paulo, Brazil3Instituto de Quımica, Universidade Federal da Bahia, Campus Universitariode Ondina, Salvador, Brazil
Measurements based on absorption, reflectance, or luminescence of molecular species or
complex ions can be carried out directly on a solid support simultaneously to the retention
of the analyte. The use of this strategy in flow-based systems is advantageous in view of the
reproducible handling of solutions in retention and elution steps of the analyte. This
approach can be exploited to increase sensitivity, minimize reagent consumption as well
as waste generation, improve selectivity or for simultaneous determination based on selec-
tive retention or differences in sorption rates of the analytes. This review focuses on the
main characteristics of direct solid-phase measurements in flow systems, including the dis-
cussion of advantages and limitations and practical guidelines to the successful implemen-
tation of this approach. Selected applications in diverse fields, such as pharmaceutical,
food, and environmental analysis are discussed.
Keywords: Flow analysis; Flow-through optosensors; Infrared spectroscopy; Luminescence; Reflectance;
Review; Solid-phase spectrophotometry
INTRODUCTION
Flow analysis in its different modalities (e.g., FIA, SIA, and MCFA) is gener-ally used for processing samples in solution, aiming mechanization, or automation ofanalytical procedures. In this sense, the main aim is to replace operations usuallycarried out by the analyst, improving precision and reducing the analysis time.Sensitivity in flow-based methodologies is usually lower than those achieved in batchprocedures, due to the dispersion process and the incomplete conversion of theanalyte to the species to be measured, due to the reduced sample residence time.
Received 13 March 2010; accepted 5 May 2010.
This paper was submitted as part of a Special Issue on Flow Injection Analysis.
Address correspondence to Leonardo S. G. Teixeira, Instituto de Quımica, Universidade Federal
da Bahia, Campus Universitario de Ondina, Salvador, Bahia, 40170-290, Brazil. E-mail: [email protected]
Analytical Letters, 44: 528–559, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0003-2719 print=1532-236X online
DOI: 10.1080/00032719.2010.500790
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However, heterogeneous systems can be also adapted to flow analysis, sometimescontributing to the improvement of the analytical performance. A typical exampleis the use of solid-phase reactors, which are exploited for analyte concentration, deri-vatization (e.g., using of immobilized reagents or enzymes), as well as on-line gener-ation of reagents (Luque de Castro 1992; Calatayud 1996).
Direct measurements on solid-phase are another interesting alternative involv-ing application of heterogeneous systems, in which the analytical signal is recordedsimultaneously to the retention of the analyte on a suitable solid-support (Teixeiraet al. 2001; Rocha and Teixeira 2004). Measurements can be carried out by electro-chemical or, more typically, by spectrometric techniques. The approach called solidphase spectrophotometry (SPS) or optosensing, which exploits measurements byUV-vis spectrophotometry (transmission or reflectance modes), is clearly the mostfrequently used detection technique for direct measurements on solid surfaces.However, luminescence and infrared spectroscopy have also been exploited (Rocha,Teixeira, and Nobrega 2009). The main advantages of this approach are: (1) increasein sensitivity and achievement of lower detection limits due to the possibility ofaccumulation of a large amount of the analyte in a reduced amount of the solid-support; (2) improvement of selectivity by separation of the analyte from the samplematrix or by exploiting differences in the rates of reaction or retention of the analyteand concomitant species on the solid-support; (3) simultaneous determinations bykinetic discrimination; and (4) minimization of reagent consumption and waste gener-ation by the reversible retention of the analyte and reuse of the solid-phase reagent.
In spite of several applications of direct measurements on solid-phase carriedout in batch, the use of flow systems present some advantages. The differences inpacking of the solid-phase in the cell between the measurements, commonly observedin batch procedures, are avoided. In addition, sample processing in flow systems areless time-consuming and precision is improved. On the other hand, enrichmentfactors are usually lower than in batch mode due to the lower sample volumes.A schematic representation of the process is presented in Figure 1.
Some requirements need be attained for direct solid-phase measurements inflow systems:
1. Thermodynamic of the retention process: The coefficient of distribution of theretained species needs to be considered, i.e., analyte retention on the solid supportneeds to be favorable. With this aim, non-polar materials are used for the reten-tion of hydrophobic species, and so on. When reaction occurs at the solid surface(e.g., complex formation with an immobilized ligand), the equilibrium constantand the reaction conditions (e.g., pH and presence of masking agents) should alsobe taken into account. However, in flow based systems the residence time isusually lower than those required to achieve the steady state, and the thermodyn-amic values are only indicative;
2. Selectivity: the solid-support and reactional conditions should allow separation ofthe analyte from interfering species. This is often achieved by chemical derivatiza-tion with a selective reagent immobilized at the solid support or previously to theretention of the species at the solid surface. Control of the composition of themedium (e.g., pH) and use of selective eluents are other commonly employedapproaches;
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3. Kinetic aspects: analyte retention needs to be compatible with the short residencetime characteristic of the flow methodologies. Fast retention of the analytein comparison to that of interfering species can also be exploited to improveselectivity.
4. Reversibility: the solid-phase needs to be efficiently regenerated by the eluent sol-ution after sample measurement by the eluent solution, making feasible the use ofthe same solid support for several measurements. One alternative is replacementof the whole solid support in each measurement cycle, which can be alsoefficiently carried out in flow systems;
5. Stability of the solid support: the solid-phase should not change significantly whensubmitted to the different media required in the retention and elution steps. This ismore critical when a reagent is immobilized on the solid support and leaching canreduce the lifetime of the solid-phase sensor. This is often minimized by properimmobilization of the reagent at the solid support and selection of suitable carrierand eluent solutions. The stability of the solid-phase under irradiation should alsobe evaluated, especially for measurements based on fluorescence, when the solidsurface is submitted to high power radiation favoring photo-degradation.
6. Compatibility between the solid support and the measurement system: materialswhich cause excessive attenuation of the radiation beam (by absorption orscattering) are not suitable for spectrophotometric measurements based on trans-mission, but can be used for measurements by reflectance or fluorescence;
7. Backpressure: the measurement cell, particle size, and packing need to be care-fully selected to avoid fluid leakage. This aspect generally limits the total flow-ratethat can be employed, also affecting system design. As an alternative, reagentscan be immobilized in porous membranes, minimizing the drawbacks related tothe increase of the hydrodynamic impedance.
Figure 1. Schematic representation of a typical FI-SPS procedure and the corresponding signal: a)
establishment of baseline and insertion of the sample aliquot; b) retention of the analyte at the solid
support yielding the analytical signal; and c) elution process and regeneration of the solid support. In
the scheme, a transient signal is obtained because the eluent reaches the solid support immediately after
the whole sample zone cross the flow cell.
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The flow manifold is designed to implement distinct conditions for analyteretention and elution. A common strategy is preparation of the sample in a mediumsuitable for analyte retention and injection on an eluent stream (Lazaro, Luque deCastro, and Valcarcel 1989). With this approach, the sample volume should be largeenough to avoid mixing with the eluent in the central part of the sample zone. Moreingenious alternatives are the sequential injection of sample and eluent (Teixeira et al.1999) or the intermittent addition of the eluent solution (Molina-Dıaz et al. 2002).Both strategies are represented in the scheme in Fig. 1, in which a transient signalis obtained because the eluent reaches the solid support immediately after the wholesample zone cross the flow cell.
Applications have exploited the direct retention of the analyte (Ruiz-Medina,Fernandez de Cordova, and Molina-Dıaz 1999; Ortega-Barrales et al. 2002), reten-tion of a reaction product formed in solution (Frenzel and Krekler 1995; Cassellaet al. 2000), or direct formation of the product on solid-phase (Ayora-Canada,Pascual-Reguera, and Molina-Dıaz 1998; Teixeira et al. 1999).
This review focuses on the main characteristics of direct solid-phase measure-ments in flow systems, including the discussion of advantages and limitations andpractical guidelines to the successful implementation of this approach. Selectedapplications in diverse fields, such as pharmaceutical, food, and environmentalanalysis are discussed.
APPLICATIONS OF DIRECT SPECTROMETIC MEASUREMENTS ON SOLIDPHASE IN FLOW SYSTEMS
UV-Vis Spectrophotometry
Most of the applications of direct optical measurements on solid-phase are basedon transmission spectrophotometry. In solid-phase spectrophotometry (SPS), the absor-bance due to the retention of the analyte is measured directly on the solid surface, whichcan be modified by immobilization of a suitable chromogenic reagent. In the pioneerwork, presented by Yoshimura et al. in 1976, an ion-exchange resin modified with1,5-diphenylcarbazide, 1,10-phenantroline, ammonium thiocyanate or Zincon wasemployed for the determination of chromium, iron, cobalt, or copper, respectively.After analyte retention, the solid support was transferred to a cuvette, resulting in a10-fold increase in sensitivity in comparison to the analogous procedures in solution.Batch procedures are usually based on dual-wavelength spectrophotometry, being mea-surements simultaneously carried out at the absorption maximum and at a wavelengthtaken as reference, in order to compensate differences in packing of the solid-phasebetween measurements. On the other hand, coupling of solid phase spectrophotometryto flow systems (FI-SPS) allows the use of simple mono-channel spectrophotometer.This coupling was first evaluated for retention of copper in a cation-exchange resinplaced at a flow cell designed for measurements in solution (Yoshimura 1987). Mostof the applications of FI-SPS are aimed at determination of metal ions (Table 1) or spe-cies of pharmaceutical interest (Table 2). Applications on this subject were previouslyrevised (Molina-Dıaz et al. 2002;Matsuoka and Yoshimura 2010). The potential of flowsystems coupled to solid-phase spectroscopy to the development of Greener Analyticalprocedures was also emphasized (Garcıa-Reyes, Gilbert-Lopez, andMolina-Dıaz 2009).
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Table
1.SelectedapplicationsofFI-SPSfordeterminationofinorgan
icspecies
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume
(mL)
LD,
CV
(%)
Reference
Copper
—Intrinsicabsorption
Cation-exchange
resin
2.0molL
�1HNO
34410
NE,4.0
Yoshim
ura
1987
Chromium
Naturalwaters
1,5-diphenylcarbazide
Ion-exchangeresin
3.0molL
�1HNO
34400
55ngL�1;
Yoshim
ura
1988
Molybdenum
Naturalwaters
androcks
Tiron
Sephadex
QAE
A-25gel
1.0molL
�1
NaNO
3
5000
3.0mg
L�1,NE
Yoshim
ura
etal.
1989
Iron
Naturalwaters
andwine
Thiocyanate
Anion-exchange
resin
NaF,EDTA,and
NaCH
3COOH
inalkaline
medium
2000
10.0mg
L�1,NE
Lazaro
etal.1989
Tin
Fruitjuices
PyrocatecholViolet
Sephadex
QAE
A-25gel
0.6molL
�1HCl
780
0.3mg
L�1,2.5
Capitan-V
allvey
etal.1994
Zinc
Humanhair,pharm
aceutical,and
cosm
etic
preparations,watersamples
PAN
Cation-exchange
resin
EDTAþbuffer
pH
4.0
2000
5.0mg
L�1,2.3
Ayora-C
anada
etal.1998
Vanadium(V
)Mussel,oyster,andtoadstooltissues,
petroleum
crudes,andwater
5-Bromosalicylhydroxamic
acid
Sephadex
QAE-A
-
25ion-exchanger
2.0molL
�1NaF,
10%
acetone
5000
14mg
L�1;1.1
Ayora-C
anada
etal.1998
Nickel
Alloys,petroleum,mineraloil,and
wastew
ater
1-(2-pyridylazo)-2-naphthol
Dowex
50W
cation-exchanger
1.0molL
�1H
2SO
4800
10mg
L�1;2.3
Ayora-C
anada,
Pascual
Reguera,and
Molina-D
ıaz
1999
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Zinc
Pharm
aceuticalpreparations
1-(2-thiazolylazo)-2-naphthol
C18
0.5molL
�1HCl
625
10mg
L�1,3.3
Teixeira
etal.1999
Nickel
and
zinc
Metalalloys
1-(2-thiazolylazo)-2-naphthol
C18
0.2molL
�1HCl
625
NE,<
2.0
Teixeira
etal.2000
Sulfide
Naturalwaters
N,N
-dim
ethyl-p-
phenylenediamine,
iron(III)
C18
60%
CH
3OH,
1.0molL
�1HCl
500
1.7mg
L�1;4.0
Cassella
etal.2000
Nickel,iron,
andzinc
—1-(2-thiazolylazo)-2-naphthol
C18
0.5molL
�1HCl
625
NE
Teixeira
etal.2002
Iron
Wine,
waters,pharm
aceutical
preparations
Ferrozine
Sephadex
QAE
A-25
Noeluent,bead
injection
approach
1000
3.0mg
L�1;4.0
Ruedas-Rama
etal.2003
Nıkel
Oil,andchocolate
Br-PADAP
Nafion1
1.0molL
�1HNO
33000
70mg
L�1;1.1
Aminiet
al.
2004
Selenium
Naturalwaters
p-amino-
p0 -methoxydiphenylamine
orvariamineblue
Nafion1
—2000
4.0mgL�1;1.8
Cooand
Martinez
2004
Copper
Urine
4-(2-pyridylazo)resorcinol
Silicate
gel
0.5molL
�1
picolinic
acid
830
3.0mg
L�1;2.0
Jeronim
oet
al.
2004
Iron
Naturalwaters
1-(2-thiazolylazo)-2-naphthol
C18
0.1molL
�1HCl
625
15mg
L�1,4.0
Teixeira
and
Rocha,2007
NE:notevaluated;C18:C18-bonded
silica;Br-PADAP:2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol.
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Table
2.SelectedapplicationsofFI-SPSfordeterminationoforganic
species
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume
(mL)
LD;
CV
(%)
Reference
Totalphenols
Naturalwaters
4-aminoantipyrine
C18
CH
3OH
5000
0.4mgL�1;3.0
FrenzelandKrekler,
1995
Caffeine,
dim
enhyd
rinate
andAcetaminophen
Pharm
aceutical
preparations
Intrinsic
absorption
C18
CH
3OH
600
NE
Ayora-C
anada,
PascualReguera,
Molina-D
ıaz,
and
Capitan-V
allvey
1999
Caffeine,
acetylsalicylicacid
andacetaminophen
Pharm
aceutical
preparations
Intrinsic
absorption
C18
CH
3OH
250
NE
Ruiz-M
edinaet
al.
1999
Ascorbic
acidor
acetaminophen
Pharm
aceutical
preparations
Intrinsic
absorption
Sephadex
QAEA-25
0.04molL
�1
acetate
buffer,
pH
5.6
or
0.05molL�1
NaCl,pH
12.5
1000
0.02mgL�1;1.3
or1.5
Ruiz-M
edinaet
al.
2000
Thiamine,
ascorbic
acid,
acetylsalicylicacid
Pharm
aceutical
preparations
Intrinsic
absorption
Sephadex
SP-C
25
0.5molL
�1
acetate
buffer,
pH
4.8
600
NE;2.56,1.85,
1.25
Ortega-Barralesetal.
2002
Thiamine,
ascorbic
acid
Pharm
aceutical
preparations
Intrinsic
absorption
Sephadex
SPC-25,
Sephadex
QAEA-25
0.15molL
�1
acetate
buffer=
0.18molL
�1
citric
acid=
K2HPO
4buffer
1000
0.14and
0.36mgL�1;
0.9
and1.2
Ruiz-M
edinaet
al.
2002
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Acetaminophen,
acetylsalicylicacid,
caffeine
Pharm
aceutical
preparations
Intrinsic
absorption
C18
0.1molL
�1
acetate
buffer,
pH
4.8
and
20%
CH
3OH
200
0.3–0.8mgL�1;
1.2–3.4
Domınguez-V
idal
etal.2002a
Sulfamethoxazole
and
trim
ethoprim
Pharm
aceutical
preparations
Intrinsic
absorption
Sephadex
SPC-25
1�10�4molL
�1
HClor
0.20molL
�1
acetate
buffer
pH
5.0
500
9.5
and
0.6mgL�1;0.4
and1.4
Fernandez
de
Cordovaet
al.
2003
Pyridoxine
Pharm
aceutical
preparations
N,N
-diethyl-p-
phenylenedia-
mine,
hexacyanoferra-
te(III)
C18
60%
methanol
and4�10�3
molL
�1HCl
860
60mgL�1;4.0
Portela,Costa,and
Teixeira
2004
Salicylamideandcaffeine
Pharm
aceutical
preparations
Intrinsic
absorption
C18
25%
CH
3OH
1500
330and
150mgL�1;2.1
and1.8
Llorent-Martınez,
Domınguez-V
idal
etal.2005
NE:notevaluated;C18:C18-bonded
silica.
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The solid support and the design of the measurement cell are critical aspects forSPS based on transmission. The solid material should ideally be transparent at themeasurement wavelength, and the thickness of the solid phase should be carefullyadjusted in order to minimize the attenuation of the radiation beam, which can occurby absorption, scattering, and reflection. On the other hand, limited amounts of thesolid support can hinder the response range, by limiting the number of sites for ana-lyte retention. Measurements by FI-SPS have been carried out with conventionalflow-cells partially filled with the solid support (Yoshimura 1987; Yoshimura,Matsuoka, and Waki 1989) or by specially designed cells. These are projected toincrease the amount of solid at the observation area and to reduce the backpressure(Reis et al. 2000).
The increase in sensitivity by SPS depends mainly on the optical path and thesample volume. Measurements in flow systems usually are carried out with samplevolumes lower than 1mL and optical paths of ca. 1mm. As a consequence, sensi-tivity is usually lower than that achieved in batch, in which sample volumes, some-times as high as 1000mL, are employed. However, mechanization of the process,improvement of precision, and an increase of the sample throughput make flowbased measurements attractive.
Direct measurements on solid phase are an effective way to increase sensitivity,as demonstrated by comparing three procedures for preconcentration in determi-nation of total phenols by the reaction with 4-aminoantipyrine (Frenzel and Krekler1995). The direct measurement of the product by FI-SPS yielded better sensitivity,with a detection limit of 0.4 mgL�1, against 8 mgL�1 for liquid–liquid extraction ofthe reaction product in chloroform and 11 mgL�1 for pre–concentration of the pro-duct on a C18-bonded silica mini-column followed by elution with methanol. The lowdetection limit by SPS in comparison to the last procedure was achieved becausesample dilution at the elution step is avoided, illustrating one of the advantages ofFI-SPS.
Several procedures have been proposed for determination of metal ions in lowconcentrations (Table 1). This can be carried out after complex formation in solutionor directly on the solid-phase. The former approach can be exemplified by the deter-mination of Sn(IV) in fruit juices after complex formation with Pyrocatechol Violet(Capitan-Vallvey, Valencia, and Miron 1994). The reaction product was retained onSephadex QAE A-25 gel and reagent concentration was selected in order to minimizethe blank values.
The complex formation with an immobilized reagent is more interesting in viewof the reuse of the ligand for sequential determinations. In this sense, a solid-phaseformed by 1-(2-pyridylazo)-2-naphthol (PAN) immobilized on a cation-exchangeresin was employed for zinc determination in water, human hair, pharmaceutical,and cosmetic preparations (Ayora-Canada et al. 1998). Variation of the sample vol-ume (100 or 2000 mL) was exploited to extend the working ranges to 20–500mgL�1
or 0.2–4.0mgL�1, with sampling rates of 38 or 15 determinations per hour, respect-ively. An EDTA solution prepared in buffered medium at pH 4.0 was used to elutethe metal ions without removing PAN from the solid support. A similar approachwas adopted for determination of nickel in alloys, petroleum crude, mineral oil,and industrial wastewater, using H2SO4 as eluent (Ayora-Canada, Pascual Reguera,and Molina-Dıaz 1999). Other examples of applications of FI-SPS for metal ions are
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the determinations of iron in natural waters and wines by formation of complex withthiocyanate (Lazaro et al. 1989); molybdenum in natural waters or rocks throughreaction with Tiron (Yoshimura et al. 1989); V(V) in mussel, oyster and toadstooltissues, petroleum crudes and water samples (Ayora-Canada, Pascual-Reguera,and Molina-Dıaz 2000); and chromium(VI) after reaction with 1,5-diphenylcarba-zide (Yoshimura 1988). In the last application, sensitivity increased 160-fold in com-parison to the measurements in solution.
Reversible analyte retention and reuse of the immobilized reagent wereachieved in applications exploiting 1-(2-tiazolylazo)-2-naphthol (TAN) adsorbedon C18 bonded silica (Baliza, Ferreira, and Teixeira 2009). A specially designedflow-cell containing the solid-phase was coupled to flow injection systems for deter-mination of zinc in pharmaceutical preparations (Teixeira et al. 1999) and iron innatural waters (Teixeira and Rocha 2007). Elution of the metal ions was implemen-ted with a low volume of a diluted acid solution without removing the TAN reagent,which was used for at least 100 determinations. This corresponds to reagent con-sumption lower than 1 mg per determination, which is 1000-fold lower than thoserequired in batch determination of zinc by conventional spectrophotometry (Ferreiraet al. 1995). The procedures are, thus, inherently more environmentally friendly, inaddition to the increase in sensitivity (e.g., 10-fold higher sensitivity for iron deter-mination in comparison to measurements in solution).
The C18-TAN system was also employed for the sequential determination ofnickel and zinc in copper-based alloys, exploiting differences in the sorption ratesof the metal ions (Teixeira et al. 2000). Zinc retention is relatively fast and inde-pendent of the flow rate in the range 0.70 to 2.2mL min�1, whereas nickel complexa-tion at the solid-phase is hindered at higher flow rates. A flow injection system withintermittent flows was employed to carry out measurements at different flow rates(0.85 and 1.9mL min�1) for sequential sample injections.
Chemometric data treatment by Partial Least-Squares (PLS-2) and the differ-ences in the retention rates at the solid support were explored for the simultaneousdetermination of iron, nickel, and zinc, whose TAN complexes yield strongly over-lapped spectra (Teixeira et al. 2002). The PLS was also applied for data treatmentaimed at the simultaneous determination of caffeine, dimenhydrinate, and acetami-nophen (Ayora-Canada, Pascual Reguera, Molina-Dıaz, and Capitan-Vallvey 1999)or caffeine, acetaminophen, and acetylsalicylic acid (Ruiz-Medina et al. 1999) by UVmeasurements directly at the solid phase.
Sulfide was determined by FI-SPS by formation of Methylene Blue in reactionwith N,N-dimethyl-p-phenylenediamine and iron(III) (Cassella et al. 2000). The reac-tion product was adsorbed on C18-bonded silica, and the elution was carried out witha mixture of methanol and diluted hydrochloric acid. A linear response was achievedfrom 5 to 50 mgL�1, with a sampling rate of 12 determinations per hour. Thesame solid support was employed for determination of pyridoxine (vitamin B6) afterderivatization with N,N-diethyl-p-phenylenediamine and potassium hexacyanoferra-te(III) (Portela et al. 2004). The reaction product showed maximum absorption at633 nm and the procedure presented linear response up to 4.0 or 10.0mgL�1 for860 or 235mL sample volumes, respectively.
An alternative to the solid materials used in FI-SPS is reagent immobilizationin porous membranes. This strategy often simplifies the coupling to the flow system
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and overcomes the drawbacks caused by backpressure. Reagent immobilization onNafion1, for example, has found several applications, such as determination ofnickel based on immobilization of 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol(Amini et al. 2004) or of selenium with p-amino-p0-methoxydiphenylamine or varia-mine blue (Coo and Martinez 2004). In the former, the chromogenic reagent wasdirectly dissolved in a 5% Nafion1 solution which originated a transparent andmechanically stable sensing membrane. The immobilization process minimizedreagent leaching and increased the sensor lifetime, which was applied to nickel deter-mination in samples of vegetable oils and chocolate. For selenium determination, thesensing phase was prepared by immersing the membrane in a concentrated reagentsolution. The optode was coupled to a flow injection system for selenium determi-nation in natural waters. Another approach was the development of an optical sen-sor based on the physical entrapment of 4-(2-pyridylazo)resorcinol (PAR) in sol–gelthin films (Jeronimo et al. 2004). The sensor was applied for copper determination inurine samples in a multicommuted flow system, yielding results in agreement withICP-MS.
Spectrophotometric determination without chemical derivatization (measure-ment of the intrinsic absorption by the analytes) can be implemented in FI-SPSdue to the capacity to separate the analyte from the sample matrix. This also contri-butes to the development of greener procedures by avoiding the use of toxic reagents.As an example, ascorbic acid and paracetamol were both determined in pharmaceu-tical formulations by absorption at 264 nm (Ruiz-Medina et al. 2000). An anionexchanger gel (Sephadex1 QAE A-25) placed in an 1-mm optical-path quartz flowcell was used as solid support for at least 200 determinations. The strategy for deter-mination of both species was the selection of suitable solutions employed as carrier=elution streams in a single line manifold. When acetate buffer at pH 5.6 was used as acarrier, only ascorbic acid was temporally retained on the solid support, which isquantified without interference from paracetamol or other components of the samplematrix. The carrier was then replaced to NaCl at pH 12.5 for the selective retentionof paracetamol in its ionic form (phenolic group ionized). Linear responses wereobserved from 0.3 to 20mgL�1 ascorbic acid and from 0.4 to 25mgL�1 paraceta-mol, with a mean sampling rate of 20 determinations per hour.
Intrinsic absorption of radiation was also exploited for the determination ofsalicylamide and caffeine in pharmaceutical preparations (Llorent-Martınez et al.2005). A multicommuted flow system was employed to reduce up to 85% the con-sumption the carrier and eluent solutions. Other applications of flow-through opto-sensors coupled to multicommutation for environmental analysis were previouslyrevised (Llorent-Martınez, Ortega-Barrales, and Molina-Diaz 2006).
An ingenious alternative was proposed for the simultaneous determination ofthiamine and ascorbic or acetylsalicylic acids in pharmaceutical preparations basedon direct measurements at UV (Ortega-Barrales et al. 2002). A mini-column and aflow-through cell, both filled with a cation-exchange gel (Sephadex1 SP-C25), wereplaced in sequence in the flow system. Vitamin B1 was retained at the mini-column,while the other analyte was carried out to the flow cell being measured in the inter-stitial solution. Thiamine was then eluted by acetate buffer yielding a transient signalfor analyte quantification. Increase of sensitivity by direct measurements onsolid-phase and reduction of the apparent absorptivity by measurements in the
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interstitial solution enabled the determination of the analytes in different concen-tration ranges. Linear responses were then observed in the ranges 3–50, 25–400,and 300–3000mgL�1 for thiamine, ascorbic acid, and acetylsalicylic acid, respect-ively. In another work, a mini-column and a flow-through cell both filled withC18-bonded silica were placed in series in a flow system for the determination ofparacetamol, acetylsalicylic acid, and caffeine. The analytes were determined in a sin-gle sample injection and discrimination was due to the different interactions with thesolid upport (Domınguez-Vidal et al. 2002a). A similar strategy was later used fordetermination of sulfamethoxazole and trimethoprim by using a mini-column anda flow-through cell filled with Sephadex SP C-25 ion-exchanger gel and different elu-ent solutions (Fernandez de Cordova et al. 2003).
In another approach for biparametric determination, flow cells packed withcationic (Sephadex SP C-25) and anionic (Sephadex QAE A-25) exchange gels wereused in a flow system for conjunct determination of ascorbic acid and thiamine(Ruiz-Medina 2002). The cells were placed in a double beam spectrophotometerand suitable solutions were used as carrier and eluent for sequential determinationof both analytes. For a sample volume of 1000 mL, linear responses were obtainedin the ranges 0.5–15 and 3–50mgL�1 for thiamine and ascorbic acid, respectively.
In FI-SPS, the eluent should efficiently regenerate the solid-phase in order toassure the same experimental conditions for analyte retention in subsequent samples.However, this is not ease to achieve for species strongly retained on the solid-phase,requiring the use of a solid support with high retention capability or renovation ofthe solid-phase in each measurement cycle. An ingenious alternative is the bead injec-tion approach, which explores the reproducible handling of suspensions in flow sys-tems for the regeneration of the solid support. The bead suspension can interact withthe sample during the transport to the flow-through cell or after retention of the solid(Ruzicka and Scampavia 1999). After measurement, the whole solid-phase is dis-carded and the process is repeated for a new sample aliquot. Bead injection was suc-cessfully implemented with a commercially available flow cell by using ferrozineimmobilized in an anion exchanger (Ruedas-Rama, Ruiz-Medina, and Molina-Dıaz2003). The procedure was applied to the determination of iron in wine, naturalwaters and pharmaceutical preparations as well as of ascorbic acid in fruit juices,pharmaceuticals and conservatives.
Luminescence
Analytical methods based on luminescence are less widely applicable thanabsorption methods because of the relatively limited number of species which pre-sents radiative relaxation. However, these techniques show attractive features suchas better sensitivity and selectivity and wide linear range when compared withabsorption spectrometry (Skoog, Holler, and West 1994). This also holds for directmeasurements in solid-phase, as demonstrated by recent applications.
As absorbance measurements directly on solid supports, the solid phase fluori-metry (SPF) have the merit of combining separation and preconcentration steps within situ surface fluorescence detection (Zhu, Chen, and Zhou 2003). The association offlow analysis with SPF combines the advantages of both approaches: the selectivityand sensitivity inherent to solid phase spectrofluorimetry and the capability for
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mechanized sample processing with high sampling rate usually achieved in flow-based methodologies. In addition, retention on a solid surface can increase the quan-tum yield of the luminescence processes by minimizing the losses of energy byvibrational relaxation. This is especially relevant for measurements of phosphor-escence at room temperatures. Table 3 summarizes the characteristics of some appli-cations of this combined approach.
A procedure based on the combination of multi-commutation and SPF hasbeen developed for the determination of thiabendazole in citrus fruits (Garcia-Reyeset al. 2006). The native fluorescence of the target pesticide, retained on-line on C18-bonded silica packed in the detection flow cell, was exploited for analytical purposes.Under the optimized conditions, the analytical response was linear in the range0.3–10mgkg�1 with a detection limit of 0.09mg kg�1 and coefficient of variationbetter than 2%.
An analytical procedure for determination of dipyridamole consisted of a sin-gle channel flow-injection system using Sephadex1 QAE A-25 resin as a sorbing sub-strate, placed into a fluorimetric flow-through cell. The fluorescence of the analyteretained on the solid phase was continuously monitored followed by elution witha phosphate buffer solution at pH 6.0 allowing resin regeneration. The systemresponds linearly in the range 10–500 mgL�1 with a detection limit of 0.94 mgL�1
and coefficient of variation of 0.82%. The method was applied to the determinationof dipyridamole in pharmaceutical preparations and human plasma (Ruiz-Medina,Fernandez de Cordova, and Molina-Dıaz 2001).
Sometimes, it is necessary the derivatization of the analyte to a fluorescent speciesbefore sorption at the solid-support. An example of this strategy is the fluorimetricdetermination of thiamine after its conversion to the fluorescent thiochrome by oxi-dation with hexacyanoferrate(III) in alkaline medium (Zhu et al. 2003). The thio-chrome formed was concentrated and separated from the sample matrix by sorptionon octadecyl-alklylated poly-[styrene=divinylbenzene] (C18-PS=DP) emitting strongfluorescence on the solid surface. Based on this observation, a sequential injection sys-tem coupled to a flow-through cell was employed for sample processing, includingchemical derivatization, sorption of the reaction product and removal of thesolid-support after measurement. Detection was realized by coupling the flow-throughcell to a spectrofluorimeter by means of optical fibers. Under the optimized conditions,a detection limit of 0.03mgmL�1 was achieved with a sample throughput of 30 h�1 andconsumption of 1mg C18-PS=DP microbeads per run. The developed approach wasapplied for the determination of thiamine contents in pharmaceutical preparations.
Another useful way to convert a non-fluorescent analyte to a fluorescent com-pound is through photochemical derivatization. The photochemically induced fluor-escence (PIF) detection was also adopted for determination of thiamine. In theprocedure, the fluorescent photoproduct generated on-line was retained on C18-bonded silica and the emission was continuously monitored on the solid surface.The elution of the analyte from the solid support was carried out with a 60% (v=v)acetonitrile solution. Concentrations of the vitamin in the range 0.09–50 ngmL�1
were determined with a detection limit of 28 ngL�1 for a 600 mL sample. The methodallowed the determination of the vitamin in pharmaceutical preparations and biologi-cal samples without chemical derivatization (Lopez-Flores, Fernandez de Cordova,and Molina-Diaz 2005).
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Table
3.Selectedapplicationsofsolidphase
spectrofluorimetry
associatedwithflow-injectionsystem
s
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume
(lL)
LD,CV
Reference
Thiabendazole
Citrusfruits
Nativefluorescence
C18
Methanol
666
0.09mgkg�1;2%
Garcia-R
eyes
etal.
2006
Thiamine
Pharm
aceutical
preparations
Hexacyanoferrate(III)in
alkalinesolution
C18-PS=DP
–400
0.03mgmL�1,1%
Zhu,Chen,and
Zhou2003
Thiamine
Pharm
aceutical
preparationsand
biologicalsamples
Photochem
ical
derivatization
C18
Acetonitrile
60%
(v=v)
600
28ngL�1;2.4%
Lopez-Flores,
Fernandez
de
Cordova,and
Molina-D
iaz2005
Dipyridamole
Pharm
aceutical
preparationsand
humanplasm
a
Nativefluorescence
Sephadex
QAE
A-25
KH
2PO
4=NaOH
buffer
solution,pH
6.0
300
0.94mgL�1;0.82%
Ruiz-M
edinaet
al.
2001
Sulfamethoxazole,
sulfanilamideand
sulfathiazole
Pharm
aceuticals,milk
andhumanurine
Nativefluorescence
and
photochem
ical
derivatization
Sephadex
QAE
A-25
NaAc=HAcbuffer
solution,pH
4.0
900
8.1,2.9
and
5.7ngmL�1;2.75,
1.06,and1.78%
Flores,Fernandez
deCordova,and
Molina-D
ıaz2007
Pyridoxine
Pharm
aceutical
preparations
Nativefluorescence
Sephadex
SP-C
25
HCl(10�3molL
�1)=
NaCl(3
�10�2
molL
�1)
2000
0.33ngmL�1;
1,31%
Ruiz-M
edinaet
al.
1999
Benomyland
carbendazim
Waters
Nativefluorescence
C18
Methanol65%
(v=v)
2000
3.0
and
7.5ngmL�1;2%
Garcia-R
eyes,
OrtegaBarrales,
andMolinaDıaz
2003
Thiabendazole
and
benomyl
Waters
andpesticide
form
ulations
Nativefluorescence
C18
Methanol20%
(v=v)
andmethanol65%
(v=v)
3200
0.06and
3.6ngmL�1;5%
Garcia-R
eyes,
Ortega-Barrales,
etal.2004
Fuberidazole,
carbaryland
benomyl
Waters
Nativefluorescence
C18
Methanol55%
(v=v)
andmethanol55%
(v=v)
2100
0.09,6and
9mgL�1;2%
Garcia-R
eyes,
Llorent-Martınez,
etal.2004
Norfloxacin
Biologicalfluids
Terbium
ion
Sephadex-SP
C-25
EDTA
(0.08molL
�1)
900
1.5ngmL�1;
1.82%
Llorent-Martınez,
Domınguez-V
idal
etal.2005
(Continued
)
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Table
3.Continued
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume
(lL)
LD,CV
Reference
a-naphthol,
o-phenylphenol
andthiabendazole
Waters
Nativefluorescence
C18
Methanol20%
(v=v)
500
–Domınguez-V
idal
etal.2007
VitaminsB2,B6,
andC
Pharm
aceuticals
Nativefluorescence
and
permanganate
ina
sulphuricacidmedium
(chem
iluminescence
for
Vitamin
C
determination)
C18(vitamins
B2,B6)and
QAE-A
25
(vitamin
C)
H2SO
41molL
�1and
methanol20%
(v=v)
600(V
itaminsB2
andB6)and400
(vitamin
C)
0.12,0.008,and
9.1mgL�1;5%
Llorent-Martınez,
Ortega-Barrales,
andMolina-D
iaz
2008
Amiloride
Biologicalfluidsand
pharm
aceutical
preparations
Nativefluorescence
Sephadex
SP-C
25
NaAc=HAcbuffer
solution,pH
4.0
600
0.33mgL�1;0.65%
Domınguez-V
idal,
Ortega-Barrales,
etal.2002
Imidacloprid
Peppersand
environmental
waters
Photochem
ical
derivatization
C18
Methanol
640
1.8mgL�1;2.4%
Flores,Diaz,
and
Fernandez
de
Cordova2007
Florfenicol
Anim
altissues
Nativefluorescence
MIP
Methanol:HAc:SDS
(88:10:2)
150
3.4�10�7gmL�1;
3.5%
Geet
al.2010
1-naphthylamine
and
2-naphthylamine
Drinkingwaters
Nativefluorescence
MIP
Acetone
2000
26ngmL�1and
50ngmL�1;3.2
Valero-N
avarro,
Salinas-Castillo,
etal.2009
C18:C18-bonded
silica.
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A flow-through optosensor combined with PIF was also employed forsimultaneous determination of binary mixtures of sulfamethoxazole=sulfanilamideand sulfathiazole=sulfanilamide. The procedure exploited the native fluorescenceof sulfanilamide and that of the photoproduct generated from sulfamethoxazole(or sulfathiazole). The strategy used for simultaneous determination was the on-lineseparation of both sulfonamides in a mini-column filled with Sephadex1 QAE A-25,placed before the flow cell containing the same resin. While one of the sulfonamidespassed through the mini-column and was determined by its native fluorescence, theother was retained and photochemically converted into a fluorescent photoproductafter elution, yielding a transient signal on the sensing support. The selected resinwas able to retain sulfamethoxazole and sulfathiazole in pH above 4.0, whereassulfanilamide only was retained in pH values above 10.0, making possible theirsimultaneous determination. The detection limits for the determination of sulfa-methoxazole, sulfanilamide, and sulfathiazole were 8.1, 2.9, and 5.7 ngmL�1,respectively, and the method was applied to pharmaceuticals, milk, and human urine(Flores, Fernandez de Cordova, and Molina-Dıaz 2007).
Simultaneous determination of thiabendazole and benomyl was possible with asolid phase spectrofluorimetric flow injection method. In the procedure, C18 silica gelbeads were placed in the flow cell and the pesticides were separated using differentretention-desorption kinetics in their interaction with the solid support. After thiaben-dazole was totally eluted from the solid phase by a 20% (v=v) CH3OH solution and itstransient fluorescence signal was obtained, a 65% (v=v) CH3OH solution was used foreluting benomyl. For a sample volume of 3200 mL, the detection limits were 0.06 and3.6 ngmL�1 for thiabendazole and benomyl, respectively, and coefficient of variationwere lower than 5% for both analytes. The proposed method was applied to pesticideformulations and water samples (Garcia-Reyes, Llorent-Martınez et al. 2004).
Other approach for simultaneous determination is the combination of a flow-through optosensor spectrofluorimetric system with partial least-squares (PLS)calibration. In this way, a procedure was developed for the resolution of mixturesof three pesticides: a-naphthol, o-phenylphenol, and thiabendazole. The sensorwas developed in conjunction with a single channel flow-injection manifold withdetection using C18 silica gel as solid support. The different retention rates of theanalytes on the sensing zone allowed the selection of a time matrix for each analyteproviding better results in the PLS calibration. The method was successfully appliedto natural water samples (Domınguez-Vidal et al. 2007; Garcia-Reyes et al. 2006).Others strategies employing fluorescence based optosensors were used for simul-taneous determination of fuberidazole, carbaryl, and benomyl (Garcia-Reyes,Llorent-Martınez et al. 2004); and benomyl and carbendazim (Garcia-Reyes,Ortega-Barrales, and Molina-Dıaz et al. 2003).
The use of molecularly imprinted polymers (MIPs) is an interesting alternativeto obtain new materials that can be used in the development of optical chemical sen-sors, considering the highly selective molecular recognition properties, physicalrobustness, and good thermal, chemical, and mechanical stability. These facts renderthem particularly suitable for use as recognition elements in sensor technology. Inthis way, advantages of fluorescent flow-through sensors were improved employinga solid matrix based on MIPs (Valero-Navarro, Damiani, et al. 2009; Valero-Navarro, Salinas-Castillo, et al. 2009).
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A MIP solid-phase extraction flow-injection fluorescence sensor was developedfor determination of florfenicol in animal tissues (Gea et al. 2010). In the procedure,after adsorption of the analyte and fluorescence response, the clean step was performedwith a methanol:acetic acid:SDS solution. Under optimum conditions, the intensity offluorescence was directly proportional to the concentration of the antibacterial agentover the range from 1.2� 10�6 to 2.6� 10�5 gmL�1. The proposed sensor was satis-factorily applied to the determination of the analyte in liver and meat samples.
As reported previously, flow systems required for developing analytical meth-ods based on the measurement of native fluorescence on a solid phase are very sim-ple, normally employing a single channel configuration (Ruiz-Medina et al. 2001).On the other hand, when non-fluorescent species can be transformed into a fluor-escent compound by means of derivatizing reactions or photochemically (Zhu et al.2003; Lopez-Flores et al. 2005), more complex manifolds (exploiting FIA, SIA, ormulticommutation) are combined with the fluorescence detection on the solid phase.
Spectrofluorimetry seems to be one of the most suitable analytical techniquesfor pharmaceuticals, due to its simplicity and sensitivity. This methodology (batchmode) has been successfully applied to the sensitive fluorimetric determination ofvarious analytes. However, a low sample throughput is obtained, making this tech-nique unsuitable for routine analysis. The combination with flow system has beenemployed to determine low concentrations of various chemical species improvingsampling throughput and providing low consumption of reagents and samples(Molina-Dıaz et al. 2002).
Others luminescence processes, as phosphorescence and chemiluminescence,can be used analytically for direct detection on a solid support. However, applica-tions are scarce in comparison to the use of fluorimetry, because phosphorescenceand chemiluminescence are less common phenomena. For example, a flow-throughoptosensor for determination of tetracyclines, based on formation of a chelate witheuropium and room temperature phosphorescence, was developed (Alava-Moreno,Diaz-Garcia, and Sanz-Medel 1993). The method was based on the transient reten-tion of the chelate on a non-ionic resin (Chelex1 l00) packed in a flow-through cell.The detection limits for tetracycline, oxytetracycline, and chlortetracycline were 0.25,0.30, and 0.40 ngmL�1, respectively, with a coefficient of variation of 1% for a 0.24p gmL�1 concentration. The procedure was applied to urine samples and pharma-ceutical preparations.
A method for determination of thiabendazole in water samples was developedbased on phosphorescence obtained when the analyte was retained in a solid support(Piccirilli and Escandar 2009). While thiabendazole does not phosphoresce in pack-ing materials commonly used in flow-through optosensors, emission signals wereobserved after retention on nylon powder in the presence of iodide and sulfite. Aflow-injection manifold coupled to a phosphorescence detector containing the solidphase packed into a conventional flow-cell was employed. After the phosphorescenceemission was registered, the analyte was eluted with a 65% (v=v) methanol–watermixture. Using a sample volume of 2000 mL, the analytical signal showed a linearresponse in the range 12.9–110 ngmL�1, with a detection limit of 4.5 ngml�1 anda sample throughput of 14 samples per hour.
A single flow-through phosphorescence optosensor for simultaneous determi-nation of the pesticideN-1-naphthylphthlamic acid and its metabolite 1-naphthylamine
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was developed (Salinas-Castillo et al. 2004). The system was based on the on-lineimmobilization of the analytes onto a non-ionic resin solid support (AmberliteTM
XAD 7) in a continuous flow system, followed by the measurement of their nativephosphorescence. Nitric acid was used to strip the retained analytes from the solidphase before proceeding with the next injection. The developed sensor was appliedto the determination of the pesticides in drinking and mineral waters. Table 4 sum-marizes the characteristics of some applications of sensors based on phosphorescencemeasurements.
The use of flow-injection system associated with chemiluminescence (CL)detection on a solid support is another way to configure a flow-through opticalsensor (Llorent-Martınez et al. 2009). There has been a deal of interests in thedevelopment of CL-based sensors due to their high sensitivity compared withphotoluminescence-based sensors. This promoted sensitivity of CL-based sensingsystem is ascribed to the avoidance of noise caused by light scattering, and featuressimpler set-up with lower background emissions in comparison with photolumines-cence detection. However, one of the major deficiencies in applying CL sensors toroutine analysis was the short lifetime and signal drift due to the irreversible con-sumption of CL reagents that have limited the application of CL sensors in practice(Zhang, Zhang, and Zhang 2005).
Ion-exchange resin has been widely used to immobilize CL reagents to developa series of CL sensors by analyte reaction with the immobilized reagents. Generally,the solid phase with immobilized reagents is packed into a flow cell and positioned infront of the detection window of a photomultiplier (Zhang et al. 2005). With thispurpose, a chemiluminescence flow-through sensor for the determination of pyrogal-lol based on the reaction between the analyte and potassium hexacyanoferrate(III) insodium hydroxide solution was developed (Chen and Bai 2008). In the procedure,potassium hexacyanoferrate(III) was immobilized on anion-exchange resin packedin a column. Pyrogallol was sensed by the CL reaction with the potassium hexacya-noferrate(III) and sodium phosphate was employed as eluent. The CL emissionallowed the determination of pyrogallol in the range 0.01–3.8 mgmL�1, with a detec-tion limit of 0.003 mgmL�1, being applied to water analysis.
As well as flow-injection fluorescence sensor applications, the use of MIPs asrecognition element in flow injection CL sensors has been also developed in recentyears, improving the selectivity of the analysis. The MIP-CL sensors have beendesigned for determination of hydralazine (Xiong et al. 2006), maleic hydrazide(Fang et al. 2009), salbutamol (Zhou et al. 2005) and isoniazid (Xiong et al.2007), for example. Applications on the developments and applications of chemi-luminescence (CL) sensors dated from 1999 to 2005 were previously revised byZhang et al. (2005).
Reflectance
Reflectance measurements are a convenient alternative to transmittance mea-surements in SPS as they circumvent the problems regarding the transparency ofthe solid support as well as the length of the optical path. This approach usuallyemploys a bifurcated optical fiber bundle (one arm to conduct the radiation fromthe light source to the sample and other to conduct the radiation diffusively reflected
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Table
4.Selectedapplicationsofsolidphase
phosphorimetry
associatedwithflow-injectionsystem
s
Analyte(s)
Sample
Reagent
Solid
support
Eluent
Sample
volume(ml)
LD,CV
Reference
Thiabendazole
Water
Iodideandsulfite
Nylon
Methanol
2000
4.5ngmL�1;
3.2%
Piccirilliand
Escandar2009
Tetracycline,
oxytetracycline
andchlortetracycline
Urineand
pharm
aceutical
preparations
Europium
Chelex
l00
0.5molL
�1
HCl
1000
0.25,0.30,and
0.40ngmL�1;
1%
Alava-M
oreno
etal.1993
N-1-naphthylphthlamic
acid(N
AP)andits
metabolite
1-nap
hthylamine
(NNA)
Drinkingand
mineralwaters
Native
phosphorescence
Amberlite
XAD
7
15molL
�1
HNO
3
2000
8.1ngmL�1(N
AP),
11.2ngmL�1
(NAPandNNA);
3.1%
Salinas-Castillo
etal.2004
Benzo(a)pyrene
Water
Native
phosphorescence
Amberlite
XAD
7
Acetonitrile
4000
12ngmL�1;
3.3–4.6%
Salinas-Castillo
etal.2005
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towards the detector). This provides higher flexibility as the optical fiber permits toplace the flow cell outside of the spectrophotometer. In addition, multiparametricmeasurements can be straightforwardly implemented, as several flow cells with solidphases selective to different analytes can be incorporated to the flow manifold.
Diffuse reflectance has been widely explored in detection systems based onoptodes (or optrodes), which employ a (selective) reagent immobilized in an appro-priate support coupled to a bifurcated optical fiber. In this area, measurements areusually performed in continuous flow systems, that is, sample is continuouslypumped through the sensor until the steady state is reached. As a consequence, ashort response time is not an essential figure of merit and values up to 15 minutescan be found in the literature. Of course, response times as high as 15min areimpracticable in flow injection analysis, thus restricting this review to optosensorsthat are appropriate as detectors in flow systems. Table 5 summarizes the applica-tions addressed in the present work.
As aforementioned, the signal intensity depends on the injected sample volume,the flow rate, as well as the mass transfer rate of the analyte from the solution to thesolid phase. As the mass transfer rate also depends on the concentration gradient,decreasing the sample dispersion can enhance the signal intensity. In this aspect,Sotomayor et al. (1998) demonstrated that a monosegmented flow system (MSFA)provides higher signals than an unsegmented flow system, reaching up to 90% ofthe steady state signal for a pH optosensor based on bromocresol purple immobi-lized on the distal end of a bifurcated optical fiber. Unfortunately, the MSFAapproach is difficult to use in SPS based on a packed reactors, as the air bubblescan perturb the arrangement of the solid particles in the measuring cell, decreasingthe repeatability of the measurements.
Plasticized PVC membranes have been used in SPS for immobilizing differentreagents. The membrane is cast (by manual deposition, spin coating or dip coating)on an appropriate support (e.g., polymeric film or glass slide) from a THF solutioncontaining PVC (usually ca. 33% m=m), a plasticizer (ca. 66% m=m) and a selectivereagent (ca. 1% m=m). Sometimes an ion-exchanger is incorporated to the mem-brane, in order to keep the charge balance as well as to decrease the response time.A PVC membrane plasticized with 2-nitrophenyl-octyl-ether (NPOE) was used toimmobilize PAN (1-(2-pyridylazo)-2-naphthol) for detection of Cu(II) in waterand ore samples (Sanches-Pedreno et al. 2000). A sample volume of 1.0mL inammonia buffer at pH 9.0 was injected in a water carrier stream at a flowrate of 0.44mLmin�1, providing a linear response from 5.0� 10�5 to 1.0�10�3mol L�1 and a detection limit of 5.0� 10�6mol L�1. A 1.0� 10�2mol L�1
EDTA solution was employed to regenerate the solid phase for the next measure-ment. A similar PVC membrane with PAN immobilized was employed for deter-mination of Zn(II) in pharmaceuticals (Albero et al. 2002). A sample solutionprepared in ammonia buffer pH 10.2 was injected (90 mL) into an acetate bufferpH 3.9 carrier stream, which also acted as eluent. A detection limit of0.10mgL�1 of Zn(II) was obtained for a linear response range from 0.16 to3.27mgL�1. The main disadvantage of optosensors based on PAN is the poor sel-ectivity of the reagent, which reacts with several metal ions. However, for specificapplications and controlled conditions, this drawback can be circumvented, asdemonstrated in the aforementioned papers.
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Table
5.Selectedapplicationsofflow
injection-solidphase
spectrophotometry
basedonreflectance
measurements
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume
(mL)
LD,CV
(%)
Reference
Copper
Waterandore
PAN
PVC=NPOE
0.01molL
�1EDTA
1000
5.0�10�6molL
�1,3.5
Sanches-Pedreno
etal.2000
Zinc
Pharm
aceuticals
PAN
PVC=NPOE
0.2molL
�1acetate
buffer
pH
3.9
90
0.10mgL�1
Alberoet
al.
2002
Perchlorate
Water
5-octadecan
oyloxy-2-
(4-nitrophenylazo)
phenol
PVC=NPOE
0.01molL
�1NaOH
200
6.2�10�5molL
�1,1.1
Garcıa
etal.
2003
Nitrite
Water
NED=SAM
(Shinn)
C18
80%
methanol
2500
0.1mg
L�1,3.2
Miroet
al.2001
1-nap
hthylamine
Water
Nitrite=SAM
(Griess)
C18
80%
methanol
2000
1.1mg
L�1,4.7
Maret
al.2006
Sulfide
Waterand
wastew
ater
N,N
-dim
ethyl-p-
phenylenediamineþ
Fe(III)
C18
80%
methanol=0.01molL
�1
HClþ
80%
methanol
2900
2.9mg
L�1,0.7
Ferreret
al.
2005a
Sulfide
Water,
suspensions
N,N
-dim
ethyl-p-
phenylenediamineþ
Fe(III)
C18
80%
methanol=0.01molL
�1
HClþ
80%
methanol
5000
1.3mg
L�1,2.2
Ferreratal.
2005b
Sulfide
Water,
wastew
ater,
grouwater
N,N
-dim
ethyl-p-
phenylenediamineþ
Fe(III)
C18
80%
methanol=0.01molL
�1
HClþ
80%
methanol
5000
4.6mg
L�1,2.1
Ferreratal.
2006
Fe(II),Fe(III)
Seawater
Ammonium
thiocyan
ate
Chelatingdisk
(iminodiacetic
acid)
2molL
�1HCl
2000
0.012mg
,2.5
Ponset
al.2005a
Fe(II),Fe(III)
Naturalwater
Ammonium
thiocyan
ate
Anionexhange
0.25molL
�1HClin
75%
ethan
ol
1000
0.0004
mg,3.6
Ponset
al.2005b
Propoxur,
carbaryl
Vegetables
Acetylcholinsterase=
chlorophenolred
Controlled
porous
glass
5.0mmolL
�1phosphate
buffer
pH
8.5
50
0.4ng(propoxur)
and
25ng(carbaryl),5.0
Xavieret
al.
2000
C18:C18-bonded
silica.
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The PVC membranes plasticized with NPOE have also been employed fordetermination of perchlorate ions in water (Garcıa et al. 2003). The membranewas used to immobilize the lipophilic pH indicator 5-octadecanoyloxy-2-(4-nitrophenylazo)phenol (ETH 2412) and the ionic additive methyltridodecylammo-nium chloride, being the response mechanism based on the co-extraction of per-chlorate and proton from a buffered solution of the anion. The extractedhydrogen ion then reacts with the pH indicator, generating a signal proportionalto the perchlorate concentration. A 1.0� 10�2mol L�1 NaOH solution wasemployed as carrier in a single line manifold, in which 200 mL of sample solutionbuffered with 5.0� 10�2mol L�1 TRIS pH 8.2 was injected. A linear response wasobtained from 7.0� 10�5 to 2.0� 10�2mol L�1, with a detection limit of 6.2� 10�5
mol L�1 ClO�4 . The optosensor presented a severe interference of bicarbonate ion,
which would be eliminated by acidifying the sample previously the addition of thebuffer solution. Recovering tests performed in tap and spring waters demonstratedthe adequate performance of the optosensor for determination of perchlorate inthis kind of matrix.
Solid phases have been used to retain a product formed in solution in order toimprove sensitivity. In this aspect, nitrite was determined in waters by injecting2500 mL of the sample solution into a Shinn reagent carrier stream (Miro et al.2001). The product formed was retained by an octadecyl covalently bonded silicagel (C18 disk) coupled to an open sandwich-shape flow cell furnished with a bifur-cated optical fiber for reflectance measurements. The product adsorbed onto theC18 disk was removed by injecting 150 mL of a 80% (v=v) methanol aqueous solution.The optimized conditions provided an enrichment factor of 140 and a detection limitof 0.1 mgL�1, with reproducibility and repeatability better than 3.2%. The samplethroughput was 11 h�1 and the membrane can be used for up to 40 runs. A C18 diskhas also been used in the determination of 1-naphthylamine in water samples by theGriess reaction performed in a multisyringe flow injection system (Mar et al. 2006).The diazonium salt formed in the reaction was retained by the disk, which wascleaned up with an 80% (v=v) methanol aqueous solution. By using a samplevolume of 2000 mL, a working linear range from 10 to 160 mgL�1 was obtained, witha detection limit of 1.1 mgL�1 and repeatability better than 4.7%. A sampling rate of14 injections per hour was also attained.
Three different strategies based on multisyringe flow injection analysis havebeen proposed for the determination of sulfide by solid phase spectrophotometry(Ferrer et al. 2005a; Ferrer et al. 2005b; Ferrer, Estela, and Cerda 2006). All of thememployed the Fischer reaction, in which sulfide reacts with N,N-dimethyl-p-pheny-lenediamine in the presence of Fe(III) as oxidizing agent. The methylene blueformed in the reaction was then adsorbed onto an octadecyl-chemically modifiedsilica (C18 disk), which was cleaned up and regenerated for a next measurementwith a 80% (v=v) methanol solution containing 0.01mol L�1 HCl, followed byan 80% (v=v) methanol aqueous solution. The first strategy was applied to the deter-mination of low concentrations of sulfide in water samples and processed waste-waters (Ferrer et al. 2005a). With the injection of 2900 mL of sample into a0.14mol L�1 HCl carrier stream at 8 samples per hour, a linear dynamic range from20 to 200 mgL�1 was achieved, providing a detection limit of 2.9 mgL�1. A repeat-ability of 0.7% and a reproducibility of 4.5% were also obtained and the addition
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of known amount of sulfide in different water samples permitted recoveries from95% to 112%, demonstrating the feasibility of the method. The same authors (Ferreret al. 2005b) expanded the use of the MSFIA system by coupling a gas diffusionunity, which permitted the determination of sulfide in more complex matrices, con-taining suspended material, without the necessity of preliminary treatment. The gasdiffusion step was performed by injecting a sample volume of 5000 mL into a0.14mol L�1 HCl carrier stream, which merged with a 1.5mol L�1 HCl solution.The H2S diffused through the membrane towards a 0.001mol L�1 NaOH acceptorsolution, which was then conducted to the reaction coil for production of methyleneblue, as described previously. The flow system allowed a linear response range from20 to 500 mgL�1, with a detection limit of 1.3 mgL�1. Finally, a fully automatedsmart MSFIA system was proposed for the determination of sulfide in a wide con-centration range (Ferrer et al. 2006). In this system, three different procedures basedon the gas-diffusion step can be automatically elected to perform the determination:(a) a direct procedure, in which absorbance measurements were carried out forthe determination; (b) a procedure involving a dilution step followed by the directmeasurements; and (c) a preconcentration procedure, which employed a C-18 disk.The optosensing procedure provided a detection limit of 4.6 mgL�1 and a linearresponse from 50 to 1000 mgL�1, values that do not differ considerably from theprevious work.
The MSFIA systems have been also employed to the determination andspeciation of iron in water samples by employing extraction disks (Pons, Forteza,and Cerda 2005a, 2005b). The use of a chelating disk modified with iminodiaceticgroups (Pons et al. 2005a) permitted to preconcentrate Fe(III), which subsequentlyreacted with ammonium thiocyanate for reflectometric determination of the com-plex in the solid phase. Total iron was determined by on-line oxidation of Fe(II)with hydrogen peroxide prior to the preconcentration step. A detection limit of0.0012 mg was achieved for Fe(III) ions, which is 10-fold better in comparison toa similar MSFIA system that employed the same chelating disk, performing, how-ever, the determining reaction in solution (Pons, Forteza, and Cerda 2004). Withthe use of an anion-exchanger disk, which retains the FeðSCNÞ3�6 previouslyformed in the MSFIA system, a detection limit of 0.0004 mg was reached (Ponset al. 2005b).
A biosensor employing a pH optical transduction was described for the deter-mination of the pesticides carbaryl and propoxur in vegetables (Xavier et al. 2000). Acontrolled pore glass was used to covalently immobilize the enzyme acetylcholines-terase, whose activity in the hydrolysis of the acetylcholine to produce choline andacetic acid is inhibited by the pesticides. The pH variation due to the formation ofacetic acid was measured by a reflectometric optosensor based on the acid-base indi-cator chlorophenol red also immobilized on CPG. A substrate solution was initiallyinjected into a 5.0mmol L�1 phosphate buffer pH 8.5 carrier solution, providing ananalytical signal proportional to the enzyme activity. Subsequently, 50 mL of samplewas injected and the flow was stopped for 6 minutes after the sample zone reachedthe enzymatic column. Afterward, another injection of the substrate was carriedout, in order to determine the percentage of inhibition of the enzyme, which wasproportional to the pesticide concentration. Detection limits of 0.008mgL�1 and0.5mgL�1 were achieved for propoxur and carbaryl, respectively.
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Vibrational Spectroscopy
The use of infrared spectrometry (MID or NIR) for direct determination oforganic compounds in aqueous samples is limited due to the low sensitivity ofthe technique as well as the intense absorption bands of water in this region of thespectrum. Thus, the extraction of the analyte from the aqueous solution seems tobe a valuable alternative, as this procedure avoids the interference of water, besidespre-concentrating the analyte in the solid phase. In addition, once the extraction isperformed, the interfering species can be separated from the analyte, improvingthe selectivity. Early procedures employed a solid phase extraction cartridge topre-concentrate the analyte, followed by its elution with an appropriate organicsolvent towards the detection cell (Garrigues 1994). The first flow-through sensorwas constructed by using a conventional MID-IR transmission cell furnished witha 55 mm polymeric spacer modified in order to hold DEAE Sephadex1 A-25anion exchange resin beads, which swelled in contact with water, forming a gel-likedisc between the two CaF2 windows (Lendl and Schindler 1999). The optosensorwas applied to the determination of acetic and malic acids in aqueous solutionsby sequential injection analysis. The addition of NaOH to the sample solutionsproduced the corresponding anions, allowing their retention by the anion exchangeresin. Thus, by injecting 500 mL of sample a linear response up to 1mmol L�1 ofacids was obtained, with standard deviations of 0.032 and 0.031mmol L�1 for aceticand malic acids, respectively.
A mid-IR flow-through sensor for determination of carbohydrates in beerwas constructed by immobilizing amyloglucosidase on agarose beads (Haberkorn,Hinsmann, and Lendl 2002). Maltose standard solutions were automaticallyprepared in the SIA system and injected towards the detector. Once the solutionreached the optosensor, the flow was stopped and spectra were run for 10min inorder to monitor the formation of the product. A water carrier stream was employedto flush the system and the content of carbohydrates in beer samples, expressed asmaltose, was successfully determined by standard addition method.
The C18 modified silica particles were employed to develop a FTIR optosensorfor determination of caffeine in soft drinks (Armenta and Lendl 2009). The C18
particles were initially activated by pumping 0.8mL of methanol, followed by3mL of water. Afterward, a reference spectrum was recorded and 1.5mL of samplesolution was impelled through the sensor. The beads were subsequently rinsed with2mL of water for elimination of all adsorbed compounds except caffeine, whosespectrum was then acquired. Finally, the optosensor was regenerated by injecting0.8mL of methanol to completely remove the adsorbed analyte, followed by3.0mL of water to prepare the system for the next determination. A linear responseup to 115mgL�1 of caffeine was obtained, with a detection limit of 1.8mgL�1 and arelative standard deviation of 4.1%. The results obtained in the determination ofcaffeine in soft drinks showed good agreement with those obtained by the HPLCreference method.
Raman spectroscopy has been also applied in SPS, presenting the advantagethat water weakly scatters Raman radiation. In this aspect, the determination ofcaffeine in energy drinks by a FT-Raman flow-through sensor based on a C18 solidphase was also described in the literature (Armenta et al. 2005). An anionic
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Table
6.Selectedapplicationsofflow
injection-solidphase
infrared
spectroscopy
Analyte(s)
Sample
Reagent
Solidsupport
Eluent
Sample
volume(mL)
LD,CV
(%)
Reference
Aceticand
malicacids
Water
–Anionexchange
0.5molL
�1
HClþ
water
500
NE,3.1
Lendland
Schindler1999
Carbohydrate
(maltose)
Beer
Amyloglucosidase
Agarose
geads
Water
Steadystate
NE
Haberkorn
etal.2002
Caffeine
Softdrinks
–C18
Methanolþ
water
1500
1.8mgL�1,4.1
Arm
enta
and
Lendl2009
Caffeine
Energydrinks
–C18
Methanolþ
water
10000
18mgL�1,3.0
Arm
enta
etal.2005
Sulfonamides
Pharm
aceuticals
–Sephadex
QAE
A-25gel
0.1molL
�1NaCl
þ0.01molL
�1NaOH
1000
0.1gL�1,4.1
Ruedas-Rama
etal.2005
Caffeine
Softdrinks
–Polystyrene-
divinylbenzene
Acetone
3000
7mgL�1,4.0
Alcudia-Leon
etal.2008
a-naphthylamine
Water
–C18
20%
methanol
17000
0.6mgL�1,5.2
Ortega-Barrales
etal.1999
NE:notevaluated;C18:C18-bonded
silica
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solid-phase reactor located before the C18 column was used to avoid matrix inter-ference, improving the selectivity of Raman measurements. The activation of thesolid support was accomplished by pumping 2mL of methanol, followed by 2mLof water. Afterward, 10mL of sample was passed through the sensor at a flow rateof 7.5mL min�1, followed by 20mL of distilled water. The solid phase was pre-viously dried with a nitrogen stream before acquiring the spectrum. The calibrationgraph was linear from 100 to 600mgL�1, providing a detection limit of 18mgL�1,with a relative standard deviation of 3%. An analytical throughput of 13 samples perhour was obtained, higher than those provided by HPLC procedure (7.0 h�1). Sepha-dex1 QAE A-25 anion exchanger gel has also been used to determination of sulfo-namides in pharmaceutical preparations (Ruedas-Rama et al. 2005) by FT-Raman.In this approach, a flow cell was packed with the gel and baseline signal was obtainedby pumping 0.010mol L�1 NaOH solution at a 1.0mL min�1. A volume of 1000 mLof sample solution (sulfathiazole or sulfamethoxazole) was injected into the systemand the flow was stopped at the maximum intensity signal for acquiring spectral data(as an average of 20 scans). Afterward, 2000 mL of a 0.10mol L�1 NaCl solution con-taining 0.010mol L�1 NaOH was injected for the elution of the analyte retained bythe solid phase, thus recovering the baseline signal. A detection limit of 0.1 gL�1 wasobtained for both sulfonamides, with a sampling rate of 10 samples per hour andrelative standard deviations lower than 4.1%.
An attenuated total reflection-based optosensor was also employed for thedetermination of caffeine in soft drinks (Alcudia-Leon et al. 2008). A flow cell wasspecially designed to accommodate a small amount of a polystyrene-divinylbenzenesorbent material at the sensitive zone of the ATR crystal, with the aid of a cotton frit.A SIA system was employed to manage the solutions. Initially, 2mL of water waspumped through the system for acquiring a reference spectrum. Three milliliters ofsample was then passed through the optosensor, followed by 2mL of water, forretention of caffeine and elution of the interfering species. Once a spectrum wasacquired, the sorbent material was regenerated with 0.25mL of acetone. The useof a flow rate of 0.5mL min�1 allowed a whole measuring cycle in 14min. A relativestandard deviation of 4% and a detection limit of 7mgL�1 were estimated.
Near infrared (NIR) was also employed as detection technique in solid phasespectroscopy for the determination of a-naphthylamine in water samples (Ortega-Barrales et al. 1999). A commercial cell of 1mm pathlength was filled with a C18-bonded silica gel solid phase, which was conditioned=regenerated with 1mL of a20% (v=v) methanol aqueous solution. For measurements, 17mL of naphthlaminesample solution was pumped through the optosensor at a flow rate of 2.1mLmin�1, providing a dynamic range from 4 to 30mgL�1, with a detection limit of0.6mgL�1 and a relative standard deviation of 5.2%. Table 6 lists the applicationsof MID and NIR flow-through optosensors.
CONCLUSIONS AND TRENDS
The combination of flow analysis with detection on optically active solid phasespacked in a flow-through cell (optosensor) has demonstrated to offer importantadvantages due to its precision, simplicity, selectivity, and high sensitivity, providedby the in situ preconcentration of analyte(s) on the solid sensing support. In addition,
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they exhibit notable analytical features that contribute to a greener analyticalchemistry by saving reagents and sample, and reducing waste generation. Furtherdevelopments of these optosensing techniques have shortened analysis time consider-ably and reduced costs, being adequate for routine control, since automated systemsaccelerated the operational sequence and can reduce manipulation by the analyst.
Despite measurements by UV-vis spectrophotometry (transmission or reflec-tance modes) is the most frequently used detection technique in SPS for inorganicor organic analytes, luminescence, infrared and Raman spectroscopies can also bethe basis of measurements on solid phase. The high sensitivity and selectivity pro-vided by luminescence-based techniques are favorable features, while vibrationalspectroscopy can be an alternative for organic analytes determination. Additionally,considering that some application of spectrometric methods is often limited byselectivity, development of new sorbents, such as MIP, with high affinity, specificrecognition, and high stability is of great significance for use in direct measurementson solid surface.
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Alava-Moreno, F., M. Diaz-Garcia, and A. Sanz-Medel. 1993. Room-temperature phosphor-escence optosensor for tetracyclines. Anal. Chim. Acta 281: 637–644.
Albero, M. I., J. A. Ortuno, M. S. Garcıa, C. Sanchez-Pedreno, and R. Exposito. 2002.Determination of zinc (II) in pharmaceuticals based on a flow-through bulk optode.J. Pharm. Biomed. Anal. 29: 779–786.
Alcudia-Leon, M. C., R. Lucena, S. Cardenas, and M. Valcarcel. 2008. Characterization of anattenuated total reflection-based sensor for integrated solid-phase extraction and infrareddetection. Anal. Chem. 80: 1146–1151.
Amini, M. K., T. Momeni-Isfahani, J. H. Khorasania, and M. Pourhossein. 2004.Development of an optical chemical sensor based on 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol in Nafion1 for determination of nickel ion. Talanta 63: 713–720.
Armenta, S., S. Garrigues, and M. de la Guardia. 2005. Solid-phase FT-Raman determinationof caffeine in energy drinks. Anal. Chim. Acta 547: 197–203.
Armenta, S., and B. Lendl. 2009. Flow through FTIR sensor based on solid phasespectroscopy (SPS) on conventional octadecyl (C18) silica. Vib. Spectrosc. 51: 60–64.
Ayora-Canada, M. J., M. I. Pascual-Reguera, and A. Molina-Dıaz. 1998. Continuousflow-through solid phase spectrophotometric determination of trace amounts of zinc. Anal.Chim. Acta 375: 71–80.
Ayora-Canada, M. J., M. I. Pascual Reguera, and A. Molina-Dıaz. 1999. An integratedreaction-retention and spectrophotometric detection flow system for the determination ofnickel. Fresenius J. Anal. Chem. 363: 59–63.
Ayora-Canada, M. J., M. I. Pascual-Reguera, and A. Molina-Dıaz. 2000. Determination ofvanadium by solid phase spectrophotometry in a continuous flow system. Intern. J. Environ.Anal. Chem. 76: 319–330.
Ayora-Canada, M. J., M. I. Pascual Reguera, A. Molina-Dıaz, and L. F. Capitan-Vallvey.1999. Solid-phase UV spectroscopic multisensor for the simultaneous determination of caf-feine, dimenhydrinate and acetaminophen by using partial least squares multicalibration.Talanta 49: 691–701.
Baliza, P. X., S. L. C. Ferreira, and L. S. G. Teixeira. 2009. Application of pyridylazo andthiazolylazo reagents in flow injection preconcentration systems for determination ofmetals. Talanta 79: 2–9.
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