Multi-commutation in flow analysis: Recent developments and

a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
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Review
Multi-commutation in flow analysis:
Recent developments and applications
Mário A. Feres a , Paula R. Fortes a , Elias A.G. Zagatto a,∗ ,
João L.M. Santos b , José L.F.C. Lima b
a
Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P.O. Box 96, Piracicaba 13400-970, Brazil
REQUIMTE, Departamento de Quı́mica-Fı́sica, Faculdade de Farmácia, Universidade do Porto,
Rua Anibal Cunha 164, Porto 4050-047, Portugal
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
The concept of multi-commutation in flow analysis is revisited, and emphasis is given
Received 12 March 2008
to recent methodological and applicative achievements. Multi-commutation is compatible
Received in revised form
with different flow patterns (unsegmented, segmented, pulsed, tandem) and amenable to
14 April 2008
concentration-oriented feedback mechanisms. Its exploitation has led to significant attain-
Accepted 15 April 2008
ments mainly in relation to versatility of the flow system. Characteristics and potentialities
Published on line 24 April 2008
of the multi-commuted flow systems are discussed, and guidelines for assisting methodological implementation are given.
Keywords:
The number of applications has experienced remarkable increase during last years; there-
Multi-commutation
fore, the applicative part of this review is focused on the recent noteworthy applications,
Flow analysis
mainly in relation to environmental, agronomical, pharmaceutical, biological, food and
Tandem streams
industrial samples.
© 2008 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
7.
∗
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The original concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternative system operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sample insertion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Addition/removal of components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1. Sample dilutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2. Titrations and related strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3. Analyte separation/concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4. Sample stopping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author. Tel.: +55 19 34294650; fax: +55 19 4294610.
E-mail address: [email protected] (E.A.G. Zagatto).
0003-2670/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.aca.2008.04.039
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a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
8.
1.
7.5. Sequential/simultaneous determinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
Analytical flow systems relying on the multi-commutation
concept [1] usually comprise discretely computer-operated
devices strategically positioned in the manifold. Enhanced
versatility is inherent to them, and different strategies
such as e.g. sample stopping, random access reagent
selection, individual sample conditioning, and exploitation of concentration-oriented feedback mechanisms can be
exploited simply by modifying the time course related to
operation of these devices or by allowing their automated
adjustment [2]. Moreover, tandem streams [1,3–6] are efficiently implemented.
This unique stream comprises initially “several neighboring small plugs of miscible solutions that undergo fast
mixing while flowing through the analytical path” [7]. Overlap between them is then rapidly attained. Tandem streams
constitute themselves in a driving force towards improved
mixing conditions. Another favorable aspect related to tandem streams is that the flow systems can be designed in the
straight configuration without the drawbacks inherent to it
[8]. In view of high number of interfaces involved, the sample volume can be increased at will without impairing the
sample/reagent interaction or causing double-peaking.
Multi-commutation is closely related to the degree of
automation of the flow analyzer. As a rule, flow systems
exploiting multi-commutation are more versatile and more
prone to accommodate a fully automated sample processing.
Multi-commutation can be exploited in relation to any modality of flow analysis (segmented flow, flow–injection, sequential
injection) and any flow pattern (unsegmented, segmented,
pulsed, tandem), being also compatible to concentrationoriented feedback mechanisms [9,10]. A literature survey
reveals that proposal of multi-commuted flow systems has
undergone amazing increasing in recent years [11], and critical reviews emphasising historical developments, concepts,
characteristics and applications of these flow systems have
been recently presented [2,8,12].
The aim of the present review is to provide upgraded
information on multi-commutation, in order to discuss novel
methodological and applicative achievements.
2.
14
14
14
14
14
With the advent of computer-controlled devices (injectors, commuters, valves, pumps), the systems exploiting
commutation became more elaborated allowing exploitation of tandem streams [7], selective directioning of sample,
reagents or carrier streams [14], addition/removal of manifold
components [15], sample stopping [10], etc., which culminated with the inception of the multi-commutation concept
[1].
This concept can be illustrated by considering a very simple
flow analyzer relying only on the sample, reagent and carrier
streams. The system can be designed with serial (Fig. 1a) or
centered (Fig. 1b) valve positioning, depending on whether
three-way or two-way valves are used. Management of the
involved solutions, including their addition to the analytical
channel and eventual stopping inside it, is then efficiently
accomplished. The fluid propelling device (usually a syringe
or a peristaltic pump) is positioned after the detection unit,
so that all the involved solutions are aspirated through the
analytical channel [16]. Solution aspiration is then inherent to
the original concept of multi-commutation. The solutions are
selected by the stream directioning valves operated at different time schedules. Consequently the system is characterized
by enhanced simplicity and versatility, as well as easy of control.
The original concept
With the ever-increasing development of flow–injection analysis, commutation manifested itself as an essential feature
towards enhanced system versatility. The main involved
aspects were synthetised in 1986 [13]. Manual operation of
the commuters was exploited in some earlier applications for
implementing, e.g. merging zones, zone sampling and sample stopping [8]. Here however applications involving manual
operation of the system are not highlighted.
Fig. 1 – Model multi-commuted flow systems designed with
serial (a) or centered (b) valve positioning involving only the
sample, reagent and carrier streams. S = sample; C = carrier
stream; R = reagent; B = reactor; D = detector; P = pumping
device with indication of flow direction; W = waste;
Vi = stream directing valves (1, 2 = three-way; 3–5 = two-way
valves); full, traced lines = actual, alternative flow paths.
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
As a consequence of the solution aspiration, the hydrodynamic pressure is lower than atmospheric; therefore efforts
have been done to avoid the eventual inlet of air through
the junctions or release of formed gaseous species as bubbles
which might affect sample dispersion and/or impair detection.
System operation relies on time-based insertion of sample, reagent or carrier stream into the main analytical channel.
Considering that only one valve is switched at a time, different
flow paths can be established depending on system architecture, valves time course and their positioning in the manifold.
The systems in Fig. 1 can be operated in order to behave
similarly as a flow–injection system [17]. A chemically inert
carrier stream is not needed; therefore the sample is inserted
directly into the reagent flowing stream and V2 valve is not
required. The reagent stream is initially added to the analytical
channel; further, it is replaced by the sample stream during a
pre-selected time interval; thereafter, the sample inlet ceases
and the reagent carrier stream is re-directed towards the analytical channel. The originated sample zone is transported by
the reagent carrier stream through the analytical path towards
detection, allowing the steps inherent to the specific analytical
application to be performed.
Tandem streams can be promptly established. Instead of
the above mentioned single sample insertion, multiple sample/reagent interchanges can be performed through fast and
repetitive alternated switching of the valves. In this way, a
number of neighboring sample and reagent plugs are inserted
into the analytical channel [1]. Coalescence of these plugs
leads to the homogeneization of the sample zone that is processed similarly as above. Exploitation of tandem streams
becomes more attractive when several solutions are required
by the analytical protocol.
Moreover, stopping of the sample zone is also attainable
simply by resorting from an additional valve (Fig. 2a) that permits the aspiration of either the solution inside the analytical
channel or that inside an alternative channel.
Alternative flow paths can be also attained by placing several stream directioning valves along the manifold (Fig. 3).
With solidary switching of these valves, any artifact such
as resin mini-columns, enzyme cartridges, reactors, etc., can
be easily inserted into and removed from the analytical
path.
3.
3
Fig. 2 – Model multi-commuted flow systems with an
additional three-way valve (V4 ) for attaining sample
stopping (a) or for minimizing sample carryover (b).
A = alternative path; other symbols (including
representation of three-way and two-way valves) as in
Fig. 1.
propelling channel. Discretely computer-operated three-way
valves direct the flowing solutions either towards the analytical channel or back to their individual flasks [19]. This fluid
delivery mode is compatible with both confluent and singleline flow systems [8].
Another modification in the original concept of multicommutation refers to the design of confluent flow–injection
system [17]. The reagent stream in Fig. 1 is continuously
added to the analytical channel thus acting as a steady confluence stream, and the sample and the chemically inert carrier
streams are managed as above described. Undesirable con-
Alternative system operation
During the development of the multi-commuted flow systems,
it was realized that system versatility allowed other flowbased strategies to be exploited. Consequently, the original
concept has undergone a continuous expansion.
A relevant modification refers to the solutions propelling
mode. In fact, pumping the involved solutions towards the
analytical channel allowed sample handling under hydrodynamic pressure higher than atmospheric. In addition, more
than one solution can be simultaneously added to the main
channel.
For operating the flow system in the propelling mode
(Fig. 4), a single peristaltic pump suffices [18]. Each solution is
then associated to a different pumping tube, thus a different
Fig. 3 – Manifold portion associated to artifact
inclusion/removal into the analytical channel. Vi = valves;
C1 = alternative path; C2 = main analytical channel;
A = artifact (mini-column for analyte
separation/concentration, enzyme cartridge, immobilized
reagent, etc.). In the situation specified, solution C1 flows
through A towards W whereas C2 is directed towards
detection. After solidary valve switching, C2 flows through
A whereas C1 flows directly towards W.
4
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Fig. 4 – Model multi-commuted flow systems involving
positive displacement of the sample, reagent and carrier
streams. Arrows = pumping application sites; other
symbols as in Fig. 1.
centration gradients that might affect sample dispersion and
impair detection [20] are then minimized.
Furthermore, merging zones are straightforwardly implementable [17]. The strategy leads to a pronounced lessening
of the reagent consumption, as the reagent is added only in
presence of the sample or, in other words, management of the
sample and reagent streams is solidary.
Regardless of the system configuration, analytical applications requiring sample stopping can be also straightforwardly
implemented. To this end, inlet of all the involved solutions
ceases during a pre-selected time interval in order to halt the
sample zone inside the analytical channel or at the detection unit. Increase in mean sample incubation time is then
attained without increasing sample dispersion. The approach
is attractive for analytical procedures relying on relatively slow
chemical reactions [21] and/or involving detection procedures
requiring larger measurement time intervals [10].
It is important to stress that the system in Fig. 1 can be
expanded in order to permit the implementation of more
elaborated analytical protocols, including e.g. those involved
in sequential spectrophotometric determinations [22], sometimes taking advantage of the random access reagent selection
[14]. When two reagent solutions are separately added to a
single sample plug, sandwich techniques [23] are promptly
implemented.
Piston or syringe pumps can be also used as fluid propeller
devices. In this context, solidary syringes and re-directing
valves constitute themselves in the essence of the multisyringe flow analyzer [24,25]. Another possibility is to exploit
discretely operated solenoid pumps, and the innovation is
inherent to the multi-pumping flow systems [26,27].
4.
Timing
Every flow system is characterized by at least one time window, the time interval available for development of given
events [28]. As timing is always precise, partial and repro-
ducible developments of some involved physicochemical
processes are feasible, and reaction kinetics can be efficiently
exploited. Earlier applications relied on the timing defined by
e.g. flow rates, analytical path length and sites of confluent
stream additions. This aspect dominated during the initial
developments of segmented flow and flow–injection analysis. With multi-commutation, there are other possibilities for
timing setting, in view of the presence of discretely operated devices in the manifold. Analytical procedures requiring
sample stopping can be selected for illustrative purposes: the
pre-defined time interval for reaction development is set by an
external timer, usually the computer, which defines the STOP
period [29].
These discreetly operated devices (valves, pumps, timers)
can be classified as passive or active ones, depending on
whether concentration-oriented feedback mechanisms are
exploited or not. Without these mechanisms, conditions for
sample handling (dispersion, interaction with other solutions,
monitoring) are defined previously to sample introduction
into the analytical path and do not vary from one sample
to another. Alternatively, the parameters needed for handling
samples in a diverse and specific way, as in e.g. the flow systems with random reagent access [30], could be provided via
keyboard.
The potentialities of the multi-commuted flow analyzer
are expanded by real-time modifying the conditions for sample handling according to a prior measurement [9]. A typical
example is the individual sample conditioning [31]: relevant
information (acidity, ion strength, presence of a potential
interfering species) is roughly gathered and the result is taken
as the basis for real-time sample conditioning by suitable
reagent additions, pH adjustment, sample dilution, etc. Moreover, the need for multiple STOP periods [32] or for modifying
the sample volumetric fraction [33] can be real-time confirmed
through concentration-oriented feedback mechanisms.
Self-optimization of the flow system can be also performed
by taking advantage of active devices. Best analytical response
can be attained by automatically varying the operational
conditions according to a previously defined optimization
algorithm [34].
5.
Sample insertion
Although loop-based injection can be used in relation to multicommuted flow systems, most of the applications exploit
time-based injection [13] for sample introduction into the
manifold. A critical comparison between these insertion
strategies in relation to a sequential injection analyzer is given
elsewhere [36].
Regarding time-based injection, the sample is directed
towards the analytical channel at a constant flow rate during a pre-selected time interval, and these parameters dictate
the volume of the inserted sample aliquot. The strategy is
attractive for process monitoring, because the sampling probe
remains into the investigated medium; therefore carryover
effects are not relevant. In relation to serial assays, this sampling strategy may present some limitations, as the sampling
probe is moved from one sample cup to another. The system becomes then more susceptible to carryover effects and
Table 1 – Applications involving high and/or variable sample dilutions
Analyte
Sample
Detection technique
Detection limit or range
Sampling rate h−1
Ref.
Dynamical range expanded by split zone
Zone sampling; indirect detection
Pioneer CL implementation in multisyringe
flow–injection analyzer
Zone sampling
Computer-assisted splitting of the flowing
sample
Large and variable dilutions with an opened-loop
configuration
Smart system; dilution degree selected according
to a feedback mechanism
Wide-range determination
[61]
[67]
[68]
Pioneer CL application in relation to tandem flow
Zone sampling for expanding the dynamical
concentration range
High and variable in-line sample dilutions
[72]
[67]
[42]
[77]
Al
Ce(IV)
Co
Fruits
Pharmaceuticals
B-12 vitamin
UV–vis
Fluor.
CL
0.1 mg L−1
34.3 ng mL−1
15 ng L−1
Variable
50
180
Creatinine
Cu, Zn
Urine
Plant materials
UV–vis
UV–vis
0.50–2.00 g L−1
0.05, 0.04 mg L−1
24
45
Fe
Wastewaters
UV–vis
0.5–10 g L−1
8–20
Glycerol
Yeast cultivation media
UV–vis
0.1–4.0 or 1.0–40.0 g L−1
14 or 12
Hg
Fish and water
reference materials
Surface waters
Pharmaceuticals
CV-AAS
5 ng L−1
44
CL
Fluor.
0.1–15.0 mg L−1
34.4 ng mL−1
103
50
FAAS/FOES
Turb.
500–3500, 50–150, 30–120,
20–40 mg L−1
10–150 mg L−1
70, 75, 70, 58
SO4
Parenteral and
hemodialysis solutions
Plant materials
Sulfide
Tin, Ni
Natural and wastewater
Brass
UV–vis
ET-AAS
0.15, 0.09 mg L−1
0.001, 0.003% (w/w)
80
30
Total acidity
Fuit juices, soft drinks
Pot.
1–100 mmol L−1
22
Turbidity
Natural water
UV–vis
1–0.1 NTU
60
Possibility of expanding the concentration
dynamical range
Two concentration dynamical ranges
Expansion of the concentration dynamical range
by valve triggering; electrolytic dissolution
Wide-range flow titration; monosegmented flow
system
In-line dilution of a single standard solution
Uric acid
Urine
Amp.
990 ␮mol L−1
(b)
2500-fold sample dilution
Hydroquinone
Isoniazid
Na, K, Ca, Mg
100
[41]
[65]
[69]
[70]
[71]
[73]
[48]
[74]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Remarks
[75]
[76]
Amp. = amperometry; CL = chemiluminescence; CV-AAS = cold vapor atomic absorption spectrometry; CV-AFS = cold vapor atomic fluorescence spectrometry; ET-AAS = electrothermal vaporization
atomic absorption spectrometry; FAAS = flame atomic absorption spectrometry; Fluor. = fluorescence or fluorimetry; FOES = flame optical emission spectrometry; FTIR = Fourier transform infrared
spectrophotometry; HG-AFS = hydride generation atomic fluorescence spectrometry; ICP-MS = inductively coupled plasma mass spectrometry; ICP-OES = inductively coupled plasma optical emission
spectrometry; NIR = near infrared spectrophotometry; Pot. = potentiometry; Turb. = turbidimetry; UV–vis = UV–vis spectrophotometry; Voltam. = voltammetry. (a) Not informed; (b) not pertinent.
5
6
[84]
[54]
[75]
[51]
[83]
[82]
[79]
[80]
[81]
Typically 16
22
22
160
12
52
Variable
5–30
(b)
(a)
Monosegmented flow titration; real-time
implementation of algorithm for end point
search
Flow titration using iodide selective electrode
No prior sample treatment
Argentimetric flow titration; monosegmented
flow system
Standard addition method for circumventing
thermal influence on the spectra
Development of an enhanced flow-batch titrator
Extraction in chloroform; sample stopping at the
detection unit
Flow-batch system
Wide-range flow titration; monosegmented flow
system
Flow titration of colored samples
[78]
to the eventual inlet of air. The drawback can be circumvented by including an additional valve (Fig. 2b) which directs
the remaining of the sampled stream towards waste, without
passing through the analytical channel [1].
Regarding loop-based injection, the aqueous sample flows
through an external loop which defines the sample inserted
volume. The loop is further inserted into the sample carrier stream. The strategy has been often implemented by
resorting from ordinary injectors, usually rotary valves [8];
the feasibility of loop-based injection in multi-commuted flow
systems comprising stream re-directing valves as the only discretely operated devices was also demonstrated [36]. In view
of the favorable characteristics of loop-based injection [13],
its exploitation in combination with other active devices for
versatility enhancement is advantageous [37]. Hydrodynamic
injection [38] is analogous to loop-based injection, and the
approach can be efficiently accomplished by resorting from
stream directing valves [35].
Multiple injections can be also exploited in multicommuted flow systems aiming at performing sequential
determinations, including those relying on optional additions of reagents [14], using different masking agents [39],
and allowing speciation [40]. Successive injections of different
aliquots taken from a dispersing sample can be also performed
[41]. This zone sampling approach has been used mainly to
widen the concentration range of an analytical procedure [42].
0.001–0.1 mol L−1
Symbols as in Table 1. (a) Not pertinent; (b) not informed.
Silage materials
Total acidity
UV–vis
5.7–8.5 g L−1
1–100 mmol L−1
Red wines
Fruit juices, soft drinks
Total acidity
Total acidity
UV–vis
Pot.
257–416 mg/tablet
0.05 mg mL−1
Drug
Tobacco, cigarette filters
Metronidazole
Nicotine
UV–vis
FTIR
0.1 mg L−1
Pesticide formulations
Hexythiazox
NIR
7.5–15.0 mmol L−1
0.6–6.0 mmol L−1
10–1200 mg L−1
Pharmaceuticals
Fruit juices, soft drinks
Milk, wine
Ascorbic acid
Ascorbic acid
Chloride
Pot.
UV–vis
Pot.
Vinegar, soft drinks
Pot.
0.4–9.0 mol L−1
6.
Acetic acid
Detection limit or range
Detection Technique
Sample
Analyte
Table 2 – Applications involving titrations, standard additions and related strategies
Sampling rate h−1
Remarks
Ref.
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Addition/removal of components
A noteworthy feature of multi-commuted flow systems is the
feasibility of adding components to the manifold or removing
it. This aspect was initially demonstrated in the spectrophotometric determination of nitrate and nitrite in natural waters
[43] where a manually operated commuter allowed a copperized cadmium mini-column to be inserted and removed
from the analytical path. Reactor replacement is also feasible,
as demonstrated in the spectrophotometric determination
of nickel and cobalt in alloys relying on differential kinetic
analysis [44]. Manual inter-changing of two reactors with different lengths defined two different time intervals for reaction
development. Analogously, mini-columns for analyte separation/concentration can be displaced from the concentration to
the elution position. Other examples include detector relocation for parallel or serial assays requiring multi-site detection
[45,46] and displacement of a filtering device after passage of
the flowing sample [15]. In this latter situation, the retained
material is efficiently discarded by moving the filtering device
to another manifold site where a different solution trespasses
it under reversal flow conditions.
All the above-mentioned strategies are more efficiently
accomplished by resorting from multi-commutation, and
Fig. 3 outlines how a given component can be displaced by
exploiting stream-directing valves.
7.
Applications
The selected applications in Tables 1–7 do not reflect the whole
available methodology, as the number of applications is very
Table 3 – Applications involving hydride generation, electrolytic dissolution and related strategies
Analyte
Sample
Detection technique
Detection limit or range
Sampling rate h−1
Nonferrous alloys
FAAS
(a)
50
As
HG-AFS
0.05 ␮g L−1
Ba, Cu, Pb, Zn
Fish and water reference
materials
Honey
ICP-MS
Bi
Cu
Milk shake
Seawater
Fe, W, V, Mo, Cr
Ref.
[85]
10
Analytical curves based on one
multi-analyte standard solution;
electrolytic dissolution
As(III)/As(IV) speciation
30
In-line isotope dilution
[87]
HG-AFS
ET-AAS
0.2–1.24, 0.49–1.23, 0.61–2.28,
0.5–1.51%
1.67 ng g−1
5 ng L−1
72
(b)
[88]
[89]
Steel alloy
ICP-OES
(a)
30
Hg
Hg
Milk
Fish
CV-AFS
CV-AFS
0.011 ng g−1
7 mg kg−1
70
(b)
Hg
CV-AAS
5 ng L−1
44
Hg
Hg
Fish and water reference
materials
Natural waters
Agro-industrial products
In-line neutralization of waste effluent
Valves operation in synchronism with an
auto sampler
External calibration, addition of the
electrolytic solution at different manifold
sites; electrolytic dissolution
Ultrasound-assisted sample preparation
Multi-commutation implemented with
six-way rotary valves
Wide-range determination
CV-AFS
CV-AAS
1.3 ng L−1
0.8 ng l−1
63
25
[93]
[94]
Pb
High-purity Cu
ICP-MS
7–70 ␮g g−1
20
Rare earth elements, Th, U
Steel alloy
ICP-MS
(b)
Se
HG-AFS
Te
Te
Natural and drinking
waters
Milk
Milk
10-fold lower than the
batch-wise procedure
50 ng L−1
HG-AFS
HG-AFS
0.57 ng g−1
0.20 ng L−1
24
85
Tin, Ni
Brass
ET-AAS
0.001, 0.003% (w/w)
30
Argon inlet modified by commutation
Solidary multi-commutation; use of a
sliding bar commuter; ion-exchange
analyte concentration
Programmable isotope dilution;
electrolytic dissolution
Versatile flow–injection system;
electrolytic dissolution
Hydride trapping onto a tungsten
filament
Free Te(IV)/total Te speciation
Environmentally friendly procedure;
exploitation of tandem flow
Expansion of the concentration
dynamical range through valve triggering;
electrolytic dissolution
40
[86]
[90]
[91]
[92]
[71]
[95]
[96]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Al, Cu, Zn
Remarks
[97]
[98]
[99]
[74]
Symbols as in Table 1. (a) Not pertinent; (b) not informed.
7
8
Table 4 – Applications involving analyte in-line separation/concentration
Analyte
Ag, Au, Te, U
Sample
Detection technique
Detection limit or range
ICP-MS
0.82, 0.64, 2.24, 0.05 pg mL
Ammonium
Anionic surfactants
B-2, B-6 vitamins
Biological materials, food,
waters
Surface and tap water
Lake water
Multivitamin complex
UV–vis
UV–vis
Fluor.
Cationic surfactants
Natural waters
Cd
−1
Sampling rate h−1
Remarks
Ref.
21 (18 for Au)
C18 immobilized on silica
[100]
50–1000 ␮g L−1
10 ng mL−1
0.003, 0.045 ␮g mL−1
20
2
(b)
[101]
[102]
[103]
UV–vis
0.08 ␮mol L−1
(b)
Plant materials
UV–vis
0.23 mg L−1
20
Cd, Pb, Ni
Plant materials
ICP-OES
1, 4, 2 ng mL−1
90
Chlorine
UV–vis
0.05 ␮g mL−1
38
Cu
Cu
Drinking and wastewater,
bleach tablets
Plant materials
Plant materials, food
Gas diffusion into a re-circulating acceptor stream
C18 immobilized on silica; enrichment factor of 65
Pioneer use of fluorescence-based flow-through
multioptosensor in a multi-commuted flow system;
in-line analyte separation
Tandem flow for liquid–liquid extraction; use of the
film on the tubing inner wall
Minimization of interferences by electrolytic
deposition
Simultaneous operation of three cation-exchange
resin mini-columns
Gas diffusion; exploitation of tandem flow
UV–vis
FAAS
50–400 ␮g L−1
1 ng mL−1
30
48
[107]
[108]
Cu, Cd, Pb, Bi, Se(IV)
Diphenylamine
Exchangeable K
Fe
Seawater
Apple, pear
Soil
Natural waters
ICP-MS
Fluor.
Pot.
UV–vis
5, 0.2, 0.3, 0.06, 5 ng L−1
0.06 mg kg−1
6–390 mg L−1
0.1–35 mg L−1
22
(b)
50
(b)
Fe
Tap and sea waters
UV–vis
8.4 ng mL−1
22
Fuberidazole, o-phenylphenol
Furosemide, triamterene
Heavy metals
Fluor.
Fluor.
ICP-MS
0.18–6.1 ng mL−1
15, 0.1 ng mL−1
(a)
12
(b)
21
Hg
NH4 , P-PO4
Nitrate, nitrite
NO2 , NO3 , Cl, P-PO4
River and well waters
Pharmaceuticals, urine, serum
River waters, urine, liver,
muscles
Seawater
Natural waters
Soil, fertilizers
Natural waters
In-line liquid–liquid extraction
Multi-purpose flow system; use of tannin resin
mini-column
C18 immobilized on silica
C18 immobilized on silica
In-line potassium extraction
Speciation; analyte concentration onto chelating
disks; expert system
Two different concentration ranges, in-line speciation
or concentration, no manifold reconfigurations
Resin mini-column as an optrode
Sephadex mini-column
C18 immobilized on silica
ICP-MS
UV–vis
UV–vis
UV–vis
5 ng L−1
1.0, 1.0 ␮g L−1
0.19 ␮mol L−1
6, 40, 400, 30 ␮g L−1
21
40
15
50
Pb
Pb
Phenols
P-PO4
Biological materials
Plant materials
Natural wasters
Plant materials
FAAS
UV–vis
CL
UV–vis
3.7 ␮g L−1
12 ␮g L−1
5 ng mL−1
24 ␮g L−1
(b)
15
12 or 60
38
P-PO4
P-PO4
UV–vis
CL
0.02 mg L−1
4 ␮g L−1
(b)
11
Salicylamide, caffeine
Soils, sediments
Mineral, ground and tap
waters
Capsule, tablet
UV–vis
0.33, 0.15 ␮g mL−1
(b)
Salicylic acid
Sulfate
Sulfide
Pharmaceuticals
Natural waters
Sea, ground and wastewaters
CL
Turb.
UV–vis
0.30 ␮g L−1
0.1–2.0 mg L−1
1.3 ␮g L−1
Warfarin
Drinking water
Fluor.
50–64,000 ng L−1
[105]
[16]
[106]
[109]
[110]
[111]
[40]
[112]
[113]
[114]
[115]
C18 immobilized on silica
Simultaneous in-line concentration
Tandem stream for sampling; in-line photo-reduction
Optional analyte concentration and nitrate reduction;
sequential determinations
Amberlite XAD-2 resin
Liquid–liquid extraction; gaseous washing stream
Optional addition of a resin mini-column
Monosegmented flow system; different extracting
fractions
Serial extractions
Flow-through solid-phase sensor
[116]
[117]
[118]
[119]
[126]
60
50
(b)
Pioneer use of the flow-through multi-optosensor in
flow analysis
In-line separation by anion exchange
Use of an anion-exchange resin mini-column
In-line gas diffusion
12
Use of octadecyl bonded on silica gel-based beads
[130]
[120]
[121]
[122]
[123]
[124]
[125]
[127]
[128]
[129]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Symbols as in Table 1. (a) Not pertinent; (b) not informed.
[104]
Table 5 – Applications involving sample stopping
Analyte
Sample
Detection technique
Detection limit or range
Sampling rate h−1
Remarks
Ref.
Tandem streams; sample stopping for
photo-induced chemiluminescence
In-line photodegradation; sample
stopping during UV irradiation
Dual sample stopping; simultaneous
processing of two samples
Sequential determinations; sample
stopping at a LED-based detector;
tandem stream
C18 silica gel sorbent for solid-phase
detection; sample stopping at the
detector
In-line photodegradation; sample
stopping during UV irradiation
Simultaneous sample stopping inside
three parallel reaction coils
Bead injection system; solenoid valves
for management of beads
Multi-pumping flow system with two
parallel reactors
Loop-based injection; PLS application;
simultaneous determination
C18 silica gel sorbent for solid-phase
detection; sample stopping at the
detector
Tandem streams; sample stopping for
photo-induced chemiluminescence
Sample stopping at the detection unit
for native fluorescence
measurements; C18 silica gel sorbent
Extraction in chloroform; sample
stopping at the detection unit.
Tandem stream; sample stopping for
UV-irradiation
C18 silica gel sorbent for solid-phase
detection; sample stopping at the
detector
[131]
Soils
CL
5–100 mg L−1
52
Aldicarb
Mineral waters
CL
0.069 ␮g L−1
17
Amiloride hydrochloride
Pharmaceuticals
UV–vis
Up to 120 ␮g mL−1
30
Anionic and cationic surfactants
Natural waters
UV–vis
0.06 and 0.05 mg L−1
60
Azoxystrobin
Grape, must, wine
Fluor.
0.021 mg kg−1 ; 18, 8 ␮g L−1
28
Azulan
Irrigation and tap waters
CL
40 ␮g L−1
30
B
Plant materials
UV–vis
0.25–6.00 mg L−1
35
Bone alkaline phosphatase
Blood serum
UV–vis
10–1000 ␮ L−1
24
Buspirone
Pharmaceuticals
UV–vis
2.8 mg L−1
55
Cr, Co
River, coastal, harbour and
wastewaters
Pharmaceuticals, human
serum, urine
CL
0.2 ␮g L−1
(b)
Fluor.
0.00112 ␮mol L−1
38
Fluometuron pesticide
Natural waters
CL
0.1–5 mg L−1
16
Naproxen, salicylic acid
Pharmaceuticals, urine,
serum
Fluor.
0.3, 1.3 ng mL−1
8
Nicotine
Tobacco, cigarette filters
FTIR
0.05 mg mL−1
12
Strychnine
Diverse
CL
2 ␮g L−1
15
Thiabendazole
Citrus fruits
Fluor.
0.09 mg kg−1
(b)
Flufenamic acid
[132]
[32]
[29]
[133]
[134]
[62]
[135]
[136]
[137]
[138]
[139]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Acrolein
[10]
[83]
[140]
[141]
Symbols as in Table 1. (a) Not informed; (b) not pertinent.
9
10
Table 6 – Sequential/simultaneous determinations, including single analyte determinations relying on parallel reactors
Analyte
Sample
Detection technique
Detection limit or range
Sampling rate h−1
Animal whole blood
UV–vis
1.5, 14, 4 mg L−1
55, 40, 40
Al, Fe
Plant materials
UV–vis
1.0–15.0, 2.0–12.0 mg L−1
60
Albumin, total protein
Animal plasma
UV–vis
Up to 15 g L−1
45
Anionic, cationic surfactants
Natural waters
UV–vis
0.06, 0.05 mg L−1
60
Azo colorants
Carbohydrates, reducing sugars
Chloride
Foods
Forage materials
River waters
Voltam.
UV–vis
UV–vis
1.0, 3.5, 1.4 ␮mol L−1
0.2–0.8% (w/v)
0.50–10.0 mg L−1
(b)
32
25
Cr(III)/Cr(VI)
Natural waters
UV–vis
10–200 ␮g L−1
67 and 105
Cr, Co
CL
0.2 ␮g L−1
(b)
Cu
River, coastal, harbour and
wastewater
Urine, serum
FAAS
0.035 mg L−1
24
Cu, Zn
Plant materials
UV–vis
0.05, 0.04 mg L−1
45
Fe, Cu, Zn
Pharmaceuticals, alloys
UV–vis
(a)
80
Hydrosoluble vitamins
Pharmaceuticals
UV–vis
(a)
60
NH4 , P-PO4
Plant materials
UV–vis
25.0–125.0, 2.5–12.5 mg L−1
80
Nitrate, nitrite
Nitrate, nitrite
Natural waters
Lake and fountain waters
UV–vis
UV–vis
5 ␮g L−1
0.19 ␮mol L−1
55
15
NO3 , NO2 , NH4
River waters
UV–vis
5, 15, 25 ␮g L−1
60
Zn, Fe, Cu, Ca, Mg
Pharmaceuticals
UV–vis
200, 200, 50, 10, 10 ␮g L−1
60
Symbols as in Table 1. (a) Not pertinent; (b) not informed.
Ref.
Polyvalent flow system;
reagents from commercial kits
In-line adjustment of a critical
parameter (pH)
Portable systems with a LED
photometer
Sample stopping at a
LED-based detector; tandem
stream
Standard addition method
Heating of a tandem stream
Accuracy assessment relying
on two methods; optional
in-line spiking
Speciation; multi-pumping
flow system; long optical path
length
Loop-based injection; sample
stopping; PLS application
Two flow systems sharing a
single spectrophotometer
Computer-assisted splitting of
the flowing sample
Single reagent and different
masking agents
Random access reagent
selection
Random access reagent
selection
Multiple flow reversals
Tandem stream for sampling
and in-line photo-reduction
Fluid propelling by gravity;
nitrogen speciation
Fluid propelling by gravity;
simultaneous determinations
[66]
[142]
[143]
[29]
[144]
[145]
[52]
[146]
[137]
[147]
[65]
[39]
[148]
[149]
[64]
[118]
[63]
[150]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
3-Hydroxybutyrate, glucose, cholesterol
Remarks
Table 7 – Other applications exploiting multi-commutation
Analyte
Sample
Detection technique
Detection limit
or range
Sampling rate h−1
Remarks
Ref.
CL
Voltam.
2 mg L−1
17 ␮mol L−1
60
24
No prior sample treatment
Use of a modified tubular electrode
[151]
[152]
UV–vis
Fluor.
Fluor.
0.2 mmol L−1
0.5 ␮g L−1
0.04 mg L−1
22
Up to 154
60
Use of a flow-through sol–gel biosensor
Micellar enhanced luminescence
Monosegmented flow system
[153]
[154]
[155]
UV–vis
UV–vis
10–200 mg L−1
0.034 mg L−1
60
60
[156]
[157]
UV–vis
Fluor.
CL
UV–vis
CL
CL
UV–vis
CL
UV–vis
UV–vis
UV–vis
7.0 ␮g L−1
0.014 mg kg−1
25.0–100.0 ␮g L−1
0.035 mg L−1
0.06 mg L−1
3.7 mg L−1
Up to 50 ␮g L−1
2.5–60 mg L−1
3 ␮g L−1
30 ␮mol L−1
0–1 g L−1
45
(b)
72
72
25
40
15
19–32
14
60
90
Diphenamid
Pharmaceuticals
Banana
Natural waters
Natural waters
Mineral waters
Animal serum
Pharmaceuticals
Pharmaceuticals
Urine
Sweetener
Parenteral and
hemodialysis solutions
Natural waters, urine
CL
1 ␮g L−1
20
Ethanol
Ethanol
Wine
Red wine
CL
UV–vis
2.5–25% (v/v)
0.05 mol L−1
23
50
Fe, B
Soils
UV–vis
34, 15
Fluometuron
Fluor.
Folic acid
Urine, pesticide
formulations, soils,
natural waters
Pharmaceuticals
0.50–10.0,
0.20–4.0 mg L−1
0,1 mg L−1
Solenoid pumps for reagents commuting
Ion-pair formation with acetyl pyridine
ion
Use of a xylenol orange sol–gel sensor
Screening analysis
Enzymatic reaction
No prior sample treatment
In-line photodegradation
Two enzyme mini-columns
Binary sampling
Binary sampling
Use of sol–gel optotrode sensor
Multi-pumping flow system
Enzymatic reagents from a commercial
kit
Aspirating mode, in-line
photodegradation
Laboratory-made luminometer
Reagentless procedure; exploitation
surface tension
Hydrodynamic injection; multi-syringe
polyvalent system
Tandem stream; prior manual
ion-exchange
Fluor.
0.1–40.0 mg L−1
25
[173]
Glucose
Animal serum
CL
50–600 mg L−1
60
Glycerol
Wine
UV–vis
0.006 g L−1
33
Hg
Hydrogen peroxide
Karbutilate
Lactate
Lactate
Natural waters
Pharmaceuticals
Human urine
Silage materials
Sugar-cane juice
Fluor.
CL
CL
UV–vis
UV–vis
0.05–2.0 ␮g L−1
2.2–210 ␮mol L−1
10 ␮g L−1
10.0–100.0 mg L−1
5.0–100.0 mg L−1
82
200
17
16
36
Photochemical reaction assisted by
UV-irradiation; exploitation of a feedback
mechanism
GOD immobilized onto porous silica
beads
Enzyme immobilized onto aminopropyl
glass beads
Tandem streams; membrane pumps
Home-made luminometer
In-line photodegradation
Cucurbita pepo from a natural source
Exploitation of tandem stream
Acetazolamide
Al
Al
Ambroxol
Anionic surfactants
Bi
Bitertanol
Carbaryl
Cationic surfactants
Chlorsulfuron
Cholesterol
Clomipramine
Clomipramine
Cu
Cyclamate
Dextrose
33
[158]
[159]
[160]
[161]
[162]
[163]
[164]
[165]
[153]
[166]
[167]
[168]
[169]
[170]
[171]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Animal serum, plasma
Pharmaceuticals, blood
serum
Tablets, capsules
Drinking waters
Plant nutrition solution,
natural waters
Pharmaceuticals
Filtered wastewater
3-Hydroxybutyrate
Acetaminophen
[172]
[174]
[175]
11
[176]
[177]
[178]
[179]
[180]
12
Table 7 (Continued )
Analyte
Sample
Detection technique
Detection limit
or range
Sampling rate h−1
Yoghurt
CL
10–125 mg L−1
55
Methyl parathion
Mn
Nitrite
Paracetamol
Phenols
Natural waters
Plant materials
Natural waters
Tablets
Natural waters
UV–vis
UV–vis
UV–vis
UV–vis
UV–vis
50 ppt
1.2 mg L−1
25 or 8 ␮g L−1
0.4 mg L−1
1.0 ␮g L−1
(b)
50
108 or 44
60
90
Phenols
UV–vis
13 ng mL−1
65
Pindolol
Piroxicam
Spring, tap, rain and
wastewaters
Pharmaceuticals
Pharmaceuticals
UV–vis
Amp.
Up to 120 ␮g mL−1
10–400 ␮mol L−1
30
20
Propanil and related herbicides
Propanolol
Natural waters
Pharmaceuticals
CL
CL
8 ␮g L−1
20–150 mg L−1
20
27
Salicylamide, caffeine, propyphenazone
Pharmaceuticals
UV–vis
Tartaric acid
Total tannin
Trimipramine
Wine
Wine, tea
Pharmaceuticals
UV–vis
UV–vis
UV–vis
2.0–40, 0.7–15.0,
1.0–20.0 ␮g mL−1
0.50–10.0 g L−1
0.5–5.0 ␮mol L−1
1.0–18.0 ␮g mL−1
Zinc
Pharmaceuticals
UV–vis
2.0 ␮g L−1
Symbols as in Table 1. (a) Not pertinent; (b) not informed.
Ref.
Enzyme immobilized onto porous silica
beads
Flow ELISA compared to plate ELISA
Exploitation of monosegmented flow
Comparison of two analytical methods
Multi-pumping flow system
Long optical path for improving
sensitivity
Green procedure, pulsed flow
[181]
Compensation of the Schlieren effect
Solution aspirations by a single burette;
carryover minimization by sample
aspiration through an alternative path
Exploitation of tandem stream
Different strategies for sample/reagent
insertion
Simultaneous determination
[187]
[188]
28
50
26
Applicable also to red wine
Additional valve for sample replacement
Exploitation of flow reversal
[192]
[193]
[194]
16
Optical sensor incorporating PAR in a
sol–gel thin film
[195]
[182]
[59]
[183]
[184]
[185]
[186]
[189]
[190]
[191]
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Lactic acid
Remarks
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
high and tends to increase. The authors apologize for any relevant contribution omitted. The selection criteria were: the
flow system should exploit multi-commutation, but it is not
necessary that author(s) emphasize(s) this aspect; the article should be easily accessible; the method should present
superior figures of merit and should be properly validated,
although for special situations recovery data were acceptable; the concentration units are presented as provided by the
author(s).
7.1.
Sample dilutions
High and variable sample dilutions can be attained by modifying the sample injected volume and/or exploiting specific
strategies such as e.g. zone sampling or split zones [47].
Regarding high dilutions, combination of low inserted volume, implementation of tandem streams involving diluent
plugs and/or exploiting zone sampling constitute itself in a
powerful tool for attaining high dilution without impairing
analytical precision, and this is efficiently accomplished by
resorting from multi-commutation (Table 1). In this way, prior
manual dilution can be avoided.
Moreover, several multi-commuted flow systems involve
processing of a single sample in such a way that two or
more analytical signals were obtained, each one associated
to a different dilution degree. Use of a set of standard solutions permits then different analytical curves to be obtained,
each one associated with a different sensitivity. Consequently,
the dynamical concentration range of a give method is
expanded. As different analytical results are obtained for the
same sample under different dilutions, an additional accuracy
assessment is readily available [48]. Another possibility for
attaining variable sample dilution is to exploit sample recirculation inside a closed loop [49].
The feasibility of having different analytical signals per
sample opens the possibility to use a single standard solution
for e.g. obtaining the analytical curve [50], implementing the
standard addition method [51] and performing spiking [52].
7.2.
Titrations and related strategies
Two kinds of titrations have been proposed, the first one
relying on the recorded peak width measurement [47]. This
procedure has been named as pseudotitration, as it does not
comply with IUPAC definitions of titration. The related analyzer is simple, robust and usually allows titrations to be
carried out at a very high sampling rate. However, a previous calibration step involving analyte standard solutions
is required. As a rule, multi-commutation has been less
exploited in relation to these pseudotitrations. Pseudotritrations may also exploit zone sampling and related techniques
relying on concentration gradients, which are accountable
for selection of the different titrant aliquots with known
concentrations to be added to the assayed sample [50]. Multicommutation has proved to be a very important tool in this
regard, as the sample and titrant solutions should be intensively handled.
Other kind of flow titration is the true titration that relies
on the analyte/titrant stoichiometry. In this way, a prior
calibration step is not required. A unique feature of the multi-
13
commuted flow systems is their ability to perform true flow
titrations. The standard solution to be managed is the titrant
and strategies for end-point determinations are applicable.
Prior system calibration is then not required. As a rule, several plugs of the titrant are precisely defined and added to the
sample, and exploitation of tandem streams is important in
the context [33,53].
Here, it should be emphasized that flow-batch systems
have been often used in relation to titration techniques [54]
relying on addition of specific titrant aliquots to the samples. These systems include a reaction chamber and present
the favorable analytical characteristics of both flow and batch
analyses. The flow-batch systems have been also used in specific analytical applications involving feedback mechanisms
for individual sample conditioning. This permits the easy
implementation of the standard addition method [55], of titration procedures relying on titrant generation [56], of individual
sample conditioning [31], etc., to be efficiently accomplished.
As different solutions are managed inside the main chamber,
exploitation of the concept of multi-commutation is worthwhile.
Analogously to titrations, other strategies such as e.g. the
standard addition method require additions of known aliquots
to the assayed samples. Therefore, applications involving
these strategies are also included in Table 2.
7.3.
Analyte separation/concentration
Intensive sample handling is inherent to in-line analyte separation concentration, and this holds also in relation to
flow-based analytical procedures involving the formation of
gaseous species. To this end, discretely operated solenoid
valves are used to permit the efficient redevelopment of the
steps of gas formation, release, collecting and monitoring. It
is important to note that several applications in Table 3 rely
on hydride generation with atomic spectrometric detection.
Another relevant separation technique involves the sample electrolytic dissolution. The solid sample (to date, alloys) is
placed on the electrolytic dissolution chamber and submitted
to a controlled direct current that dissolves a reproducible analyte amount to be in-line handled [57]. In this way, metal alloys
can be analysed without prior treatment. The innovation however, requires intensive sample manipulations and has been
better implemented in multi-commuted flow systems.
Another possibility for in-line analyte separation/
concentration relies on use of mini-columns of different
materials such as “e.g. ion-exchange resins, C18 immobilized
on silica, polyurethane foams, PTFE turnings, extracting disks,
monolithic columns and molecularly imprinted polymers”
[58]. A pre-selected sample volume is allowed to pass through
the mini-column and the species of interest are retained;
after optional column washing, the retained species are
eluted towards detection. Again, multi-commutation is very
attractive in the context. Analyses of Table 4 reveals the dominance of detection techniques involving chemiluminescence,
fluorescence and similar. In fact, the number of applications
relying on UV–vis spectrophotometry or electroanalytical
techniques is scarce, perhaps in view of the unavoidable
formation of undesirable concentration gradients that might
impair proper sample monitoring.
14
7.4.
a n a l y t i c a c h i m i c a a c t a 6 1 8 ( 2 0 0 8 ) 1–17
Sample stopping
The processed sample is halted either inside the main reactor or at the detection unit, thus long residence times are
efficiently attained without increased sample dispersion. The
approach was initially referred to as stopped-flow [21], and
has been often exploited in relation to analytical procedures
involving relatively slow chemical reactions [59] or requiring
longer time intervals for proper detection [60].
Sample stopping can be accomplishing either by stream
re-directing or by pump stopping. The STOP period is usually
set before the analyses of a given sample lot; therefore all the
assayed samples are generally subjected to the same analytical conditions. Implementation is then better accomplished
by rersorting from multi-commutation. Consequently, especial strategies such as e.g. dual sample stopping [32], selection
of different STOP periods for widen the concentration dynamical range [61], sample stopping inside parallel channels for
improving sampling rate and/or sensitivity [62] can also be
efficiently accomplished (Table 5).
7.5.
Sequential/simultaneous determinations
Flow manifolds for accomplishing sequential or simultaneous
determinations are normally more complex, and exploitation
of multi-commutation is beneficial in the context. Different
strategies have been proposed (Table 6), most of them involving more than one sample insertion. In this way, the originated
sample zones are handled under different conditions, and the
different analytes are sequentially monitored.
The most often used strategy is to replace the reagent
merging streams according to the aimed determinations, and
the random access reagent selection [14] is beneficial in this
regard. Other possibilities involve multi-site detection [46],
exploitation of concentration gradients [51], sandwich techniques [23], addition of a discriminating reagent for speciation
[63], flow reversals [64], stream splitting [65], etc.
Design of polyvalent systems exploiting multicommutation is also feasible [66]. Different analytes can
be determined simply by varying the reagent constitution
and eventually the detector operating conditions (usually
wavelength setting). These systems are useful for industrial
purposes, as no modifications in the manifold architecture
are needed.
7.6.
Miscellaneous
Multi-commutation has also been exploited in specific situations, in order to improve system performance, analytical
figures of merit and methodological enhancement. Some of
these situations are highlighted in Table 7.
8.
Conclusions
The number of applications of multi-commuted flow-based
analytical procedures is increasingly and this aspect has
becoming more pronounced during last years [11]. A literature survey of the recent published innovations reveals also
the increased number of articles reporting novel flow systems
with a high degree of automation.
The aspect is probably due to the high versatility inherent to the multi-commuted flow systems as well as the easy
implementation of concentration-oriented feedback mechanisms. These mechanisms have been more evident in relation
to expert flow systems, especially those designed for titrations.
The flowing streams as well as some manifold components
are often added or removed from the manifold; therefore,
transient variations in baseline may occur. Especial attention
should then be given during the monitoring step, as undesirable concentration gradients may impair detection, especially
in relation to potentiometric or spectrophotometric (Schlieren
effect) analytical procedures.
Time-based injection has been preferred in multicommuted flow system, and it is difficult to understand this
preference. The first replication handling is usually different
relatively to the other ones, causing a different system conditioning. This is perhaps the reason why the first analytical
signal is sometimes different from the other ones recorded for
the same sample. This limitation, as well as those related to
carryover effects and sampling rate becomes less relevant in
relation to loop-based sample insertion.
The multi-commuted concept has been recognized as relevant in the field of flow analysis, and this can be verified
by checking the referenced quotations in international data
bases. This recognition will certainly increase by incorporating
the concept in the commercially available flow analyzers.
Acknowledgements
Partial support from FAPESP (proc. 06/03859-9, 06/03718-6,
06/07309-3) and from a bi-national scientifical consortium
(CAPES/GRICES) is greatly appreciated.
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