**3. On-line stripping analysis**

Flow analysis methodologies are based on the measuring of a transient non-steady signal, allowing a high sampling rate without the need for segmentation to limit analyte dispersion. This concept has simplified the measuring systems, and has resulted in a rapid increase in the interest of these techniques (Ruzicka & Hansen, 1988).

Continuous flow methodologies are the common approach for analysis by flow systems coupled to stripping analysis. This coupling mode uses a selection valve (SV), different streams of solutions are selected by the valve and pumped unidirectionally through the electrochemical flow cell for electrode modification/conditioning, pre-concentration and stripping (Fig.2). On-line medium exchange is an alternative to minimize the interference of the analytical matrix. The continuous flow mode is based on simple instrumentation, but its greatest drawback is the high consumption of sample and reagent solutions (Muñoz & Palmero, 2004).

Fig. 2. Continuous flow system. S, sample; CS, conditioning solution; SS, stripping solution; PP, peristaltic pump; SV, selection valve; D, electrochemical flow cell; W, waste.

Flow injection analysis (FIA) is based on the injection of a known amount of sample in a flowing carrier solution stream via an injection valve (IV); the flowing carrier transports the sample to the detector. The main advantages of FIA compared with continuous flow systems are the operational simplicity and the lower consumption of sample and carrier (Bryce et al., 1995). However, the reduction in the amount of analyte deposited on the electrode surface as a result of the decrease in the contact time between the sample and the working electrode and the dispersion of the sample in the flow manifold results in a decrease of the analytical signal. A typical FIA system is illustrated in Fig. 3.

Sequential injection analysis (SIA, Fig. 4.) is other flow methodology that has been coupled to on-line stripping analysis. The heart of SIA manifold is the multiport selection valve; solutions are aspired and transported as zones using a bidirectional pump. SIA advantages are the low consumption of sample and reagents, the flexibility and the potential for automated sample manipulation (Ivaska & Kubiak, 1997). The sample volumes used in SIA are smaller than those employed for continuous flow systems and FIA, the amount of analyte deposited is lower, thus yielding a decreased signal.

Flow analysis methodologies are based on the measuring of a transient non-steady signal, allowing a high sampling rate without the need for segmentation to limit analyte dispersion. This concept has simplified the measuring systems, and has resulted in a rapid increase in

Continuous flow methodologies are the common approach for analysis by flow systems coupled to stripping analysis. This coupling mode uses a selection valve (SV), different streams of solutions are selected by the valve and pumped unidirectionally through the electrochemical flow cell for electrode modification/conditioning, pre-concentration and stripping (Fig.2). On-line medium exchange is an alternative to minimize the interference of the analytical matrix. The continuous flow mode is based on simple instrumentation, but its greatest drawback is the high consumption of sample and reagent solutions (Muñoz &

**SV**

**W**

**W**

Fig. 2. Continuous flow system. S, sample; CS, conditioning solution; SS, stripping solution;

Flow injection analysis (FIA) is based on the injection of a known amount of sample in a flowing carrier solution stream via an injection valve (IV); the flowing carrier transports the sample to the detector. The main advantages of FIA compared with continuous flow systems are the operational simplicity and the lower consumption of sample and carrier (Bryce et al., 1995). However, the reduction in the amount of analyte deposited on the electrode surface as a result of the decrease in the contact time between the sample and the working electrode and the dispersion of the sample in the flow manifold results in a

Sequential injection analysis (SIA, Fig. 4.) is other flow methodology that has been coupled to on-line stripping analysis. The heart of SIA manifold is the multiport selection valve; solutions are aspired and transported as zones using a bidirectional pump. SIA advantages are the low consumption of sample and reagents, the flexibility and the potential for automated sample manipulation (Ivaska & Kubiak, 1997). The sample volumes used in SIA are smaller than those employed for continuous flow systems and FIA, the amount of

PP, peristaltic pump; SV, selection valve; D, electrochemical flow cell; W, waste.

decrease of the analytical signal. A typical FIA system is illustrated in Fig. 3.

analyte deposited is lower, thus yielding a decreased signal.

**D**

**Position 2**

**SV**

**3. On-line stripping analysis** 

Palmero, 2004).

**PP**

**S CS SS**

the interest of these techniques (Ruzicka & Hansen, 1988).

**D**

**Position 1**

Fig. 3. Flow injection analysis system. a) insertion of S into CS, b) dispersion phenomena. S, sample; CS, carrier solution; PP, peristaltic pump; IV, injection valve; R, reactor; D, electrochemical flow cell; W, waste.

Fig. 4. Sequential injection analysis system. a) sample and reagents aspiration, b) mixture dispense. CS, carrier solution; LR, loading reactor; SV, selection valve; S, sample; R1 and R2, reagents; RR, reaction coil; D, electrochemical flow cell; W, waste.

Another critical part of flow methods is the detector (electrochemical flow cell). An electrochemical detector uses the electrochemical properties of analytes for determination in the flowing stream. Electrochemical detection is usually performed by controlling the potential of the working electrode at a fixed value and monitoring the current as a function

Sequential Injection Anodic Stripping Voltammetry

(Kopanica & Novotny, 1998).

CS

R1

MC

SV

at Tubular Gold Electrodes for Inorganic Arsenic Speciation 209

L-cysteine 1x10-3 M (HOOC-CH(NH2)-CH2-SH, Sigma) was used as reducing agent. A

Figure 6 shows a scheme of the sequential injection anodic stripping voltammetry system (SI-ASV) used for inorganic arsenic speciation in water samples. The system consisted of a MicroBu 2030 multisyringe burette with programmable speed (CS, Crison, Spain) used to aspire and dispense the reagent solutions, an eight-way selection valve (SV, Crison), a

The tubular electrochemical detection cell was made up of a Perspex body in which working and auxiliary electrodes were placed. These electrodes were built from gold and carbon discs (7.0 mm diameter) with length of 1.0 and 2.0 mm, respectively. Both have a tubular channel (0.8 mm diameter) in the centre of the electrode. These electrodes were used in connection with a saturated Ag/AgCl reference electrode (Metrohm, Switzerland). The instrumental devices were controlled by means of the Autoanalysis 5.0 software (Sciware, Spain). All tubing connecting the different components of the flow system was made of Omnifit PTFE with 0.8 mm (i.d.). Electrochemical experiments were performed with an Autolab PGSTAT10 potentiostat/galvanostat (EcoChemie) equipped with GPES 4.6 software. Unless otherwise stated, a frequency (*f*) of 25 Hz, pulse amplitude (Esw) of 50 mV, step height (ΔEs) of 8 mV, and deposition potential (Ed) and time (td) of −0.4 V for 40 s were chosen as the square wave anodic stripping voltammetry (SWASV) parameters. A conditioning potential and time (2 s at 2.0 V) was added to increase the reproducibility

S

**D**

<sup>b</sup> <sup>c</sup>

Fig. 6. Schematic set up of the SI-ASV flow system: CS, carrier solution; R1, holding coil; R2, reaction coil; SV, selection valve; R, reductant ; S, sample; MC, mixer chamber; D, detector; W, waste. Components of the electrochemical cell: a, reference electrode; b, tubular gold

R2

f

electrode; c, glassy carbon counter electrode; d, connector, e, O-ring.

Tubular electrochemical

Flow

cell

R

**W**

a

d e

supporting electrolyte solution of 2.0 M HCl was used for all the experiments.

home-made tubular electrochemical cell (D), and a mixer chamber (MC).

of time. The current response thus generated is proportional to the concentration of the analyte. During on-line stripping analysis the analyte is pre-concentrated on the surface electrode in flowing conditions, whilst the stripping step can be done in flowing conditions or stopped flow (Economou, 2010).

Different cell designs have been used for electrochemical detection. The cell design must fulfil the requirements of high signal-to-noise ratio, low dead volume, well defined hydrodynamics, small ohmnic drop, high contact area and easy of construction and maintenance. In addition, the reference and counter electrodes should be located downstream next to the working electrode, so that reaction products at counter electrode or leakage from the reference electrode do not interfere with the working electrode detection.

The most widely used detectors are based on the wall-jet, thin-layer, and tubular configurations (Fig. 5.) (Trojanowicz, 2009):

Fig. 5. Schematic representation of front and side view of electrochemical flow cells configurations. a) thin-layer, b) wall-jet and c) tubular. WE, working electrode.

In the wall-jet design, the stream flows perpendicularly to the working electrode surface, and then spreads radially over it improving the contact between the analyte and the electrode. The thin-layer cell consists on a thin layer of solution that flows parallel to the planar electrode surface, the main disadvantage is the small contact area. Tubular configuration provides minimal flow disturbance and a higher contact area, compared with thin-layer configuration. This feature has enabled the application of tubular configuration in flow injection systems with sequential determination in which the detector is relocated inside the flow manifold (Catarino et al., 2002).
