**2. Arsenic speciation**

In-house laboratory assays are generally required to accurately measure arsenic in environmental samples at the µg l-1 level in waters. The preferred laboratory methods for measurement of arsenic involve sample pre-treatment, either with acid addition or acidic digestion of the sample. Pre-treatment transfers all the arsenic in the sample into an arsenic acid solution, which is subsequently measured using techniques such as graphite furnace atomic absorption spectroscopy (ETAAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS), high performance liquid chromatography (HPLC) coupled to ICP-MS, X-ray fluorescence (XRF), neutron activation analysis (NAA) and capillary electrophoresis (CE) (B'Hymer & Caruso, 2004; Burguera & Burguera, 1997; EPA, 1999; Gong et al., 2002; Melamed, 2005).

The most commonly used speciation techniques often involve a combination of chromatographic separation with spectroscopic detection. HPLC is the most used in the ionpairing and ion exchange modes (B'Hymer & Caruso, 2004; Gong et al., 2002). Such techniques are expensive to operate and maintain and require fully equipped and staffed laboratories.

Anion- and cation-pairing chromatography techniques have been developed for separation of arsenic species. Tetrabutylammonium is the common pairing cation for separating As(III) and As(V) using reverse phase columns for the separation. The resolution depends on the concentration of ion-pair reagent, the flow rate, ionic strength and pH of the mobile phase (Guerin et al., 1999). Anion-exchange chromatographic techniques have been used for inorganic arsenic speciation analysis. A gradient elution using ammonium phosphate as mobile phase allows the resolution of As(III) and As(V) from organoarsenic species (Terlecka, 2005). Speciation of trace levels of arsenic in environmental samples requires high sensitivity, then the use of HPLC-MS with electrospray ionization, or HPLC-ICP-MS are often needed (B'Hymer & Caruso, 2004).

On the other hand, electrochemical assays, in particular stripping analysis, have demonstrated to be useful for detection of arsenic traces in water samples. Cathodic stripping voltammetry (CSV) or adsorptive cathodic stripping voltammetry (AdCSV) using hanging mercury drop electrodes (HMDE) was used in the past for arsenic analysis (Ferreira & Barros, 2002; Sadana, 1983). In the last years the analytical use of mercury has been discouraged due to its toxicity. Different materials have been reported for the determination of arsenic, including platinum (Williams & Johnson, 1992), gold (Forsberg et al, 1975), bismuth (Long & Nagaosa, 2008), carbon substrates (Sun et al., 1997) and boron doped diamond (Ivandini, et al. 2006).

analogue of phosphate and inhibits oxidative phosphorylation, the main energy generation system. As(V) is most frequently present in surface water while As(III) is commonly found in anaerobic groundwaters. Redox potential, pH and organic matter control the species

To determine the potential transformation and risk of arsenic in the environment, the analysis of arsenic should include identifying and quantifying both, the total quantity of arsenic present and the specific chemical forms, a procedure known as speciation (Bednar et

In-house laboratory assays are generally required to accurately measure arsenic in environmental samples at the µg l-1 level in waters. The preferred laboratory methods for measurement of arsenic involve sample pre-treatment, either with acid addition or acidic digestion of the sample. Pre-treatment transfers all the arsenic in the sample into an arsenic acid solution, which is subsequently measured using techniques such as graphite furnace atomic absorption spectroscopy (ETAAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS), high performance liquid chromatography (HPLC) coupled to ICP-MS, X-ray fluorescence (XRF), neutron activation analysis (NAA) and capillary electrophoresis (CE) (B'Hymer & Caruso, 2004; Burguera & Burguera, 1997; EPA,

The most commonly used speciation techniques often involve a combination of chromatographic separation with spectroscopic detection. HPLC is the most used in the ionpairing and ion exchange modes (B'Hymer & Caruso, 2004; Gong et al., 2002). Such techniques are expensive to operate and maintain and require fully equipped and staffed laboratories.

Anion- and cation-pairing chromatography techniques have been developed for separation of arsenic species. Tetrabutylammonium is the common pairing cation for separating As(III) and As(V) using reverse phase columns for the separation. The resolution depends on the concentration of ion-pair reagent, the flow rate, ionic strength and pH of the mobile phase (Guerin et al., 1999). Anion-exchange chromatographic techniques have been used for inorganic arsenic speciation analysis. A gradient elution using ammonium phosphate as mobile phase allows the resolution of As(III) and As(V) from organoarsenic species (Terlecka, 2005). Speciation of trace levels of arsenic in environmental samples requires high sensitivity, then the use of HPLC-MS with electrospray ionization, or HPLC-ICP-MS are

On the other hand, electrochemical assays, in particular stripping analysis, have demonstrated to be useful for detection of arsenic traces in water samples. Cathodic stripping voltammetry (CSV) or adsorptive cathodic stripping voltammetry (AdCSV) using hanging mercury drop electrodes (HMDE) was used in the past for arsenic analysis (Ferreira & Barros, 2002; Sadana, 1983). In the last years the analytical use of mercury has been discouraged due to its toxicity. Different materials have been reported for the determination of arsenic, including platinum (Williams & Johnson, 1992), gold (Forsberg et al, 1975), bismuth (Long & Nagaosa, 2008), carbon substrates (Sun et al., 1997) and boron doped

al., 2004; Burguera & Burguera, 1997; Gong et al., 2002).

present in water.

**2. Arsenic speciation** 

1999; Gong et al., 2002; Melamed, 2005).

often needed (B'Hymer & Caruso, 2004).

diamond (Ivandini, et al. 2006).

Anodic stripping voltammetry (ASV) provides an alternative technique for measuring inorganic arsenic in water samples. ASV at gold film electrodes (Sun et al. 1997) or solid gold electrodes (Kopanica & Novotny, 1998) have been extensively used for inorganic arsenic speciation as they allow to determine separately As(III) and total As. The analysis by ASV involves three major steps (Figure 1). First, the electrode surface is conditioned for analysis (cleaning the surface of the solid electrode and/or plating a gold film). The As(III) is then deposited as elemental arsenic on the working gold electrode by electrochemical reduction. After the deposition step, the elemental arsenic is electrochemically oxidized (stripped) back to As(III). As(V), the most stable form of the element in oxidizing environments, is determined after chemical or electrochemical (Muñoz & Palmero, 2005) reduction to As(III), total As is then determined and As(V) is calculated by difference between total As and As(III). The Environmental Protection Agency (EPA) has approved an analytical method (EPA, 1999) for arsenic determination in water samples based on the use of ASV at gold film electrodes.

Fig. 1. Anodic stripping voltammetry: the potential-time waveform with the resulting voltammogram.

The remarkable sensitivity, broad scope and low cost of stripping analysis have led to its application in the determination of arsenic in water, soils and food samples. From early years of stripping analysis two main different research areas have been considered. The use of microelectrodes and disposable electrodes (Gibbon et al., 2010), and the development of hyphenated techniques using flow manifolds (Economou, 2010). On-line stripping analysis using flow analysis has demonstrated the viability and potentialities of this coupling such as: a) lower consumption of sample and reagents, b) higher precision and accuracy and c) higher degree of automation.

Sequential Injection Anodic Stripping Voltammetry

**CS**

electrochemical flow cell; W, waste.

**CS**

**S**

**PP**

at Tubular Gold Electrodes for Inorganic Arsenic Speciation 207

**IV**

**R**

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,

**LR**

**(a)**

reagents; RR, reaction coil; D, electrochemical flow cell; W, waste.

**CS S R1**

**D W**

**D**

**W**

**R1**

**R2**

**S**

**SV**

**RR**

**(b)**

**CS S R1**

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,

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

CS

**(a)**

<sup>3</sup> <sup>4</sup>

L

D

<sup>5</sup> <sup>1</sup>

S

6

W

2

**(b)**
