## 2 Flow-based electrochemical detection
### Amperometric detection  
In this technique, a constant potential is applied at the working electrode, then the oxidation or reduction reaction of an analyte takes place. A three-electrode system is required for conventional amperometric detection. Metal working electrodes such as Au, Ag, and Pt can be used in this technique. The current responses are directly proportion to the concentration of the analyte. A major drawback of amperometry is the lack of reproducibility due to some adsorption of by-products and/or impurities on the electrode surface (electrode fouling). To obtain reproducible results during analysis, the electrode surface needs to be cleaned frequently by either mechanical polishing or an electrochemical cleaning procedure. FIA with amperometric detectors has been widely used in pharmaceutical applications including food and drink analysis. The main advantages are the low consumption of reagents and samples, better repeatability, high sample throughput, reduction of the risk of contamination during the analysis step, and the relatively low-cost of the instrumentation. Figure 1: Schematic diagram of flow system: A) Segmented flow analysis (SFA), B) Flow injection analysis (FIA), C) Sequential injection analysis (SIA), D) Multisyringe flow analysis (MSFIA), E) Multipumped flow injection analysis (MPFIA), and F) Multicommutated flow injection analysis (MCFIA): PP = peristaltic pump; A = Air; S = Sample; R = Reagent; MC = Mixing Coil; DB = Debubbler; D = Detector; W = Waste; CA = Carrier; SP = Syringe Pump; HC = Holding Coil; MV = Multi-Ports Valve; RC = Reaction Coil; MSP = Multisyringe Pump; SV = Solenoid Valve; CF = Control Interface; MP = Micro Pump. As mentioned above, the main disadvantage of amperometry is a deposition of the by product or solution impurities on the electrode surface. Another technique, pulsed amperometric detection (PAD), has been introduced to overcome that kind of problem. PAD offers the possibility of cleaning and reactivating the electrode surface effectively after the measuring cycle without mechanical polishing. In the simplest implementation of PAD, the potential of the working electrode is stepped between the potentials for detection, \(E_{\text{det}}\); cleaning, \(E_{\text{exd;}}\) and reactivation, \(E_{\text{red}}\). All three steps of PAD require the following: (a) the oxidation of analyte during the detection step; (b) the oxidative desorption of adsorbed detection products or solution impurities during the cleaning step; and (c) the cathodically dissolution of inert oxide product during reactivation step. Pulsed amperometric detection has been used for the sensitive detection of numerous compounds such as tetracycline [13] and chlortetracycline [14]. Another interesting system is a gas-diffusion flow injection (GD-FI) which can be used for the determination of trace levels of Hg (II) in aqueous samples [15]. The analytical procedure was carried out by injection of Hg(II) samples and standards into an acid stream which was subsequently merged with a reagent stream of SnCl\({}_{2}\) to reduce Hg(II) to metallic mercury. The mercury vapor diffused across a Teflon membrane into an acceptor stream, and then it was amperometrically detected at a gold electrode. This system was successfully applied for the determination of mercury in freshwater samples. Combination of the FIA system and immobilized cell reactor can be used for dopamine determination. PC 12 cells were immobilized on to the lumen of fused silica microbore tubing. At the optimized conditions, the cells could be attached to the poly-L-lysine layer in approximately 2 h, after which they could be maintained inside of the tubing for a period of up to 5 days. A release of dopamine by Ca\({}^{2+}\)-induced can be amperometrically detected at a Nafion\({}^{\circledR}\) modified platinum ultramicro electrode [16]. Non-metallic electrodes such as carbon paste, glassy carbon, and boron-doped diamond (BDD) electrodes can also be used as well.