## 3 Flow-based optical detection  
### Atomic absorption spectrometry (AAS)  
Due to the short analysis time and feasibility for on-line system, flow-based systems have become one of the most power analytical tools in recent years. Various types of detection modes are applied to flow analytical techniques, and atomic adsorption spectroscopy is one that is most commonly used sensors. Flow analytical techniques coupled with AAS comprise those involving flow separation and/or preconcentration.  
For the ideal of on-line preconcentration, [PERSON] and coworkers [57] have described an on-line flow injection preconcentration-electrothermal AAS for the determination of traces of cobalt in drinking water. The minicolumn packed with activated carbon was used as the sorbent. In this approach, an enrichment factor of 190 for a sample volume of 10 mL was obtained using 10% (v/v) nitric acid as an eluent. The detection limit for the proposed method was as low as 5 ng/L. In addition, [PERSON] and coworkers [58], have also recently used this kind of material for on-line sorption trace speciation of chromium in drinking water before ETAAS. An enrichment factor of 35 was obtained for the same conditions as the previous work. This improvement can be attributed to the low detection limit of 3 ng/L.  
A flow injection on-line preconcentration-electrothermal AAS coupled with a co-precipitation method for the determination of lead in seawater was developed by [PERSON] and colleagues [59]. The preconcentration procedure was comprised of coprecipitatiion with iron (III) hydroxide and solid phase extraction with a lead-selective resin. This method allowed the determination of lead at the ng/kg level. For the flow-injection manifold of this method has been shown in figure 3. [PERSON] et. al. [60] have also described the on-line preconcentration technique for the determination of silver using a flow system incorporating an immobilized DDTC microcolumn coupled with flame AAS. The immobilization of DDTC on surfactant-coated alumina offers a good deposition of silver that permits effective on-line preconcentration for determination of silver at \(\mu\)g/L level in different matrices. The enrichment factor of 125 was obtained and the precision at 20 \(\mu\)g/L was 4%. For determination of zinc in seawater, [PERSON] and coworkers [61] have presented a field sampling preconcentration method. The sample was pumped into a minicolumn packed with a chelating resin to enrich the zinc. Then, the minicolumn was inserted into the flow injection system for on-line zinc determination by FAAS. This system showed great advantages over the other existing methods for zinc preconcentration such as analysis time are reduced because several samples can be preconcentrated at the same time.  
Recently, the use of SI-LOV became more popular because of the renewable surface. The advantage of this technique over the previous works is that it has no adsorption of interferences on the sorbent surface. [PERSON] et. al. have described the sequential injection lab-on-valve coupling to the hydrophobic materials prior to ETAAS detection for the determination of cadmium [62]. C\({}_{18}\)-PS/DVB beads were used to achieve the goal for a renewable surface because of the homogeneity in bead size, and the perfect shape results in improving in the reproducibility compared to the PTFE beads (3.4% via 4.3%). The potential of the renewable concept of SIA-LOV lead to future use of the C\({}_{18}\)-polymeric bead. Moreover, octadecyl immobilized silica surface renewable columns in lab-on-valve have beenalso used to enrich the trace cadmium and coupled to an electrothermal atomic absorption spectrometry [63].