## 3 Flow-based optical detection
### Chemiluminescence  
Chemiluminescence (CL) detection plays an importance role for flow injection systems due to its high sensitivity and selectivity. When chemiluminescence was coupled to flow injection systems, the obtained results were reproducible. Moreover, this method was cheap, rapid, and simple. In recent work [PERSON] et. al. [122] studied flow injection chemiluminescence for the determination of carbaryl in vegetables and natural water. The enhancement in CL intensity was linear for carbaryl over the concentration range of 5-100 ng mL-1 with a detection limit of 4.9 ng mL-1. [PERSON] and [PERSON] [123] also presented the method for the determination of nitrite at the femtogram level using flow injection chemiluminescence based on luminol-myoglobin catalyzed by nitrite. Linear enhancement of the chemiluminescence intensity was observed in the presence of concentrated of nitrite ranged from 0.05 pg mL-1 to 10 ng mL-1 and had a detection limit of 20 fg mL-1. Moreover, [PERSON] [124] described the CL of the peroxyoxalate reaction for the detection of gentamicin in pharmaceutical formulation. In addition, [PERSON] and coworkers [125] reported the use of the tris(2,2'-bipyridyl)ruthenium(III)-NADH system to increase the CL signal for the determination of thyroxine. A limit of detection of 1.0x10-10 was achieved. Specific detection of some compounds can be achieved by enzymatic chemiluminescence methods coupled to flow injection analysis. [PERSON] and [PERSON] [126] presented the micro-flow injection horseradish peroxidase-catalyzed chemiluminescence method for the determination of hydrogen peroxide. The detection limit of hydrogen peroxide using ethyleneurea (100 mmol L-1) was 0.6 pmol per injection and is still sufficient for the determination of hydrogen peroxide in clinical laboratories. In recent years, a sequential injection analysis using micro-magnetic beads was introduced for many immunoassay studies. [PERSON], et. al. [127] developed a sensitive immunoassay method for the determination of vittellogenin. The system involved a sequential injection analysis system coupled with a chemiluminescence detector and a samarin-cobalt magnet. The micro-magnetic bead and sequential injection was also used for the determination of anionic surfactants [128]. A schematic diagram of the sequential injection system using magnetic microbeads is shown in figure 5. Moreover, [PERSON] et. al. have also utilized the sequential injection-ELISA based system for online determination of hyaluron [129]. To design portable and low cost equipment, a flow injection system consisting of a set of solenoid micropumps coupled to a chemiluminescence detector was developed by [PERSON], et. al. [130]. The system was applied for the determination of hydrogen peroxide using the oxidation of luminal. The linear range of 0.1--80 mmol L-1 with a detection limit of 400 nmol L-1 was achieved. [PERSON] et. al. also developed a micro-flow injection [131] coupled to chemiluminescence using a micropump. The luminol/hexacyanoferrate/hydrogen peroxide CL reaction was used as the model for testing the performance of the system. A schematic diagram of the microfluidic flow injection system is shown in figure 6. The results provided a linear range of 0.2-1.4 mM. To facilitate the automation system, a multicommutated flow system was coupled to chemiluminescence detection for the determination of clomipromine [132]. This method is based on the CL oxidation of sulphite by Ce(IV) and sensitized by clomipramine using the single pumping channel. The method is simple, fast, precise, and suitable for the routine analysis of clomipramine in bulk drug and pharmaceutical preparations. Figure 5: Schematic flow diagram of the sequential injection system using magnetic microbeads. [PERSON], et. al. [133] proposed the multisyringe flow injection chemiluminescence for the determination of ultratrace glucose in sugar. This method is well suited for the enzymatic-based CL methodologies in a homogeneous (renewable) solution.