## 3 Flow-based optical detection
### Spectrophotometry  
Spectrophotometry is widely used as the detection technique for the flow injection, sequential injection, MCFIA, MSFIA, and SI-LOV for the on-line determination of either inorganic or organic species in a wide range of applications. For example, spectrophotometry based on the Griess-Saltzman reaction was applied to determine nitrile and nitrate concentrations [74],[75] or flow injection/stopped flow coupled to kinetic catalytic spectrophotometry [76]. For the kinetic catalytic spectrophotometry, the catalytic reaction was utilized to improve the sensitivity and capability for the trace or ultratrace level determinations. [PERSON], et. al. [77] have presented the determination of copper and iron in river water, tap water, and natural water using the oxidation coupling of p-anisidine with N,N-dimethylaniline in the presence of hydrogen peroxide. [PERSON] et. al. [78] have proposed the method using the acceleration effect of Se(IV) on the reaction of etheenediamanine tetraacetic acid disodium salt and sodium nitrite with ammonium iron (II) sulfate hexahydrate in acidic media for the determination of trace amounts Se(IV) in seawater. Moreover, the total and dissolved amounts of iron based on the catalytic reaction of Fe(III) and Fe(II) in the oxidation of N,N-dimethyl-p-phenylenediamine in the presence of hydrogen peroxide was described by [PERSON] [79]. The LOD and LOQ were 0.02 and 0.06 \(\upmu\)g L\({}^{\text{-1}}\), respectively. The combination of kinetic catalytic reactions with stopped-flow is also suitable because it allows the rapid and automatic mixing of sample and reagents and a short time for obtaining measurements. [PERSON], et. al. [80] have employed the procedure based on the reaction of nitrite with perphenazine using kinetic spectrophotometry coupled to stopped-flow technique for the determination of nitrite in natural and drinking water. Another interesting flow-based technique related to spectrometry is absorptiometry[81; 82]. For example, silicic acid in highly purified water was successful determined by [PERSON] et al [83] using on-line and gel-phase absorptiometry with molybdate and malachite green. This group also used ion-exchanger phase absorptiometry combined with flow system to determine trace amounts of iron in highly purified water[84]. Flow analysis and solid phase absorptiometry was also used to analyzed micro amounts of chromium(III) and (IV) in natural water[85]. Sequential injection is another system that is widely used. The conjugation of sequential injection with spectrophotometry has been used for many applications. [PERSON] and colleagues have published the determination of speciation of Mn(II) and Mn(VII) using on-line spectrophotometry sequential injection analysis [86]. The method is based on the reaction between Mn(II) with 4-(2-pyridylazo) resorcinol (PAR) to form a red complex in alkaline media. In addition, sequential injection has been applied for the titration for the determination of a number of analytes. [PERSON] and [PERSON] [87] have purposed this method for the determination hydrogencarbonate by acid-base titration using sequential injection. The results were agreement with the potentiometric titration. Sequential injection based on acid-base titrations was also applied to evaluate the acidity of fruit juices using sodium hydroxide and phenolphthalein [88]. Monolithic column is the new tool for the separation. The incorporation of a monolithic column into a SIA system or SIC system with spectrophotometry has been introduced for online sample pretreatment. A monolithic column was utilized for separation and determination of some pharmaceutical components including ambroxol hydrochloride, methylparaben, and benzoic acid [89]. In the past few years, a number of studies focused on the use of MCFIA and MSFIA for the on-line determination of various samples. [PERSON] et. al. [90] have developed the method for determination of trimipramine in pharmaceutical preparations by a multicommutated reverse-flow system.