## 3 Flow-based optical detection
### ICP-AES, ICP-OES, and ICP-MS  
ICP-AES and ICP-OES ICP-MS techniques have some limitations because of the error caused by the matrix effects. Moreover, the direct determination of trace elements in saline sample is limited by the highly dissolved solid elements in the sample. To overcome these problems, sample preparation procedures are required. Solvent extraction is an effective method for the preconcentration and removal of the sample matrix, moreover, the presence of solvent may improve the nebulization and transportation efficiencies. [PERSON] et. al. [64] have designed the glass gravity phase separator for flow injection liquid-liquid extraction to determine rare earth elements. The proposed method was applied for the simultaneous determination of multi-rare earth elements in geological samples with good accuracy and reproducibility. Figure 3: Flow-injection manifold for the determination of lead by an online-preconcentration – ETAAS analysis. (a) Sample loading; (b) elution, C\({}_{1}\)-C\({}_{2}\), column; V\({}_{1}\)-V\({}_{5}\), three-way solenoid valve; P\({}_{1}\)-P\({}_{2}\), peristaltic pump; GF, graphite furnace; W, waste; D, autosample disk. Hydride generation coupled to ICP-MS is a sensitive method for the determination of various elements. For example, FI-HG-ICP-TOFMS was used for the simultaneous determination of As, Bi, Ge, Sb, Se, and Sn with the pre-reduction treatment [65]. The pre-reduction treatment was used to avoid the problem from the differences between the oxidation states of the element of interest in the standard and sample. In the system, the pre-reduction was conducted to reduce analytes with different oxidation states to the same form. A detection limit of the 0.08-0.54 ng mL-1 was obtained. However, the method based on the chemical hydride generation provided a large amount of hydrogen that is extinguished or destabilized in the plasma. To overcome these drawbacks, electrochemical hydride generation was introduced. As, Bi, Ge, Hg, Sb, and Se were determined using electrochemical hydride generation inductively coupled plasma time-of-fight mass spectrometry [66]. The new design for low sample volume electrolysis cells was used. The method successively evaluated the 12 isotopes in the single transient signal. Under the optimum conditions, the detection limit of 0.07-5.6 ng mL-1 was obtained. Cloud point method is also used for the preconcentration of metals. [PERSON] et. al. [67] have combined the cloud point preconcentration-flow injection coupled plasma optical emission spectrometry for trace aluminum determination. The preconcentration procedure was based on the cloud point extraction of aluminum with non-ionic surfactant polyethyleneglycolmono-p-nonylpheylether (PONPE 7.5). The detection limit was 0.25 \(\upmu\)g L-1, and a linear range up to 200 were obtained for the preconcentration of aluminum in 50 mL parenteral solution. [PERSON] and coworkers have reported the on-line preconcentration of copper [68] and chromium [69] in parateral solution using a minicolumn packed with activated carbon and subsequently determination using inductively coupled plasma optical emission spectrometry. The detection limit for the preconcentration of 25 mL of sample was 5 \(\upmu\)g L-1 and 29 ng L-1 for copper and chromium, respectively. The linear range was near the detection level up to at least 200 \(\upmu\)g L-1 for copper and 60 \(\upmu\)g L-1 for chromium. Speciation of chromium (III) and Cr(VI) in fresh water was introduced based on the on-line preconcentration of small-sized thin solid phase (STSP) column resin reactions coupled to flow injection ICP-OES [70]. In this study, anion-and cation-exchange resins (disk type) were packed in the line-filters. Then, they were fixed and serially connected to the loop of each six-way valve. Cr(III) was collected on the cation-exchange while anionic chromate ion, Cr(VI), was collected on the anionic-exchange. Detection limits of 0.04 and 0.02 \(\upmu\)g L-1 were obtained for Cr(VI) and Cr(III), respectively.