## 1 Introduction  
Heavy metal ion monitoring in different matrices is a significant task because of the harmful effects that such ions have on organisms [1; 2; 3]. Lead (Pb) is considered as one of the highly hazardous species that affect both the ecological environment and human health in a dangerous way. It can cause cardiovascular effects, renal failure, neurologic damage, and venereal toxicity [4]. Exposure to severe Pb intoxication causes acute failure to the renal and nervous systems, liver, and brain, in addition to sterility, abortion, and neonatal death [5]. The environmental and health problems arise fundamentally from the use of gasoline anti-knock products and paint pigments [6]. As an outcome, the WHO (World Health Organization) has affirmed the ultimate acceptable limit of Pb in drinking water to be 10 ng/mL [7]. For such causes, Pb assessment at low concentration becomes an urgent and important concern. Different pre-concentration methods have been presented, such as liquid-liquid microextraction (LLE) [8], opecipitation [9], ion exchange [10], cloud point extraction( CPE), and solid phase extraction (SPE) [11; 12]. CPE is considered now as one of the most efficient separation techniques used for the excellent pre-concentration of very low amounts of different analytes [13; 14; 15; 16; 17; 18; 19]. CPE offers many excellent features such as rapidity, simplicity and environmental compatibility. The process in CPE is phase separation of an aqueous solution containing non-ionic surfactant in addition to the metal-chelate complex. This solution is heated to a certain temperature (i.e., cloud point temperature) after which the solution becomes turbid. The solution produces two phases: one phase which has only the surfactant (surfactant-rich phase), and the other phase which contains a very little amount of the surfactant with critical micellar concentration (CMC) [13]. Several advantages are presented by using CPE, such as high pre-concentration factors, lower cost, fast operation, higher safety and simplicity, no need for large amounts of toxic organic solvent, and ease of coupling to analytical instruments [20; 21; 22]. The main applications of CPE are the extraction of metal-chelates using a surfactant with posterior quantification using atomic spectrometry techniques.  
The most widely used technique is flame atomic absorption spectrometry (FAAS) [23; 24; 25; 26; 27]. FAAS offers different features, such as a short time of analysis and high accuracy, but has many limitations in terms of sensitivity. Coupling CPE to FAAS could improve the limit of detection and allow accurate results to be obtained in samples down to ng/mL or even lower levels.  
Herein, a new pre-concentration method based on CPE is presented and optimized for the pre-concentration and quantification of Pb\({}^{2+}\) ions. Pb\({}^{2+}\) reacts with methyl 4,20-diisobutyl-2,5,8,16, 19,22-hexaoxo-7,17-dipropyl-3,6,9,15,18,21-hexaaza-1(2,6)-pyridinacyclo-docosaphane-10-carboxylate (DLNL) giving a chelate followed by extraction in octylphenoxypolyethoxyethanol (Triton X-114) as a non-ionic surfactant. After optimizing the essential parameters that affect the CPE process, the presented protocol is introduced for the pre-concentration of Pb before its analysis by FAAS.