## 2 Results and Discussion
### Optimization of the BA\(\mu\)E-\(\mu\)LD Efficiency
#### 2.2.1 Selection of Sorbent Coatings and Back-Extraction Conditions  
As usual in the BA\(\mu\)E technique, we started by selecting the best sorbent for the six TCAs and, for the present study, we proposed to test new and conventional sorbents. Thus, four new ACs (AC1, AC2, AC3 and AC4) prepared from biomaterials waste, as well as six commercial polymers (SX, HLB, C18, SDVB, SCN and DVBM) were assayed under the following experimental conditions; extraction stage: 5 mL of ultrapure water spiked with 100 \(\mu\)L of TCAs mix solution (500.0 \(\mu\)g L\({}^{-1}\)), pH 5.5, 3 h (990 rpm); back-extraction stage: 90 \(\mu\)L of MeOH, 30 min under ultrasonic treatment (42 \(\pm\) 2.5 kHz, 100 W). In a first approach, it was evaluated the adsorptive properties of the four new ACs prepared from coconut fiber (AC1), coffee residue (AC2), sugarcane chaff (AC3) and sugarcane bagasse (AC4) wastes, for the six target analytes. These biomaterials have already been successfully tested to remove metal ions from aqueous solutions [38] and proven to be ideal sorbents in environmental applications, namely for water cleaning treatment. Preliminary assays from these new AC phases showed that the recoveries yields were maximized at 12% for the six target TCAs (data not shown). Although these biomaterials have shown remarkable performance for adsorption of metal ions from aqueous media, based on the exploratory data obtained for organic compounds, we postulated that the observed lack of efficiency can be attributed to the back-extraction stage, despite the highly probable efficiency of the previous microextraction stage. Therefore, to evaluate this hypothesis, additional experiments were carried out to verify the ability of the ACs to extract the six TCAs from the aqueous medium. For this purpose, we used similar conditions for microextraction, although assaying a solution of ultrapure water (5 mL) plus 25 \(\mu\)L of the TCAs mix solution (100.0 mg L\({}^{-1}\)) and 16 h of equilibration time. Subsequently, a portion of the aqueous sample resulted from the microextraction stage was removed and extracted with dichloromethane (50:50, \(v\):\(v\)), followed by ultrasonic treatment (42 \(\pm\) 2.5 kHz, 100 W, 5 min) and analysis of the organic phase by LVI-GC-MS(SIM). The results obtained and depicted in Figure 2 shows, apart from the AC3 sorbent, that all the remaining carbon-based coatings (AC1, AC2 and AC4) presented remarkable microextraction efficiencies, since the resulting aqueous media do not present levels of TCAs higher than 15%. In this sense, we found that the proposed back-extraction stage conditions would not be able to efficiently remove the adsorbed TCAs, due to the very strong interactions established with the surface of these novel biosorbents, leading to low overall recovery yields. It should be emphasized that the analytical process of the BA\(\mu\)E technique is always characterized by a two-stage balancing process (adsorption-desorption equilibrium), in which the adsorption phenomena must be sufficiently effective during the microextraction stage, but not too strong that could compromise the effectiveness of the subsequent back-extraction stage. Like this, from the preliminary data achieved, we can deduce that the four biosorbents tested herein seem very effective from the adsorption point of view, ideal for removal processes (e.g., water decontamination, etc. ), yet presenting great limitations as coating phases for microextraction-based techniques. This finding may also be associated due to the large surface area and small pore size (<20 A) presented by the four ACs under study, which may promote strong interactions with the target compounds and may hinder the back-extraction stage. For this reason, we decided to discard these biomaterial sorbents and assayed the six commercial polymeric phases (SX, HLB, C18, SDVB, SCN and DVBM), since they seem to be more suitable for the chemical structures of the target molecules involved.