## 2 Results and Discussion  
### Optimization Procedure  
In order to maximize the extraction efficiencies for KET and NKET in aqueous media, different parameters affecting HT-BA\(\upmu\)E-\(\upmu\)LD/LVI-GC-MS(SIM) procedure have been investigated and optimized using a univariate approach, were the best values for each parameter were chosen for the next optimization assay, in accordance to previous similar works [17; 18]. The initial conditions consisted in spiking 1.0 mL of ultrapure water (pH 5.5) with 100 \(\upmu\)L of a working mixture containing KET and NKET resulting in a final concentration of 181.8 \(\upmu\)g L\({}^{-1}\). Afterwards, the microextraction was performed for 90 min at 1000 rpm followed by \(\upmu\)LD using 100 \(\upmu\)L of MeOH containing 1.0 mg L\({}^{-1}\) of IS under sonication (30 min, 42 +/- 2.5 kHz, 100 W). The parameters affecting the developed analytical approach were evaluated in a sequential order, starting from sorbent selectivity using 6 polymeric phases, which were chosen for their good performance for the extraction of polar to nonpolar compounds in aqueous media [17]. Next, the desorption solvent (MeOH, ACN and MeOH/ACN, 1/1, _v/v_; 100 \(\upmu\)L) and time (from 5 to 60 min), as well as matrix pH (from 2.0 to 11.0), ionic strength (salt content from 0 to 20%, _w/v_) and polarity (organic modifier from 0 to 20%, _v/v_) were assayed. Finally, shaking speed (from 600 to 2200 rpm) and microextraction time (from 5 to 120 min) were evaluated. Figure 1 depicts all the data from the optimization assays. The results clearly demonstrate that Strata-X presented higher recovery yields for KET and NKET than the other assayed sorbent coatings (Figure 1a). This result was expected since Strata-X promotes reverse-phase type mechanisms, such as  
\(\pi\)-\(\pi\) and hydrophobic interactions, which normally favors the retention of non-polar compounds through its phenyl and polystyrene groups. Moreover, this sorbent coating also allows dipole-dipole interactions, which normally favors the extraction of the more polar analytes through its pyrrolidone groups [19; 20]. In general, these types of sorbents present higher selectivity for semi-polar to non-polar compounds (log P > 2.5). According to their chemical structures and polarity (Table 1), both KET and NKET present non-polar (phenyl) and polar (ketone) groups, resulting in semi-polar to non-polar characteristics (2.91 < log P < 3.35), which would favor its retention by Strata-X. From the desorption optimization assays, it can be seen that no significant gain is achieved with either using MeOH, ACN or MeOH/ACN (1/1, _v_/_v_) to desorb KET and NKET from the NVP-DVB sorbent phase (Figure 1b). On the other hand, the optimum conditions for \(\mu\)LD were obtained by using 15 min of sonication time (Figure 1c). The matrix pH, usually plays an important role in the microextraction process, where normally the non-ionized form of the target compounds seems to promote higher recovery yields from the aqueous media, since it favors the reverse-phase type interactions with the sorbent phases [17; 18]. KET and NKET present weak basic characteristics (Table 1), being fully non-ionized at matrix pH > 9.5. For this reason, the recovery yields were maximized at the most alkaline pH assayed (pH 11.0)--Figure 1d. By changing the solubility (Figure 1e) and the ionic strength (Figure 1f) of the aqueous matrix, the recovery greatly decreased for the former (especially with 20% MeOH, _v_/_v_) and that the recovery remained partially unchanged for the later (with successive additions of NaCl). These results can be explained by the fact that increased matrix solubility normally favors the extraction of more nonpolar or very low polarity compounds (log P > 3.5) and the increased ionic strength normally favors the extraction efficiency for compounds with high polarity (log P < 2.0) [17].