## 3 Results and Discussion
### GC-IT-MS Method Validation  
Using optimized parameters, all the analytical data were investigated. Table 6 shows the correlation coefficients (R\({}^{2}\)) in the range 1-100 \(\upmu\) L\({}^{-1}\), along with the limits of detection (LODs) and limits of quantification (LOQs), repeatability (as intra-day precision) and reproducibility (as inter-day precision), and EFs of each NSAID considered. LODs and LOQs were determined according to [PERSON]'s definition [56; 57], i.e., an analyte concentration that produces a chromatographic peak equal to three times (LOD) and seven times (LOQ) the standard deviation of the baseline noise. All the compounds show a good linearity in the investigated range (\(\geq\)0.995) and LODs and LOQs between 0.1-0.2 \(\upmu\)g L\({}^{-1}\) and 4.1-4.7 \(\upmu\)g L\({}^{-1}\), respectively, with high intra- and inter-day precision (\(\leq\)3.8 and \(\leq\)4.5, respectively). The EFs, defined as the ratio between the analyte concentration in the sediment phase (C\({}_{\text{sed}}\)) and the initial analyte concentration (C\({}_{0}\)) in the sample (EF = C\({}_{\text{sed}}\)/C\({}_{0}\)) [35], were also studied, ranging between 350-450. Finally, for a complete analytical methodology evaluation, the recoveries have been studied in the investigated matrices, i.e., animal urine, at two different spiked NSAID concentrations (20 ng mL\({}^{-1}\) and 50 ng mL\({}^{-1}\)). Table 7 shows these data: recoveries in animal urine samples between 93.8 and 102 with RSDs \(\leq\)3.2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & **R\({}^{2}\)** & **LOD** & **LOQ** & **Intra-day** & **Inter-day** & **EF** \\ \hline ASA & 0.9950 & 0.2 & 4.1 & 3.8 & 4.5 & 350 \\ IBP & 0.9972 & 0.1 & 4.7 & 3.2 & 4.0 & 450 \\ NAP & 0.9987 & 0.1 & 4.7 & 3.5 & 4.3 & 385 \\ KPF & 0.9981 & 0.1 & 4.5 & 3.3 & 4.2 & 412 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Correlation coefficients (R\({}^{2}\)) calculated in the range 1–100 \(\upmu\)g L\({}^{-1}\), limit of detection (LOD; \(\upmu\)g L\({}^{-1}\)) and limit of quantification (LOQ \(\upmu\)g L\({}^{-1}\)) and inter- and intra-day precision (expressed as RSD, %) of each NSAID determined by GC-IT-MS. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & \multicolumn{5}{c}{**Recovery (\%)**} \\ \hline  & **5 g L\({}^{-1}\)** & **10 g L\({}^{-1}\)** & **13 g L\({}^{-1}\)** & **15 g L\({}^{-1}\)** & **20 g L\({}^{-1}\)** & **25 g L\({}^{-1}\)** \\ \hline ASA & 80.1 (4.0) & 89.5 (3.9) & 93.2 (3.6) & 93.8 (3.5) & 85.2 (4.1) & 83.5(3.9) \\ IBP & 92.5 (3.7) & 97.2 (3.5) & 99.4 (3.5) & 99.5 (3.7) & 92.5 (3.8) & 90.1 (4.0) \\ NAP & 94.2 (4.1) & 97.1 (4.2) & 100.6 (3.9) & 101.0 (4.0) & 91.6 (4.2) & 89.2 (4.1) \\ KPF & 95.2 (3.9) & 97.5 (4.0) & 99.8 (3.9) & 100.1 (7.8) & 93.2 (4.0) & 90.2 (3.8) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Effect of different NaCl amounts on the NSAID recoveries (%). The conditions were as follows: 9 mL of simulated urine samples spiked with NSAIDs (20 ng mL\({}^{-1}\) of each), pH 3, 1 mL of acetone, 250 \(\upmu\)L of CH\({}_{2}\)Cl\({}_{2}\), and 5 \(\upmu\)L of IS (60 ng \(\upmu\)L\({}^{-1}\)). In brackets are reported the RSDs (%); each analysis was in triplicate. Finally, Table 8 shows a comparison among different methods present in literature [58; 59; 60; 61; 62; 63] for analyzing NSAIDs. The extraction methods were different: three papers were based on hollow-fiber liquid microextraction [59; 60; 62], whereas two papers were on rotating disk sorptive [63] and liquid-liquid extraction [61]. According to the analytical techniques, three studies used HPLC with ultraviolet (UV) [58; 61; 63] and one the diode array detection (DAD) [59], one used the ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS) [62], and one used GC equipped with a flame ionization detector (FID) [60].