## 3 Results and Discussion
In response to this problem, other analytical options using the known analytical techniques and commercially available means were considered. In the first step of direct analysis by FAPA-MS, different amounts of analytes were placed on a mini crucible and their mass spectra were recorded. For ibuprofen, two signals corresponding to _m/z_ 207 [M+H]\({}^{+}\) and _m/z_ 224 [M+H\({}_{2}\)O]\({}^{\bullet+}\) were recorded (Figure 5). For naproxen, signals _m/z_ 231 [M+H]\({}^{+}\) and _m/z_ 248 [M+H\({}_{2}\)O]\({}^{\bullet+}\) and a signal from the fragmentation ion _m/z_ 185 (Figure 6) were found. For diclofenac, the _m/z_ 296 [M+H]\({}^{+}\) signal and the signal resulting from the loss of the water molecule _m/z_  
Then, the dependence of the analytical signal intensity ([M+H]\({}^{+}\) and [M+H\({}_{2}\)O]\({}^{+}\) for ibuprofen and naproxen and [M+H]\({}^{+}\) and [M-H\({}_{2}\)O]\({}^{+}\) signals for diclofenac) on the amount of analyte was plotted (Figure S4). The range of measurement linearity and method detection limits for all compounds were determined. The linear dependence of the intensity of analytical signals on the amount of drugs is in the range of 0.2 \(\upmu\)g to 1 g and the MDL for all drugs is 0.2 \(\upmu\)g. In the next step, the scope of applicability of the FAPA-MS analytical method for the determination of NSAIDs was defined for samples containing a mix of drugs. During the measurement, the heating was started at 50 \({}^{\circ}\)C with a gradual increase to 300 \({}^{\circ}\)C. In Figure 8, the first signal from ibuprofen appears when the temperature of the heating source is around 70 \({}^{\circ}\)C because the melting point of ibuprofen varies between 75 \({}^{\circ}\)C and 78 \({}^{\circ}\)C. Then, as the temperature rises, a signal from naproxen appears, which is intensively released at about 150 \({}^{\circ}\)C, because its melting point is 152-155 \({}^{\circ}\)C. The release of diclofenac occurs at the same time, due to its melting point of 156-158 \({}^{\circ}\)C.  
Figure 6: The FAPA-MS spectrum of naproxen. Figure 7: The FAPA-MS spectrum of diclofenac. Then, the FAPA-MS spectra obtained directly from the AFFINILUTETM MIP-NSAIDs column after absorbing NSAIDs were recorded. In the initial heating phase, we observed signals from ibuprofen (I), which was released first due to it having the lowest melting point (Figure 9). In the first phase of measurements, we also observed a low-intensity signal from naproxen (N). In the second phase of measurements, when the crucible temperature per sample reached approximately 150 \({}^{\circ}\)C, we recorded the most intense signals from naproxen at \(m\)/\(z\) 231 and \(m\)/\(z\) 248. Then, with increasing temperature, the signals from ibuprofen begin to weaken, which indicates that most of this analyte had already been desorbed. A low-intensity signal appears from diclofenac (D) \(m\)/\(z\) 296, which desorbs the latest due to it having the highest melting point (Figure 10). Figure 8: Total ion current chromatogram for FAPA-MS analysis of ibuprofen, naproxen, and diclofenac. Figure 9: FAPA-MS spectrum recorded in the initial heating phase from 20 \({}^{\circ}\)C to 120 \({}^{\circ}\)C.  
In the next analytical phase, we recorded one spectrum of all analytical signals from the three compounds analyzed (Figure 11). It confirms that ibuprofen, naproxen, and diclofenac were bound in the MIP structure and underwent successive thermodesorption during the analysis. In the last stage of measurements, as expected, the signal from ibuprofen disappears, the signal from naproxen decreases significantly, and the signal from diclofenac is the most intense for the longest time (Figure 12).