## 3 Results and Discussion  
### Photodegradation in Quartz Containers  
The DHP standard solutions were first tested in quartz containers to ascertain their sensitivity to light in the absence of any filter, given the known transparency of the quartz at the set wavelengths. UV spectra were recorded for 140 min at very close intervals because of the high degradation rate of some DHPs. The processing of the large amount of spectral data collected was performed by MCR. Figure 1 shows, as an example, the sequence of spectra recorded on the NF sample (\(\mathbf{D_{NF}}\)), lead of these drugs, the resolved spectra of the drug and its photoproducts (\(\mathbf{S_{NF}}\)), and the concentration profiles of all the species involved in the process (\(\mathbf{C_{NF}}\)).  
The kinetic photodegradation rate for all the DHPs was calculated by applying the routine HS-MCR-ALS and the kinetic constraint to the spectral data. The decrease in DHP concentration is shown in Figure 2, in which the curves were made linear by the log transformation of the concentration values. Reaction pathways were resolved through single or consecutive first-order reactions. MCR modeling identified in most cases the formation of a single photoproduct, resulting from the oxidation of the dihydropyridine ring. This process was characterized by the lowering of the peak in the area of 350-370 nm, typical of the DHP structure, and by the simultaneous raising of a new signal in the area of 260-280 nm, characteristic of the pyridine group. For some DHPs, MCR resolved a second photoproduct, in accordance with the results reported in previous works [4].  
Figure 1: Sequence of the UV spectra of NF solution (20.0 ug/mL) along the forced irradiation (\(\mathbf{D_{NF}}\)); and spectra (\(\mathbf{S_{NF}}\)) of the species involved in the photodegradation process predicted by MCR processing and their concentration profiles (\(\mathbf{C_{NF}}\)).  
Some DHPs showed an impressive degradation rate. Among these, NS and NT degraded completely in less than 5 min; and NM, CL, NC, MN, NF, and LC within 30 min. In contrast, IS, AM, and above all FL proved to be more stable, with a significant residual content of drug even after 120 min of forced irradiation.  
The same solutions were kept completely in the dark to monitor any degradation processes not caused by light. The differences detected in the UV spectra recorded at zero time and after 10 days of storage showed, in any case, nonsignificant differences.  
In order to quickly compare the light stability of the analytes, the parameter t\({}_{10}\) was adopted as the irradiation time necessary to obtain a 10% degradation of the initial drug concentration. The use of this parameter was justified by the fact that the drug or a formulation containing it should no longer be used if its concentration reaches a value below 90% of the starting value. The results obtained demonstrated the high photosensitivity in solution of the tested drugs, of which 9 out of 11 showed a t\({}_{10}\) value of less than 3 min (Figure 3).  
FL and AM, which showed the highest photostability values, have in common the lack of the nitro group on the benzene ring. Indeed, the drugs that have a nitro group in R\({}_{2^{\prime}}\) or R\({}_{3^{\prime}}\) showed lower stability. The influence of the nitro group on the stability of these  
Figure 3: Irradiation time (min) causing a 10% degradation (t\({}_{10}\)) of the hydroalcoholic DHP solutions in quartz containers, under a radiant power corresponding to a dose energy of 21 kJ (min m\({}^{2}\))\({}^{-1}\).  
Figure 2: Kinetic curves resulting from the degradation test on hydroalcoholic DHP solutions in quartz containers: amlodipine (AM), cilmidipine (CL), felodipine (FL), isradipine (IS), lacidipine (LC), manidipine (MN), nicardipine (NC), nifedipine (NF), nimodipine (NM), nisoldipine (NS), and nitrendipine (NT).  
molecules had already been highlighted in previous works [17]. An exception is given by LC, which, although not having nitro groups, showed low photostability.