3226 in diameter. Throughout the whole study, cryomilled materials stayed in that capillary, with access to fresh air. We suppose that when different storage conditions were adopted during monitoring the degree of crystallinity, the stability of cryomilled furosemide might be completely different. For example, when the samples were exposed to greater RH conditions or we provided the greater free surface, the crystal growth might be orders of magnitude faster. Chemical Instability of Cryomilled Furosemide As mentioned in the Introduction, due to its poor solubility and bioavailability, crystalline furosemide is a challenging molecule for the pharmaceutical industry. Still more so, furosemide has been known to be unstable during adverse storage conditions. It undergoes acid-catalyzed hydrolysis in water solutions, photochemical degradation in the solid state and thermal decomposition at a melting point. In all three cases, the same degradation product is formed, namely 4-chloro-5-sulfamoylanthranilic acid (CSA) (23,30). As reported, both furosemide and its main degradation product possess toxic potentials (31) and might undergo further photodechlorination to N-furfuryl-5-sulfa- moylanthranilic acid (FSA) (32). For this reason, it seems that the crystalline furosemide itself is strongly susceptible to Adrjanowicz et al. degradation when exposed to inappropriate storage or manufacture conditions. The milling procedure of furosemide was carried out at liquid nitrogen temperature where, as suggested, no chemical degradation of the milled material should occur. Still more so, some of our previous studies concerning amorphous pharmaceuticals (like telmisartan or ziprasi- done) formed by cryomilling have clearly demonstrated that the amount of impurities formed during that type of mechanical treatment varies (irrespective of the milling time) only from 0.2% to 0.5% (29,33). This certainly implies that cryomilling seems to be a really safe method of solid-state amorphization. To check whether this is also true for the drug under study, we used the ultra performance liquid chromatogra- phy (UPLC) technique, as an infallible tool in the purity analysis of active substances and their final dosage forms. A chromatogram of the crystalline furosemide used for further analysis as a reference is displayed in Fig. 4, as a black solid line. As seen, it consists of one very well-pronounced peak assigned to furosemide and the other, very small one with the relative retention time of 0.26. The unknown impurity assigned to this peak was denominated as Imp.1. The area under the mother peak was found to be equal 99.95%. After 30, 60, 120 and 180 min of effective cryomilling of the furosemide (1 g each time), next UPLC chromatograms Fig. 4 Overlaid chromatograms of crystalline furosemide (black) and that cryomilled for 30 min (navy blue), 60 min (green), 120 min (blue) and 180 min (purple).