For street samples suspected of containing the phosphodiesterase-5 inhibitors sildenafil (Viagra(®)) and/or vardenafil (Levitra(®)), including powders or adulterated herbal supplements, a chemical analysis is needed to provide confirmatory identification of these illegally procured substances. Sildenafil and vardenafil are structurally similar and it is difficult to differentiate between them, as previous mass spectrometric studies have shown the two drugs to produce similar fragmentation patterns. The use of tandem mass spectrometry can produce confirmatory data, but the technique requires a high level of technical expertise. We have developed an electrospray ionization-liquid chromatography-mass spectrometry (ESI-LC-MS) method that allows differentiation between these two structurally similar molecules via in-source fragmentation in combination with an ion trap mass spectrometer. A very stable gas phase ion is formed during in-source fragmentation of vardenafil; the combination of the stability of this ion and the longer residence time for the ion in the ion trap results in a very strong signal. This feature results in a method that can provide clear differentiation between sildenafil and vardenafil while at the same time requiring less expertise from the routine analyst to confirm the presence or absence of the two compounds.
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