Development and implementation of a pass/fail field-friendly method for detecting sildenafil in suspect pharmaceutical tablets using a handheld Raman spectrometer and silver colloids

- Handheld Raman spectrometers and silver colloids are used to detect sildenafil in suspect tablets.
- The method is based on a pass/fail platform that can be taught to non-experts in about 20 min.
- The method involves measuring sample solutions directly through glass vials (no drying required).
- The method proved to be 92.6% effective on average for 117 counterfeit and unapproved drugs.
- The method yielded detection limits in samples as low as 10 ppm.

A simple, fast, sensitive and effective pass/fail field-friendly method has been developed for detecting sildenafil in suspect Viagra and unapproved tablets using handheld Raman spectrometers and silver colloids. The method involves dissolving a portion of a tablet in water followed by filtration and addition of silver colloids, resulting in a solution that can be measured directly through a glass vial. Over one hundred counterfeit Viagra and unapproved tablets were examined on three different devices during the method development phase of the study. While the pass/fail approach was found to be 92.6% effective on average, the efficacy increased to 97.4% on average when coupled with the software's “Discover Mode” feature that allows the user to compare a suspect spectrum to that of a stored sildenafil spectrum. The lowest concentration of sildenafil in a water/colloid solution that yielded a “Pass” was found to be 7.6 μg/mL or 7.6 parts per million (ppm). For the analysis of suspect tablets, this value was found to be as low as 10 μg/mL and as high as 625 μg/mL. This variability was likely related to the tablet formulation, e.g., concentration of sildenafil, presence and concentration of water-soluble and/or water-insoluble ingredients. However, since most counterfeit Viagra and unapproved tablets contain >50 mg sildenafil per tablet, such low concentrations will not be encountered often. Limited in-lab and in-field validation studies were conducted in which analysts/field agents followed the procedure outlined in this study for small sample sets. These individuals were provided with written instructions, a ∼20 min demonstration regarding how to perform the procedure and use the instrument, and a kit with field-friendly supplies (purified bottled water from a local grocery store, disposable plastic pipettes, eye-dropper with a silver colloid solution, etc.). The method proved to be 98.3% and 91.7% effective for the in-lab and in-field validation studies, respectively, which demonstrated the ruggedness, simplicity and practicality of the method.

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