Viagra cheats exposed by TLC

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  • Published: Jun 1, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
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Health supplements often adulterated with PDE5 inhibitors

There is a huge global market for health supplements, many of which are based on herbal remedies. Some of these are claimed to act as aphrodisiacs or are euphemistically labelled with ‘Enhances male vigour’. In practice, adulteration of such supplements by PDE5 inhibitors, such as sildenafil (Viagra) and vardenafil (Levitra), is common. The side-effects of these drugs include headaches and nasal congestion, along with possible serious cardiovascular complications in susceptible people. More dangerously, in some cases unapproved PDE5 inhibitors, which are likely to have poor or unknown safety profiles, may be used as adulterants, increasing the risk of dangerous side effects.

Detection of PDE5 inhibitors in supplements is possible by LC-MS or GC-MS. However, these methods are costly and require well-trained staff. Simpler methods that can be used with relatively little training are required to quickly monitor suspect supplements. TLC is a quick and simple technique, but can easily give false positives if other compounds with the same Rf value to a PDE5 inhibitor are present in a sample. The Chinese researchers looked at combining TLC with surface-enhanced Raman spectroscopy (SERS) in order to detect PDE5 inhibitors. SERS involves enhanced Raman signals occurring from organic molecules absorbed onto a rough metal surface (typically silver). The aim was to distinguish molecules by their Rf values and Raman spectra.

Combination of TLC and SERS used to detect PDE5 inhibitors

Samples of six PDE5 inhibitors were used to test the method: sildenafil, hydroxyhomosildenafil (i.e. lodenafil), thioaildenafil, acetildenafil, vardenafil and pseudovardenafil. These were used to prepare standard solutions and also to spike samples of unadulterated health supplements bought locally. The spiked samples were extracted using a mixture of methanol, water and ammonium hydroxide (75:25:1 by volume) under sonication. Undissolved solids were removed by centrifugation.

TLC was carried out with standard UV silica plates (GF254). The samples were spotted on the plates, which were run using a mixture of ethanol, n-hexane and ammonium hydroxide (70:30:1 by volume), which was found to be the best eluting mixture. The TLCs of standard solutions were run 10 times to obtain ranges of Rf values. The spots were well separated, with only a slight overlap between the Rf ranges of vardenafil and thioaildenafil.

A silver colloid was prepared using silver nitrate and trisodium citrate solutions, which were mixed together and then heated in a microwave. Some colloid was added to the TLC spots detected by UV. It was found best to add the colloid a little at a time rather than all at once. SERS spectra were obtained for each spot using a Renishaw Raman spectrometer with a 785 nm laser, while Renishaw WiRE 3.4 software was used to process the data.

In spiked samples, the LOD values for the PDE5 inhibitors ranged from 5 to 10 mg/kg, which is adequate for practical purposes since lower levels would have little physical effect. At very low levels the spots were invisible under UV, but still could be detected if silver colloid was added at the expected position and the SERS spectrum run.

A BP (backpropagation) neural network using Matlab software was used to predict the identity of spots. 40 SERS spectra were used as training spectra and the network was used to predict the identity of 10 samples. However, owing to similarities in some of the Raman spectra, the results were unsatisfactory. It was found that if the Rf values were also taken into account, good identification was possible, with >95% accuracy.

TLC, SERS and BP neural network combine to detect adulteration

The combination of TLC, Raman and a neural network allows accurate detection of adulteration with PDE-5 inhibitors in health supplements. Less operator training and less expensive equipment is required compared to LC-MS and GC-MS. This technique could also be used to detect other adulterants in health supplements, foodstuffs, or cosmetics.

Related Links

Journal of Separation Science, 2017, Early View paper. Hu et al. Rapid detection of six phosphodiesterase type 5 enzyme inhibitors in healthcare products using thin-layer chromatography and surface enhanced Raman spectroscopy combined with BP neural network.

Wikipedia, Backpropagation

Wikipedia, Surface-enhanced Raman Spectroscopy

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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