A solution for contact lens germicide
- Published: Nov 16, 2009
- Author: Steve Down
- Channels: HPLC
One common, very popular germicide that most people will have come across but will not have heard of is polyhexamethylene biguanide, abbreviated for ease to PHMB. It is applied to the water in swimming pools, industrial processes, personal care products, cosmetics, fabric softeners, hand washes, wound dressings and contact lens solutions. It is also applied to medical utensils and equipment as a disinfectant.
PHMB has been approved for use by the US EPA and has been declared a low risk product, except for workers in the drilling industry who handle drilling muds and workover fluids. For the general public at the levels normally encountered, it is a safe chemical.
Nevertheless, concentrations of PHMB must be measured in this broad range of consumer products for quality control purposes. Various methods of quantitation have been published but they failed to reach down to the levels normally encountered in contact lens solutions, at 1 ppm. The best performances have given detection limits of 4-5 ppm.
Regardless of any method of separation, the compound is difficult to detect. It has no chromophores, so UV detection is poor. The mixture of groups at either end of the polymeric chain complicates methods involving chemical derivatisation or binding.
Now, researchers from the Center for Devices and Radiological Health of the US FDA at Silver Spring, MD, have developed a method based on HPLC with evaporative light scattering detection (ELSD) which overcomes the sensitivity limitations of other methods. Anne Lucas, Edward Gordon and Melvin Stratmeyer examined commercial multipurpose contact lens solutions bought locally. Six contained PHMB, two contained hydrogen peroxide and one contained the preservative Polyquad, a polymeric quaternary ammonium compound.
The contact lens solutions were extracted by SPE using a weak cation exchange cartridge, eluting with aqueous acetonitrile containing trifluoroacetic acid. This cartridge was preferred over a strong cation exchange cartridge because it required less acidic conditions for elution.
A C8 column was fitted for HPLC and PHMB was eluted with a gradient of aqueous acetonitrile containing triethylamine and formic acid. Under these conditions, the retention time was approximately 5.5 minutes.
ELSD is known to be less reproducible than UV detection and the analyte signal can vary with a mobile phase gradient. However, in this case, a sharp, isolated peak was observed for PHMB, with no interfering peaks. With a UV detector, the PHMB peak appeared as a shoulder alongside a much broader peak, so could not be used.
The ELSD detection limits were 0.1 ppm, well below the normal levels encountered in contact lens solutions and repeatabilities and reproducibilities were 4-11% r.s.d. and less than 17.5% r.s.d, respectively. The recoveries were quantitative.
The levels of PHMB in the six contact lens solutions containing PHMB as the active ingredient were all 1 ppm. The hydrogen peroxide solution contained no PHMB.
The Polyquad solution displayed an HPLC peak 0.05 minutes ahead of the retention time for PHMB, and was three-fold larger. In theory, this peak could be mistaken for PHMB but the researchers declared that this is not a problem for contact lens solutions because the two agents are not used together.
The method was used to test the stability of PHMB in one contact lens solution. Samples were stored in polypropylene test tubes for up to 28 days at temperatures ranging from -20°C to 25°C. The germicide proved stable under all conditions.
Conversely, storage in a glass container gave a 50% drop in analyte signal, attributed to sorption of PHMB on the glass walls. This method of storage should be avoided as it will diminish the germicidal power of the solutions.
The HPLC-ELSD method for measuring PHMB in multipurpose contact lens solutions is simple and reliable and has an adequate detection limit for this medium.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.