Earlier studies using methylparaben, propylparaben, and several types of flexible tubing showed that the parabens are sorbed by a number of commonly used tubing. The sorption increased with increase in time of contact and the tubing surface area. The temperature and pH ranges tested did not affect sorption. Current studies evaluated other commonly used antimicrobial preservatives used in oral, parenteral, opthalmic, nasal, and other pharmaceutical products. These preservatives include benzoic acid, sorbic acid, benzyl alcohol, and benzalkonium chloride. Results show that all preservatives, except benzalkonium chloride, are sorbed by several types of flexible tubing. After 24 hr, some types of tubing sorbed over 40% of benzoic acid, benzyl alcohol, and about 30% of sorbic acid. Significant losses were observed within a few hours. No sorption of benzalkonium chloride occurred in the tubing tested. Three different types of fluoropolymer resin tubing, Teflon(R) (flourinated ethylene propylene (FEP), Teflon(R) perfluoroalkoxy (PFA) and Teflon(R) polytetrafluoroethylene (PTFE, NXT)), fluoropolymer FEP laminated Tygon(R) tubing, and Zelite(R) do not sorb any of the preservatives. Silicone tubing shows the highest sorption of preservatives. After 120 hr, as much as 64% of the preservative is sorbed by some tubing. Detailed studies using silicone tubing show that the sorption increases with increase in surface area at 25degreesC, and increasing the temperature to 40degreesC shows little change in sorption. At the concentration range generally used in formulations, the concentration of the preservative has no significant effect on sorption. Refilling the tubing with fresh preservative solution after contact with preservative solution for 168 hr still causes extensive sorption of the preservative. Desorption studies indicate that the sorbed preservatives are desorbed in water to a limited extent over 120 hr. Results are important in situations where short or extended interruptions occur in the processing and filling of products. This can lead to decreased and variable results due to residence of the solution in the tubing.
