Just what do you mean by “dry?”

I read with interest the article “VHP takes its place in room decontamination” (CleanRooms, November, 2001), but would like to point out some misconceptions that it perpetuates. The most important of which is that condensation can be avoided. If it cannot be avoided then the process cannot be described as “dry.”

In February, 1996, in Pharmaceutical Technology Europe, M. A. Marcos-Martin stated that in hydrogen peroxide decontamination process “condensation is a phenomena that cannot be avoided, according to the laws of physics.” This view was confirmed in a paper presented at the Washington ISPE conference in June, 2001, by Scot Pool who showed a relationship between 'D' value and the rate of condensation.

Over the past six years we have undertaken a considerable amount of experimental and theoretical work in an effort to understand the process. In the present context, the theoretical work is of more importance because it allows us to calculate where condensation will occur and the mass of condensate produced. The calculation is complex but based on the equations published by Scatchard in the J. Chem Soc., and subsequently reviewed and published in an American Chemical Society monograph in 1955 titled “Hydrogen Peroxide.”

The equation allows for the calculation of the gas concentration at which the first bead of condensation will form. Because the hydrogen peroxide gas is generated by flash evaporation of an aqueous solution of hydrogen peroxide into a room with air of a known RH, it is possible to calculate, at any time, the total mass of both water and hydrogen peroxide.

An allowance must be made in this calculation for the water and hydrogen peroxide that is returned to the gas generator and extracted from the system. Once this has been done the mass of water and hydrogen peroxide may be calculated at any moment in time.

Condensation will form when the concentration of hydrogen peroxide and water vapor are those which would be exerted by a solution where the sum of the mol fractions is unity. This point can only be found using an iterative calculation.

Putting the data provided in the article and making an appropriate allowance for the masses of water and hydrogen peroxide returned to the generator, we find that condensation would have occurred after no more than 50 minutes into the gassing phases, or 35 minutes into the 100 minute sterilization phase.

The reason that condensation is not visible is that the film is very thin, at most only a few microns thick. But just because it is not visible does not mean that it does not exist.

A proper understanding of the physical chemistry of hydrogen peroxide decontamination is essential to optimize the process.

Since condensation must have occurred in the tests reported by the authors their comments on the material compatibility need to be reconsidered.

David Watling Ph.D.
Bioquell Pharma


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