Compound measurement is not simple

08/01/2001

by Hank Rahe

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To determine the exposure limits for highly potent compounds, we must have the ability to detect and measure the amount of the compound that escapes during manipulations. The ability to measure the compound's activity is impacted by several factors, including analytical techniques to detect a compound, the ability to recover the compound from both airborne and surface areas and whether the study has been properly designed.

The ability of the sampling technique to recover the active compound is also a critical factor because it is very difficult to recover 100 percent. The ability of the analytical tests to measure a small quantity of the active compound determines what we call "zero." The term "zero" exposure limit may be misleading to the less informed, but it simply means the compound is active below our ability to measure. Those who understand testing also understand that "zero" does not exist.

Sampling of the areas in which the potent compound is being processed, be it the laboratory, hospital pharmacy or manufacturing facility, requires professional expertise.

The sampling program must be correctly designed to represent the actual environmental conditions of the process area. The correct number of locations and samples taken and the method of sampling, either airborne or surface, are critical to obtaining accurate results.

Sampling the air for the presence of potent compounds requires that those doing the test have an understanding of which locations to sample, the proper air sampling devices and filters to use to assure that the amount of material captured is representative of what is actually present in the environment.

Air sampling requires totally different techniques and equipment to capture airborne solids, liquids and gasses. If the form of the material is a solid, then the particle size is important due to the ability of the filter media to not only capture but also recover the material from the filter for testing. Liquids can be present in large droplets or smaller aerosols, each presenting a challenge in terms of capture requirements for filters and the uniformity of the airborne droplets. Gasses require an understanding of the specific compound and capture tools. Gasses will pass through a filter intended for solids and must be measured with special instruments designed to capture and analyze the specific gas.

Surface sampling is also challenging because the potent compound must be recovered from a surface. Factors impacting the recovery are the surfaces themselves and the means used to remove the material. Pre-testing should be conducted to determine the percentage of the material that can be recovered from a specific surface material. The smoothness of the surface and its ability to hold static charge can also impact the recovery rate. Without the recovery rate testing beforehand, information regarding the amount of potent compound present can be underestimated, resulting in a potentially dangerous situation.

The design of the study used to sample for the potent compound should include the sampling techniques, location and number of samples. Each of these factors can significantly impact the results. If the sampling techniques are not appropriate, the potent compound may not be detected—for example, sampling for a gas using a surface sample, or surface sampling for small quantities on a surface—that has a very low recovery rate.

In some cases, it is more appropriate to use a surrogate material to determine the concentration of the potent compound present while performing a given operation. This can have several advantages in that the material may be less dangerous to work with and have better detection characteristics. If this approach is chosen, it is important that the surrogate material and the active compound's physical characteristics are as similar as possible.

Hank Rahe is director of technology at Contain-Tech in Indianapolis. He has over 30 years' experience in the healthcare industry. He is the past chairman of the board of the International Society of Pharmaceutical Engineers, and is on the CleanRooms Editorial Advisory Board.