Collection efficiency tests on adsorption media such as TEDA Impregnated Carbon and Silver Impregnated Zeolite, are important indicators of what one might expect to collect in actual use.
What effects collection of iodine?
Iodine usually is found as sublimed volatilized Iodine. This specie will condense on almost any surface or dust particle and will always have very high collection efficiency.
Methyl Iodide is used for standard testing because it is the smallest organo-molecular form of Iodine and is gaseous at ambient temperature. Because it is gaseous, it can be used to test the collection efficiencies on a 'worst case scenario' basis. Methyl Iodide is not normally found in nature, or as a by-product of nuclear power generation.
What test conditions are used?
The standard industrial test procedure is: ASTM D-3803-98, with test conditions of P=1ATM, T=30°C, Velocity=40 ft/min, R.H.=95%, CH3I concentration=1.75 mg/M3, Pre-Equilibration = 18 hours, Pollutant load = 1 hour, Elution = 1 hour. The nuclear industry uses these test procedures for comparative testing of collection media.
Awareness of environmental collection conditions.
Environmental scientists will not normally use the analytical cartridges under the specified ASTM conditions, but rather at the conditions found at the sampling site. For example, the sample cartridge will be used immediately in the air sampler and not swept with inert gas, or pre-equilibrated for 18 hours. The iodine loading rate or concentration cannot be controlled. Nor does one have control of the sampled environmental air conditions such as temperature and relative humidity. The environmental scientist should be aware that air sampling for radioiodine is strongly affected by: The species of Iodine, flow rate (face velocity), collection media, mesh size of the selected media, relative humidity, temperature, and sampling duration.
As a general guideline, and per the U.S. Department of Energy in their publication Environmental Regulatory Guide for Radiological Effluent
The 'AGX' Series, Silver impregnated Zeolite Cartridges contain a highly efficient inorganic adsorbent for the collection and removal of elemental and organic forms of radioactive Iodine. Laboratory test indicate that radioactive Xenon, Krypton, and other Noble Gases are not retained to any significant degree by Silver impregnated Zeolite cartridges (approximately 1/15,000th, or less, than that retained by activated Carbon). The media is nonflammable and operates at a very high efficiency at elevated temperatures. These cartridges are the preferred and specified type for use in post accident standby monitoring systems. All AGX-Series cartridges are individually heat-sealed in an airtight 6-mil polyethylene package to prevent contamination before use.