Preparation and characterization of a charcoal reference material for the determination of atmospheric iodine

Full text: Activated charcoal is a well-known effective absorber of gaseous iodine and is routinely employed in environmental sampling. Post collection processing for iodine isotope ratio mass spectrometry can be challenging if sample preparation does not include a sampling media combustion step. One gap in an analysis protocol is the absence of a charcoal matrix reference material for method development and quality control use for analysis. Preparing a suitable laboratory reference material can be challenging due to the granular nature of the collection media and the high partition coefficients for molecular and alkyl iodides. Air flow heterogeneity across charcoal grains during sampling efforts negatively impacts aliquot uniformity and is a difficult parameter to control. In preparing a reference material corresponding to lesser amounts of iodine, representative sampling becomes troublesome. The Idaho National Laboratory has established a method for producing an iodine on charcoal laboratory standard which is tunable for iodine content. Acceptable aliquot uniformity has been achieved through sampling design and sampler air flow analysis of variance. The media produced has been characterized for total iodine concentration and ¹²⁹I / ¹²⁷I ratio by ICP-MS and AMS. A reference material corresponding to 100 hours of exposure time has been found to have a total ¹²⁷I concentration of 0.059 ± 1.9% μg 127I / g charcoal with ¹²⁹I / ¹²⁷I atom ratio of 4.1 x10^-8 ± 2.1% at the 95% confidence level.