Concentrations of iodine determined by pre-irradiation combustion and neutron activation analysis in powdered grass as a function of particle size

Pre-irradiation combustion of a sample to liberate iodine, trapping the iodine on charcoal and quantifying the element by neutron activation analysis (NAA), has proven to be a reliable technique for the determination of iodine in biological samples. This method has been applied to the determination of iodine content of a powdered grass material that was prepared by another laboratory for use as an in-house quality assurance material. Analyses of aliquots of the material yielded results that ranged from 0.45 to 0.96 μg/g of iodine, so additional studies were undertaken in order to determine material homogeneity. Visual inspection indicated that the material was inhomogeneous with respect to particle size. To determine whether various sizes of particles contained different amounts of iodine, the material was sieved into four differently sized fractions and a portion of each fraction was analyzed. Variations in iodine concentration as a function of particle size were observed. The fraction containing particles of sizes greater than 350 μm, the woody and fibrous plant parts, contained the least iodine (0.43 ± 0.09 μg/g, average ± expanded uncertainty), while iodine concentrations in the subsequent three fractions increased with decreasing particle size with a maximum concentration of 0.89 ± 0.13 μg/g for particles less than 75 μm. These results support conclusions of many researchers that trace element concentrations are inhomogeneously distributed in various cells and structural parts, and emphasize the importance of achieving uniform particle sizes during sample grinding and blending in the preparation of reference materials.