CONCENTRATIONS OF IODINE DETERMINED BY PREIRRADATION COMBUSTION AND NEUTRON-ACTIVATION ANALYSIS IN POWDERED GRASS AS A FUNCTION OF PARTICLE-SIZE

Pre-irradiation combustion of a sample to liberate iodine, trapping th e iodine on charcoal and quantifying the element by neutron activation analysis (NAA), has proven to be a reliable technique for the determi nation of iodine in biological samples. This method has been applied t o the determination of iodine content of a powdered grass material tha t was prepared by another laboratory for use as an in-house quality as surance material. Analyses of aliquots of the material yielded results that ranged from 0.45 to 0.96 mu g/g of iodine, so additional studies were undertaken in order to determine material homogeneity. Visual in spection indicated that the material was inhomogeneous with respect to particle size. To determine whether various sizes of particles contai ned different amounts of iodine, the material was sieved into four dif ferently 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 mu m, the woody and fibrous plant parts, contained the least iodine ( 0.43 +/- 0.09 mu g/g, average +/- expanded uncertainty), while iodine concentrations in the subsequent three fractions increased with decrea sing particle size with a maximum concentration of 0.89 +/- 0.13 mu g/ g for particles less than 75 mu m. These results support conclusions o f many researchers that trace element concentrations are inhomogeneous ly distributed in various cells and structural parts, and emphasize th e importance of achieving uniform particle sizes during sample grindin g and blending in the preparation of reference materials. (C) 1997 Els evier Science B.V.