ANALYSIS OF SUBMICROGRAM SAMPLES BY INAA

Procedures have been developed to increase the sensitivity of instrumental neutron activation analysis (INAA) so that cosmic-dust samples weighing only 10(-9)-10(-7) g are routinely analyzed for a sizable number of elements. The primary differences from standard techniques are: (1) irradiation of the samples is much more intense, (2) gamma ray assay of the samples is done using long counting times and large Ge detectors that are operated in an excellent low-background facility, (3) specially prepared glass standards are used, (4) samples are too small to be weighed routinely and concentrations must be obtained indirectly, (5) sample handling is much more difficult, and contamination of small samples with normally insignificant amounts of contaminants is difficult to prevent. In spite of the difficulties, INAA analyses have been done on 15 cosmic-dust particles and a larger number of other stratospheric particles. Two-sigma detection limits for some elements are in the range of femtograms (10(-15) g), e.g. Co = 11, Sc = 0.9, Sm = 0.2. A particle weighing just 0.2 ng was analyzed, obtaining abundances with relative analytical uncertainties of less than 10% for four elements (Fe, Co, Ni and Sc), which were sufficient to allow indentification of the particle as chondritic interplanetary dust. Larger samples allow abundances of twenty or more elements to be obtained.