Instrumental neutron activation analysis (INAA) consists of placing each sample (generally weighing 200 mg to 500 mg) into a pre-cleaned polyethylene bag and placing each packaged sample into a polyethylene irradiation vessel. The vessel is irradiated and the elements comprising each sample undergo neutron capture resulting in the formation of radioactive product nuclides. The energy of the radiation emitted from the product nuclei during radioactvive decay indicates from which element the product nuclide was formed and the amount of radiation at a given energy is directly proportional to the amount of that element. The length of irradiation and subsequent gamma-ray counting depends on the half-life of the isotope of interest. Samples are irradiated and counted for a short time, seconds or minutes, to activate elements for which the products of neutron capture are relatively short-lived (e.g., F, Na, Mg, Cl, K, Ca, Ti, V, Mn, Cu, Br, I). Similarly, samples are irradiated for hours and counted for hours, after a decay times sufficient to allow for the decay of the short-lived components of the sample, to determine elements for which the products of neutron capture are long-lived (e.g., Sc, Cr, Fe, Co, Zn, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, La, Ce, Sm, Eu, Tb, Hf, Ta, Au, Hg, Th, U).
Advantages of INAA:
INAA does not require sample dissolution and depends on the nuclear rather than atomic properties of the element of interest so that any sources of uncertainty or bias in INAA are independent of those associated with most other techniques of chemical analysis. As almost all of the elements comprising the sample will yield radioactive products, INAA can be used to provide a great deal of information from one relatively small amount of sample.
This technique is used routinely for certification analyses and homogeneity assessment of Standard Reference Materials (SRMs). During the past year, NMG has used INAA to provide values for the certification of eight SRMs. INAA is also used routinely in the analysis of environmental specimens for the National Biomonitoring Specimen Bank (NBSB). For many of these archived environmental materials, only a limited amount of material has been included in the NBSB. INAA provides a great deal of information and uses only 200 mg to 400 mg of material.
Research to Establish INAA as a Primary Method (UNDER DEVELOPMENT; contact: Robert R. Greenberg, 301-975-6285, Robert.Greenberg@nist.gov ) Contacts:
Last modified 21-July-2003 by website owner: NCNR (attn: Bill Kamitakahara)