Trace Elements

Trace element concentrations are determined by instrumental neutron activation analysis (INAA) using standard techniques (for example, Gordon et al., 1968). Materials we have analyzed by this technique include rock and mineral samples, soil, coal, atmospheric aerosols, human hair, film negatives, archaeological samples, tree rings, and process sludge. Elements determined by INAA, and typical detection limits, are listed in Table 1.

Table 1. INAA detection limit (DL). Absolute amount in ng.
Element	DL	Element	DL	Element	DL
Na	200	Br	10	Gd	50
K	2000	Rb	500	Tb	1
Sc	0.5	Sb	5	Tm	1
Cr	50	Cs	10	Yb	1
Fe	500	Ba	3000	Lu	5
Co	10	Sr	3000	Hf	5
Zn	100	La	5	Ta	1
Ag	3	Ce	10	W	20
Cd	200	Nd	100	Th	10
As	20	Sm	5	Ir	0.5
Se	3	Eu	2	U	10

Approximately 100 to 200 mg of sample material is weighed into an acid-cleaned polyethylene vial (Fig. 1), although as little as 0.5 to 1 mg has been used in certain applications. After heat sealing the material is subjected to a neutron flux (Fig. 2) for several hours. During the irradiation neutrons are captured by various stable isotopes and radioactive isotopes are formed.

Figure 1. Irradiated sample and counting vial.

Figure 2. Core of research reactor.

The gamma rays emitted by these radioactive isotopes when they decay to stable forms are used to both identify the isotopes and to determine their absolute concentrations. International rock and mineral standards are used as reference standards. Gamma ray analysis is done using a broad energy germanium detector (BEGe) and a fully automated gamma ray spectroscopy counting system (Fig.3). The resulting gamma ray spectra are analyzed for gamma ray energies and intensities. Following corrections for geometry, flux variations, decay times, and fission products, absolute elemental concentrations are determined by reference to known standards. Typically precision and accuracy vary from +/- 2% to +/- 10% depending on the element, the nature of the sample matrix, and the absolute concentration of the element.

Figure 3. INAA counting laboratory.

Further information:

Instrumental Neutron Activation Analysis (INAA) overview

Supplemental materials:

INAA Parameters. Listing of isotopes produced by thermal neutron interactions, capture cross-sections, half-lives, and gamma ray energies.

Activity Calculation Spreadsheet. Calculate activity of various nuclides at the end of thermal neutron irradiation and after a specified decay time.

Links:

Capture Cross-sections. Interactive page that can be used to find neutron capture cross-sections. Lawrence Berkeley National Laboratory.

Chart of the Nuclides. Interactive chart of the nuclides that can be used to find x-ray and gamma ray energies for all the nuclides. Brookhaven National Laboratory.

Nuclear Data Search. Interactive pages that can be used to search by nuclides, gamma ray energies, etc. Lund/LBNL Nuclear Data Search.

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