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

Queen's University INAA presentation

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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Publications:

Eby, N. and Eby, S. (2008) Instrumental Neutron Activation Analysis (INAA): practice and potential forensic applications. In: Donnelly, L. J. 2008 (ed). Geoscientific Equipment & Techniques at Crime Scenes. The Geological Society Forensic Geoscience Group. FGG 2008 conference, booklet of abstracts, 17 December 2008, Geological Society of London, pp. 28-30.

Eby, G. N. (2008) Instrumental neutron activation analysis (INAA) - practice and application. National Organization of Test, Research, and Training Reactors 2008 Annual Meeting Program with Abstracts, p. 21.

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Instrumental Neutron Activation Analysis (INAA): Practice and Potential Forensic Applications

Eby, N. and Eby, S.

Instrumental Neutron Activation Analysis (INAA) is a relatively straightforward technique for determining elemental abundances in a wide range of materials. The method utilizes the interaction between a thermal (or higher energy) neutron and a nucleus to produce a radioactive nuclide that emits characteristic gamma rays. The energy of the emitted gamma rays is used to identify the nuclide and the intensity of the radiation can be used to determine abundance. Solid state detectors are used to sense the emitted gamma rays, and after suitable corrections and comparisons with standards, an elemental concentration is determined.

The advantages of INAA are (1) it is a relatively cheap analytical method, a state-of-the-art facility can be acquired for significantly less than $100,000 compared to the much higher costs of competing analytical methods; (2) the method is non-destructive hence the same sample can be used for other measurements; (3) sample size can be very small, often as little as a milligram; (4) detection limits for many elements are in the nanogram range; (5) no chemical preparation is required, samples are analyzed as is; and (6) on the order of 40 elements can be measured essentially simultaneously. The major disadvantage of INAA is that there are elements that may be of interest in the periodic table that cannot be analyzed by INAA. For this reason INAA laboratories often partner with laboratories that do X-ray fluorescence (XRF) analysis, which is a complementary technique to INAA. The combined methods can produce high quality data for about 60 elements in the periodic table. The elements that can routinely be determined by INAA, and their detection limits, are listed in Table 1.

Table 1. Detection limits (DL) for elements that can be determined by INAA
DL (nanograms)	Elements
0.01-0.1	Au, Eu, Ho, Ir, Sm, Lu
0.1-1	Ag, As, Co, Cs, Hf, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, W, Yb
1-10	Ba, Br, Ce, Cr, Gd, Mo, Na, Nd, Ni, Rb, Sr, Zn, Zr
10-100	K
100-1000	Fe

There are numerous potential applications for INAA in forensic investigations. Here we give two examples – determining the source of maple syrup and identifying the region of origin of grass samples.

1) Maple syrup – during the production of maple syrup there are several opportunities for the introduction of characteristic elemental signatures – initial elemental signatures in the sap due to differences in the underlying soil chemistry, trace elements introduced during the tapping of the tree and transport to the sugar house, and trace elements introduced during the boiling down of the sap to produce maple syrup. In Table 2 we list selected elements and elemental ratios for maple syrup from various sources that allow us to distinguish between these different sources.

Table 2. Elemental characteristic of maple syrup. Numbers in bold are characteristic of the particular sample.
	Quebec	Newton	Winsor	Parker	Gale
Sc	0.030	0.010	0.009	0.004	0.006
Cr	1.67	0.67	0.71	0.83	0.87
Co	0.119	0.094	0.064	0.073	0.057
Zn	19.4	9.3	13.1	50.6	76.3
Rb	9.0	7.5	3.1	10.2	15.7
Sr	17.5	28.6	13.7	10.7	8.3
As	0.016	0.029	0.014	0.022	0.010
Sb	0.009	0.018	0.010	0.034	0.010
Se	8.72	ppb
Zn/Cr	11.6	13.9	18.5	61	88
Rb/Cs	419	642	363	433	175
Ba/Sr	0.37	0.59	0.18	0.76	1.29
As/Sb	1.91	1.59	1.50	0.64	2.24

2) Serengeti grasses – grass samples were collected from a variety of locations in a several hundred square kilometer area of Serengeti National Park, Tanzania. The major genera are Digitaria, Pennisetum, Sparobolus, and Themeda. The samples were analyzed for a number of trace elements by INAA. Many elements were determined in the ppb to 10s of ppm range. Grass samples collected from different areas show different abundances and abundance ratios for a number of the trace elements. These variations are in part due to variations in the chemistry of the underlying soils. Hence, trace element abundances can be used to identify the geographic location of a grass sample. It should be noted that 50 years ago mineral exploration geologists used the chemistry of plant materials to explore for ore deposits. Elevated abundances of the elements of interest in the plant material suggested a possible exploration area. Ecologists have also shown for the more common elements that there is a relationship between soil chemistry and plant chemistry. Hence the relationship between soil chemistry and the chemistry of plant material is well established and can be used to differentiate between grass samples collected from different areas.

Electronic version

Nature Blog - Geological Society of London Forensic Geology Meeting 2008

Back to publications.

Instrumental Neutron Activation Analysis (INAA) - Practice and Application

Eby, G. N.

Instrumental Neutron Activation Analysis (INAA) is a relatively straightforward technique for determining elemental abundances in a wide range of materials. The method utilizes the interaction between a thermal (or higher energy) neutron and a nucleus to produce a radioactive nuclide that emits characteristic gamma rays. Solid state detectors are used to sense the emitted gamma rays, and after suitable corrections and comparisons with standards, an absolute elemental concentration is determined.

The advantages of INAA are (1) it is a relatively cheap analytical method, a state-of-the-art facility can be acquired for significantly less than $100,000 compared to the much higher costs of competing analytical methods; (2) the method is non-destructive hence the same sample can be used for other measurements; (3) sample size can be very small, often as little as a milligram; (4) detection limits for many elements are in the nanogram range; (5) no chemical preparation is required, samples are analyzed as is; and (6) about 40 elements can be measured essentially simultaneously. The major disadvantage of INAA is that there are a number of elements of interest in the periodic table that cannot be analyzed by INAA. For this reason INAA laboratories often partner with laboratories that do X-ray fluorescence (XRF) analysis, which is a complementary technique to INAA. The combined methods can produce high quality data for about 60 elements in the periodic table. For a wide range of analytical problems the major competing method is Inductively Coupled Plasma Mass Spectrometry (ICPMS). This method involves significant sample preparation steps, which depending on the material can be very problematic, and high entry costs (on the order of $500,000 to $1,000,000). However, ICPMS can be used to determine elemental abundances for most of the elements in the periodic table, which is its major advantage.

Some examples of application of INAA (and when needed XRF) are (1) trace element analyses of minerals and rocks; (2) partitioning of metals between phases in coal; (3) origin of archaeological artifacts; (4) metals in hair, nails, etc.; (5) chemistry of atmospheric aerosols; (6) metals in tree rings as a proxy for environmental pollution; (7) chemistry of Serengeti grasses as a factor in animal behavior; (8) correlation of tephra (ash) layers for archaeological projects; and (9) characterization of trace constituents in nanotech materials.

Electronic version

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