Advantages and Disadvantages of
Instrumental Neutron Activation Analysis (INAA)

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INAA Advantages

 Many elements can be determined simultaneously.

We routinely determine concentrations of 25-30 elements in rock samples. See “Typical Results...”

 INAA is highly sensitive to some trace elements.

Sb, Cs, La, Sm, Eu, Tb, Lu, Ta, Au, and Ir can be determined at sub-ppm levels with 10% precision, or better.

 High accuracy and precision can be obtained.

In “homogeneous” rock powders of basalts or shales analyzed over many years, we typically achieve 1% precision (one standard deviation) for Na, Sc, Fe, Co, La, Sm, and Eu, and better than 3% for Ce, Yb, and Hf. [See: Korotev (1996) A self-consistent compilation of elemental concentration data for 93 geochemical reference samples. Geostandards Newsletter 20, 217-245.]

 Many samples can be analyzed in one batch.

We typically analyze samples in batches of 50-100.

 Little sample preparation is necessary.

We usually analyze samples "as is." No dissolution is necessary. Rock samples are usually analyzed as powders to ensure representative subsampling, but rock "chunks," glass fragments, and mineral grains are easily accommodated.

 Contamination and blank problems are usually negligible.

Samples ground in tungsten carbide, however, are usually seriously contaminated with W, Co, and Ta.

 Small sample can be analyzed.

For terrestrial whole-rock analysis, we usually analyze 100-300 mg subsamples. For our lunar work, we routinely analyze 1-50 mg samples. We have analyzed individual glass spheres as small as 50 µg. (We cannot determine a concentration value any more precisely that we can measure the mass of the sample.)

 Matrix effects are minimal.

Most samples are effectively transparent to both neutrons and gamma rays, thus certain types of matrix effects are negligible. There are some exceptions: we cannot analyze samples with high concentrations of B, Cd, or rare earth elements because isotopes of these elements absorb a significant fraction of the neutron flux.

 The technique is mostly nondestructive.

After analysis, the sample is moderately radioactive. In most samples, radiation levels are low after a year (<0.05 mR/hr @ 30 cm). Isotope ratios are noticeably altered for some elements. However, we routinely make thin sections of analyzed rock chips for petrographic study and subsequent analysis by electron and ion microprobe.

INAA Disadvantages

 Some elements of interest cannot be determined or are determined imprecisely.

Some trace elements that we do not determine are P, Mn, Cu, Ga, Ge, Y, Nb, Mo, Pr, Gd, Dy, Er, Hg, and Pb. Some of these elements (Mn, Mo, Dy) are determined by other INAA labs that are capable of radioassaying short-lived isotopes and others (Mo, Hg) we can determine when they occur in much higher concentrations than are typical of common rocks.

 The techniques is slow.

Because INAA relies on radioactive decay of elements with half lives up to 13 years the technique is inherently slow. It typically takes eight weeks from the time we begin sample preparation until we have final data. For many elements, less-precise "preliminary" data are available after 2-3 weeks.

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