Detection Limits

Elemental detection limits for NAA are variable because some elements become very radioactive–, and can be determined at very low levels while other elements do not become very radioactive or have very short half–lives (less than 10 seconds). Activation analysis determines the total mass of an element in a sample. A certain amount of an element, like arsenic–, is needed in the sample for detection. For arsenic, under ideal conditions, 5 ng is required. To determine 5 ppb of arsenic, 1 g of sample is sufficient. To determine 0.5 ppb of arsenic, 10 g of sample is necessary, etc. The production of radioactive nuclides depends on the cross sections of the specific elements. Also important is the number of gamma rays that are emitted by a radionuclide. In some cases, only a small fraction of the total emissions from a specific nuclide is in the form of gamma rays. Samples with high concentrations of elements that are readily activated and emit a considerable number of gamma rays, such as Na and Sc, can generate high background count rates and raise the detection limits for the element(s) of interest. The following table shows the best case minimum detection levels (MDL) for the 70 elements that NAA is capable of identifying.

Text version of data available at https://www.usgs.gov/data-tools/detection-limits