Determining Elemental Composition with Neutron Activation Analysis
Jun. 26 2019
Neutron Activation Analysis (NAA) is a sensitive and accurate trace element analytical method. Samples are irradiated in a nuclear research reactor to analyze the characteristic gamma rays that were emitted during radioactive decay. These distinct energy signatures provide positive identification and quantification of the targeted elements present, while their intensity is proportional to the concentration of the element in the sample. Samples are measured without any preparation, thereby avoiding problems that are common with other techniques: incomplete dissolution, loss of volatile elements or contamination from laboratory chemicals. Neutron activation is not subject to calibration uncertainties associated with chemical methods of trace elemental analysis, further enhancing accuracy.
NAA is considered a referee method, based solely upon well-characterized nuclear vs. chemical properties. The chemical forms of elements present in a sample have virtually no effect on the results. This sensitive technique delivers such high quality analytical data that it is used by the National Institute of Standards and Technology (NIST) and other metrology institutes to certify their standards. NAA is also used to verify the homogeneity of reference materials given its multi-element capability and dynamic range of analysis. The accuracy of NAA makes it valuable for certification of elemental composition and for comparison with other trace element analytical techniques.
When should Neutron Activation be considered?
- When accuracy is important
- To verify the results of chemical methods of analysis
- For quality control programs testing for high purity
- For matrices that are difficult to analyze using digestion procedures
- For elements prone to losses or contamination
What are the advantages of NAA?
- Results independent of the chemical form of an element
- Free of contamination from lab chemicals
- None or minimal sample preparation
- Limited matrix effects
- Applicable for most matrices: liquids, air filters, and solids – quartz, graphite, textiles, plastics/polymers, food, soil and rock, pottery and oil/petroleum products
- Cost effective analysis of 30+ elements*
- Ability to analyze large (40g) samples, minimizing subsampling error
- Sensitivity to parts-per-billion for specific elements
- Effective for limited sample quantities where only milligrams of precious material are available
- Total sample analysis, not just extractable or surface analysis as with some other analytical techniques
*NAA is not suitable for the analysis of lead, bismuth, or phosphorus.
What are the common applications of NAA?
Geochemical Exploration Applications
Geological surveys and mining companies use NAA to test stream and lake sediments for gold and associated elements, such as arsenic and antimony.
Table 1: Gold +33 package (typical detection limits)
NAA is applied for measurement of total Chlorine and Bromine in rocks and ores because dissolution and contamination are a problem with other techniques due to the volatility of the elements and their use in mineral acids.
Platinum Group Elements (PGE)
NAA provides results for all six PGE compared to traditional fire assay techniques that commonly determine Platinum and Palladium as shown in Table 2.
Table 2: Platinum Group Elements
Table 3: Key Elements Analyzed in Vegetation and Humus**
|Gold||0.2 ppb||Arsenic||0.1 ppm||Selenium||0.5 ppm|
|Antimony||0.02 ppm||Mercury||0.05 ppm||Tungsten||0.5 ppm|
**No ashing required
The unusual abundance of Iridium in a rock layer may indicate a meteor impact. NAA is used to investigate this Iridium anomaly because of its capability of measuring Iridium in the sub-ppb range.
Quartz and Carbon Materials
NAA easily measures ultratrace impurity levels in quartz sands and silica wafers, as shown in table 4. Similarly, NAA can be applied to analysis of graphite, carbon and related nano-materials. There is no requirement to get the difficult matrix into solution and large sample sizes can be processed. Measurement of impurities by NAA is used for quality control during the manufacture of semi-conductors, solar power cells, lenses, optical fiber and lighting.
Table 4: Typical Detection Limits Commonly Measured in Quartz
^Other elements are available
Iodine in Food and Pet Food
NAA is used to measure iodine levels added to food and pet food, as shown in table 5. As samples are analyzed directly without heating and dissolution, total iodine is reported as there is no loss of this volatile element. In addition to iodine, chlorine, bromine and many other elemental concentrations can be reported as they are analyzed simultaneously with iodine.
Table 5: Typical Detection Limits of Impurities Commonly Measured in Food and Pet Food
^Other elements are available
Medical Products and Research
Silver nanoparticles have been added to certain products to prevent bacterial growth, reduce the risk of infection and speed up healing time. Silver-containing products include bandages, dressings, syringes, urinary catheters, breathing tubes, contact lenses and lens cases. Research is also being conducted on the application of gold and silver nanoparticles for treatment of cancer.
NAA is the preferred method of analysis for gold and silver nanoparticles because it measures the sample directly, providing a sensitive, total concentration of the element present. This is valuable for quality control, assuring the appropriate amount of nanoparticle has been applied to the product to ensure biocide efficacy.
Oils & Plastics
NAA is used for the trace elemental analysis of plastics, oils, and polymers to determine additives and contaminants. There is no requirement for sample preparation with NAA, making it amenable to direct analysis of matrices that can be challenging to work with using other techniques.
Table 6: Typical Detection Limits of Elements Commonly Measured in Polyethylene**
NAA is used for characterization of archaeological artefacts (pottery, obsidian, chert, basalt and limestone) and relating the elemental profiles to that of the geographic source material. In doing so, archaeologists have learned about prehistoric trade routes and movement of people.
Only a small portion of the precious archaeological artefact is required for analysis by NAA, which measures 34 elements simultaneously providing an informative elemental profile for comparison.
The volatile nature of fluorine makes it challenging to measure in the laboratory using some methods as the element can be lost during sample preparation. Depending on the matrix, NAA has the advantage of improved sensitivity (1 ppm) compared with measurement e.g., by ion selective electrode (50-60 ppm).
For these reasons, NAA is used to measure the quantity of fluorine in various products for quality control:
- Textile industry – fluorinated compounds added to carpet fiber and apparel to improve water-, oil- and stain-resistance and improve durability
- Polymers – dissolution of polymers can be extremely difficult for other test methods
- Oral hygiene products – fluoride added to toothpaste and mouthwash
- Pharmaceuticals – about 20% of pharmaceuticals contain fluorine
Luminescence dating measures the last time an object was exposed to sunlight and is an important method for dating archaeological artefacts.
Bureau Veritas provides precise measurements of the elements that produce ionizing radiation (uranium, thorium, potassium and rubidium). See Table 7 for the detection limits. This data is used in the age calculation along with the luminescence reading. Precision (uncertainty) can be reported with results.
Table 7: Luminescence Dating