Radiochemical methods use the specific properties of radionuclides for analytical purposes. There are three main types of radiochemical analysis: radiometric analysis, isotope dilution, and activation analysis. Radiometric analysis uses a radioactive reagent to isolate the analyte. Isotope dilution introduces a known quantity of a radioactive isotope of the analyte. Activation analysis induces radioactivity in sample elements through neutron bombardment, then identifies elements by their characteristic gamma ray emissions. These methods provide sensitive, specific quantitative and qualitative analysis of elements in samples.
1. Radiochemical Methods
Ch 14, 7th e, WMDS)
Radiochemical methods of analysis depend on the
specific properties of certain radionuclides. These
properties include the type and energy of the radiation
emitted, the half-life (t1/2), and the decay schemes of that
particular nuclide.
2. Radiochemical Methods
Ch 14, 7th e, WMDS)
Radiochemical methods are both sensitive and
specific. There are three general types of radio-
analytical methods of analysis:
1. Radiometric analysis
2. Isotope dilution, and
3. Activation analysis
We will consider each of these methods.
3. Radiochemical Methods
Ch 14, 7th e, WMDS)
Before we enter into a discussion of these methods it is probably
helpful to review some of the features of radioactive decay.
1 - Particles may be emitted by radioactive isotopes include
alpha, α, the He-4 nucleus, 42He
electrons, β, 0-1e
positrons, β+, 01e
gamma, γ, or 00γ
X-rays, similar to gamma, differ only in their source;
γ from nuclear events
– X-rays from electronic transitions external to the nucleus.
4. Radiochemical Methods
Ch 14, 7th e, WMDS)
2 – Decay processes of radionuclides are first-order process, i.e.,
dN/dt = kM or,
ln(N/N0)= -kt
Radionuclides are characterized by 1) the particles emitted, and
2) their half-lives, t1/2
The half-life, t1/2 is defined as the time for N0 to decay to one-half
its original value; at t1/2 , N = ½ (N0). The decay is a random
process, so one deals with a statistically large population of nuclei.
5. Radiochemical Methods
Ch 14, 7th e, WMDS)
Decay curve for a First-order Process; t1/2 ~ 2.9 days
6. Radiochemical Methods
Ch 14, 7th e, WMDS)
For example, note the t1/2 values of following radionuclides:
H-3 (31H), β, 12.33 y
C-14 (146C), β, 5730 y
Co-60 (6027Co), β, γ, 5.27 y
Te-99m (99m43Tc), γ, 6.01 h
I-131 (13153I), β, 8.02 d
H-6 (62He), β, 0.86 s
Ra-226 (22688Ra, α, γ, 1620 y
8. Radiochemical Methods
Radiometric Analysis
Ch 14, 7th e, WMDS)
Radiometric analysis – the use of a radioactive reagent of
known activity to isolate the analyte from the other
components of the sample. The activity of the product is
directly proportional to the amount of the analyte.
Example: Chromate has been determined by precipitating it
with radioactive Ag+ (Ag-111, β, γ, 7.5d) of a known activity.
The limited reactant is the analyte, here CrO4-2. Determining
the activity of the precipitate of Ag2CrO4 allows for the
determination of the amount of chromate.
9. Radiochemical Methods
Radiometric Analysis
Ch 14, 7th e, WMDS)
A second example is the determination of magnesium or
zinc by precipitation with phosphate that contains a known
activity due to the presence of the radionuclide P-32 (β,
14.2 d). The known formulas of the precipitates are
Mg3(PO4)2 and Zn3(PO4)2. The determination of the
activity of the precipitate is related to the amount of
phosphate present in the final product.
10. Radiochemical Methods
Isotopic dilution
Ch 14, 7th e, WMDS)
Isotopic dilution analysis was introduced by von Hevesy and Hofer
in 1934. It involves the preparation of the analyte in a radioactive
form. A known weight of this compound labeled with isotope
(such as an acid with O-18 or a hydrocarbon with H-2) is then
mixed with the mixture containing the compound to be analyzed.
After treatment to ensure homogeneity between the labeled and
unlabelled species, a portion is recovered as a chemically pure
substance.
11. Radiochemical Methods
Isotopic dilution
Ch 14, 7th e, WMDS)
The pure substance is weighed and its radioactivity
measured. The extent of the dilution of the
radioactive sample may then be calculated and
related to the amount of the nonradioactive substance
in the original sample. Quantitative recovery (100%
yield) is not required for a successful analysis.
12. Radiochemical Methods
Isotopic dilution
Ch 14, 7th e, WMDS)
The mathematical relationship for calculating the amount of
the material in the original sample is
Wm / Wa = Ai / Af - 1 or
Wm = Wa { Ai / Af – 1}
where Wm is the mass of the analyte, Wa is the mass of the
radioactive compound added, Ai is the activity of the added
compound, and Af is the activity of the final purified
compound.
13. Radiochemical Methods
Isotopic dilution
Ch 14, 7th e, WMDS)
Isotopic dilution is especially useful in the analysis of complex
biochemical substances, such as vitamins D and B, insulin,
steroids, that occur a complicated matrix such that methods of
separation and analysis are difficult. In a recent work
(www.wcaslab.com/tech/isotope_dilution.htm), the isotopes are
detected by a highly sensitive mass spectrometer; the radioactive
form of the compound then acts as an internal standard for the
analysis because the mass of the isotope used for labeling is
different than the non-radioactive form of the element.
14. Radiochemical Methods
Activation Analysis
Ch 14, 7th e, WMDS)
Neutron activation analysis (NAA) was discovered in
1936 when Hevesy and Levi found that samples
containing certain rare earth elements became highly
radioactive after exposure to a source of neutrons.
From this observation, they recognized the potential
of employing nuclear reactions on samples followed
by measurement of the induced radioactivity to
facilitate both qualitative and quantitative
identification of the elements present in the samples.
15. Radiochemical Methods
Ch 14, 7th e, WMDS)
There are several ways of inducing the radioactivity in the
atoms present in the sample for analysis. The most
common is neutron activation in which the sample is
irradiated with neutrons. After the irradiation, the gamma
or beta spectrum is obtained, depending on the type of
emission produced by the irradiated element. For
quantitative work, both may be used. The energies of the
spectral peaks allow for the identification of the elements
present and the areas of the peaks define the amounts of
each element as shown in the next slide.
16. Radiochemical Methods
Ch 14, 7th e, WMDS)
Gamma ray spectrum of a sample after neutron induced radioactivity
17. Radiochemical Methods
Ch 14, 7th e, WMDS)
For many elements and applications, neutron
activation analysis offers sensitivities that are superior
to those attainable by other methods - on the order of
parts per billion or better. In addition, because of its
accuracy and reliability, it is often recognized as the
"referee method" of choice when new procedures are
being developed or when other methods yield results
that do not agree.
18. Radiochemical Methods
Ch 14, 7th e, WMDS)
The basic essentials required to carry out an analysis of
samples by NAA are a source of neutrons,
instrumentation suitable for detecting gamma rays, and a
detailed knowledge of the reactions that occur when
neutrons interact with target nuclei.
19. Radiochemical Methods
Ch 14, 7th e, WMDS)
The sequence of events occurring during the most common
type of nuclear reaction used for NAA, namely the neutron
capture or (n, gamma) reaction, is illustrated in Figure 1.
When a neutron interacts with the target nucleus via a non-
elastic collision, a compound nucleus (metastable) forms
in an excited state. The excitation energy of the metastable
nucleus is due to the binding energy of the neutron with the
nucleus. This nucleus will almost instantaneously de-excite
into a more stable configuration through emission of one or
more characteristic prompt gamma rays.
20. Radiochemical Methods
Ch 14, 7th e, WMDS)
Fig. 1. Diagram illustrating the process of neutron capture by a
target nucleus followed by the emission of gamma rays. The above
figure is from http://www.missouri.edu/~glascock/nna_over.htm
21. Radiochemical Methods
Ch 14, 7th e, WMDS)
In many cases, this new configuration yields a radioactive
nucleus which also decays by emission of one or more
characteristic delayed gamma rays, but at a much slower
rate according to the unique half-life of the radioactive
nucleus. Depending upon the particular radioactive
species, half-lives can range from fractions of a second to
several years.
22. Radiochemical Methods
Ch 14, 7th e, WMDS)
Although there are several neutron sources such as
reactors, accelerators, and radioisotopic neutron emitters,
nuclear reactors with their high fluxes of neutrons from
uranium fission offer the highest available sensitivities for
most elements. Different types of reactors and different
positions within a reactor can vary considerably with
regard to their neutron energy distributions and fluxes due
to the materials used to moderate the primary fission
neutrons.
23. Radiochemical Methods
Ch 14, 7th e, WMDS)
There are 3 types of neutrons, classified according to their
energies:
1) thermal (low energy, < 0.5eV)
2) epithermal (mid energy, 0.5eV to 0.5MeV) , and
3) fast (> 0.5 MeV) An NAA technique that employs
nuclear reactions induced by fast neutrons is called fast
neutron activation analysis (FNAA).
24. Radiochemical Methods
Ch 14, 7th e, WMDS)
Measurement of Gamma Rays
The instrumentation used to measure gamma rays from
radioactive samples generally consists of a semiconductor
detector, associated electronics, and a computer-based,
multi-channel analyzer (MCA/computer). Most NAA labs
operate one or more hyperpure or intrinsic germanium
(HPGe) detectors which operate at liquid nitrogen
temperatures (77 degrees K) by mounting the germanium
crystal in a vacuum cryostat, thermally connected to a
copper rod or "cold finger".
25. Radiochemical Methods
Ch 14, 7th e, WMDS)
Gamma-ray spectrum showing several short-lived elements measured in a sample of
pottery irradiated for 5 seconds, decayed for 25 minutes, and counted for 12 minutes
with an HPGe detector.
26. Radiochemical Methods
Ch 14, 7th e, WMDS)
Gamma ray spectrum of a sample after neutron activation analysis. The energy of
the emitted γ ray serves as qualitative analysis; the area of the peak is related to
the amount of that element.
27. Radiochemical Methods
Ch 14, 7th e, WMDS)
Sensitivities Available by NAA
The sensitivities for NAA are dependent upon the irradiation parameters (i.e., neutron flux, irradiation and decay times), measurement
conditions (i.e., measurement time, detector efficiency), nuclear parameters of the elements being measured (i.e., isotope abundance, neutron
cross-section, half-life, and gamma-ray abundance).
For more details regarding the sensitivity of NAA see http://nucleus.wpi.edu/Reactor/labs/R-naa.html
• Possible Applications for NAA
• 1. Archaeology
• The use of neutron activation analysis to characterize archaeological specimens (e.g., pottery, obsidian, chert, basalt and limestone) and to
relate the artifacts to sources through their chemical signatures is a well-established application of the method. Over the past decade, large
databases of chemical fingerprints for clays, obsidian, chert and basalt have been accumulated through analysis of approximately thirty
elements in each of more than 42,000 specimens. The combination of these databases with powerful multivariate statistical methods (i.e.,
principal components analysis, factor analysis, discriminant analysis, and Mahalanobis distance probabilities) allows many archaeological
materials to be sourced with a high degree of confidence. The sourcing information can help archaeologists reconstruct the habits of prehistoric
peoples. For example, the "fingerprinting" of obsidian artifacts by NAA is a nearly 100 percent successful method for determining prehistoric
