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7. ANALYTICAL METHODS
Table 7-1. Analytical Methods for Determining Plutonium in Biological Materials
Sample matrix Preparation method
Analytical
method
Sample
detection limit
Percent
recovery Reference
Fecal matter
Wet ash; filter; extract;
electrodeposition on
platinum disk
α spectro
metry
No data
No data Singh and
Wrenn 1988
Bones
Dry ash; reduce valence
state; extract; electro
deposition
on platinum disk
α spectro
metry
No data
No data Singh and
Wrenn 1988
Milk
Dry ashed; dissolution in
HCl; extraction with
triisooctylamine;
coprecipitate with lanthanum
fluoride; filtration
α spectro
metry
No data
No data EPA 1984
(Method
00-09)
Plant
Dissolve starch; filter; wet
ash; extract; electro
deposition on platinum disk
α spectro
metry
0.0027 pCi
(0.1x10
-4
Bq)
No data Bunzl and
Kracke 1987
ICP-MS = inductively coupled plasma-mass spectrometry
PLUTONIUM
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7. ANALYTICAL METHODS
with a high content of total dissolved solids, requiring chemical separation of the analyte from the
dissolved flux material (Wolf 2006).
Plutonium solutions that contain: (1) other alpha-particle emitters (e.g., americium and neptunium); or
(2) large amounts of fission products (e.g., cesium), or interfering amounts of other substances such as
iron, calcium, uranium, and phosphorous need to undergo additional chemical separation procedures.
Non-radioactive carriers, such as lanthanum fluoride (LaF
3
), neodymium fluoride (NdF
3
),
and zirconium
phenylphosphate (ZrC
6
H
6
PO
4
), are used to selectively precipitate the lanthanides. Solvent extraction and
ion exchange separation methods are preferred methods because of better separations. In addition, they
do not involve the addition of nonvolatile substances resulting in an easier preparation of the co-
precipitation source used for alpha-particle counting.
These extraction techniques can be made very efficient and selective by adjusting the oxidation state of
the plutonium and other sample constituents. Common extraction methods specific for plutonium use
2-thenoyltrifluoroacetone (TTA), tetrapropylammonium trinitrate in isopropylacetone or triisooctylamine,
cupferron in chloroform, tributylphosphate, and tri-octylphosphine dioxide.
Anion exchange methods
with either nitric or hydrochloric acid solutions are commonly used. Cation exchange column methods
are less frequently used (Brouns 1980).
Prior to measurement, the separated and purified plutonium is typically deposited as a very thin layer on a
highly polished metal plancet. Two techniques that are commonly used are: (1) electrodeposition; and
(2) co-precipitation with a carrier. In electrodeposition, the plutonium is electrodeposited on a polished
stainless steel, or platinum disk. In the
co-precipitation technique, actinides can be co-precipitated from a
large volume of solution using anions such as fluorides, hydroxides, and phosphate. Actinides in the tri-
or tetravalent state can be removed from solution by the addition of lanthanide fluoride carriers, such as
NdF
3
or LaF
3
, which are used to co-precipitate the separated and purified plutonium from solution. Iron
hydroxide can also be used to co-precipitate actinides from a carbonate-free solution.
The precipitate is
then prepared for counting by either filtration or by evaporation of a slurry of the precipitate onto a
stainless steel disk (Hindman 1983; Mitchell 1960; Sill and Williams 1981; Talvitie 1972; Wolf 2006).
The U.S. Department of Energy Environmental Measurement Laboratory Procedures Manual and the U.S.
Transuranium and Uranium Registries Radio Analysis Procedures Manual provide techniques for the
determination of plutonium in biological samples using alpha spectroscopy (DOE 1997; USTUR 2001).
PLUTONIUM
200
7. ANALYTICAL METHODS
The other two alpha-particle emitting plutonium isotopes,
236
Pu and
242
Pu, are normally not found in
environmentally significant quantities, and are not common constituents of nuclear fuels or waste waters.
Therefore, they can be used as tracers to aid in the analysis of other isotopes. In this calibration
procedure, a known quantity of a tracer is added to the sample being analyzed in order to determine the
yield. This is the percentage of the total amount of plutonium in the sample that is actually measured in
the electrodeposited amount after the separation, purification, and preparation of the source (Brouns
1980).
The most critical step in the analysis of biological samples is complete dissolution of the
sample to assure
solubilization of all plutonium compounds. Biological samples are generally dissolved by wet ashing or a
combination of wet and dry ashing. High temperatures (700–1,000 °C) during ashing should be avoided
in order to prevent the formation of an insoluble form of plutonium dioxide (Nielson and Beasley 1980;
Sill 1975). Plutonium that has been distributed to urine, blood, or soft tissue as a result of metabolic
processes is usually in a readily soluble form. Lung tissue, feces, and excised tissue from wound sites
will likely contain insoluble forms of plutonium and will require treatment with
HF and repeated ashings
to effect solubilization. Tissues, feces, and vegetation require repeated treatment with a mixture of
concentrated nitric acid (HNO
3
), perchloric acid (HClO
4
), and sulfuric acid (H
2
SO
4
) in order to oxidize
the large amount of organic materials in these samples. If an insoluble residue remains after repeated
ashings, then fusion of the residue with gram quantities of an inorganic flux (e.g.,
sodium carbonate,
sodium pyrosulfate) can be used to effect solution. Known amounts of a plutonium isotope are
commonly added subsequent to the dissolution step so that the percentage of plutonium recovered after
separation and purification (i.e., the yield) may be determined. This added plutonium must be in the same
chemical form as the plutonium in the sample or the yield estimates will not reflect the percentage of
plutonium recovered from the dissolved sample (EPA 1976a; Nielson and Beasley 1980).
Methods used for concentrating plutonium in a sample by a carrier are often specific to one oxidation
state of the plutonium. For example, the classical bismuth phosphate-lanthanum fluoride method of
concentrating plutonium from urine samples is specific to plutonium in the tri- and tetravalent states and
will leave plutonium(VI) in solution. The fate of the various oxidation states of plutonium in humans is
not well understood and analysis procedures must insure reduction or oxidation
of plutonium into
appropriate oxidation states. Liver and kidney samples may contain metals (e.g., iron) that may greatly
reduce chemical yields during the final electrodeposition step (EPA 1976a).