Chemchek Instruments, Inc.
KINETIC PHOSPHORESCENCE ANALYZER (KPA)
URANIUM ASSAY OF URINE
Proper preparation of samples is of utmost importance to the analysis. Raw urine cannot be analyzed
without pretreatment except at levels well above 20 μg/L. A large dilution, >1000, is necessary to
dilute away the matrix effects. The reasons for this are: the organic constituents fluoresce, may
complex uranium, and along with chloride, quench uranyl phosphorescence. Sample treatment, i.e.,
wet-ashing, becomes increasingly important as the needed detection limit is lowered. Residual
unsaturated organics in a poorly ashed sample may raise the detection limit by a factor of ~100.
Aliquots of samples from the same bottle may ash differently.
This procedure is written for urine analyses near the KPA detection limit of about 0.02 μg/L. The
requirements for ashing are less stringent at higher levels. However, good ashing technique is still
needed for good precision.
Reagents
Deionized Water: Preferably over 10 megaohms.
URAPLEX: uranium complexant.
Concentrated
nitric
acid
and
30%
hydrogen
peroxide.
Equipment
Acid Dispenser - Such as 500 mL Repipet; set the dispensed volume to 1.5 mL. Glass reservoirs
should be acid-leached; plastic reservoirs should be immersed in boiling water before use.
Liquid Scintillation Vials, For analyses below ~0.2 μg/L, vials must be leached in either 1 M
phosphoric or 4 M nitric acid at sub-boiling temperature for two or more days to remove leachable
uranium. Rinse well with deionized water.
Hot Plate.
Heating Block, optional - Made from 40 - 45 mm (1.5 inch) thick aluminum with a number of holes
such as 50 holes to accept vials, 20 mm (0.75 inch) deep, 28 mm (1.1 inch) diameter. Handles for
carrying may be placed at each end.
Muffle Furnace.
Procedure
1. Add weight or volume aliquot of sample (2-10 mL) to vial. NOTE: Samples with solids must be
homogenized
(vigorously
stirred),
because
uranium
concentrates
in
solids.
2. Add 2-3 mL 16 M nitric acid with swirling and 0.5 mL of 30% hydrogen peroxide.
3. Place vials on hot plate at moderate heat and NEAR boiling for several hours. Hydrogen peroxide
may be effectively added after partially cooling the samples.
4. Increase heat to boil dry. The dry residue should be pale yellow to white in color. If the residue
is dark, repeat the procedure form step 2, replenishing the oxidants 1-2 times until the residue is pale
yellow to white.
Cool samples somewhat before adding hydrogen peroxide.
5.
When the vials are dry, place in muffle furnace at 500-550°C for at least 1/2 hour.
NOTE: WET-ASHING MUST BE COMPLETE TO AVOID RESIDUAL ORGANICS.
FURNACE TREATMENT WILL NOT COMPENSATE FOR POOR WET-ASHING.
The presence of carbon black after the furnace step indicates incomplete wet-ashing. For samples less
than 0.2 μg/L, the ashing should be restarted with a fresh aliquot.
6. When cool, dissolve the urine salts in 1/2 to 1 mL of 4 M nitric acid with warming. Dilute to
desired volume with deionized water. Swirl to mix. Final volume may be determined by weight.
NOTE: For efficiency, steps 2-6 can be accomplished in a heating block.
7. For low-level samples, it is beneficial to let solutions set overnight to let micro- particulates settle
out. Centrifuging is an alternative.
8. Analyze resulting solutions with the KPA. Avoid picking up solids with the sample aliquot.
Alternative Ashing Procedure: Perchloric acid (HClO4), 0.5 mL, may be used instead of 30%
hydrogen peroxide. Step 4 can be eliminated.
NOTE: The following article may be of interest for the analysis of urine, soil, and milk samples:
"A Rapid Method For Determining Nanogram Quantities Of Uranium In Urine Using The Kinetic
Phosphorescence Analyzer", by Linda L. Moore and R. L. Williams, Journal of Radioanalytical and
Nuclear Chemistry, Articles, Volume 156 (1), 1992, pages 223-233.
This paper describes the procedure used for the analysis of urine with the KPA. The authors reached
some interesting conclusions: 1) That large amounts of several elements such as Na, K, Mg, and Ca,
are found in urine, soil and milk samples. 2) When too much Na or K was present, the observed
concentration of uranium (measured by KPA) was higher than the actual concentration in the sample
(positive error in the KPA readings). 3) Too much Mg and Ca caused the observed concentration to
be lower than the actual concentration (negative error in the KPA readings). 4) The transition metals
Cr, Mn, Fe, Co, Ni and Cu, also caused negative interferences, although the concentrations of these
elements in urine, soil, and milk are lower than those of the alkali and alkaline earth metals.
In a separate study where KPA data and "-spectrometry data on milk samples were compared, a
positive error was obtained from the analysis of wet-ashed milk samples with the KPA. However, the
results from the KPA and "-spectrometry agreed when the wet-ashing of the milk samples was
followed by chemical separation (i.e. ion exchange).
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