Proper preparation of samples is of utmost importance to the analysis. Raw urine can not 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 mg/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).