Kinetic Phosphorescence Analyzers for trace level detection of Uranium & Lanthanides

Seawater Sample Prep


The presence of chloride in seawater samples is a major interferant during measurements of uranium with the Kinetic Phosphorescence Analyzer (KPA). Treatment of the seawater samples before performing KPA measurements is required under these circumstances: Chloride will quench the uranyl phosphorescence, causing a short luminescence lifetime and a low response. Also, seawater samples may contain sediments and vegetation residues that may cause curvature of the luminescence decay plot and short luminescence lifetimes. When quenching from the interferants in vegetation and sediments is very strong, the results will be close to the detection limit, with apparently long lifetimes.

Elimination of chloride from seawater samples can be effectively accomplished by boiling to dryness an aliquot of the sample with concentrated nitric acid. Add 2-3 ml of concentrated nitric acid to an aliquot (2-10 ml) of seawater. Heat the sample to sub-boiling temperature, making sure no spattering occurs, which would cause loss of sample. All the liquid must be eliminated and only a white, dry residue will be left at the end of the treatment. Redissolve theKPA - Kinetic Phosphorescence Analyzers for trace level detection of Uranium & Lanthanides residue in 4 M nitric acid while heating gently, then dilute to the final volume, making sure the final concentration of nitric acid in the sample does not exceed 2M.

To eliminate the interference due to vegetation residues, wet-ashing of the sample to destroy the organic components is necessary. Wet-ashing can be carried out using concentrated nitric acid and 30% hydrogen peroxide until a pale yellow or white residue is obtained.

KPA Measurements of a Seawater Sample

The amount of uranium present in seawater samples will vary from region to region. The following measurements are meant only to give a range of expected values for the luminescence lifetimes and R².

TREATMENT DILUTION URANIUM CONC. 
(µg/L) (µs)
LIFETIME (µs)
None None 0.038 419-492 0.928
HNO3 None 0.610 255-263 0.997
HNO3 None 0.615 261-270 0.998
Ashed None 0.637 255-267 0.997

Results are the average of 10 measurements. Without sample treatment (first row of data), the seawater sample gave a luminescence response close to the detection limit. In this situation, lifetimes longer than the expected value (250-300 µs) were observed. The two treatments with nitric acid were carried out on two different aliquots of the same sample. Results of the duplicates agree within 0.8%. The wet-ashed aliquot agreed with the nitric acid analyses within 4%. Luminescence decay plots for all treated aliquots were linear as indicated by the R² values. Lifetimes are in the expected range for uranium in the presence of URAPLEX™.