Determination of uranium in solution at the ng 1−1 level by time-resolved laser-induced spectrofluorimetry: application to geological samples

Time-resolved laser-induced spectrofluorimetry was used for the determination of uranium at the ng 1^?1 level in alkaline water with high chloride and sulphide contents in order to obtain a better understanding of uranium mobilization under natural reducing conditions. The optimum complexing medium, limit of detection and results obtained for several waters from different localities in the Pyrenees are presented. These data seem to confirm that even at very low uranium concentrations, thermodynamic control by a mineral phase takes place.     

Determination of low-molecular-weight carboxylic acids in aqueous samples by gas chromatography and nitrogen-selective detection of 2-nitrophenylhydrazides

A gas chromatographic method was developed for the determination of low-molecular-weight carboxylic acids in aqueous samples based on a derivatization procedure compatible with aqueous solutions. The technique uses nitrogen-selective detection with a thermionic-specific detector after derivatization of carboxylic acids as 2- nitrophenylhydrazides. The hydrazides were extracted with ethyl acetate prior to injection into the gas chromatograph. The derivatives appear to be stable in ethyl acetate at 0–5° C for long periods and, therefore, can be stored for analysis at a later date. The detection limits of different short-chain acids are in the range 0.8–1.4 pmol per injected sample. The relative standard deviation is less than 10% at the 1 μM level. Examples of the use of the method are given for the determination of carboxylic acids in anoxic marine sediment pore waters, coastal sea water and Black Sea water samples.     

Determination of dissolved inorganic phosphorus in natural waters at nanomolar concentrations using a long capillary cell detector

A high sensitivity spectrophotometric system using a long capillary cell (LCC) detector was developed to determine dissolved inorganic phosphorus (DIP) at nanomolar concentrations in natural waters. The colorimetric chemistry used is based on the classical molybdophosphate blue technique. The radiation source is a near-infrared light-emitting diode and a silicon phototransistor is used as a detector. Method parameters were optimized for the final LCC design. With a 600-mm LCC, a detection limit (twice the standard deviation of the blank) of 1 nmol l^?1 was obtained with a relative standard deviation of 6%. The working range of the instrument is 1–500 nmol l^?1 DIP, and a sample volume of 10 ml is required for each analysis. The technique was applied to both fresh and marine water. The instrument is compact, relatively simple and easy to use in the laboratory and the field.     

Determination of gold in natural waters by neutron activation and γ-spectrometry after preconcentration with tributyl phosphate as solid extractant

A method for the preconcentration of gold in natural waters at the sampling site using tributyl phosphate as a solid extractant [Se(TBP)] was developed as a preliminary step prior to the determination of gold by neutron activation and γ-spectrometry. The SE(TBP) was saturated with gaseous chlorine for extracting all gold species. In batch experiments gold was quantitatively retained on the SE(TBP) in 10 min. After extraction and washing, the SE(TBP) was ashed or back-extracted. Gold was quantitatively eluted with hot, neutral 0.025 M thiourea. The gold content of residues of ashing or eluents after evaporation was determined by neutron activation and γ-spectrometry. The detection limit for the overall procedure was 0.2 ng 1^?1. The efficiency was tested on ‘equilibrated’ solutions prepared from river water and tracer solutions of gold. For comparison, the gold content of natural water samples was determined using preconcentration on activated charcoal.     

Rapid ion-exchange technique for the separation and preconcentration of chromium(VI) and chromium(III) in fresh waters

A technique for the separation and preconcentration of Cr(VI) and Cr(III) in fresh waters is presented. The analytical procedure involves the use of anion- and cation-exchange columns. The columns are eluted and the eluate is analysed for chromium using a graphite furnace atomic absorption spectrometer. The recovery of Cr(VI) and Cr(III) is 97.86 ± 1.31% and 102.36 ± 1.25% (95% confidence), respectively. The detection limits are 0.019 and 0.020 μg 1^?1 for 200 ml of sample (twice the standard deviation of eleven replicate blanks). The method is rapid and the need for minimum sample handling avoids contamination problems.     

Field method for determination of traces of thiols in natural waters

Derivatization with 2,2′-dithiobis(5-nitropyridine) (DTNP) was used to stabilize thiols in field samples and to determine these compounds by liquid chromatography (LC) with ultraviolet detection. Alternatively, the thiols can be regenerated from DTNP derivatives using tributylphosphine (TBP) and derivatized with o-phthalaldehyde (OPA). The DTNP derivatives were stable at pH 5 and 6 for more than 2 weeks in the reaction mixture, but the stability was considerably lower at pH values > 8. Derivatization at pH 6 was routinely done and the reaction was complete within 5 min. The derivatives can be extracted quantitatively on commercial C₁₈ cartridges, which allows several-fold sample enrichment. The cartridge-adsorbed derivatives are stable at 0–5 °C for long periods and, therefore, can be stored for LC separation at a later date. DTNP derivatives of ten low-molecular-weight thiols were separated on a C₁₈ microbore column using gradient elution and a flow-rate of 200 μl min^?1. The relative standard deviation based on repeated analyses of standards is about 5% in the range 0.5–2 nM. Without C₁₈ cartridge enrichment, the detection limits of the studied thiols are in the range 50–100 nM. These limits are further reduced by about 100-fold by using the OPA method after regeneration of thiols from the DTNP derivatives using TBP. Applications of the method to coastal sediment pore water and anoxic Black Sea water are illustrated.     

Development of a pulsed-laser,fiber-optic-based fluorimeter: determination of fluorescence decay times of polycyclic aromatic hydrocarbons in sea water

A fiber-optic-based system for remote measurement of time-resolved fluorescence emission spectra is described and characterized. A pulsed nitrogen laser is used to induce fluorescence and a time-gated, one-dimensional photodiode array is used to measure the decay of the fluorescence emission spectra. The results compare favorably with reported values for well characterized compounds having fluorescence decay times in the range 4–50 ns. The potential of using time-resolved fluorimetry (TRF) over fiber-optic cables as a means of improving the specificity of remote fluorescence determinations of spectrally similar polycyclic aromatic hydrocarbons in sea water is demonstrated.     

Spectrophotometric determination of uranium in natural waters

A method for the spectrophotometric determination of uranium in samples of natural water is described. Ion exchange with Amberlite IR-120 (H⁺) to concentrate the metal was used. The absorption properties of the complex formed between uranium and the chromogenic reagent Arsenazo III, its stability over several hours, the effect of the pH on the ability of the resin to retain uranium, the reproducibility of the method and the effects of ionic interferences were considered. The sensitivity was 0.67 and 0.05 μg l^?1 of uranium for the direct and the addition methods, respectively. Uranium concentrations for the samples analysed were between 0.10 and 0.50 μg l^?1.     

Synthesis of novel chitosan resin derivatized with serine moiety for the column collection/concentration of uranium and the determination of uranium by ICP-MS

A chitosan resin derivatized with serine moiety (serine-type chitosan) was newly developed by using the cross-linked chitosan as a base material. The adsorption behavior of trace amounts of metal ions on the serine-type chitosan resin was systematically examined by packing it in a mini-column, passing a metal solution through it and measuring metal ions in the effluent by ICP-MS. The resin could adsorb a number of metal cations at pH from neutral to alkaline region, and several oxoanionic metals at acidic pH region by an anion exchange mechanism. Uranium and Cu could be adsorbed selectively at pH from acidic to alkaline region by a chelating mechanism; U could be adsorbed quantitatively even at pH 3–4. Uranium adsorbed on the resin was easily eluted with 1 M nitric acid: the preconcentration (5-, 10-, 50- and 100-fold) of U was possible. The column treatment method was used prior to the ICP-MS measurement of U in natural river, sea and tap waters; R.S.D. were 2.63, 1.13 and 1.37%, respectively. Uranium in tap water could be determined by 10-fold preconcentration: analytical result was 1.46±0.02 ppt. The resin also was applied to the recovery of U in sea water: the recovery tests for artificial and natural sea water were 97.1 and 93.0%, respectively.     

A very sensitive flow system for the direct determination of copper in natural waters based on spectrophotometric detection

A very sensitive flow injection method with spectrophotometric detection has been developed for the on-line determination of copper in natural waters. The method exhibits a limit of detection three times lower than the most sensitive direct spectrophotometric method previously described and then allows the direct and simple in situ determination of copper in most natural waters.The method was based on the measurement of the absorbance of the coloured complex formed by copper with the chromogenic reagent di-2-pyridyl ketone benzoylhydrazone (dPKBH) in an alkaline medium. This complex presents stoichiometry 1:2 (Cu:dPKBH), and exhibits maximum absorbance at 370 nm. The manifold used was very simple, and consisted of two channels. The first one contained the sample while the second one contained the colorimetric reagent (3.3×10⁻⁴ M dPKBH in 10% ethanol), in a 1.6×10⁻² M phosphate buffer solution at pH 8. The performance of the system was optimised by using both univariate and modified simplex methodologies. When modified simplex was used, the best signal was obtained for a sample injection volume of 529 μl, a reaction coil length of 1.29 m, and a reagent flow rate of 4.8 ml min⁻¹. Under optimum conditions, the response was linear up to 3 mg l⁻¹ copper, the equation of the straight line being y=0.314x+5.2×10⁻⁴ (r²=0.998). The method allowed a sampling frequency of 40 samples per hour and exhibited a precision of 2.11% (as R.S.D., n=11). The limit of detection was 4.6 μg l⁻¹ (calculated as 3s_b/m, where s_b is the standard deviation of the y-intercept and m represents the slope of the straight line), and was therefore more sensitive than all the direct continuous methods reported previously.The method was successfully applied to the analysis of real water samples, with an average relative error of 5.32%.