Narrow Results

A simple but precise method for the PIXE analysis of trace heavy-metals in aqueous samples was developed, in which the PIXE targets were prepared by pre-concentrating heavy metals on a cellulose phosphate ion-exchange filter paper and no additional chemical treatment was required. Heavy metals in trace concentrations were quantitatively retained up to 16.7 μ-equivalent on a sheet of filter paper due to the excellent selectivity for heavy metals and ion-exchange kinetics of phosphate groups in cellulose matrix. Heavy metals of less than 1 μg on one filter paper are precisely and rapidly determined by PIXE analysis using 3-MeV proton beams. The present method is compared with the method preparing a PIXE target for each dissolved species of an element contained in aqueous samples. It will be resulted that the newly developed method enables an on-line PIXE analysis for river waters.

In this study, we carried out PIXE analysis of raw and treated water at five water treatment plants as well as of tap water from several houses located in each supply route. We used a simple fast procedure for preparing thin uniform targets of inorganic components in both soluble and insoluble fractions of aqueous samples in combination with preconcentration of trace heavy metals; that is, Nuclepore filtration targets for coarse particles, preconcentration targets for heavy metal ions and deposit targets for fine particles and soluble major constituents. The target preparation and the PIXE measurement are not time-consuming, and a broad range of concentrations (several tenths of ppb to a few tens of ppm) of 19 elements from Na to Pb is determined simultaneously with a precision sufficient to reveal the elemental distribution in the soluble and insoluble fractions. Tap water quality was examined as a function of elemental distribution in untreated water at the plants. We confirmed the increase of insoluble components of some heavy metals in untreated water taken from river due to heavy rainfall and the elution of Cu, Zn and Pb in drinking water by corrosion of the piping in some part of the water distribution systems.

A workshop on practical problems in biological applications of micro-PIXE analysis was held in one of the evening sessions at the 8th International Conference on Nuclear Microprobe Technology and Application which was held in Takasaki, Japan, throughout September 8-13, 2002. We summarize the participants' exchange of observations and suggestions on technical aspects of biological sample target preparation and specimen damage due to beam irradiation, which occur in micro-PIXE analysis under vacuum.

The aim of this study is to develop a method for monitoring water pollution caused by dissolved sulfides and other harmful elements by means of PIXE offering the advantage of multielemental nature, high sensitivity and speed of analysis for a wide variety of samples. Sulfide ions dissolved in an aqueous sample were converted to insoluble CuS compounds and then collected on a thin organic filter suitable as a backing foil for PIXE measurements. The standard method for scavenging CuS precipitate is founded on an investigation of the pH-dependence of the recovery of dissolved sulfides and the calibration curve covering the concentration range from 10 to 1000 ppb. The recovery-factor of CuS showed a maximum value at around pH 8 although it changed with the intrinsic pH of samples. The recovery factor for pH adjusted to 7.50 was kept constant in a wide concentration range of sulfide ions from 10 to 1000 ppb. The concentration of tens of ppm sulfur compound was detected in both drain water from the industrial waste disposal and in peripheral agricultural water, but the fraction of dissolved sulfide was only about 1%. The low concentration of dissolved sulfides was discussed from the viewpoint of the oxidation condition originated from the soluble and insoluble components detected in samples by PIXE analysis.

An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate chromium ions of different valence in water samples. Cr³⁺ ions are selectively adsorbed by ferric hydroxide colloids generated in the solution, and a PIXE target for analyzing the total concentration of chromium of hexavalent and trivalent states is prepared by depositing 0.15 ml of sample solution on a user-made thin polycarbonate film. PIXE analyses for the two kinds of targets reveal the fractions of chromium of different oxidation state. The standard method for collecting the colloids adsorbing Cr³⁺ ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved Cr³⁺ ions and the calibration curve is measured. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beams (0.7-5 nA beam currents). The lower detection limit of chromium in a 25 ml aquatic sample is 1 ppb for the Cr³⁺-precipitated targets and 3.4 ppb for the CrO₄^2--deposit targets based on the 3σ error of background counts integrated over the FWHM of chromium peak in the PIXE spectrum. The applicability of PIXE using these sample-preparation techniques for determining chromium oxidation states was confirmed using river water samples to which Cr³⁺ and CrO₄^2- ions were added in 50 ppb concentrations.

An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate arsenic ions of different oxidation states in water samples. Arsenate ions are separated from arsenite ions by co-precipitating into 10 ppm indium hydroxide colloids that are generated at pH 4.0 in a 25 ml solution containing 1 ppm phosphate ions and 25 ppm sulfate ions. Arsenite ions are oxidized to the pentavalent state with permanganate ions and adsorbed by indium hydroxide colloids generated afterwards in solution. The standard procedure for collecting the colloids adsorbing arsenic ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved arsenic ions and the obtained standard calibration curve covers the concentration range from 1 to 100 ppb for arsenic ions. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beam (0.7-4 nA beam currents). The lower detection limit of arsenic in a 25 ml aquatic sample is 0.3 ppb for the arsenic-precipitated targets based on the 3σ error of background counts integrated over the FWHM of arsenic peak in the PIXE spectrum. This sample preparation technique was then applied to analyze concentrations and oxidation states of arsenic in a river basin where hot springs are located upstream being possible sources for releasing arsenic in the river.

A new apparatus is made to prepare a uniform and thin target for PIXE analysis by atomizing sample solution hydrolyzed with nitric acid at 105ºC for 2 hrs. When concentration of NIST Bovine Liver (SRM 1577b) is determined, values of almost all elements heavier than Na, except for Cl and Br, which are volatile under acidic condition, are roughly same as those certified by NIST.

A paste spreading method for target preparation of biological materials for PIXE analysis by proton beams from a small cyclotron is developed. Quantitative results obtained for NIST Bovine Liver (SRM 1577b) are consistent with values certified by NIST for almost all elements heavier than Mg except for Ca and Rb. Concentrations of Cl and Br, which are volatile under acidic condition, can also be reasonably determined. Effect of wet ashing on determination of Se is examined, then recovery of Se, which means a ratio of determined value divided by that of Se added to NIST Bovine Liver, falls to less than 50% because of vaporization during drying, and especially decreases to 22% when the targets were dried by lyophilization. On the other hand, the recovery for targets prepared by the paste spreading method reaches about 100%. As an example of target preparation for a real sample material by the paste spreading method, elemental concentrations in the liver of Long-Evans Cinnamon (LEC) rats, where Cu spontaneously accumulates, and of Sprague-Dawley (SD) rats as a control are determined. Among 12 elements determined, the concentrations of hepatic S, Fe, Cu, Zn, and Se of LEC rats are significantly higher than those of SD rats (P<0.01).

PIXE technique has been applied to quantitative analysis of thorium and uranium ions in drainage from a radioisotope laboratory of Tohoku University. Two kinds of targets were prepared and analyzed with an in-air vertical PIXE system of 3 MeV protons. The concentrations of thorium and uranium higher than 40 ppb were easily determined by 3-μC irradiation on targets which are made from a 0.12-ml solution containing Ga-internal standard evaporated on a hand-made polycarbonate film. Uranium(VI) ions in a 25-ml sample were preconcentrated into a thin uniform target containing Zr or Pd as an internal standard by means of dibenzyldithiocarbamate complexation with subsequent condensation into dibenzylidene-D-sorbitol gels, and low concentration of 10 ppb was precisely determined by PIXE measurement. This method does not work for concentrating thorium ions. The PIXE analysis for these two kinds of targets has good sensitivity and precision enough to determine concentrations of thorium and uranium lower than their permissible concentration limits in drainage from a radioisotope laboratory.