RU2338186C1 - Method of preparing samples for detecting content of uranium isotopes in oils - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to analytical chemistry, namely, to method of preparing samples for detecting elements and their isotopes in hydrocarbon, mineral and synthetic, in particular, vacuum oils, oil products and fuels and lubricants. Method of preparing samples for detecting content of uranium isotopes in oils by method of weight-spectrometry with inductively-connected plasma includes selection of oil samples and their decomposition with concentrated acid at high temperatures and pressure. Sample decomposition is carried out till full transparency according to multi-step programme in microwave oven by means of nitric acid in amount not less than 5 cm³ per each sample with weight from 0.4 g to 0.6 g. High frequency radiation power is maximum, pressure within autoclave being within the range from 500 to 1200 kPa, with exposure time on sample from 5 to 15 min at each decomposition stage.

EFFECT: ensuring full decomposition of hydrocarbon oils samples to state of transparency, reduction of sample decomposition time and lowering limits of detecting uranium isotopes in hydrocarbon oils samples.

1 dwg, 1 ex, 3 tbl

Description

The invention relates to analytical chemistry, and in particular to methods of sample preparation for determining the content of elements and their isotopes in hydrocarbon, mineral and synthetic, in particular, vacuum oils, petroleum products and fuels and lubricants.

The method is of interest to research and production laboratories of the nuclear industry engaged in the production and processing of materials containing uranium with an isotopic composition different from the natural composition of the isotopes.

Known methods for determining uranium isotopes in various materials by inductively coupled plasma mass spectrometry (ICP MS), for example, a method for determining the content of uranium isotopes in soils [1], include two successive stages: decomposition (mineralization) of the sample material and subsequent determination of the content isotopes of uranium in solution on a mass spectrometer with ICP. The greatest difficulty, especially when analyzing organic materials, is the procedure of complete decomposition of the sample to obtain a clear solution. In the case of incomplete decomposition of organic matter, its residues change the viscosity of the solution, settle inside the sample-passing communications, on the internal parts of the device and thereby distort the analysis results.

A known method for determining uranium isotopes in materials containing organic compounds, for example in urine [2], comprising oxidizing the sample with nitric and hydrochloric acids when heated on an electric stove to dissolve uranium and destroy the organic matrix, followed by determining the content of uranium isotopes in solution by ICP MS. This method is very long, does not provide complete decomposition of samples of hydrocarbon oils to obtain a clear solution and introduces an error in the analysis results.

Known methods for the decomposition of samples of hydrocarbon oils with concentrated acids at high pressure and high temperature. The closest in technical essence is the method of preparing oil samples by decomposition with acids at high pressure and high temperature to determine traces of 28 elements (uranium is not among them) by atomic spectroscopy and mass spectrometry with ICP, atomic absorption spectroscopy with cold vapor and graphite cell and radiometric methods [3]. According to this method, a weighed sample of oil weighing 0.2-0.7 g is placed in a glass, poured 5 ml of nitric acid, 2.5 ml of hydrochloric acid, covered with a film of polytetrafluoroethylene, a hole is pierced in the film, covered with a lid, wrapped with foil and installed in the oven high pressure (High Pressure Asher). The furnace is filled with inert gas, argon, or nitrogen, the temperature program is set, and the decomposition process, which consists of several stages, is started. Each decomposition stage has its predetermined temperature and duration. During decomposition, the pressure in the glasses and inside the furnace is equalized. Monitoring the pressure level in the furnace by this method is not carried out. Table 1 presents the temperature program for the decomposition of oils and oily waste by the considered method.

Table 1 Temperature program for the decomposition of oils and oily waste [3] Stage Initial temperature, ° С Time min Final temperature, ° С one one hundred thirty 125 2 125 60 300 3 300 60 300 four 25 thirty 25 5 0 0 0

The main disadvantage of this method of sample preparation is that it does not allow to determine the content of the uranium-238 isotope in hydrocarbon oils by ICP MS at a level below 0.01 mg / g and the content of the uranium-235 isotope below 0.001 mg / g, since the sample solution leaves traces of undecomposed oil, which settles inside the communications of the mass spectrometer and thereby dramatically increases the analysis error and the memory of the device, especially for the uranium-235 isotope.

In addition, this method is very long. The net decomposition time of the sample is 3 hours.

The objective of the invention is to provide such a method of sample preparation, which would simultaneously ensure complete decomposition of samples of hydrocarbon oils to a state of transparency, reducing the time of decomposition of samples and reducing the detection limits of uranium isotopes in samples of hydrocarbon oils.

The problem is achieved in that in the known method, including sampling oil and decomposing them with concentrated acid at high temperature and pressure, the decomposition of the samples is completely transparent according to a multi-stage program in a microwave oven with nitric acid in an amount of at least 5 cm ³ for each sample weighing from 0.4 g to 0.6 g at the maximum power of high-frequency radiation and pressure in the autoclaves in the range from 500 to 1200 kPa at a time of exposure to the sample from 5 to 15 minutes at each stage of decomposition.

As a result of the changes made, the process of decomposition of hydrocarbon oil samples in determining the content of uranium-238 and uranium-235 isotopes in it became more efficient, decomposition of samples to the degree of transparency of hydrocarbon oil became possible with the addition of only one concentrated nitric acid, the time for decomposition of oil samples was reduced, and the limits of detection of uranium isotopes.

The optimal conditions for the decomposition of samples of hydrocarbon oil in order to achieve full mineralization were selected on vacuum oil samples by selecting the values of the decomposition mode parameters (pressure inside the autoclaves and exposure time of microwave radiation) at 100% radiation power and a maximum load of 12 autoclaves, due to the design of the device used. A portion of the oil in all cases was approximately equal to 0.5 g, the volume of concentrated nitric acid - 5 cm ³ . The completeness of the decomposition process was monitored visually by the absence (or presence) of oil in the mineralized solution. The values of the established parameters of the decomposition mode and the results of a visual assessment of the completeness of the decomposition processes are shown in table 2.

table 2 The values of the parameters of the decomposition mode and the evaluation of the quality of decomposition of vacuum oil samples Decomposition mode option Variable Settings Decomposition stage Decomposition Quality Assessment one 2 3 four 5 one Radiation power,% one hundred 0 one hundred - - Undecomposed sediment left Pressure kPa 500 0 800 - - Time min 8 fifteen fifteen - - 2 Radiation power,% one hundred 0 one hundred 0 one hundred Undecomposed sediment left Pressure kPa 500 0 800 0 900 Time min 8 fifteen fifteen fifteen fifteen 3 Radiation power,% one hundred 0 one hundred 0 one hundred No draft Pressure kPa 500 0 800 0 1200 Time min 8 12 8 5 fourteen four Radiation power,% one hundred 0 one hundred 0 one hundred No draft Pressure kPa 500 0 800 0 1200 Time min 8 5 5 5 fifteen

The drawing shows graphs of the dependence of pressure in the autoclaves on the time of exposure to microwave radiation for different versions of the decomposition modes corresponding to modes 1, 2, 3, 4 from table 2.

Based on the experimental data, decomposition mode No. 4 was chosen.

An example of preparing a sample of hydrocarbon oil.

From a sample of oil after thoroughly mixing it with a glass rod for 1-2 minutes using a microdoser with a capacity of 200-1000 mm ^3, two samples of oil weighing about 0.5 g (560 mm ³ ) are taken, placed in high-pressure fluoroplastic autoclaves and weighed on an analytical balance with an error of ± 0.2 mg. Then, 5 cm ^{3 of} concentrated nitric acid are poured into each sample of oil using a microdoser with a capacity of 1-5 cm ³ and autoclaves are placed in a microwave oven. Then, the oil samples are decomposed using five stages of the decomposition mode with regulated pressure inside the autoclaves and the duration of each stage.

The values of the parameters of the decomposition regime of vacuum oil samples at its various stages with a full load of the microwave oven (12 autoclaves) are presented in table 3.

Table 3 Values of parameters for the decomposition mode of hydrocarbon oil samples Parameter Decomposition Mode Stage one 2 3 four 5 Radiation power,% one hundred 0 one hundred 0 one hundred Pressure, kPa. 500 0 800 0 1200 Time min 8-10 5-12 5-8 5-8 14-15

When using fewer autoclaves, the microwave radiation power is programmed in accordance with the number of autoclaves used in the following relationship:

for 10 autoclaves - 90%, for 8 - 80%, for 6 - 70%, for 4 - 50%, for 2 - 40%.

After the decomposition of the samples, the autoclaves are allowed to cool directly in the microwave for 30 minutes. Then the autoclaves are removed from the microwave and opened. Nitric acid solutions (basic solutions) obtained after microwave mineralization of oil samples are transferred from autoclaves to volumetric flasks with a capacity of 100 cm ³ . The autoclaves are washed 2-3 times with distilled water in portions of about 5 cm ³ and the washing solutions are connected to the main ones. Then, 5 cm ^{3 of} concentrated nitric acid are added to the autoclaves, the autoclaves are washed with acid, and the contents of the autoclaves are quantified in volumetric flasks with basic solutions. Then the autoclaves are washed 2-3 times with distilled water in portions of about 5 cm ³ and the washing solutions are connected to the main ones. The resulting solutions were adjusted to the mark with water and mixed.

Before measuring, samples are diluted 100 times with distilled water. Diluted solutions are analyzed on an inductively coupled plasma mass spectrometer with optimal instrument performance.

Thus, as a result of the proposed method for preparing oil samples, the decomposition time of the samples was reduced to 60 minutes, the detection limits of the uranium-238 isotope in hydrocarbon oils decreased to 0.001 mg / kg, and the uranium-235 isotope to 0.0001 mg / kg.

Information sources

1. Standart Test Method for Analysis of Total and Isotopic Uranium and Total Thorium in Soils by Inductively Coupled Plasma-Mass Spectrometry. ASTM, C 1345-96 (Reapproved 2001).

2. Standart Test Method for Analysis of Urine for Uranium-235 and Uranium-238 Isotopes by Inductively Coupled Plasma-Mass Spectrometry. ASTM, C 1379-04.

3. Standart Practice for Preparation of Oils and Oily Waste Samples by High-Pressure, High-Temperature Digestion for Trace Element Determinations. ASTM, C 1234-98 (Reapproved 2004).

Claims (1)

A method of preparing samples for determining the content of uranium isotopes in oils by inductively coupled plasma mass spectrometry, including sampling the oil and decomposing it with concentrated acid at high temperature and pressure, characterized in that the decomposition of the samples is completely transparent according to a multi-stage microwave program furnaces with nitric acid in an amount of at least 5 cm ³ for each sample weighing from 0.4 to 0.6 g with a maximum power of high-frequency radiation and pressure in autoclaves in the range from 500 to 1200 kPa when the exposure time for the sample is from 5 to 15 minutes at each stage of decomposition.

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