CN113075014A - Tritium analysis sample preparation system - Google Patents
Tritium analysis sample preparation system Download PDFInfo
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- CN113075014A CN113075014A CN202110167972.XA CN202110167972A CN113075014A CN 113075014 A CN113075014 A CN 113075014A CN 202110167972 A CN202110167972 A CN 202110167972A CN 113075014 A CN113075014 A CN 113075014A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/40—Concentrating samples
- G01N2001/4038—Concentrating samples electric methods, e.g. electromigration, electrophoresis, ionisation
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Abstract
The invention belongs to the technical field of tritium analysis, and particularly relates to a tritium analysis sample preparation system, which comprises the following steps: distilling the collected water sample to remove salt; carrying out electrolytic concentration on the desalted water sample; neutralizing the concentrated electrolyte; putting the electrolytic cell filled with the electrolyte into a thermostat for vacuum distillation; preparing and measuring a sample; calculating; tritium elements with low concentration in underground water can be analyzed, and the method is high in accuracy and precision.
Description
Technical Field
The invention belongs to the technical field of tritium analysis, and particularly relates to a tritium analysis sample preparation system.
Background
The successful application of the environmental isotope tritium in various fields such as evaluating the age of shallow groundwater, researching the migration rule of underground and surface water and the like has made the development of the environmental isotope tritium an important means in the research of hydrology and geology, the electrolytic concentration-liquid scintillation counting method is the most important technical method for analyzing low-level tritium in water, and the tritium analysis sample preparation system is a key technology for testing the content of tritium.
Tritium has a half-life of only 12.32 years, so the tritium content in natural water is generally not very high. Particularly, since the atmospheric nuclear test is forbidden in 1964, the concentration of tritium in natural water is reduced year by year, for example, the concentration of tritium in atmospheric precipitation mostly falls back to 5-20 TU (concentration of tritium due to cosmic origin), and the concentration of tritium in underground water is mostly less than 10 TU.
Tritium is directly measured by a liquid scintillation counting method, the uncertainty of a synthetic standard of an analysis result is about 8TU, and the analysis precision cannot meet the application requirement of tritium as an environmental isotope in the related research field. The gradual fall-back of the tritium content of the land circulating water reduces the uncertainty of the synthetic standard of the method to be within 0.5TU, which becomes the premise that the method is continuously applied as a hydrology research means.
Tritium is concentrated by electrolyzing a water sample, and then measurement is performed by a liquid scintillation counting method, so that it becomes necessary and important to improve the analysis accuracy. The conventional sample preparation system for analysis by an electrolytic concentration-liquid scintillation counting method is not suitable for analyzing low-level tritium in water.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a tritium analysis sample preparation system, which has high accuracy and precision, can be widely applied to environmental geology and hydrology researches, and obtains good social benefit and economic benefit.
In order to solve the problems, the invention adopts the following technical scheme: tritium analysis sample preparation system, including the following steps:
A. distilling the collected water sample to remove salt;
B. carrying out electrolytic concentration on the desalted water sample;
C. neutralizing the concentrated electrolyte;
D. putting the electrolytic cell filled with the electrolyte into a thermostat for vacuum distillation:
1) inserting the electrolytic cell neutralized by the electrolyte into a heating thermostat, and connecting the electrolytic cell to a vacuum system through a metal joint;
2) the glass collection bottle is arranged on a vacuum distillation system, a liquid nitrogen cup is sleeved on the cold trap, and the modem regulates the temperature of the heating belt to 60-80 ℃;
3) starting a vacuum pump, opening all vacuum valves of a vacuum system, starting vacuumizing, and pumping air in the vacuum system, the electrolytic cell and a glass collecting bottle;
4) when the vacuum degree is less than 300Pa, sleeving the collection bottle on a liquid nitrogen cup;
5) continuously vacuumizing until the vacuum degree is less than 30Pa, and closing a vacuum valve of a unit consisting of the electrolytic cell and the sample collecting bottle to keep the unit consisting of the electrolytic cell and the sample collecting bottle in a vacuum state;
6) closing a vacuum valve of the vacuum system to keep the vacuum system in a vacuum state;
7) turning off the vacuum pump, presetting a temperature control temperature of 100 ℃ on a digital temperature controller of an electric heating system of the heating thermostat, and presetting distillation time for 60 minutes on a time timer;
8) starting an electric heating system of the heating thermostat;
9) when the working time preset by the time timer is up, the electric heating system of the heating thermostat automatically closes the main power switch;
10) taking down the liquid nitrogen cup sleeved on the collecting bottle, and taking down the collecting bottle after a water sample distilled from the electrolytic cell in the collecting bottle is melted to prepare a measuring sample;
E. sample preparation and measurement
1) Sample preparation
Firstly, transferring a water sample collected and distilled in a collecting bottle into a weighed 20mL counting bottle, weighing again, and controlling the amount of a concentrated water sample in the counting bottle to be 8 +/-0.01 g;
tritium-free water is supplemented when the water sample is insufficient;
adding 12.00mL of scintillation fluid, shaking up, and placing into an instrument sample chamber;
2) and measurement of the sample
The instrument should be started up 4 hours before the sample is measured and set the measurement parameters;
the sample is kept in the instrument sample chamber in a dark place and is kept still for 12 hours, and the sample is sequentially sent into the instrument for automatic measurement;
measuring a standard sample every 3-5 samples to be measured according to the drift condition displayed by the operation of the previous instrument, wherein the background sample and the standard electrolysis sample are measured once at the beginning of the same batch of samples, and the samples are measured once again after the end of the measurement; each measurement time of each sample is more than or equal to 400 minutes;
3) background sample
Adding 8.00g of tritium-free water into a 20mL counting bottle, adding 12.00mL of scintillation liquid, shaking to emulsify, and preparing three parts in parallel;
4) blank sample
Replacing a water sample with 250ml of tritium-free water, and preparing three blank samples according to the water sample testing steps;
5) standard specimen
Taking 8.00g of standard tritium water with known concentration to be put into a 20mL counting bottle, adding 12.00mL of scintillation fluid, shaking to emulsify, and preparing three parts in parallel;
6) calibration sample (electrolytic standard sample)
Absorbing 8.00mL of standard tritium water with known concentration into a weighed 250mL dry volumetric flask, and weighing to determine the accurate dosage of the standard tritium water;
then adding 242ml of tritium-free water to the volumetric flask to replace the water sample, and preparing three standard electrolytic samples according to the operation of the water sample testing step;
F. computing
1) Calculating the background counting rate:
in the formula: CRb-background count rate, times/min;
Cb-background count, times;
tbbackground measurement time, min;
2) calculating a standard net count rate:
in the formula:
-standard net count rate, times/min;
cst — standard count, times;
tst-Standard measurement time, min;
CRb-background count rate, times/min;
3) calculating the counting efficiency of the instrument:
in the formula: CE-Instrument count efficiency;
a0-standard use of tritium water initial specific activity, Bq/g;
m is the mass g of tritiated water used as a standard in measurement;
λ -decay constant, (ln2/12.32) y-1;
t-interval time from using tritium water for preparation standard to using tritium water for separation measurement standard, y;
60-convert the standard net count rate to a value in units of "times/s";
4) calculating the fractionation factor
The fractionation factor for each cell can be determined by electrolyzing a standard sample; before the electrolytic cell is used for electrolyzing a water sample for the first time, the electrolytic cell firstly measures the fractionation factor through experiments, each batch of experiment later carries three standard samples, and different electrolytic cells are used by the standard samples in batches in turn, so that the fractionation factor of each electrolytic cell is continuously corrected;
calculating the blank sample counting rate
In the formula: CRB-blank count rate, times/min;
CB-counting blank samples, times;
tB-blank measurement time, min;
calculating the net counting rate of the standard electrolytic sample:
in the formula:
-the net counting rate of standard electrolysis samples, times/min;
Ce,st-counting the number of standard electrolysis samples;
te,st-standard electrolysis sample measurement time, min;
CRB-blank count rate, times/min;
calculating the tritium recovery rate of the standard electrolytic sample:
in the formula: rt,stTritium recovery from standard electrolysis samples;
CE-Instrument count efficiency;
mu-preparing a standard water sampleStandard use mass of tritiated water, g;
λ -decay constant, (ln2/12.32) y-1;
t is the interval time from the preparation of standard tritium water to the preparation of standard water sample, y;
60-converting the net count rate of the standard electrolytic sample to a value in units of "times/s";
calculating fractionation factor (separation coefficient)
In the formula: beta-fractional distillation factor;
(V0/Vf)ststandard water sample concentration ratio, where V0 and Vf are the electrolysis initial and terminal volumes (mL), respectively;
Rt,st-tritium electrolysis recovery of a standard water sample;
5) calculating the tritium electrolysis recovery rate of the water sample
In the formula: rt,sa-electrolyzing and recovering tritium in a water sample;
(V0/Vf) sa-water sample concentration ratio, where V0 and Vf are the initial and final volumes (mL) of electrolysis, respectively;
beta-fractional distillation factor;
6) calculating the tritium content of the water sample:
calculating the net counting rate of the electrolyzed water sample:
in the formula:
-net counting rate of electrolyzed water sample, times/min;
Ce,sa-counting of the electrolyzed water sample;
te,sa-measuring time of electrolyzed water sample, min;
CRb-background count rate, times/min;
calculating the tritium content of the water sample:
in the formula: TU-tritium content of water sample, TU;
CE-Instrument count efficiency;
(V0/Vf)sawater sample concentration ratio, where V0 and Vf are the electrolysis initial and terminal volumes (mL), respectively;
Rt,sa-electrolyzing and recovering tritium in a water sample;
λ -decay constant, (ln2/12.32) y-1;
t-the time interval from the sampling date to the analysis date, y;
k-unit conversion factor, 0.11919 (Bq. L-1)/TU
7) Calculating the uncertainty of the analysis result
Calculating the relative standard deviation of the analysis result:
in the formula: sr-analyzing the relative standard deviation of the results;
CRb-background count rate, times/min;
te,sa-measuring time of electrolyzed water sample, min;
tbbackground measurement time, min;
calculating the standard deviation of the analysis result:
sTU＝TU×sr
in the formula: sTU-standard deviation of the analysis results, TU;
TU-tritium content of water sample, TU;
sr-analyzing the relative standard deviation of the results;
calculating the uncertainty of the analysis result:
U＝2×sTU
in the formula: u-uncertainty of analysis result, TU;
sTU-standard deviation of the analysis results, TU;
2-coverage factor value with confidence level of about 95%.
Preferably, the step A comprises the following steps:
1) weighing 300mL of water sample, pouring the water sample into a distillation flask, adding a plurality of glass beads, adding 0.1-0.2 g of potassium permanganate, covering a ground stopper, and placing the distillation flask on an electric furnace;
turning on circulating water, starting the electric furnace, and turning a temperature-adjusting knob of the electric furnace to 700-800W for distillation;
2) and discarding a few milliliters of water sample distilled initially, collecting the distilled water sample in a ground glass bottle or a high-density polyethylene bottle with a plug, and sealing and storing.
Preferably, the step B comprises the following steps:
1) weighing the cleaned and dried empty electrolytic cell and accessories;
2) weighing 1.20g of sodium peroxide in a cleaned and dried beaker, and measuring 250ml of desalted water sample by using a volumetric flask;
3) slowly adding about 100ml of desalted water sample in the volumetric flask into the beaker, shaking while adding to ensure that sodium peroxide is completely dissolved, and transferring into a cathode electrolytic cell of an electrolytic cell;
washing the residual water sample by using sodium hydroxide solution in the beaker for three times, and transferring the water sample into a cathode electrolytic cell of an electrolytic cell to obtain an initial water sample weight for electrolysis;
4) assembling the electrolytic cell, and installing the capillary air guide joint on the air outlet of the inner pole;
then inserting the electrolytic cell into a specified hole position of the liquid bath circulation low-temperature constant-temperature bath, connecting 24 capillary air guide joints by using exhaust pipes, and leading the exhaust pipes to the outside;
5) connecting each electrolytic cell in a series connection mode that an external pole is connected with a negative pole to form a cathode and an internal pole is connected with a positive pole to form an anode by using a special cable with a wiring terminal, and forming a loop with a direct current power supply, an ammeter and an ammeter;
6) starting a liquid bath circulation low-temperature constant-temperature system, and setting the electrolysis control temperature to be 0.5 ℃;
after the temperature of the liquid bath circulation constant-temperature low-temperature tank is reduced to 0.5 ℃ for about half an hour, a direct-current power switch is turned on, the direct current is slowly adjusted from 0A to 10A, an ampere-hour meter starts to measure the ampere-hour value, and an exhaust pipe enables H generated by electrolytic reaction to be discharged2And O2Leading to the outside.
Preferably, said C comprises the following steps:
1) when the ampere-hour meter reaches a set ampere-hour value, stopping the electrolytic reaction;
closing the direct current power supply and the liquid bath circulating low-temperature constant-temperature system, detaching the connecting cable, the exhaust pipe and the capillary air guide joint, taking out the electrolytic cell from the liquid bath circulating low-temperature constant-temperature tank, and drying the surface of the outer electrode of the electrolytic cell;
2) drying the electrolytic cell on the surface of the external pole in the air, and weighing to obtain the sample weight of the final water for electrolysis;
3) taking out the magnesium perchlorate drying tube which is heated and dehydrated in advance from the dryer, and installing the magnesium perchlorate drying tube on CO2A lead-in device;
introducing CO2The air duct of the importer is inserted into the inner pole of the electrolytic cell and rotates CO clockwise2The connector of the leading-in device is connected for two weeks, so that the leading-in device is firmly connected with the inside of the electrolytic cell and the vent hole is ensured to be smooth;
opening of CO2Regulating valve, introducing CO2And (5) gas is used for 3-5 minutes to neutralize the electrolyte in the electrolytic cell.
The invention has the beneficial effects that: detection limit (95% confidence level): less than or equal to 0.6 TU; synthesis standard uncertainty: less than or equal to 0.5TU (when the concentration of tritium is less than 20 TU), high accuracy and precision, and can analyze tritium element in underground water with low concentration.
Detailed Description
The electrolytic concentration-liquid scintillation counting method is the most important technical means for analyzing low-level tritium in water. With the gradual fall-back of the content of tritium in the land circulating water after nuclear explosion is controlled, the uncertainty of the synthetic standard of the method is reduced to be within 0.5TU, and the method becomes a premise for continuously applying the method as a hydrology research means. Although theoretically, the analysis uncertainty can be infinitely reduced by prolonging the counting time of the sample (and the standard and background of the simultaneous measurement), when the counting time exceeds 500min, the improved amplitude of the analysis uncertainty is rapidly reduced by prolonging the counting time, so that the cumulative counting time of the sample is generally set to 400-600 min in practical application, and further improvement of the analysis uncertainty is realized by improving the enrichment factor and the reproducibility of the electrolytic concentration system.
Determining design index in advance
Liquid scintillation counting method after introduction of electrolytic concentration procedure, if the specific radioactivity a of tritium is calculated by using the following formulat(hereinafter referred to as tritium concentration):
the net sample count rate N can be calculated as followssaContribution u (a) to uncertainty of analysis resultT,Nsa)：
In the formula ast、NstD, E and u (N)sa) Standard tritium concentration, standard net count rate, sample tritium decay correction factor, sample tritium enrichment factor, and standard uncertainty of sample net count rate, respectively. The tritium enrichment factor is calculated using the formula:
in the formula mI、mFQ, F and beta are respectively the initial quality of water sample electrolysis, the termination quality of water sample electrolysis, the electric charge quantity consumed by electrolyzing water samples, the Faraday constant and the separation coefficient of the electrolytic cell.
The formula shows that if the uncertainty u (N) of the net counting rate of the sample is assumedsa) U (a) independently of the change in the concentration of tritium before and after electrolysis (which is not completely consistent with the actual condition)T,Nsa) The only control component of standard uncertainty (often true for low levels of tritium) is synthesized for the analysis results, then the introduction of enrichment factor E will reduce the standard uncertainty of the analysis results by about 1/E. The standard uncertainty of a water sample directly measured by a liquid scintillation counting method is about 8TU, and the E value of an electrolytic concentration system is required to be not lower than 16 in order to reduce the standard uncertainty of an analysis result to 0.5 TU.
From the third expression, there are 3 ways to increase the value:
increase the concentration ratio mI/mF；
Increase Q/(m)I-mF)；
The separation coefficient beta is improved;
theoretically only by making the concentration ratio mI/mFSufficiently large to obtain an E value of any size. The requirement of the counting measurement on the sample dosage limits mFIncrease mI/mFThe approach can only be to increase m by increasing the volume of the electrolytic cellI. Increase mIWill prolong the electrolysis time and increase the volume of the thermostatic bathReduction in the number of large or electrolytic cell operations, increased labor intensity, etc., i.e., mIToo large will render the process inoperable. Comprehensively considering the factors of uncertainty analysis requirement, operability of the method, experiment cost and the like, and determining mIAnd was 250 g.
At mIAfter determination, to further reduce mI/mFThen m should be madeFThe specified sample amount is maximally approached to the counting measurement, for which purpose an accurate control of the electrolysis end point and a loss during the sample transfer process is maximally avoided by means of in-situ secondary distillation.
mIAnd/mFAfter determination, Q/(m) is increasedI-mF) The method reduces evaporation and spray loss in the water sample electrolysis process to improve the Q value, and therefore, the electrode reaction temperature should be reduced and other technical measures for inhibiting the evaporation and spray loss of the water sample should be developed.
The value of beta is related to the cathode material and the electrode reaction temperature of the electrolytic cell, and the cathode is generally made of flexible steel which can obtain larger beta value. On the other hand, the lower the electrode reaction temperature is, the larger the β value is, obviously, it is most desirable to control the reaction temperature to be close to 0 ℃, but limited by technical difficulty, and the temperature control point is set to 5 ℃ in the constant temperature bath adopted in most laboratories at present.
Analysis of the sample Net count Rate component of uncertainty u (a)T,Nsa) Is a controlling component and the other significant uncertainty component is u (a)Tβ). Experiments show that the tritium separation coefficients beta of all electrolytic cells are often greatly different, the beta value obtained by the same batch of standard cells cannot be used for calculating water samples of other sample cells in the batch, and otherwise, larger deviation is generated. The beta value of each electrolytic cell cannot be obtained in the same batch of the electrolyzed water sample, and the beta value of each electrolytic cell needs to be measured in advance. Thus, the difference of experimental conditions of different running batches inevitably leads to the fluctuation of the beta value, and the uncertainty of the beta value caused by the fluctuation is superposed into the uncertainty of the analysis result by a significant uncertainty component, especially for water samples with high tritium concentration, and is often used as the control of the uncertainty of the synthesis of the analysis resultOne of the braking component or the control component. Therefore, attention should be paid to the control of the uncertainty β value component of the analysis result, which can be achieved in two ways:
firstly, evaluating the average beta value of each electrolytic cell in long-term experiment, and calculating the analysis result of electrolytic water sample of each electrolytic cell according to the value
)。
And secondly, the uncertainty of the beta value is reduced by controlling the consistency of the electrolysis conditions of each operation batch.
The electrolysis conditions include electrode reaction temperature, electrolyte alkalinity, electrolysis rate and the like. Wherein the electrode reaction temperature should be taken into consideration at the design stage of the electrolytic concentration system. The electrode reaction temperature is controlled at a lower temperature level by continuously reducing the temperature of the outer electrode (cathode) of the electrolytic cell through a liquid bath circulation low-temperature constant-temperature device (called constant-temperature bath for short). The consistency of the electrode reaction temperature of each operation batch is controlled, and the stability and the uniformity of the liquid bath temperature of the constant-temperature bath are required to be realized.
Like most analytical experiments, contamination should be one of the factors affecting the uncertainty of the analytical results and the lower limit of the assay. Tritium contamination comes from moisture in indoor air and should be controlled by necessary technical measures.
Through the above analysis, to achieve the analysis result quality objective:
lower detection limit (95% confidence level): less than or equal to 0.6 TU;
synthesis standard uncertainty: less than or equal to 0.5TU (when the tritium concentration is less than 20 TU).
Meanwhile, considering the analysis capacity of batch samples to be formed by the designed tritium analysis electrolysis concentration system, and determining design indexes or parameters as follows:
initial mass m of water sample electrolysisI：250g。
Terminating mass m of water sample electrolysisF: 9g (8 g required for near scintillation counter measurements).
Anode material: 316L stainless steel; cathode material: and (4) softening the steel.
The thermostatic bath accommodates the number of electrolytic cells: 24 pieces of the Chinese herbal medicine are taken;
bath liquid medium of constant temperature bath: pure water.
Temperature control point of the thermostatic bath: 0.5 ℃; temperature control precision: better than plus or minus 0.1 ℃; temperature uniformity: plus or minus 0.1 ℃; cooling rate: not less than 12.8 ℃/h.
Secondary distillation mode: neutralizing with CO2, and in-situ vacuum distilling; the distillation temperature is less than or equal to 100 ℃; the distillation is completed by 24 electrolytic cells simultaneously;
direct-current power supply voltage: is more than 72V; and (3) charge metering precision: better than plus or minus 0.5 percent.
Electrolysis termination control mode: the ampere-hour meter is automatically controlled.
Electrolytic cell and cell water weighing precision: + -0.01 g.
Weighing precision of a secondary distilled water sample: + -0.1 mg.
The pond water has no environmental tritium pollution.
Preparation of Di, tritium analysis samples
The method comprises the following steps:
A. distilling the collected water sample to remove salt;
B. carrying out electrolytic concentration on the desalted water sample;
C. neutralizing the concentrated electrolyte;
D. putting the electrolytic cell filled with the electrolyte into a thermostat for vacuum distillation:
1) inserting the electrolytic cell neutralized by the electrolyte into a heating thermostat, and connecting the electrolytic cell to a vacuum system through a metal joint;
2) the glass collection bottle is arranged on a vacuum distillation system, a liquid nitrogen cup is sleeved on the cold trap, and the modem regulates the temperature of the heating belt to 60-80 ℃;
3) starting a vacuum pump, opening all vacuum valves of a vacuum system, starting vacuumizing, and pumping air in the vacuum system, the electrolytic cell and a glass collecting bottle;
4) when the vacuum degree is less than 300Pa, sleeving the collection bottle on a liquid nitrogen cup;
5) continuously vacuumizing until the vacuum degree is less than 30Pa, and closing a vacuum valve of a unit consisting of the electrolytic cell and the sample collecting bottle to keep the unit consisting of the electrolytic cell and the sample collecting bottle in a vacuum state;
6) closing a vacuum valve of the vacuum system to keep the vacuum system in a vacuum state;
7) closing the vacuum pump; presetting a temperature control temperature of 100 ℃ on a digital temperature controller of an electric heating system of a heating thermostat, and presetting distillation time for 60 minutes on a time timer;
8) starting an electric heating system of the heating thermostat;
9) when the working time preset by the time timer is up, the electric heating system of the heating thermostat automatically closes the main power switch;
10) taking down the liquid nitrogen cup sleeved on the collecting bottle, and taking down the collecting bottle after a water sample distilled from the electrolytic cell in the collecting bottle is melted to prepare a measuring sample;
E. sample preparation and measurement
1) Sample preparation
Firstly, transferring a water sample collected and distilled in a collecting bottle into a weighed 20mL counting bottle, weighing again, and controlling the amount of a concentrated water sample in the counting bottle to be 8 +/-0.01 g;
tritium-free water is supplemented when the water sample is insufficient;
adding 12.00mL of scintillation fluid, shaking up, and placing into an instrument sample chamber;
2) and measurement of the sample
The instrument should be started up 4 hours before the sample is measured and set the measurement parameters;
the sample is kept in the instrument sample chamber in a dark place and is kept still for 12 hours, and the sample is sequentially sent into the instrument for automatic measurement;
measuring a standard sample every 3-5 samples to be measured according to the drift condition displayed by the operation of the previous instrument, wherein the background sample and the standard electrolysis sample are measured once at the beginning of the same batch of samples, and the samples are measured once again after the end of the measurement; each measurement time of each sample is more than or equal to 400 minutes;
3) background sample
Adding 8.00g of tritium-free water into a 20mL counting bottle, adding 12.00mL of scintillation liquid, shaking to emulsify, and preparing three parts in parallel;
4) blank sample
Replacing a water sample with 250ml of tritium-free water, and preparing three blank samples according to the water sample testing steps;
5) standard specimen
Taking 8.00g of standard tritium water with known concentration to be put into a 20mL counting bottle, adding 12.00mL of scintillation fluid, shaking to emulsify, and preparing three parts in parallel;
6) calibration sample (electrolytic standard sample)
Absorbing 8.00mL of standard tritium water with known concentration into a weighed 250mL dry volumetric flask, and weighing to determine the accurate dosage of the standard tritium water;
then adding 242ml of tritium-free water to the volumetric flask to replace the water sample, and preparing three standard electrolytic samples according to the operation of the water sample testing step;
F. computing
1) Calculating the background counting rate:
in the formula: CRb-background count rate, times/min;
Cb-background count, times;
tbbackground measurement time, min;
2) calculating a standard net count rate:
in the formula:
-standard net count rate, times/min;
cst — standard count, times;
tst-Standard measurement time, min;
CRb-background count rate, times/min;
3) calculating the counting efficiency of the instrument:
in the formula: CE-Instrument count efficiency;
a0-standard use of tritium water initial specific activity, Bq/g;
m is the mass g of tritiated water used as a standard in measurement;
λ -decay constant, (ln2/12.32) y-1;
t-interval time from using tritium water for preparation standard to using tritium water for separation measurement standard, y;
60-convert the standard net count rate to a value in units of "times/s";
4) calculating the fractionation factor
The fractionation factor for each cell can be determined by electrolyzing a standard sample; before the electrolytic cell is used for electrolyzing a water sample for the first time, the electrolytic cell firstly measures the fractionation factor through experiments, each batch of experiment later carries three standard samples, and different electrolytic cells are used by the standard samples in batches in turn, so that the fractionation factor of each electrolytic cell is continuously corrected;
calculating the blank sample counting rate
In the formula: CRB-blank count rate, times/min;
CB-counting blank samples, times;
tB-blank measurement time, min;
calculating the net counting rate of the standard electrolytic sample:
in the formula:
-the net counting rate of standard electrolysis samples, times/min;
Ce,st-counting the number of standard electrolysis samples;
te,st-standard electrolysis sample measurement time, min;
CRB-blank count rate, times/min;
calculating the tritium recovery rate of the standard electrolytic sample:
in the formula: rt,stTritium recovery from standard electrolysis samples;
CE-Instrument count efficiency;
mupreparing a standard water sample, wherein the mass g of the standard tritium water is used;
λ -decay constant, (ln2/12.32) y-1;
t is the interval time from the preparation of standard tritium water to the preparation of standard water sample, y;
60-converting the net count rate of the standard electrolytic sample to a value in units of "times/s";
calculating fractionation factor (separation coefficient)
In the formula: beta-fractional distillation factor;
(V0/Vf)ststandard water sample concentration ratio, where V0 and Vf are the electrolysis initial and terminal volumes (mL), respectively;
Rt,st-tritium electrolysis recovery of a standard water sample;
5) calculating the tritium electrolysis recovery rate of the water sample
In the formula: rt,sa-electrolyzing and recovering tritium in a water sample;
(V0/Vf) sa-water sample concentration ratio, where V0 and Vf are the initial and final volumes (mL) of electrolysis, respectively;
beta-fractional distillation factor;
6) calculating the tritium content of the water sample:
calculating the net counting rate of the electrolyzed water sample:
in the formula:
-net counting rate of electrolyzed water sample, times/min;
Ce,sa-counting of the electrolyzed water sample;
te,sa-measuring time of electrolyzed water sample, min;
CRb-background count rate, times/min;
calculating the tritium content of the water sample:
in the formula: TU-tritium content of water sample, TU;
CE-Instrument count efficiency;
(V0/Vf)sawater sample concentration ratio, where V0 and Vf are the electrolysis initial and terminal volumes (mL), respectively;
Rt,sa-electrolyzing and recovering tritium in a water sample;
λ -decay constant, (ln2/12.32) y-1;
t-the time interval from the sampling date to the analysis date, y;
k-unit conversion factor, 0.11919 (Bq. L-1)/TU
7) Calculating the uncertainty of the analysis result
Calculating the relative standard deviation of the analysis result:
in the formula: sr-analyzing the relative standard deviation of the results;
CRb-background count rate, times/min;
te,sa-measuring time of electrolyzed water sample, min;
tbbackground measurement time, min;
calculating the standard deviation of the analysis result:
sTU＝TU×sr
in the formula: sTU-standard deviation of the analysis results, TU;
TU-tritium content of water sample, TU;
sr-analyzing the relative standard deviation of the results;
calculating the uncertainty of the analysis result:
U＝2×sTU
in the formula: u-uncertainty of analysis result, TU;
sTU-standard deviation of the analysis results, TU;
2-coverage factor value with confidence level of about 95%.
The method A comprises the following steps:
1) weighing 300mL of water sample, pouring the water sample into a distillation flask, adding a plurality of glass beads, adding 0.1-0.2 g of potassium permanganate, covering a ground stopper, and placing the distillation flask on an electric furnace;
turning on circulating water, starting the electric furnace, and turning a temperature-adjusting knob of the electric furnace to 700-800W for distillation;
2) and discarding a few milliliters of water sample distilled initially, collecting the distilled water sample in a ground glass bottle or a high-density polyethylene bottle with a plug, and sealing and storing.
The B comprises the following steps:
1) weighing the cleaned and dried empty electrolytic cell and accessories;
2) weighing 1.20g of sodium peroxide in a cleaned and dried beaker, and measuring 250ml of desalted water sample by using a volumetric flask;
3) slowly adding about 100ml of desalted water sample in the volumetric flask into the beaker, shaking while adding to ensure that sodium peroxide is completely dissolved, and transferring into a cathode electrolytic cell of an electrolytic cell;
washing the residual water sample by using sodium hydroxide solution in the beaker for three times, and transferring the water sample into a cathode electrolytic cell of an electrolytic cell to obtain an initial water sample weight for electrolysis;
4) assembling the electrolytic cell, and installing the capillary air guide joint on the air outlet of the inner pole;
then inserting the electrolytic cell into a specified hole position of the liquid bath circulation low-temperature constant-temperature bath, connecting 24 capillary air guide joints by using exhaust pipes, and leading the exhaust pipes to the outside;
5) connecting each electrolytic cell in a series connection mode that an external pole is connected with a negative pole to form a cathode and an internal pole is connected with a positive pole to form an anode by using a special cable with a wiring terminal, and forming a loop with a direct current power supply, an ammeter and an ammeter;
6) starting a liquid bath circulation low-temperature constant-temperature system, and setting the electrolysis control temperature to be 0.5 ℃;
when the temperature of the liquid bath circulation constant-temperature low-temperature tank is reduced to 0.5 ℃ for about half an hour, a direct-current power switch is turned on, the direct current is slowly adjusted from 0A to 10A, an ampere-hour meter starts to measure the ampere-hour value, and an exhaust pipe conducts electrolysis reaction to generate H2And O2Leading to the outside.
The step C comprises the following steps:
1) when the ampere-hour meter reaches a set ampere-hour value, stopping the electrolytic reaction;
closing the direct current power supply and the liquid bath circulating low-temperature constant-temperature system, detaching the connecting cable, the exhaust pipe and the capillary air guide joint, taking out the electrolytic cell from the liquid bath circulating low-temperature constant-temperature tank, and drying the surface of the outer electrode of the electrolytic cell;
2) drying the electrolytic cell on the surface of the external pole in the air, and weighing to obtain the sample weight of the final water for electrolysis;
3) taking out the magnesium perchlorate drying tube which is heated and dehydrated in advance from the dryer, and installing the magnesium perchlorate drying tube on CO2A lead-in device;
introducing CO2The air duct of the importer is inserted into the inner pole of the electrolytic cell and rotates CO clockwise2Connector of introducerTwo weeks to ensure that the electrolytic cell is firmly connected with the electrolytic cell and the vent hole is smooth;
opening of CO2Regulating valve, introducing CO2And (5) gas is used for 3-5 minutes to neutralize the electrolyte in the electrolytic cell.
Third, experimental data
Table 1 shows the analysis results of all 6 samples in the laboratory and the evaluation results of the detection ability given by IAEA for this system:
table 1:
the data in table 1 are from the global laboratory of the international organization for atomic energy (IAEA) and preliminary evaluations of the detection capabilities of the system are given in the activity of low-level tritium analysis in water versus activity (tri 2008).
Table 2 shows the final assessment conclusion published by IAEA on the accuracy and precision of the analysis results of this laboratory system, all of which were confirmed to be acceptable, and which can satisfy the requirements of the hydrological study on the quality of the analysis results.
Table 2:
the data in table 2 are from the final assessment conclusion given in the global laboratory low-level tritium analysis comparison activity in water (TRIC2008) of the international agency for atomic energy (IAEA) organization in 9 months 2009, and the code for this laboratory to participate in this activity is 48.
Claims (4)
1. Tritium analysis sample preparation system, characterized by, including the following step:
A. distilling the collected water sample to remove salt;
B. carrying out electrolytic concentration on the desalted water sample;
C. neutralizing the concentrated electrolyte;
D. putting the electrolytic cell filled with the electrolyte into a thermostat for vacuum distillation:
1) inserting the electrolytic cell neutralized by the electrolyte into a heating thermostat, and connecting the electrolytic cell to a vacuum system through a metal joint;
2) the glass collection bottle is arranged on a vacuum distillation system, a liquid nitrogen cup is sleeved on the cold trap, and the modem regulates the temperature of the heating belt to 60-80 ℃;
3) starting a vacuum pump, opening all vacuum valves of a vacuum system, starting vacuumizing, and pumping air in the vacuum system, the electrolytic cell and a glass collecting bottle;
4) when the vacuum degree is less than 300Pa, sleeving the collection bottle on a liquid nitrogen cup;
5) continuously vacuumizing until the vacuum degree is less than 30Pa, and closing a vacuum valve of a unit consisting of the electrolytic cell and the sample collecting bottle to keep the unit consisting of the electrolytic cell and the sample collecting bottle in a vacuum state;
6) closing a vacuum valve of the vacuum system to keep the vacuum system in a vacuum state;
7) turning off the vacuum pump, presetting a temperature control temperature of 100 ℃ on a digital temperature controller of an electric heating system of the heating thermostat, and presetting distillation time for 60 minutes on a time timer;
8) starting an electric heating system of the heating thermostat;
9) when the working time preset by the time timer is up, the electric heating system of the heating thermostat automatically closes the main power switch;
10) taking down the liquid nitrogen cup sleeved on the collecting bottle, and taking down the collecting bottle after a water sample distilled from the electrolytic cell in the collecting bottle is melted to prepare a measuring sample;
E. sample preparation and measurement
1) Sample preparation
Firstly, transferring a water sample collected and distilled in a collecting bottle into a weighed 20mL counting bottle, weighing again, and controlling the amount of a concentrated water sample in the counting bottle to be 8 +/-0.01 g;
tritium-free water is supplemented when the water sample is insufficient;
adding 12.00mL of scintillation fluid, shaking up, and placing into an instrument sample chamber;
2) and measurement of the sample
The instrument should be started up 4 hours before the sample is measured and set the measurement parameters;
the sample is kept in the instrument sample chamber in a dark place and is kept still for 12 hours, and the sample is sequentially sent into the instrument for automatic measurement;
measuring a standard sample every 3-5 samples to be measured according to the drift condition displayed by the operation of the previous instrument, wherein the background sample and the standard electrolysis sample are measured once at the beginning of the same batch of samples, and the samples are measured once again after the end of the measurement; each measurement time of each sample is more than or equal to 400 minutes;
3) background sample
Adding 8.00g of tritium-free water into a 20mL counting bottle, adding 12.00mL of scintillation liquid, shaking to emulsify, and preparing three parts in parallel;
4) blank sample
Replacing a water sample with 250ml of tritium-free water, and preparing three blank samples according to the water sample testing steps;
5) standard specimen
Taking 8.00g of standard tritium water with known concentration to be put into a 20mL counting bottle, adding 12.00mL of scintillation fluid, shaking to emulsify, and preparing three parts in parallel;
6) calibration sample (electrolytic standard sample)
Absorbing 8.00mL of standard tritium water with known concentration into a weighed 250mL dry volumetric flask, and weighing to determine the accurate dosage of the standard tritium water;
then adding 242ml of tritium-free water to the volumetric flask to replace the water sample, and preparing three standard electrolytic samples according to the operation of the water sample testing step;
F. computing
1) Calculating the background counting rate:
in the formula: CRb-background count rate, times/min;
Cb-background count, times;
tbbackground measurement time, min;
2) calculating a standard net count rate:
in the formula:
-standard net count rate, times/min;
cst — standard count, times;
tst-Standard measurement time, min;
CRb-background count rate, times/min;
3) calculating the counting efficiency of the instrument:
in the formula: CE-Instrument count efficiency;
a0-standard use of tritium water initial specific activity, Bq/g;
m is the mass g of tritiated water used as a standard in measurement;
λ -decay constant, (ln2/12.32) y-1;
t-interval time from using tritium water for preparation standard to using tritium water for separation measurement standard, y;
60-convert the standard net count rate to a value in units of "times/s";
4) calculating the fractionation factor
The fractionation factor for each cell can be determined by electrolyzing a standard sample; before the electrolytic cell is used for electrolyzing a water sample for the first time, the electrolytic cell firstly measures the fractionation factor through experiments, each batch of experiment later carries three standard samples, and different electrolytic cells are used by the standard samples in batches in turn, so that the fractionation factor of each electrolytic cell is continuously corrected;
calculating the blank sample counting rate
In the formula: CRB-blank count rate, times/min;
CB-counting blank samples, times;
tB-blank measurement time, min;
calculating the net counting rate of the standard electrolytic sample:
in the formula:
-the net counting rate of standard electrolysis samples, times/min;
Ce,st-counting the number of standard electrolysis samples;
te,st-standard electrolysis sample measurement time, min;
CRB-blank count rate, times/min;
calculating the tritium recovery rate of the standard electrolytic sample:
in the formula: rt,stTritium recovery from standard electrolysis samples;
CE-Instrument count efficiency;
mupreparing a standard water sample, wherein the mass g of the standard tritium water is used;
λ -decay constant, (ln2/12.32) y-1;
t is the interval time from the preparation of standard tritium water to the preparation of standard water sample, y;
60-converting the net count rate of the standard electrolytic sample to a value in units of "times/s";
calculating fractionation factor (separation coefficient)
In the formula: beta-fractional distillation factor;
(V0/Vf)ststandard water sample concentration ratio, where V0 and Vf are the electrolysis initial and terminal volumes (mL), respectively;
Rt,st-tritium electrolysis recovery of a standard water sample;
5) calculating the tritium electrolysis recovery rate of the water sample
In the formula: rt,sa-electrolyzing and recovering tritium in a water sample;
(V0/Vf) sa-water sample concentration ratio, where V0 and Vf are the initial and final volumes (mL) of electrolysis, respectively;
beta-fractional distillation factor;
6) calculating the tritium content of the water sample:
calculating the net counting rate of the electrolyzed water sample:
in the formula:
-net counting rate of electrolyzed water sample, times/min;
Ce,sa-counting of the electrolyzed water sample;
te,sa-measuring time of electrolyzed water sample, min;
CRb-background count rate, times/min;
calculating the tritium content of the water sample:
in the formula: TU-tritium content of water sample, TU;
CE-Instrument count efficiency;
(V0/Vf)sawater sample concentration ratio, where V0 and Vf are the initial and final volumes (m) of electrolysis, respectivelyL)；
Rt,sa-electrolyzing and recovering tritium in a water sample;
λ -decay constant, (ln2/12.32) y-1;
t-the time interval from the sampling date to the analysis date, y;
k-unit conversion factor, 0.11919 (Bq. L-1)/TU
7) Calculating the uncertainty of the analysis result
Calculating the relative standard deviation of the analysis result:
in the formula: sr-analyzing the relative standard deviation of the results;
CRb-background count rate, times/min;
te,sa-measuring time of electrolyzed water sample, min;
tbbackground measurement time, min;
calculating the standard deviation of the analysis result:
sTU＝TU×sr
in the formula: sTU-standard deviation of the analysis results, TU;
TU-tritium content of water sample, TU;
sr-analyzing the relative standard deviation of the results;
calculating the uncertainty of the analysis result:
U＝2×sTU
in the formula: u-uncertainty of analysis result, TU;
sTU-standard deviation of the analysis results, TU;
2-coverage factor value with confidence level of about 95%.
2. A tritium analysis sample preparation system according to claim 1, characterized in that: the method A comprises the following steps:
1) weighing 300mL of water sample, pouring the water sample into a distillation flask, adding a plurality of glass beads, adding 0.1-0.2 g of potassium permanganate, covering a ground stopper, and placing the distillation flask on an electric furnace;
turning on circulating water, starting the electric furnace, and turning a temperature-adjusting knob of the electric furnace to 700-800W for distillation;
2) and discarding a few milliliters of water sample distilled initially, collecting the distilled water sample in a ground glass bottle or a high-density polyethylene bottle with a plug, and sealing and storing.
3. A tritium analysis sample preparation system according to claim 1, characterized in that: the B comprises the following steps:
1) weighing the cleaned and dried empty electrolytic cell and accessories;
2) weighing 1.20g of sodium peroxide in a cleaned and dried beaker, and measuring 250ml of desalted water sample by using a volumetric flask;
3) slowly adding about 100ml of desalted water sample in the volumetric flask into the beaker, shaking while adding to ensure that sodium peroxide is completely dissolved, and transferring into a cathode electrolytic cell of an electrolytic cell;
washing the residual water sample by using sodium hydroxide solution in the beaker for three times, and transferring the water sample into a cathode electrolytic cell of an electrolytic cell to obtain an initial water sample weight for electrolysis;
4) assembling the electrolytic cell, and installing the capillary air guide joint on the air outlet of the inner pole;
then inserting the electrolytic cell into a specified hole position of the liquid bath circulation low-temperature constant-temperature bath, connecting 24 capillary air guide joints by using exhaust pipes, and leading the exhaust pipes to the outside;
5) connecting each electrolytic cell in a series connection mode that an external pole is connected with a negative pole to form a cathode and an internal pole is connected with a positive pole to form an anode by using a special cable with a wiring terminal, and forming a loop with a direct current power supply, an ammeter and an ammeter;
6) starting a liquid bath circulation low-temperature constant-temperature system, and setting the electrolysis control temperature to be 0.5 ℃;
after the temperature of the liquid bath circulation constant-temperature low-temperature tank is reduced to 0.5 ℃ for about half an hour, a direct-current power switch is turned on, the direct current is slowly adjusted from 0A to 10A, an ampere-hour meter starts to measure the ampere-hour value, and an exhaust pipe enables H generated by electrolytic reaction to be discharged2And O2Leading to the outside.
4. A tritium analysis sample preparation system according to claim 1, characterized in that: the step C comprises the following steps:
1) when the ampere-hour meter reaches a set ampere-hour value, stopping the electrolytic reaction;
closing the direct current power supply and the liquid bath circulating low-temperature constant-temperature system, detaching the connecting cable, the exhaust pipe and the capillary air guide joint, taking out the electrolytic cell from the liquid bath circulating low-temperature constant-temperature tank, and drying the surface of the outer electrode of the electrolytic cell;
2) drying the electrolytic cell on the surface of the external pole in the air, and weighing to obtain the sample weight of the final water for electrolysis;
3) taking out the magnesium perchlorate drying tube which is heated and dehydrated in advance from the dryer, and installing the magnesium perchlorate drying tube on CO2A lead-in device;
introducing CO2The air duct of the importer is inserted into the inner pole of the electrolytic cell and rotates CO clockwise2The connector of the leading-in device is connected for two weeks, so that the leading-in device is firmly connected with the inside of the electrolytic cell and the vent hole is ensured to be smooth;
opening of CO2Regulating valve, introducing CO2And (5) gas is used for 3-5 minutes to neutralize the electrolyte in the electrolytic cell.
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