Count the samples for the required time to meet the detection limit.
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Step 14
If there is time and the counting batch is low count all samples a second or third time and average the counts for each sample. This will reduce statistical uncertainty.
Step 15
Determine the concentration of tritium according to the following expression and complete Worksheet El:
C -
- ~
Where
Cn-3 = tritium concentration [Bq/L]
N = sample counts
t = sample counting time [s]
Nb = background counts
tb = background counting time [s]
z = counting efficiency (expressed as a decimal) V = volume of the sample aliquot [L].
Use a standard solution of high accuracy with a total uncertainty of ±3% at 95% confidence level.
Equipment/Supplies
^ Liquid scintillation counter
> Worksheet El
Reagents
> Tritium standard solution
^ Scintillation cocktail
Stepl
Know what size vials are going to be used, and what liquid scintillation cocktail will be used for sample analysis.
Step 2
Prepare a counting standard for use in the counting batch. Use a commercially available tritium source which is diluted with low background distilled water so that about 10 000 counts will be obtained in about 100 minutes.
Step 3
Pipette the correct quantity of standard solution into a vial, according to what the liquid scintillation cocktail will hold. This is best done by weight. Precisely determine the activity.
Step 4
Add the correct quantity of liquid scintillation cocktail, coming to a final volume of 20 mL for a 20 mL vial.
StepS
Put on the cap, label the vial on the cap only, and shake the vial to mix the contents. Do not add smudges or fingerprints to the exterior of the vial. Put the vial aside and allow the vial to sit at least 1 hour before counting. Gloves or tissues are recommended to hold the vial.
Step 6
Once the sample counting batch is prepared, add this standard to the batch. Start counting
Calibration of liquid scintillation counter Procedure Ela, Pg. 2 of 2
Step?
Calculate counting efficiency using the following expression:
Where
s = LSC counting efficiency for tritium Ns = counts from a standard aliquot
ts - counting time for standard aliquot [s]
Nb = background counts
tb = background counting time [s]
A = activity of standard aliquot [Bq].
StepS
Record counting efficiency in Worksheet El. Keep records of all relevant data in LSC logbook.
A quench correction curve must be obtained according to the manufacturer's instructions and periodically updated. It is also desirable to have software available for chemoluminescence detection.
For best counting results, obtain the 3H standard and background sample supplied by the manufacturer.
Equipment/Supplies
^ Liquid scintillation counter
^ Tritium standard solution
^ Scintillation cocktail
Stepl
Determine the width of the counting window or the number of channels for the tritium peak and either manually select those channels or have the software determine this.
Stepl
To control: put a set of 2 standards (unquenched background and tritium) into the LSC and count it with "open window". Repeat counting the standards at least 10 times with the preset time of 1-2 minutes. Calculate counting efficiency (must be more than 60%) and perform chi-square test.
Liquid scintillation counter QC checks Procedure Elb, Pg. 2 of 2
Step 3
Count the manufacturer supplied standard and background samples daily and plot the results.
If the standard and background counts exceed 1 standard deviation derived by simple statistics, a problem may have occurred in the detectors.
Step 4
Run and update the quench curve stored in the liquid scintillation software on a weekly basis.
StepS
Periodically run the anti-coincidence software check to make sure the photomultiplier tubes are balanced. If the PM tubes do not pass the test, consult the manufacturer for service.
Step 6
Periodically run the background sample with all channels selected. Plot the results. If the background counts exceed 1 standard deviation derived by simple statistics, a problem may have occurred in the detectors.
Step?
When counting samples, randomly pick one sample in 10 and recount it.
StepS
Keep records (results) of all relevant data in LSC logbook.
Measurement of 89Sr is performed by Cerenkov counting and 90Sr and 89Sr combined by liquid scintillation counting.
In Table El sample sizes, typical minimum detectable activity concentrations (MDAs), based on the 95% confidence level, and approximate times of analysis are shown [17, 18].
TABLE El. CHARACTERISTIC PARAMETERS OF STRONTIUM ANALYSES.
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Strontium analysis Procedure E2, Pg. 2 of 3
Prepare sample for purification using Procedure E2c.
Stepl
Perform purification of strontium using Procedure E2d or E2e depending on your laboratory practice.
Step 3
Put the sample in LSC and count at the previously determined Cerenkov settings for 20 minutes.
Step 4
Add 10 mL of scintillant and recount at the previously determined 90Sr/89Sr settings. Record the mid-count time to calculate the ingrowth of 90Y.
StepS
Calculate concentrations of 89Sr + 90Sr using the following expressions:
N _ N b _
= concentration of 89Sr and 90Sr together [Bq/unit]
= sample counts
= sample counting time [s]
= background counts
= background counting time [s]
= mean counting efficiency for 89Sr
= chemical yield of radiochemical separation
= quantity of the sample [kg, L, m3].
Calculate concentration of Sr as follows:90 <
N Nv
C -f l tb f'
9
°
sr Ys^n-Q
Where
CMcSr = concentration of 90Sr [Bq/unit]
N = sample counts
t = sample counting time [s]
Nb = background counts
tb = background counting time [s]
SY = counting efficiency for 90Y
TI = chemical yield of radiochemical separation Q = quantity of the sample [kg, L, m3]
fy = build up factor for °Y fy =l-eln2/T"2-At
Tin = half-life of 90Y
At = time interval from separation of 90Y to middle of counting period [s]
f Y = decay factor for 90Y IY = e
Step?
Calculate concentration of Sr by subtraction:RQ
StepS
Complete Worksheet E2.
Purpose
To determine counting efficiency of LSC at the respective energy levels and to find 89Sr/90Sr settings.
Discussion
For calibration use a standard solution of high accuracy with a total uncertainty of ±3% at 95% confidence level.
Equipment/Supplies
> Liquid scintillation counter
> 89Sr standard solution
> 90Sr standard solution
> Scintillation cocktail
> Worksheet E2
Step 1
Pipette a certain amount of a liquid 90Sr standard (^Y in equilibrium) into a counting vial and add distilled water up to 2 ml.
Step 2
Put the vial in the LSC and count with open tritium window until good statistic has been achieved (at least 3%). Record the result of counting and other relevant data in LSC logbook.
Step 3
Pipette a certain amount of a liquid 90Sr standard (^Y freshly separated) into a counting vial, add distilled water up to 2 mL and mix with 10 mL cocktail.
Step 4
Start counting the vial in an open window immediately. Do not count longer than 1 h. Record the result of counting. This counting is needed for determining counting efficiency for 90Sr.
Step 5
Count the vial again after 5 days (ingrowth of 90Y). Subtract the counting result from step 4.
Record the result in LSC logbook. This counting is needed for determining counting efficiency for 90Y.
Step 6
Independently pipette a certain amount of a liquid 89Sr standard into another counting vial, add distilled water up to 2 mL.
Step?
Count the vial in an open window until good statistic has been achieved (at least 3%). Record the result of counting. This counting is needed for determining the Cerenkov counting
efficiency for 89Sr.
StepS
Add 10 mL liquid scintillation cocktail and mix well.
Step 9
Count the vial in an open window until good statistic has been achieved (at least 3%). Record the result of counting. This counting is needed for determining the liquid scintillation
counting efficiency for 89Sr.
Step 10
Calculate counting efficiency of LSC for 89Sr, ^Sr and 90Y using the following equation:
th b
Where
ex = counting efficiency for radionuclide x NX = counts for standard aliquot of radionuclide x tx = counting time for radionuclide x [s]
Mb = background counts
tb = background counting time [s]
Ax = activity of radionuclide x [Bq].
Step 11
Keep records of counting efficiencies in LSC logbook.
Step 12
Complete Worksheet E2.
Purpose
To check proper functioning of LSC and to control optimal counting conditions.
Discussion
The QC checks of the LSC is carried out using the tritium unquenched standard and
background samples obtained from the suppliers (±3% accuracy at 95% confidence level). In addition, checks of the counting efficiency of freshly prepared strontium and yttrium
standards are made periodically.
Equipment/Supplies
> Liquid scintillation counter
> Unquenched background and tritium samples
> 89Sr standard solution
> 90Sr standard solution
> Scintillation cocktail Step 1
Put the unquenched background and tritium standards into the LSC, and count in an open window.
Step 2
Repeat counting of the standards 10 to 100 times (preset time 1 to 2 min).
Step3
Calculate counting efficiencies (must be more than 60%) and execute a chi-square-test.
Step 4
Repeat Steps 1-3 each month.
li^
StepS
Check the calibration of the LSC periodically using Procedure E2a.
Step 6
Keep records of all relevant data in LSC logbook.
Discussion
Pretreatment of the various sample types produces a uniform matrix for further analysis.
Equipment/Supplies
> Furnace
> Ultrasonic bath
> Glassware, beakers, volumetric flasks, glass rods, PTFE beakers etc.
> Filter papers; Whatman No. 42, Whatman GF/B (glass fibre)
> Hotplate
> Rubber policeman
Reagents
> Nitric acid; concentrated, 8M
> Hydrofluoric acid; concentrated (40%)
> Aluminium nitrate; solid
> Strontium nitrate; solid
> Strontium carrier; 10 mg Sr++ per mL. Dissolve 2.24 g of strontium nitrate in water, add 1 mL of concentrated nitric acid; make up to 100 mL with water in a volumetric flask
> Hydrochloric acid; concentrated
> Sodium hydroxide; 5% wt/vol in water
> Ammonium carbonate; solid, 5% wt/vol in water
> Aqua regia; 1:4 mixture of concentrated nitric acid: concentrated hydrochloric acid; make as required — do not store