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Measurements and Calculations |
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Radiochemicals |
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Measurement of Radioactivity
| Liquid Scintillation Counting |
| Used for the measurement of beta emitting nuclides, such as, tritium and carbon-14. This technique involves dissolving the sample containing a radionuclide in a suitable scintillation solution and the use of a liquid scintillation counter. The solution normally consists of an aromatic organic solvent containing a fluor and a detergent to make the whole solution miscible when counting aqueous samples. The energy of the emitted beta particles is transferred via the solvent to the primary fluor and sometimes to a secondary fluor, which then emits energy as light photons. These photons are detected using a photomultiplier. Only a small proportion of the available energy is liberated as light. The residue is dissipated as vibrational and rotational energy in the solvent. |
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| Sigma Provides Various Scintillation Cocktails: |
| L8286 LSC Cocktail for non-aqueous samples, Packard Ultima Gold LSC Cocktail |
| L8411 LSC Cocktail for non-aqueous samples, Packard Ultima Gold xr LSC Cocktail |
| S3898 LSC Cocktail for non-aqueous samples, Sigma-Fluor™ LSC Cocktail |
| S4023 LSC Cocktail for aqueous samples, Sigma-Fluor™ High Performance LSC Cocktail |
| S4273 Universal LSC cocktail for aqueous samples, Sigma-Fluor™ Universal LSC cocktail |
| S4398 Universal LSC cocktail for aqueous samples, Sigma-Fluor™ Universal LSC Cocktail |
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Specific Activity
The specific activity of labeled compounds refers to the total amount of radioactivity per unit mass, and is commonly quoted in terms of µCi/mg, mCi/mg, Ci/mmol and Bq/mmol. The mass is usually determined by direct weighing or spectroscopic measurements. When there is sufficient mass of a radiolabeled compound for a small sample to be accurately weighed and counted by liquid scintillation counting, the specific activity is expressed as (e.g.) µCi/mg. The conversion from µCi/mg to mCi/mmol is by multiplying the molecular weight and dividing by 1000. When the specific activity is greater than 1 Ci/mmol there is often insufficient material present to be weighed.
If the specific activity is greater than 1 Ci/mmol, the specific activity may be calculated by relating the radioactive concentration (determined by liquid scintillation counting) to the chemical concentration, and then converting the figure obtained to Ci/mmol (Bq/mmol). The chemical concentration is commonly determined by U.V. spectroscopy or an appropriate colorimetric method.
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| The maximum value of a specific activity at 100% isotopic incorporation is: |
For carbon 14: 62.4 mCi/milliatom
For tritium: 28.8 Ci/milliatom |
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| Specific activities for tracer experiments can be determined by the application and the quantity of unlabeled material available. For many metabolism studies, carbon-14 compounds of specific activities 0.37-2.2 GBq/mmol, 10-60 mCi/mmol are used. For radioimmunoassays or protein binding studies, the range of specific activites of tritium compounds used would be 10-85 Ci/mmol. |
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Useful numbers when using liquid scintillation counting to determine specific activity or activity concentration:
a millicurie equals 2.22 x 109 dpm (disintegrations per minute)
a microcurie equals 2.22 x 106 dpm (disintegrations per minute) |
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| Examples: |
| Solid Sample |
| SA (mCi/mmol)= DPM x Molecular Weight (mg/mmol) x Dilution Factor (mL) / 2.22x 109 DPM x quantity counted (mL) x sample weight (mg)
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| Neat Liquid |
| SA (mCi/mmol)= DPM x Molecular Weight (mg/mmol) x Dilution Factor (mL) / 2.22x 109 DPM x quantity counted (mL) x Density (mg/mL)
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| Calculation of specific activity |
| The following formulas can be rearranged to determine the chemical concentration from the radioactive concentrations and the specific activity: |
| Chemical concentration (mmol/mL) = radioactive concentration (mCi/mL) / specific activity (mCi/mmol) |
| Radioactive concentration (mCi/mL) = chemical concentration (mmol/mL) x specific activity (mCi/mmol) |
| Specific Activity(mCi/mmol) = total activity (mCi) x molecular weight (mg/mmol) / mass (mg) |
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Radioactive Concentrations
The radioactive concentration is the radioactivity per unit quantity of the material in which the radionuclide is present, normally expressed as activity per unit volume.
If counting a 1 mL sample of a solution containing a radionuclide, then the concentration in mCi/mL is: sample concentration = dpm / 2.22 x 109 dpm/mCi
For Example:
You are counting a 1 µL sample of Glucose-UL-14C using a liquid
scintillation counter. The sample has dpms equal to 222300. Divide 222300 dpm/µL by 2.22 x 106 dpm/µCi which gives 0.1 µCi/µL or 0.1 mCi/mL (1 mCi=1000 µCi; 1 mL=1000 µL).
A specific activity or concentration can be used to determine the amount of material you would need or would receive.
Solid Materials
Equation: packaged quantity = pkg quantity (mCi ) x molecular weight (mg/mmol) / specific activity (mCi/mmol) = Mass (mg)
Example: For a 0.500 mCi package of Atrazine-UL-14C with a specific activity of 20 mCi/mmol and a molecular weight of 215.7 mg/mmol:
Solution: weight in package=0.5 mCi x 215.7 mg/mmol / 20 mCi/mmol = 5.39 mg
Neat Liquids
Equation: volume in package = package size in mCi x molecular weight in mg/mmol / specific activity in mCi/mmol x density in mg/mL = Volume (mL)
Product supplied as a solution
Equation: volume packaged = package size in mCi / conc. in mCi/mL = Volume (mL)
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SI unit of Radioactivity
Sigma's radiochemical items are sold by units of radioactivity. The basic unit used in our catalog is the Curie (Ci), which is defined as "the quantity of any radioactive nuclide which undergoes 3.7 x 1010 disintegrations per second."
Units are as follows:
A Curie is 1000 millicuries (mCi)
A millicurie is 1000 microcuries (µCi)
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| The SI unit of radioactivity is the becquerel (Bq) |
| 1 Bq |
= |
1 disintegration per second = 2.7 x 10-11 curies (Ci) |
| 1 Ci |
= |
3.7 x 1010 Bq = 37 GBq |
| Therefore: |
| 1 mCi |
= |
3.7 x 107 Bq = 37 MBq |
| 1 µCi |
= |
3.7 x 104 Bq = 37 kBq |
| Common Prefixes for SI units |
10-3 |
milli |
m |
103 |
kilo |
k |
| 10-6 |
micro |
µ |
106 |
mega |
M |
| 10-9 |
nano |
n |
109 |
giga |
G |
| 10-12 |
pico |
p |
1012 |
tera |
T |
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