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LSC FAQ
Frequently Asked LSC Questions:
Principle of Radionuclide Detection ?
Energy of a nuclear decay event is either directly or indirectly transfered to a scintillant, which in turn produces light. Light is detected by photomultiplier tube within the instrument. Both the number and intensity of individual photons are recorded by the multichannel analyzer (MCA).
What is Quenching ?
Anything which interfers with the conversion of decay energy to electronic signal in the photomultiplier tubes (PMT). This usually results in a reduction in counting efficiency, e.g. chemical - and/or color quenching.
Solvent Quench Effect
Nitro Groups Highest
Sulfides
Halides
Amines
Ketones
Aldehydes
Organic Acids
Esters
Water
Alcohols
Ethers
Hydrocarbons Least
The higher the energy of the nuclear decay, the less effect quench has on the counting efficiency.
Calculating DPM and Efficiency ?
DPM (unknown) = . Net CPM * 100
% Counting Efficiency
% Efficiency = . Net CPM . * 100
Disintegration per Minute (DPM)
Counting Efficiency is variable (the greater the quench, the lower the counting efficiency)
How Do You Determine Efficieny ?
1.) Internal Standard
2.) Create a Quench Curve using: 1) External Standard, e.g. tSIE
2) Sample Spectrum, e.g. SIS or tSIS
1) Internal Standard Method: a) Count the sample (CPM)
b) Add a known activity of standard (DPM)
c) Count the sample again (CPM+iSt.)
d) Equation: % Eff. = CPM+iSt - CPM * 100
DPM
2) Generating Quench Curves: Prepare stock solution, e.g. 120 mL LSC cocktail at 100’000 DPM / 10 mL
cocktail. Prepare 10 standards by adding 10 mL of stock solution to each vial.
Add quenching agent to each sample, e.g. Chloroform (CHCl3) 0 / 0.1 / 0.2 / 0.3 /
0.5 / 0.7 / 0.9 / 1.1 / 1.3 mL CHCl3 per vial.
Count the samples and the LSC will calculate the QIP and create the quench curve.
Typical Tritium and Carbon 14 Quench Curves ?
Due to its low energy, 3H is easily effected by small changes in quench.
Due to its higher energy, 14C is effected only by moderate to high quench.
Interferences with Detection ?
a) Quench Decreases counting efficiency (CPM)
b) Background Increases CPM
c) Luminescence Increases CPM
d) Static Charge Increases CPM
Origin of Luminescence ?
Chemiluminescence: Random single photon events being generated as a result of the chemical interaction of sample components. The coincidence circuit excludes most chemiluminescent events except at high rates. Decays fairly slow, e.g. alkaline samples.
Photoluminescence: UV light interacting with the scintillation cocktail or the vial (decays quickly, i.e. 30 min.)
Result: Usually a single photon event between 0 to 4 keV.
Origin of Electrostatic Charge ?
Caused by two nonconductive objects being separated., e.g. hands from gloves. Plastic vials or microplate seals
can be effected.
Result: Non-reproducible high counts in samples. (more easily recognizable in low activity samples)
tSIE and SIS as QC Parameter ?
CPM 2S% SIS tSIE %Lum Cause
1000 6.6 105 490 2
1000 6.6 103 492 2
1000 6.6 32 491 45 Lum
1000 6.6 105 490 2
1000 6.6 103 492 1
1000 6.6 34 491 1 Statics
10 66 20 491 5 Statistics
Sample Preparation Considerations ?
The detector is in the vial (or plate). Make sure that you choose the right LSC cocktail. The final sample must be homogeneous, i.e. clear, not milky / no phase separation / 4p counting geometry.
Components of a LSC Cocktail ?
Aromatic solvent, e.g. PXE, DIN, Pseudocumene
Surfactant, e.g. Ethoxylated Alkylphenols (Non-ionic detergent, e.g. Triton)
Primary scintillator, e.g. PPO (2,5-diphenyloxazole / 5 – 7 g/L / Fluorescence maximum: 375 nm )
Secondary scintillator, e.g. bis-MSB / 1.5 g/L / Fluorescence maximum: 425 nm)
Nuclear Statistics (2s%) ?
The nuclear decay is a random event. Statistics is allowing us to describe the average behaviour of all nuclear decays within a sample. The counting error (2s%) describes the accuracy of the counting results. The counting error is determined by the count time and count rate of the sample.
Count time and count rate do determine the accuracy.
The theoretical standard deviation is used for this purpose: %SD = . 100 .
v Counts
Total Counts 1 s 2 s Uncertainty Percentage
Accumulated (Counts) (Counts) 1s 2s
10 (+/-) 3.3 (+/-) 6.6 33.00% 66.00%
100 (+/-) 10 (+/-) 20 10.00% 20.00%
1000 (+/-) 33 (+/-) 66 3.30% 6.60%
5000 (+/-) 70 (+/-) 140 1.40% 2.80%
10000 (+/-) 100 (+/-) 200 1.00% 2.00%
20000 (+/-) 141 (+/-) 282 0.70% 1.40%
50000 (+/-) 224 (+/-) 448 0.45% 0.90%
100000 (+/-) 316 (+/-) 632 0.32% 0.64%
160000 (+/-) 400 (+/-) 800 0.25% 0.50%
When you program the instrument for radioisotopic detection, you have to define the level of acceptable error for your experiment. Total Counts = CPM * Count Time.
Liquid Scintillation General Considerations ?
a) Strive for good sample preparation, e.g. homogeneous samples / sample geometry
b) Use a quench curve based on the external standard , e.g. tSIE. This is the best method for converting CPM to DPM.
c) Be aware of counting interferences and how to deal with them
d) Define acceptable counting error for your experiment and chose counting time accordingly.
Vials vs. Microplate Counting ?
Pro Microplate: Faster Counting (Up to 12 detectors)
Higher throughput and sample capacity
Less tedious sample handling
Less waste
Pro Vial: Best counting results (DPM with ext. Std)
Best flexibility (20 µL to 20 mL)
Can do Dual Lable properly
Perfect for Wipe Tests (3 regions & spectrum)
LSC Counting Windows and Counting Efficiency ?
Radionuclide Energy A Window B Window Efficiency
(kev) LL UL LL UL (%)
3H 18.6 0 18.6 2 18.6 60
14C 156 0 156 4 156 95
35S 167 0 167 4 167 95
33P 220 0 220 4 220 96
45Ca 257 0 257 4 257 96
59Fe 462 0 462 4 462 98
90Sr 544 0 544 4 544 97
36Cl 714 0 714 4 714 98
32P 1710 0 1710 5 1710 98
125I 80 0 80 3 80 78
129I 153 0 153 4 153 90
131I 606 0 606 4 606 98
51Cr 160 0 160 4 160 34
60Co 318 0 318 4 318 98
22Na 545 0 545 4 545 95
Dual Lable Counting Considerations ?
SEP ratio between the low – and high energy isotope has to be > 1.3
Use low energetic isotope in excess, so plan your experiment accordingly.
Use AEC to keep spill-down constant.
Activity ratios > 1:100 are not doable anymore.
Dual Lable Quench Curves
Radioactive Units & Curie to Becquerel Conversion ?
DPM/60 = DPS 1 DPS = 1 Bq 1 Bq = 2.7 * 10¯¹¹ Ci
1 µCi = 37 kBq 1 mCi = 37 MBq 1 Ci = 1 GBq
Gamma Counting Aphorisms ?
Gamma counters use a solid scintillant being usually a NaI (Tl) crystal detector, thus no liquid scintillant needs to be added to your sample as in a LSC (Beta counter). Gamma counting is an easy technique to use and virtually free of artifacts. One caveat however remains, i.e. counting geometry. This means that changes in sample volume make a difference and the higher the energy of the nuclide the worse the situation becomes.
In this context it is advisable to chose a Gamma Counter with a “Through-hole” detector system rather than a “Well-type” detector. The “Through-hole” detector allows to center the sample within the detector to compensate for the changing geometry.
Gamma Counting Windows and Counting Efficiency with a 2” Detector ?
Radionuclide Energy A Window Efficiency
(keV) LL UL (%)
59Fe 1400 940 1400 5
125I 80 15 80 82
22Na 1417 433 1417 31
57Co 165 80 165 80
60Co 1550 1050 1550 10
51Cr 400 240 400 2.8
131I 470 260 470 20
129I 40 15 40 61
137Cs 760 500 754 30
32P 900 15 900 5