PerkinElmer QuantaSmart 1694267

CHAPTER 12
204 PerkinElmer Life and Analytical Sciences
The performance in any given laboratory will depend on many factors. First, in
counting environmental water, the source of water used for the background
sample is extremely important. The water must be free of contamination by
radionuclides, especially
3
H. When counting very low levels of
3
H, even the
smallest amount of contamination will increase the count rate of the background
sample. This cannot be discriminated out since the contamination comes from the
sample itself. Therefore, it is necessary to secure a source of water for the
background samples which is “dead’’ in terms of radioactivity.
For radiocarbon dating samples, it is important to ensure that the reagents used
in the process of sample preparation are free from contaminating
14
C. This
includes the actual scintillators and any reagents used to convert the sample into
benzene.
Overall, the preparation of samples for extremely low level counting must be
conducted with extreme care to provide contamination free samples for counting.
If the background samples or the unknown samples contain extraneous sources
of radionuclides, performance will be degraded.
In Super Low Level counting as in other low level count modes, it is important to
tailor the scintillation fluor (scintillator) to the counting conditions. Secondary
scintillators produce significantly higher counting efficiencies than are possible
using only a primary scintillator. These secondary scintillators include BBOT, DPA,
POPOP, Me2POPOP, and bis-MSB.
Although there are many potential combinations of primary and secondary
scintillators which are suitable for use with the TR-LSC, a combination of 6 grams
per liter PPO and 0.2 grams per liter bis-MSB works well for many sample types
and can be used reliably. This is the commonly used combination of scintillators
for a wide variety of counting applications.