PerkinElmer QuantaSmart 1694267, Reference Manual

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THEORY
PerkinElmer Life and Analytical Sciences 201
High Sensitivity and Low Level Counting
In the PerkinElmer TriCarb series of liquid scintillation analyzers, the patented TR-
LSC burst counting circuitry quantitates the number of afterpulses following a
coincident event. This information is then used to discriminate background pulses
from pulses arising from actual scintillation events in the sample. Thresholds have
been established and are used to reject events which produce significant numbers
of afterpulses. The thresholds vary with the energy of the coincident pulses due to
the afterpulsing phenomena in the photomultiplier tubes. Since the
Photomultiplier tubes can produce energy-dependent afterpulses, this must be
factored into the discrimination of background noise from sample activity.
The PerkinElmer Tri-Carb series liquid scintillation analyzers use this time-resolved
discrimination of afterpulses at three levels. These levels are:
Normal Count Mode. This mode employs a minimum amount of discrimination.
High Sensitivity Count Mode (HSCM). This mode applies an intermediate
amount of discrimination.
Low Level Count Mode (LLCM). This mode provides maximum discrimination.
Progressing from NCM to LLCM yields increasing efficiency to background ratios
(E
2
/B). The improvement is demonstrated in the table of
14
C data below.
PerkinElmer also offers the Tri-Carb Model 3170TR/SL analyzer, which achieves
super low level sensitivity through use of a slow scintillating BGO detector guard.
In the 3170TR/SL analyzer, this special material surrounds the sample in the form
of a special detector assembly to further increase the number of photons in the
trailing burst from background radiation. The detector assembly replaces the
conventional sample changer and provides a considerable increase in E
2
/B
performance as compared to the burst counting system alone. This configuration
is referred to as the Super Low Level Count Mode (SLLCM).
For environmental water samples, it is generally necessary to count as much
water as possible, since the amounts of
3
H found are extremely low and the larger
volume permits shorter counting times. Typical water samples are made up using
from 9mL to 12mL of water in a total volume of 20mL to 22mL.
Radiocarbon dating samples are generally made up of pure benzene from the
conversion of the raw sample through a benzene synthesis procedure. Such
samples are generally only 3mL to 5mL total volume. Comparative data are shown
in the following tables for
14
C samples and
3
H water samples.