PerkinElmer QuantaSmart 1694267

CHAPTER 8
156 PerkinElmer Life and Analytical Sciences
High Sensitivity and Low Level Counting
In liquid scintillation counting, the limits of detection are dependent on an
instrument’s detection efficiency and it’s ability to detect background counts. The
TriCarb line of instruments uses Time Resolved Liquid Scintillation Counting (TR-
LSC™), an electronic background discrimination technique (burst counting
circuitry), to enhance background discrimination and thus increase sensitivity.
Sensitivity may be further enhanced through the use of the optional BGO
(Bismuth Germanium Oxide) detector guard (available on the TriCarb 3170TR).
The following table describes the differences between the counting modes
available on the TriCarb line of instruments:
Note: Do NOT use High Sensitivity or Low Level Count Modes to
count alpha samples in normal CPM or DPM counting assays.
These TR-LSC discrimination modes are not appropriate for
counting alpha samples. Extremely low alpha count rates will be
observed.
The increase in sensitivity obtained with the various counting modes is the result
of varying the amount of electronic background discrimination. For High
Sensitivity and Low Level Count Modes, the criteria employed for discriminating
true Beta counts from background counts becomes more stringent. Some loss of
counting efficiency may occur as background discrimination is increased. The
reduction in background, however, is usually far greater than the loss of counting
efficiency, resulting in an increase in counting efficiency as measured by E
2
/B
(Efficiency
2
/Background). Counting high energy beta emitters like
90
Sr or
90
Y in
High Sensitivity or Low Level Count Mode requires optimization of TR-LSC
parameters. This is accomplished by changing the Delay Before Burst value in the
Count Corrections tab of Assay Definition. The Delay Before Burst feature works
by delaying the time that afterpulse discrimination begins. The effect of this delay
is to preserve high beta counting efficiency at the lowest possible background.
Type of
Counting
Background
Discrimination
Instrument
Count Mode
Typical Count
Rate
Availability
Normal Minimal Normal >500 CPM Standard
Low Activity Intermediate High Sensitivity 50-500 CPM Optional
Ultra Low Level High Low Level
1-20 CPM above
background
Optional
Super Low Level Maximum Low Level
1-20 CPM above
background
Optional with
BGO Detector
Guard only