Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 1. 226 Ra spectrum by liquid scintillation (only 6 × 10 -4 Bq).
Vertical dashed lines indicate the approximate counting windows.
efficiency reached when counting all R events should be 300%.
This is another advantage of R versus γ spectrometry, where final
counting efficiencies for 226 Ra, depending on the counting
configuration, normally do not usually exceed a few percent.
The magnitudes used for optimization were those related to
counting precision.
• Counting efficiency E: this is the ratio between the observed
number of counts and the 226 Ra decays. The maximum
observed efficiency is close to 300% (Figure 1) because with
this method we simultaneously count R particles from 222 Rn,
218 Pb, and 214 Pb in equilibrium.
• Minimum detectable activity (MDA): 25 we used the following
expression
2.71 + 4.65√Bt
MDA)
tVE
where B is the background count rate, t is the counting time,
and V is the volume of solution used (in the case of sediment
analysis, V was substituted by m, the mass of the aliquot
analyzed).
• Figure of merit: this is a magnitude commonly used to
compare methods, which emphasizes counting efficiency (and
therefore sample throughput). FM was calculated as FM )
E 2 /B.
In this section, we describe the results of the optimization
process for the relevant counting parameters.
Optimal Admixture Composition. Once the scintillation vial
and total volume were fixed, the optimal “scintillator-to-water” ratio
was optimized. We prepared and counted several composition
mixtures with tracer solutions and background solutions, by using
1, 5, 10, 15, and 19 mL of scintillant (Figure 2).
Not unexpectedly, the highest efficiency was observed for the
maximum volume of scintillant, as this maximizes radiation
interaction with the detector (the scintillant). However, as the
scintillant itself is an important source of background, this also
increases the MDA value. This behavior is well-captured with the
FM, which shows a maximum value for a 10:10 mL admixture.
Therefore, we used this proportion in all experiments.
Optimal PSA Parameter. The PSA circuit of Quantulus 1220
sends the signals to one of the two multichannel analyzers
Figure 2. Determination of the optimal admixture composition.
Figure 3. Change of background and 226 Ra counting efficiency with
PSA.
depending on the result of the PSA analysis. However, this method
is not error-free and shows interference as (i) R and � events can
be wrongly assigned and (ii) the efficiency of this method depends
on the energy of the incident radiation particles. 26 In order to
minimize the interference, tracer solutions of 226 Ra activity 5.34
Bq and background solutions, with a 10:10 mL admixture
composition, were counted by LSC during 12 h and with the
PSA parameter ranging from PSA ) 0 (no R-� discrimination)
to PSA ) 256 (maximum R-� discrimination) with a step
increment of 5 (Figure 3).
The background signals assigned to the R spectrum have a
varied nature and may include γ interactions with the scintillant,
� events wrongly assigned to the R spectrum, R emission from
impurities in the vial, and a variety of cosmic-ray originated signals.
Therefore, the background spectrum does not show prominent
peaks. The background count rate versus PSA plot (Figure 3)
shows an almost constant value until PSA ∼ 90, as all events are
recorded in the R spectrum, and then its value decays smoothly
to a value close to 0 cpm, when all background events are recorded
in the � spectrum.
On the other hand, the 226 Ra spectrum shows well-resolved
peaks within a quite narrow energy window (Figure 1), and
the effect of the R particle energy on the interference is clearly
shown in the efficiency versus PSA curve (Figure 3). Until PSA
∼ 110, the total efficiency exceeds the maximum value of 300%
because of the misclassification of background and � events in
the R spectrum. In the PSA range of 120-160, most background
(25) Currie, L. A. Anal. Chem. 1968, 40, 586–593. (26) Pujol, L.; Sanchez-Cabeza, J. A. Analyst 1997, 122, 383–385.
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