Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 4. Variation of MDA and FM with respect to PSA for 226 Ra
standard solutions.
and � events are correctly discriminated and a counting plateau,
of almost constant efficiency (ca. 265%), is observed.
In order to choose the optimum value within the efficiency
plateau, both MDA and FM were plotted (Figure 4). In broad
terms, MDA followed the efficiency curve, suggesting that, from
a limit of detection perspective and within the plateau, best results
would be achieved at higher PSA, as the background slowly
decreases with the PSA parameter and stabilizes for the PSA
ranging from 145 to 190. It is worth noticing that, as from PSA )
135 the background rate is very low, the MDA is correspondingly
low. Higher PSA values are discarded from the discussion as the
efficiency there tends rapidly to zero. On the other hand, the FM
shows a clear peak at PSA ) 145, and shows the optimum
combination of efficiency and background, taking advantage of a
high efficiency (and therefore sample throughput) and providing
the lowest possible MDA.
Counting Window. Due to the presence of many substances
in the digestate, which may be partially soluble in the scintillator,
the sample may show different levels of quenching, which also
affect the position of the peaks. In order to compensate for the
peak shift, we opted to define counting windows with a fixed width
but a variable position in the spectrum, manually set to comprise
the peaks of interest.
The first strategy was to define a wide counting window of
150 channels. This window allowed to include, irrespective of the
quenching level (quantified through the SQP(E) parameter), the
three R peaks (Figure 1). The main advantage of this strategy is
that counting efficiency is large, up to the maximum theoretical
level of 300%, and above 200% for the real samples commonly
analyzed. However, as the spectrum resolution does not allow us
to distinguish other R impurities present in the sample and some
� interferences, this method might lead to slight activity
overestimation.
In order to minimize the effect of background from impurities
and interferences, the second strategy was to define a 50 channel
wide counting window to only comprise the higher energy R peak
( 214 Po), which stands in an area of much smaller background
and interference, 15 and is easily identified. However, in this
case the maximum theoretical efficiency is only 100%.
Calibration. Quenching is the only parameter affecting the
counting configuration that is sample-dependent and cannot be
easily controlled. Therefore, several 226 Ra standard solutions were
quenched with CCl4 to values covering the expected SQP(E)
6850 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
Figure 5. Effect of quenching (SQP(E)) on R counting efficiency.
Figure 6. Time stability test for sample quenching (SQP(E)).
value in environmental samples (Figure 5). For both counting
windows, efficiency is close to zero for SQP(E) < 575. Therefore,
the calibration curves, obtained by regression analysis between
the counting efficiency (y) and the SQP(E) parameter (x), were
150 channels:
y ) (0.74 ( 0.06)x - (435 ( 51) and r ) 0.95
50 channels:
y ) (0.21 ( 0.02)x - (119 ( 19) and r ) 0.91
Usually, LSC techniques involve the measurement of a batch
of samples in cycles. As the typical activities expected in this type
of work are low (of the order of 10 mBq), long-term counting, of
the order of days for a single batch, is required. We successfully
checked the stability of the scintillator through the monitoring of
SQP(E) for a quenched tracer with an activity of 10 mBq during
4 days (Figure 6).
Detection Limits. Each counting configuration (including
counting window and quenching) shows different detection limits.
In Table 1, we show the counting properties for a standard solution
quenched to SQP(E) ) 850, which is an average value of sediment
samples analyzed in our laboratory. The combination of the
indirect detection of 226 Ra by 222 Rn emanation, the high resulting
efficiency, and the ultralow background of Quantulus 1220
result in an extremely low MDA (0.29 Bq kg -1 ) for samples as
low as 250 mg, a typical amount used when analyzing 210 Pb by
R spectrometry. 10 Although both MDA are very low, and
therefore any of the windows can be chosen, the larger
efficiency (and therefore precision of measurements for the
same counting time) favors the use of the largest window,
which is the recommended value in this work. From a