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