Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Table 3. Analysis of 226 Ra in Sediment Reference Materials (RM)
reference material no. of analyses method median (Bq kg-1 ) RM median (Bq kg-1 ) RM 95% confidence interval deviation
IAEA-384 15 2.2 2.4 2.0–2.9 -8.3
IAEA-385 15 23.5 21.9 21.6–22.4 +7.3
IAEA-313 5 372 343 307–379 +8.5
Figure 10. Time stability test for 226 Ra concentrations in the IAEA-
384 reference material.
226Ra reaches equilibrium in the core bottom sections. An
important finding was that, although a constant 226Ra is
commonly estimated from core bottom sections, where 210Pb and 226Ra are assumed to be in equilibrium, this is not the case
in the DYFAMED core, where 226Ra ranged from 22.6 to 41.7
Bq kg-1 . This reinforces the need for paired 210Pb/ 226 Figure 11.
Ra
210Pb and 226Ra profiles in a sediment core from the
DYFAMED site (Ligurian Sea).
measurements and methodologies to accomplish it accurately,
as does the technique described in this paper. This may have
important implications on the dating results for this particular
sediment core, which will be discussed elsewhere.
CONCLUSIONS
The accurate 210 Pb dating of sediment cores requires the
determination of 226 Ra in all sections. Although this can be done
by γ spectrometry, the large sample size required (>5 g dry
weight in well detectors, >20 g in coaxial detectors) might be
impossible to obtain when sediments are used for the analysis
of multiple magnitudes (such as grain size, elemental composition,
trace metals, organic substances, biomarkers, ...) and
counting times (typically >2 days per sample) might be a
limiting factor for many laboratories. The proposed strategy
in this work is the determination of 210 Pb through 210 Po in
equilibrium by R spectrometry 10 followed by 226 Ra by LSC
without any further radiochemical processing.
We optimized the counting parameters for an ultralow background
scintillation system with R-� separation capabilities
(Quantulus 1220, Wallac) and propose the use of a PSA parameter
of 145. The system was calibrated with a series of quenched 226 Ra
standard solutions. For a typical sediment sample quenching
(SQP(E) ) 850), the efficiency was (173 ± 12)% in the wide
energy counting window (150 channels) due to the simultaneous
counting of three radionuclides in equilibrium ( 222 Rn,
218 Po, and 214 Po). When analyzing 250 mg dw sediment samples,
the MDA was as low as 0.29 Bq kg -1 , which is about 2 orders
of magnitude lower than typical sediment concentrations,
showing the usefulness of the technique for many other
environmental applications. The method was validated with
three reference materials spanning 3 orders of magnitude of
concentration. The proposed method can greatly improve the
reliability of 210 Pb chronologies of sediment cores, and can also
be tested for 226 Ra/ 210 Pb dating of carbonates such as corals
and speleothems.
ACKNOWLEDGMENT
The authors thank Mr. Jacobo Martín (IAEA) for providing
samples of a sediment core from the DYFAMED site and helpful
comments. The IAEA is grateful for the support provided to its
Marine Environment Laboratories by the Government of the
Principality of Monaco.
Received for review March 31, 2010. Accepted June 30,
2010.
AC1008332
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
6853