Supplementary Information
A Dispersive Liquid-Liquid Microextraction Technique for the Determination of Lead-210
in Drinking Water Samples
Baki. B. Sadi, Jae Y Lee and Jing Chen
Radiation Protection Bureau, Health Canada, Ottawa, ON, K1A 1C1, Canada
Optimization Procedure for α/β Pulse Separation
In order to determine the appropriate pulse decay discriminator (PDD) settings for α/β pulse
separation, an optimisation was carried out using an α emitting radionuclide standard (210Po) and
a β emitting radionuclide standard (90Sr/90Y). A toluene layer was collected from the top of the
aqueous phase after carrying out a DLLME experiment (as described above) on a blank
deionized water sample. A small volume of an aqueous solution of 210Po standard (2000 Bq) was
added to the toluene layer, so that the organic phase remains the major component (≥ 95%, v/v).
Similarly, a
90
Sr/90Y standard (2000 Bq) was prepared in the toluene layer after carrying out a
DLLME experiment on a blank deionized water sample. Since the α/β pulse discrimination is
sensitive to the matrix composition, the
210
Po and the
90
Sr/90Y standards were prepared in the
toluene layers in order to carry out α/β pulse discrimination in a very similar matrix where 210Pb
would be extracted after conducting the DLLME on a water sample. Each of the standards was
homogenized with 19 mL of a liquid scintillation cocktail (OptiPhase HiSafe 3) using a vortex
mixer. The
210
Po standard and the
90
Sr/90Y standard were counted sequentially on a Tri-Carb
3180TR/SL liquid scintillation counter (LSC) at various PDD settings. At each PDD setting the
LSC selects and sends all pulses of duration longer than the PDD setting to an α-multichannel
analyzer (α-MCA) and pulses of shorter duration to a β-multichannel analyzer (β-MCA).
Subsequently, for each PDD setting the spillover of the α-event into the β-MCA was expressed
as % α spillover (using the 210Po standard) and similarly the spillover of the β-event into α-MCA
was expressed as % β spillover (while counting the 90Sr/90Y standard). A curve of the % spillover
versus the PDD settings was constructed for both the 210Po and 90Sr/90Y standards. The optimum
PDD setting for α/β pulse discrimination is where the two spillover curves intersect.
Alpha/β pulse separation for measurement of 210Pb by LSC
A liquid scintillation spectrum of the extracted toluene layer after DLLME can be used to
evaluate the efficiency, as well as the selectivity (of
210
Pb over other radionuclides), of the
extraction process. However, in liquid scintillation counting, the pulse height spectrum of an α
particle often overlaps with that of a β particle. Discrimination of liquid scintillation signals
resulting from α and β emitting radionuclides can be carried out by the difference in the lengths
of the pulse decay events [R1]. An optimization for the pulse decay discriminator setting was
undertaken for the separation of the gross α and β pulses into two different multichannel
analyzers. Since the α/β discrimination is sensitive to the matrix composition, the
210
Po and the
90
Sr/90Y standards used for the optimization of the PDD setting needed to be prepared in a matrix
very similar in composition to the toluene layer into which the 210Pb would be extracted after the
DLLME. Fig. S1 shows the curves of the % spillover versus the PDD settings constructed for
both the
210
Po and
90
Sr/90Y standards. The optimum PDD setting for α/β discrimination was at
175 where the two spillover curves intersect.
Fig. S1 Optimisation of PDD settings for α/β pulse discrimination resulting from a α alpha
emitter (210Po, 2000 Bq) and a pure β emitter (90Sr/90Y, 2000 Bq) prepared in the DLLME
extractant
In order to assess the α/β pulse separation capability at the optimum PDD setting, a
DLLME was carried out on a blank deionized water sample. A very small volume of a
210
210
Pb-
Bi-210Po standard (1 Bq, in secular equilibrium) was added to the toluene layer collected after
DLLME. Fig. S2 shows the liquid scintillation spectrum of this toluene layer where the
measurement was carried out at the optimum PDD setting of 175. As shown on Fig. 2, the α
(resulting from
210
Po) and the β (resulting from
210
Pb and
210
Bi) pulses nicely separated into the
α-MCA and β-MCA, respectively demonstrating successful α/β separation.
210Po
α-MCA
210Pb
210Bi
β-MCA
Energy (keV)
Fig. S2 Liquid scintillation spectrum of a DLLME extractant spiked with
210
Pb-210Bi-210Po (1
Bq) at the optimum PDD setting of 175 demonstrating gross α/β pulse separation
As shown in the β-MCA panel of Fig. S2, due to the minimum overlap between the
energy distribution of
210
Pb and
210
Bi liquid scintillation spectrum, a reasonable quantitative
estimation for
210
Pb and
210
Bi can be carried out by integrating the counts over an appropriate
region of interest (ROI) for the corresponding radionuclide. For quantitative calculation, a ROI
of 0-50 keV was used for
210
efficiencies of
Pb and
210
210
Pb while a ROI of 50-2000 keV was used for
210
Bi. The counting
Bi within the above mentioned ROIs were 95 ± 5 %. The counting
efficiency for 210Po within a ROI of 180-350 keV in the α-MCA was 96 ± 3 %. Obviously, there
will be a small positive interference contribution on 210Pb from 210Bi if the later is co-extracted in
the toluene layer.
Minimum detectable activity
The detection threshold (Lc) and minimum detectable activity (MDA) for the measurement of
210
Pb was calculated according to Currie’s formula as shown in equations S1 and S2,
respectively:
𝐿𝐶 = 2.33√𝜇𝐵
MDA =
(S1)
2.71+2𝐿𝐶
(S2)
ε·Y·t·V
where μB is the average counts within the ROI (0-50 keV) for 210Pb from reagent blanks, t is the
counting time in seconds, ε is the counting efficiency, Y is the chemical recovery, and V is the
sample volume in litres. The minimum quantifiable concentration (MQC) for which the relative
standard deviation will be 10 % was also calculated according to Currie’s formula as shown in
equation S3 below:
MQC =
1/2
μ
50{1+(1+ B ) }
12.5
ε.Y.t.V
(S3)
Accuracy and precision
Measurement accuracy and precision were determined by the relative error (Er) and relative
standard deviation (RSD) at each spiked activity level according to equations S4 and S5,
respectively.
𝐸𝑟 =
𝐶𝑚 −𝐶𝑠
𝐶𝑠
× 100%
(S4)
𝑆𝐷
𝑅𝑆𝐷 = 𝑀𝑒𝑎𝑛 × 100%
(S5)
Where, Cm and Cs are the measured and spiked activity of 210Pb.
References:
R1. L’Annunziata MF, Kessler MJ (2003) In: L’Annunziata MF (ed) Handbook of
Radioactivity Analysis, 2nd edn. Academic Press, California