Determination of 210Pb
The granddaughter of 210Pb is 210Po. 210Po decays with a half-life of 138 days. It emits an alpha particle which can be detected by alpha spectrometry. By using isotopic dilution with 208Po or 209Po during the sample processing, we can increase the accuracy of the alpha spectrometry can be combined with isotope dilution using 208Po or 209Po to increase the accuracy of the analysis. With a counting efficiency of 20-30%, low backgrounds, and the low cost of the instrumentation, alpha spectroscopy is frequently the selected method of choice when trying to construct chronological profiles for lake sediment cores.
The analysis of either the daughter or granddaughter isotopes of 210Pb requires that the analyst make an additional assumption that 210Pb is present in equal concentration with the daughter isotopes. This assumption can be assured by storing the samples sealed in a container for about two-three weeks so that that the 210Pb isotopes cannot exchange and secular equilibrium is achieved. At secular equilibrium the activity of the parent isotope is equivalent to that of the daughter isotope.
Sediment systems provide a natural storage system because Pb, Bi and Po are immobile or only mobilized together in these systems. In all but the top few cm of most sediment cores the sediment is old enough to ensure that the concentration of 210Pb will be equivalent to that of its shorter-lived daughter isotopes. Tests of the top few cm and of surficial sediment indicate that the Po and Pb concentrations are quite similar, hence the analysis of the concentration of 210Po is equivalent to the measurement of 210Pb until the shorter lived isotope is chemically separated from the 210Pb.
This assumption is not valid in many other systems (e.g. biota) where the isotopes of 210Pb and the uranium series are very mobile or a geo-chemical process (such as different rates of bioaccumulation) fractionates the isotopes and produces different behaviour.
Radiochemical Separation of 210Po
For analysis by alpha spectroscopy, 210Po must be chemically separated from the sediment matrix and from other alpha emitting isotopes. Briefly, the method is as follows:
1. Dried, ground samples are weighed into glass vials.
2. A known amount of 209Po is added to each sample. The mass of the tracer is precisely determined by weight.
3. A mixture of nitric (HNO3) and hydrochloric (HCL) acids is added to the sample and heated at about 80 C for > 16 hours.
4. The solution is decanted into a centrifuge tube and the residual siliceous solids are separated from the solution by centrifugation.
5. The solution is evaporated to dryness three times with the addition of small quantities of HCl added after each drying.
6. Ascorbic acid is added to the dilute HCl solution.
7. A small silver plate is added to the solution with one side covered with adhesive glue.
8. The Po isotopes are electroplated onto silver disks.
9. The disks are gently wiped cleaned, dried and stored for analysis by alpha spectroscopy.
Measurement of 210Po by Alpha Spectroscopy
210Po is measured using isotope dilution with alpha spectrometry. Each radioactive decay of 210Po emits an alpha particle that has energy of 5.3MeV. The energy of the 209Po alpha particle is about 5.1 MeV. Particles emitted at these two energies are measured using an alpha spectroscopy system. The system consists of a surface barrier detector, preamplifier, amplifier, router, analog to digital converter, multi-channel analyzer, and appropriate computer software.
The sample disks are counted for periods of 0.25 to 2 days depending upon the activity of 210Po. The activity of 210Po in the sample is determined via the ratio of the total counts of 209Po to 210Po and from the quantities of sediment and 209Po added to the sample. Blanks and standards are measured to verify the performance of all aspects of the procedures and the instrumentation. The 209Po standard that is added to each sample also serves as an excellent internal standard to monitor the quality of the analysis.