Samples for Analysis Home
In order to perform an accurate analysis MyCore Scientific requests the following points are considered when samples are submitted for analysis.
1. Samples of at least 100 milligrams (0.1 gram) DRY weight are preferred for the sections between 0 to 10 cm and samples of at least 1000 milligrams (1.0 gram) are needed for deeper samples. Smaller samples can be used (e.g. as little as 10 mg). However the precision of the results may suffer and you should consult with us before submitting very small samples.
2. Samples must be a sub-sample from a homogenized section of the core. Large pieces of extraneous material (e.g. pieces of wood etc) should be removed prior to preparing the samples for 210Pb analysis.
3. Samples should be dried and ground to pass through a screen with about 100 mesh size. If samples are not ground then they should be sufficiently friable that they can be physically desegregated in strong acid.
4. Samples should be pre-weighed in 50 ml plastic centrifuge test tubes.
5. Wet samples can be analyzed but often they contain such a small amount of dry sediment that the results of the analysis are not very accurate.
6. Samples cannot be ashed or subjected to other high temperature combustion or leaching. The Pb and Po isotopes can be volatilized from the samples at temperatures in excess of 150C.
7. Clients should submit 12 to 20 sections of a sediment core must be analyzed before the core can be dated. Two sampling schemes can be used. Sections can be analyzed individually and results interpolated to derive data for the missing values or some samples can be composited together for analysis. The latter approach is beneficial because it minimizes the amount of interpolation that must be performed. However it may decrease the amount of temporal resolution that can be obtained in the dating.
The sections of the core should be selected for analysis so that the depth(s) or sections of the core of primary interest are analyzed most intensely. However 210Pb dating requires information from the entire concentration-depth profile as well as from a background sample. Therefore the surface sample and a deep sample from near the bottom of the core must be analyzed. In cores with an unknown stratigraphy we recommend using an exponential depth sequence. A two phase approach in which six or eight samples are analyzed and then more samples are selected based on the initial analysis is also useful. Samples should be below the Canadian Council of Ministers of the Environment (CCME) guidelines for our personnel’s safety. If you need more information on CCME please cut and paste the info below into your browser:.
Accurate interpretation of 210Pb concentration versus depth profile requires additional pieces of information about the cores. This information should be provided by email when the cores are sent. The 210Pb measurements on the cores cannot be interpreted until the information is received. A data sheet from MyCore is available. When completed, this sheet provides us with the information we need to complete analysis of the core.
1. Date the core was collected. This is used to ascertain whether the isotopes of 210Pb and 210Po are in secular equilibrium and to correct for radioactive decay that has occurred since the core was collected.
2. Area of the core. This is used to calculate the rate of sediment accumulation in the core per unit area.
3. Dry Weight of each section. This is needed to correct for the amount of compaction in the core and to date the core using the CRS model. Cumulative dry weight over the entire core is calculated so that the weight is needed for each section; not just those on which dating are performed.
This information can be obtained by several techniques, such as:
1. Drying the entire section of the core and measuring the weight of the section
2. Measuring the total wet weight of each section and determine the dry weight of a sub-sample of known wet weight. This enables one to calculate the dry weight of the section from the dry weight to wet weight ratio of each section and leave most of the material untouched.
3. Measuring (or calculating) the volume of each section and determining the wet weight from the volume of the section. Then proceed as in b. The conversion from wet to dry weight and wet weight must be measured very accurately since only a small fraction of the surface sections of the core are dry material.
4. Measuring (or calculating) the volume of each section and using the bulk density of the sediment to calculate the dry mass in the section. This method is very inaccurate unless the bulk density is known with good precision.
5. Depth of sample(s). The top and the bottom of the core section that the sample originated from.
6. Metal concentrations or Pollen counts. This information is not essential. However it is very useful to have since it enables one to examine the top ~5cm of the cores and to determine whether the core has been mixed (homogenized). If the core has been mixed then the age of the sections may need to be adjusted.
7. Organic matter content. This information is not essential. It is very useful to check and ensure that the material in the core is of similar origin and whether the background concentration of supported Pb-210 is constant in the core.