Preparing soil and sediment samples for exploration is substantially different than rock sample preparation. One of the key differences is the drying temperature. To ensure that soil and sediment samples can be effectively sieved drying is required, and it is critical to dry at a low the temperature (<60ᵒC) to prevent the loss of volatile elements. These sample types are also screened (sieved) rather than pulverised. The process of sieving soil and sediment will remove larger grains, often quartz sand. By using the finer material for geochemical analysis, a larger number of grains form part of the sample aliquot making it more representative.
Soil and sediment samples are commonly sieved to -180 microns to remove a large portion of the quartz sand in the sample. The -180 micron screen is often referred to as 80 mesh as this corresponds to the Tyler and US mesh number for this sieve size. The most commonly used preparation package for soil and sediments is PREP-41 which includes low temperature drying (to retain volatile components), and an 80-mesh (180 micron) screen size. Both fractions of the sample are retained, with analysis most commonly carried out on the minus fraction.
The distribution of elements in a soil or sediment can vary dramatically between size fractions. It is therefore important to be aware of the distribution in your project to select the best fraction to identify anomalies. A large range of alternative screen sizes are available at ALS preparation laboratories as required. If you would like to use a different screen size indicate the size on the sample submittal form. It is best to identify the size using the metric system rather than using the screen number which varies between the Tyler and US numbering systems.
The highly reactive clay-size fraction in soil and till is a key trap site for the labile elements responsible for geochemical anomalies transported into cover over mineralisation. Although partial leaches that target these labile elements in soils are useful in exploration, a method to fully isolate the <2 micron fraction can open a larger range of analytical options. Results from multi-element geochemical surveys that use the clay size fraction for analysis show enhanced geochemical response compared to traditional bulk soil analysis, allowing for better precision and greater contrast in geochemical signal.
ALS Geochemistry's clay-size fraction extraction method is performed in a clean lab setting with rigorous contamination control, ensuring your results are accurate and reliable. The amount of clay extracted from a 300g screened soil samples is generally in the order of 2g. Therefore, geochemical analytical methods which use large aliquot sizes are not applicable for clay fraction separated samples. To determine if clay-sized fraction separation is right for your project, we suggest using an orientation survey or a subset of a closely spaced local survey to test one or more digestions paired with super-trace ICP-MS analysis on the clay-sized separate.
Pulverisation uses metal bowls which may impart elements to a sample during grinding. Typically, sieving adds less carryover as the contact between the sample and the metal has less force. Pulverisation will produce a more homogeneous sample but will break down all grains, even those that are generally not of interest in exploration samples, like quartz.
The Ionic Leach™ method aims to digest weakly bound ions from a sample. As the bonding of these ions can change dramatically with drying, often making them less available for digestion, we recommend that samples for this analytical method are not dried or sieved. It is preferable that samples are preserved at the time they are collected by double bagging in plastic to prevent drying. Large rocks and twigs should be removed at the time of collection and the samples submitted as is.
Guidance on how to prepare biogeochemical and hydrogeochemical samples are given in our Generative Exploration section.
|Sieve Size (mm)||Tyler #||US #|
Soil & sediment samples prepared by sieving & clay separation can be analysed using a variety of multi-element trace methods.MORE INFORMATION