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Soil is a heterogeneous material with significant variations possible within a single sampling site due to different topography, farming procedure, soil type for an example is clay content, drainage and the underlying geology. Soil sampling can be done, for example, by using an auger, spade or trowel. A hand auger like corkscrew-type allows a sample to be acquired from a reasonable depth whereas a trowel is more appropriate for surface material. As all three devices are made of stainless steel the risk of contamination is reduced. However, great care needs to be taken to avoid cross-contamination from one sampling position to another.
Once the sample has been obtained, it should be placed inside a suitable container like a geological soil bag, sealed and clearly labelled with a permanent marker pen. After obtaining the soil sample, replace any unwanted soil and cover with a grass sod, if appropriate. The sample will then be transported back to the laboratory for pretreatment.
In the laboratory, the soil sample will be dried either by air drying or in a drying cabinet. The duration of drying and temperature are variable, but typically air drying at less than 20℃ may require 7 days whereas in a drying cabinet at 40℃ it may be 48 hours. Consideration needs to be given to the potential loss of contaminants due to the use of raised temperature.
After drying, the sample should be sieved through a nylon filter or stainless steel. Typically, the soil samples would be sieved to less than 2 mm particle size.
It may be necessary to reduce the overall quantity of the sample required for the subsequent sample treatment/analysis while still retaining the sample homogeneity. This may be done using a process called ‘coning and quartering’. The process involves decanting the soil sample onto an inert and contamination-free surface. For an example, a clean sheets of polythene, to form a cone. Then the cone is divided into 4 quarters manually by using stainless steel trowel. Then, two opposite quarters of the cone are remove and reformed into a new substance, but smaller than the original cone. By repeating the process as many times as necessary a suitable sized sub-sample is now ready for sample extraction or digestion.
Diagram show the handheld auger
Preparation of environmental solid samples for inorganic analysis
Flame atomic absorption spectrometer
Flame atomic absorption spectrometer is used for detection of metals presents in soil which is common technique for the land analysis. In this technique, a light beam of the correct wavelength to be specific to a particular metal is directed through a flame. The flame atomizes the sample, producing atoms in their lowest electronic energy state which is also known as ground state. For flame atomic absorption spectrometer (FAAS) measurement to excite ground state atoms external light source is needed These are capable of absorbing radiation from lamp. The nebuilizer is one of the part of FAAS and this part is very important and complicated to this spectrometer. The combination nebuilizer and burner provide the atomic aerosol for FAAS. The sample in aqueos solution is sucked up by the Venturi effect. Pressurized air is passed through a tube causing the sample solution to be drawn into the burner as a fine mist where it is mixed with a combustible gaseous mixture to produce a flame, which finally contains the atomic aerosol The radiation then enters a monochromator, which isolates the line of interest. The light is then measured by a photomultiplier tube which is also known as detector. The signal is then processed and the computer system prints the output on screen.
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