Thanks for asking this as I am also curious about Hg, As, Cd etc in sediments.
Soil should be dried, processed and sieved using 2 mm sieve. This samples can be used for analysis of all physical, chemical and physico-chemical analysis of soil. However for the analysis of organic carbon soil should be sieved in 0.2mm sieve. After processing, soil can be kept for upto 6 months for analysis of all soil parameters. Beyond that it is not advisable because long storage will affect the values of different properties
Soil should be dried, processed and sieved using 2 mm sieve. This samples can be used for analysis of all physical, chemical and physico-chemical analysis of soil. However for the analysis of organic carbon soil should be sieved in 0.2mm sieve. After processing, soil can be kept for upto 6 months for analysis of all soil parameters. Beyond that it is not advisable because long storage will affect the values of different properties
If you are not interested in speciation, you can store indefinitely... dried and sealed. Metals will not degrade over time.
Dave, Thanks, now if I am interested in speciation, then I must consider.
For bulk chemical analysis you can keep samples in two ways:i) dried and powdered , and ii) in a cold storage (generally 4 deg.C.). For speciation analysis it is good to store the samples in cold storage (~0 deg.C) immediately after collection in an anoxic medium, if you plan to do analysis within one week or remove the pore water from samples and then store at (~4 deg. C) again in anoxic medium. But NOTE: the result of speciation analysis is purely time dependent. for example: a long storage of samples in an oxic medium will increase the fraction of iron bound species and decrease the organic bound species.
As advised, dried soil sample can be kept in cold storage for months before analysis.
After collecting the soil sample, keep as such in plastic/ polythene container in normal room temperature . There is no time limit for storage. Practically no variation in concentration was observed in case of Kaolinite , when Arsenic was measured periodically for one year with a interval of one months.
I think, you should modify your question -
a) the term Heavy Metals should be avoided (see Duffus, J. H. (2002): “Heavy Metals” – A meaningless term? – Pure Appl. Chem., 74(5):793-807. http://bit.ly/Duffus2002) and
b) “As” is not a metal at all
Correct would be:
“How long can I keep soil samples before analyzing (semi-)metals, particularly arsenic”
All the answers are then correct - but keep in mind that when you store a soil sample too long, micro-organisms can change the chemistry (keyword: Hg Methylation). Therefore: keep the samples at or below 4 °C.
The problem with soil samples is the microbial community and stability of some compounds such as organic matter and redox sensitive reduced phases. Some compounds can such as MeHg or FeS-like precipitates can oxidize during treatment procedures such as drying and sieving. So in perfect World I would recomend to freeze the wet sample for these purposes. The oxidation of soil or sediment sample can be avoided by working in glove box filled with N2 or Ar. Of course if you are analysing Ca you wont worry with oxidation etc. you air dry, sieve and store the sample which will stay usable for a long time.
In case of microbial community in your soil sample,its activity will be ceased by drying but in case of rewetting they could start being active again. This can be avoided by sterilizing the soil sample with gamma rays, which is what companies do for example with the reference materials. To make sure and extend the sample stability.
To me, soil samples for the measurement of heavy metals should not be kept so long more than one month. You may digest and then store for up to six month before reading, so far the extracts are kept in the refrigerator.
No problem for anytime you can measure Heavy metal , because they are not change during in time. Of course if you are measuring by atomic absorption.
soil samples for analysis of arsenic after drying and sieving, must be digested in acid as per methods and then the solution may be stored at 4 degree centigrade, and can be analysed within 4 months.
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