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Introduction to environmental analysis - Reeve R.

Reeve R. Introduction to environmental analysis - Wiley publishing , 2002. - 312 p.
ISBN 0-471-49295-7
Download (direct link): introductiontoenvironmental2002.pdf
Previous << 1 .. 52 53 54 55 56 57 < 58 > 59 60 61 62 63 64 .. 111 >> Next Drying and Homogenization DQ 5.4
Which two factors do you consider will determine the temperature and drying time of a biological sample?
The temperature and duration of drying must be a balance between too low a temperature over a protracted period promoting biological activity and too high a temperature over a shorter time period leading to loss of volatile components.
A typical drying procedure would be to blow a current of dry air over the sample for a period of up to 12 h. The temperature should not be in excess of 50oC. Alternatively, the sample may be freeze-dried, i.e. deep-freezing the sample, reducing the pressure and removing the water by sublimation.
DQ 5.5
Due to the risk of potential losses caused by drying, why do you think such treatment is necessary at all?
I suspect your answer will be so that the analytical result can be referred to a dry weight. There is, however, no reason why you could not calculate the dry weight on a second sample. Drying the samples lessens the possibility of change due to biological activity. A second advantage is that homogenization of the bulk sample, necessary if sub-samples are to be taken, is made easier if the sample is dry.
Homogenization of the dried sample is often by the use of a high-speed grinding mill. Care should be taken, once again, to ensure that you are not introducing contaminants during the grinding process.
Introduction to Environmental Analysis
5.3.3 Extraction Techniques for Organic Contaminants
If present, organic contaminants are likely to be in the ^g kg-1 concentration range or below. The simplest method for the extraction of organics is to shake a sub-sample with an extracting solvent (e.g. hexane or petroleum ether for neutral organics) and to leave the two phases in contact for several hours. An alternative method is to use Soxhlet extraction. The apparatus employed for this is shown in Figure 5.1. In this method, fresh solvent is continuously refluxed through the finely divided sample contained in a porous thimble and a syphon system removes the extract back into the refluxing solvent. The net effect is continuous extraction by fresh solvent. A typical extraction takes about 12 h and uses about 300 ml of solvent. The technique is only applicable to analytes which can withstand the reflux temperature of the solvent.
Extraction thimble" containing sample
Heating mantle
Extraction solvent
Figure 5.1 Schematic of a Soxhlet extraction system.
Analysis of Land, Solids and Waste
DQ 5.6
The use of hexane or petroleum ether as extraction solvents assumes a dried sample. What problems could you foresee if for any reason the sample was not dried?
The two solvents suggested are immiscible with water and would easily form an emulsion. In some cases, it may be difficult for the solvent to penetrate the sample.
A desiccant is often mixed with the sample during the extraction - sodium sulfate is commonly used for this purpose. The solvent can be modified by the addition of a polar solvent such as acetone. Alternatively, the extraction solvent can be changed completely to a solvent which is miscible with water. Acetonitrile is often used in this case. You must, of course, be certain that the solvent is still appropriate for the extraction of the analyte. If the sample will not allow the solvent to penetrate the structure, conditioning the sample with a polar solvent which is miscible with both water and the extraction solvent may overcome the problem. Isopropanol has been used for this purpose.
5.3.4 Ashing and Dissolution Techniques for Trace Metals
Trace metals are likely to be in the mg kg-1 concentration range. The concentrations, however, will vary from species to species and throughout the growing season. In order to extract metals, the organic matter is decomposed by dry or wet ashing.
Dry ashing consists of heating the sample in a muffle furnace, typically at 400-600oC for 12-15 h. The resulting ash is then dissolved in dilute acid to give a solution of the metal ions. Inaccuracies can arise both from volatilization of metals and the retention of metals in a insoluble form in the crucible.
Wet ashing consists of heating the sample with oxidizing agents to break down the organic matter. A typical procedure would be heating first with concentrated nitric acid, followed by perchloric acid. Alternative combinations include sulfuric acid/hydrogen peroxide and nitric/sulfuric acids.
An advantage of wet digestion is lower losses from volatilization (due to lower temperatures and liquid conditions), but it can give rise to higher metal blanks from impurities in the acids. Great care has to be taken with methods involving perchloric acid. This acid, in the presence of metals, has a tendency to detonate on drying! Small sample sizes should always be used and the liquid in the flask should never be allowed to dry out.
Introduction to Environmental Analysis
5.3.5 Analysis of Animal Tissues
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