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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
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• Analytical time of the technique - compare with urgency of result.
• Instrument time required for the technique - have you sufficient instruments?
• Analyst’s time required for the technique. This can often be significantly different from the instrument time.
• Time required to set up any instrumentation, or in preparation of reagents. This time becomes more significant with small numbers of samples.
• Throughput of the laboratory. There may be instruments available for rapid analysis in a high-throughput laboratory (see Section 3.4.1).
• Number of analytes to be determined - note that some methods can determine more than one analyte.
• Availability of equipment.
• Relative cost of instrumentation/labour.
All of the criteria except the last two will be quite independent of the country in which you work. It would be possible for an instrumental method to be favoured in one part of the world where labour costs are high, whereas more labourintensive methods are favoured in other parts of the world, and where, perhaps, instrumentation is less readily available.
Chapter 4
Response 4.1
First of all, let us decide which are the volatile compounds:
toluene, methylene chloride, chloroform and benzene (1, 4, 5, 10) These will be analysed by the purge-and-trap technique.
260
Introduction to Environmental Analysis
There are two phenols in the list:
2,4,6-trichlorophenol and phenol (3, 7)
These would need to be extracted under acid conditions. (Under basic conditions, they would be in the form of non-extractable salts.)
The other compounds would form the base-neutral group:
anthracene, 1,2-dichlorobenzene, naphthalene, hexachlorobenzene (2, 6, 8, 9)
Do not be too concerned if you do not have all ten correct, but I hope you were able to think your way through most of them.
Response 4.2
Solid-phase extraction can be used in the field. You could perhaps load the column with the sample by using a syringe. Microfibres, with care, can be immersed directly in water in the field. A third method, which you may have considered, is purge-and-trap. This has been occasionally used. This method would give a concentration averaged over the purge period rather than an instantaneous value.
Response 4.3
(a) The pesticides will probably be present at the lower end of the trace-level range. Even after extraction and clean-up, a large number of compounds may still be present in the sample. A narrow-bore capillary column would give the required high resolution and detection sensitivity. A medium-polarity silicone polymer column would be a good initial choice.
(b) The components of interest would probably be part of a much higher concentration of waste chemicals. A wide-bore capillary column would be better suited than a narrow-bore column since it has a higher sample loading capacity. It would also be more tolerant of any non-volatile impurities in the sample. The column could also be coupled directly to the purge-and-trap system which you would almost certainly have used in the sample preparation. A medium-polarity silicone polymer column would again be a good initial choice.
(c) There would be a strong likelihood of non-volatile residues in the oil extract. A wide-bore capillary column would be more tolerant of contamination than narrow-bore ones. A non-polar silicone polymer column would be a good initial choice.
Response 4.4
N-methylcarbamates are analysed by HPLC using post-column derivitization or UV detection. Although not specifically stated, you should be able to deduce that MS detection could also be used. The polarity of the molecules and their thermal lability makes this technique preferable to GC.
Responses to Self-Assessment Questions
261
The techniques specifically mentioned for atrazine in the present chapter are immunoassay and HPLC. HPLC will produce a result specific to the compound. With the immunoassay there may be cross-reactivity from other triazine pesticides.
If individual phenols were not required, the analysis could be performed spec-trometrically. For individual phenols, HPLC could be used. This may be after the formation of a fluorescent derivative. Once again, the polarity of the molecules can make GC analysis difficult.
Individual PAHs can be determined by HPLC with fluorescence detection. Their inherent fluorescent properties makes this an ideal method. GC may also be used. If individual PAHs were not required, they can be determined by immunoassay.
Malathion is not mentioned in this chapter, although you should by now realize that GC is in fact the first choice technique for all organics unless there are specific reasons (such as in the examples above) why other techniques are preferable.
Response 4.5
Loss of analyte at each stage is possible. This will be a particular problem if it is present in low concentration. For the determination of common pollutants, contamination of the sample may also occur. The problem increases with the number of stages of pretreatment, and the number of reagents involved. Common metal ions are universally found at low concentration in all reagent solids and traces of pesticides are common in organic solvents. Low-molecular-mass organic materials (e.g. solvents) are themselves commonly in use within laboratories. All materials in contact with the sample should be regarded as potential sources of contamination.
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