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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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DQ 3.5
How would you sample snow and ice?
Answer
As the snow and ice are both frozen they should be treated in the same way as any other solid. You might therefore consider using an auger to sample them. You should also remember that unless you are doing this in ‘cold climates’ you might also need to sample the melt (liquid) fractions of both the snow and ice.
Summary
This chapter has focused on the different methods of sampling solids, liquids and gases. Upon its completion, you should feel confident enough to be able to sample any of these different forms and be ready to consider the storage conditions described in the next chapter.
References
1. Tan, K. H., Soil Sampling, Preparation and Analysis, Marcel Dekker, New York, 1996, p. 13.
2. Jones, A., Duck, R., Reed, R. and Weyers, J., Practical Skills in Environmental Science, Prentice Hall, Harlow, UK, 2000, pp. 102-103.
Methods for Environmental Trace Analysis. John R. Dean
Copyright © 2003 John Wiley & Sons, Ltd.
ISBNs: 0-470-84421-3 (HB); 0-470-84422-1 (PB)
Chapter 4
Storage of Samples
Learning Objectives
• To understand the issues associated with the storage of samples.
• To understand the concept of sample storage.
• To appreciate the different methods available for sample preservation for metals and organics.
• To appreciate the difficulty associated with maintaining chemical species information, i.e. speciation.
4.1 Introduction
In an ideal situation, samples would be analysed in situ without the need for sampling, storage and transport to the laboratory, before the analysis. However, this is rarely the case. We have seen in the previous chapter that sampling is crucial to obtaining a representative sample. Once this has been achieved, it then becomes essential that the acquired sample is stored and transported to the laboratory in the same state as it was sampled.
DQ 4.1
What problems may occur when a sample is stored?
Answer
The concern with the storage of samples is that losses can occur, due to adsorption to the storage vessel walls, or that potential contaminants can enter the sample, from desorption or leaching from the storage vessels.
40
Methods for Environmental Trace Analysis
4.2 Methods
The problems can all lead to the analyst getting the wrong answer or at least an unexpected answer after the analysis has taken place. It is probably true to state that complete and unequivocal preservation of samples is impossible. However, the use of preservation methods should be able to retard any chemical or biological changes that will inevitably continue after sampling has occurred. The goal therefore is to store samples for the shortest possible time-interval between sampling and analysis. Indeed, in some instances where analytes are known to be unstable or volatile it may be necessary to perform the analysis immediately upon receipt, or even not at all! Methods of preservation are relatively few and are generally intended to fulfil the following criteria:
• to retard biological action
• to retard hydrolysis of chemical compounds and complexes
• to reduce volatility of constituents
• to reduce adsorption effects
Similarly, the preservation methods available are equally limited and constitute the following approaches:
• pH control
• addition of chemicals
• refrigeration
• freezing
It is important to note how long the sample has been stored and under what conditions storage has been carried out.
DQ 4.2
Why might the nature and type of storage vessel be important?
Answer
For example, if it is known that the analyte is light sensitive it is then essential that the sample is stored in a brown glass container to prevent photochemical degradation. For volatile species, it is also desirable that the sample is stored in a well-sealed container. In most cases, the use of glass containers is recommended as there is little opportunity for contamination to result as a consequence of the vessel itself. It is also important that the appropriate sized container is used. It is better to completely fill the storage container rather than leave a significant headspace above the sample. This acts to reduce any oxidation that may occur. In addition
Storage of Samples
41
to glass containers, polyethylene or polytetrafluoroethylene (PTFE) containers are appropriate to use for solid samples. Plastic containers are not recommended for aqueous samples as plasticizers, e.g. phthalates, are prone to leach from the vessels which can cause problems at later stages of the analysis.
Whatever method of storage is chosen it is desirable to perform experiments to identify that the analyte of interest does not undergo any chemical or microbial degradation and that contamination is kept to a minimum. Some selected examples of methods of preservation for water samples are shown in Table 4.1.
SAQ 4.1
How would you preserve aqueous samples for the analysis of (a) total lead, (b) sulfate, and (c) dieldrin?
A recent review has highlighted the stability of chemical species (in speciation studies) with respect to environmental matrices [1]. These results are summarized in Table 4.2. It should be noted that samples must be analysed as quickly as possible after collection. The times given in this table are the maximum times that samples should be stored before analysis. It should also be noted that the guidelines given are general in nature. For example, many organophosphorus pesticides (not shown in Table 4.2) can be preserved by the addition of hydrochloric acid. However, as an exception to this, the (organophosphorus) pesticide diazinon breaks down when acidified.
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