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The necessity of ensuring purity of solvents and cleanliness of apparatus has been discussed earlier in Sections 2.9 and 4.2 and needs to be re-emphasized here. All batches of solvents and reagents need to be frequently checked to confirm lack of contamination. Pesticide-free or distilled-in-glass grade solvents should be used. Extreme care should be taken with respect to known sources of dioxins. Cigarette smoke and ash can contaminate the laboratory. Extraction thimbles used for solids can be a second source, with dioxins potentially being formed by the bleaching process during their manufacture. The thimbles should be pre-extracted with solvent prior to use in the analysis.
Introduction to Environmental Analysis
8.3.2 Gas Chromatography
The chromatography column has both to separate the components of the mixture and be compatible with mass spectroscopic detection.
From your knowledge of the analytical problem, what can you say about
the type of column required?
1. First of all, in order to resolve the total number of components and to interface with the mass spectrometer, narrow-bore capillary columns are necessary. A programmed temperature gradient will optimize the separations.
2. The stationary phase would have to be compatible with high-tempe-rature operation in order to elute the lower-vapour-pressure compounds. A silicone stationary phase would be the obvious choice.
3. With a mixture of compounds of varying polarity (according to the degree of substitution and substitution pattern), a medium-polarity stationary phase would be a good first try.
4. For mass spectroscopic detection, it would be advantageous for the column to group the eluted compounds into isomer groups. This aids peak identification, as well as allowing a fairly simple ion sequence to be used for detection.
The separation of all 210 dioxins and furans and their division into separate isomer groups are exacting demands for a single capillary column. You may not be surprised to find that two columns and multi-dimensional GC/GC were used in the analytical scheme (see Table 8.2).
Imagine that you were about to analyse a large number of samples by a method such as that described in Section 8.3. What features would you include in your scheme to ensure analytical quality throughout the programme?
There are at least two areas of uncertainty in the analytical procedures for the determination of dioxins in solid samples as exemplified above. What are these?
Some species (e.g. dioxins and related compounds) have such a great ability to bioaccumulate and such a high degree of toxicity that monitoring their presence at ng 1-1 or ng kg-1 concentrations is necessary. The analyses not only require highly sensitive and selective instrumentation but also a large degree of analytical skill and expertise. Gas chromatography-mass spectrometry is most often used. This technique has been discussed, along with the necessary clean-up and concentration stages.
Introduction to Environmental Analysis Roger Reeve
Copyright © 2002 John Wiley & Sons Ltd ISBNs: 0-471-49294-9 (Hardback); 0-470-84578-3 (Electronic)
Responses to Self-Assessment Questions
1. You could write many pages on this first question. The particular problem mentioned here is the disproportionate proportion of the world’s resources used in the developed world, together with a similar proportion of the waste produced and pollution of the earth.
2. Although volcanic emissions are natural phenomena, at intervals they put into the atmosphere large amounts of gases, vapour and dust, and could be considered a natural source of pollution.
3. The production of methane by cows is again a natural phenomenon. However, since the total cattle population on the earth is largely controlled by mankind, then so is the quantity of methane emitted to the atmosphere from this source.
4. This will directly lead to an increase in the concentration of nitrate above the naturally occurring levels in water supplies surrounding the farms. There may, however, be other consequences. Since all of the species in the nitrogen cycle (see Figure 1.1) are linked, changes in nitrate concentrations may lead to changes in concentrations of other species in the cycle, thus leading to further pollution problems (see Table 1.1).
These are all examples of current concern. As countries become developed, more
of the earth’s resources are used. The volcano of Mount Etna in Italy is thought
to be a significant contributor to atmospheric mercury concentrations in Europe.
The rising atmospheric methane concentration has, in part, been attributed to
increasing cattle populations.
Introduction to Environmental Analysis
Concern has been expressed over the effect of increased nitrate fertilizer usage in the production of nitrous oxide, a greenhouse gas and potential ozone-layer depleter.
Analysis of the discharge before dispersal into the river will monitor the pollutant being discharged, but will only be related to the final concentration in the river when used in conjunction with river flow data.