Download (direct link):
(ii) (a) An absorption train, perhaps cooled in ice-water to prevent evaporation, and a trap to protect the subsequent pump. Alternatively, you could have:
(b) A method for rapid cooling of the gases at the same time preventing condensation of water (dilution, drier or chiller), followed (after a pump and flow meter) by an instrument measuring gases at ambient temperatures. A further possibility could be:
(c) An instrument capable of taking measurements at high temperatures, followed by a cooling/drying system to protect the subsequent pump.
(iii) A pump.
(iv) A gas meter or flow meter.
There would also need to be a pitot tube to measure the gas flow at the sampling point within the duct to ensure isokinetic sampling.
From a practical point of view, routine analysis will often be performed in small laboratories close to the workplace being monitored and with limited facilities. Under such circumstances, ultraviolet/visible spectrometry may be a more appropriate method. As well as a greater capital investment for an atomic absorption spectrometer, adequate ventilation is necessary, and also a regular gas cylinder supply. There would probably be an insufficient throughput of samples to justify the additional cost of ICP-OES or ICP-MS.
From an analytical point of view, particulate samples from one workplace will be of relatively constant (and known) composition. Potential interferences, which limit the use of ultraviolet/visible spectrometry for samples of unknown composition, can be readily assessed.
The small sample masses of atmospheric particulates (mg or below) may mean that you are working close to the limits of detection of the available techniques. The limits of detection of each technique are different for each element (see Table 7.2) and so the most appropriate technique may differ for each analysis. In other areas of environmental analysis, the sample size may not be such a restriction and preconcentration may be used to decrease the lower limit of detection.
Responses to Self-Assessment Questions
With such a general question, I cannot put the criteria in any rank order, but they
should include the following:
1. Ease of solubility of the analyte. If the analyte is soluble in water or dilute acid, solution analytical techniques are usually the most convenient to use.
2. Number of elements being analysed. You should re-read the description of the techniques to determine which are most suitable for multiple-element analysis.
3. Availability of equipment. Many of the solid-state techniques will only be found in laboratories dedicated to solid-state analysis.
4. Sensitivity. Often, you will be working close to the limits of detection of the methods. The most sensitive technique will differ for each element.
5. Compliance with specified method. Some legislation requires the use of specific procedures for the analysis. Other legislation accepts that alternative techniques may be used if they have suitable accuracy and reliability for the application. The validation of an alternative method may, however, be a long and costly process.
1. Each of the components will have different physical and chemical properties, which in turn leads to different bioconcentration ability, rates of degradation and toxicity. If a value for the total toxicity of a sample is required, this would involve determining individual concentrations and compensating for their different toxicities, for example, by using toxic equivalent factors.
2. The majority of analytical schemes for ng kg-1 concentrations of organic compounds would involve chromatographic separation of the interfering compounds (after extensive pretreatment), which in turn provides at least a partial separation of the PCDDs and PCDFs.
3. As you might expect, the relative quantities of each of the compounds will be different from each production source. Under favourable circumstances, estimation of the relative concentrations can give an indication of their likely origin.
The assumption in the use of internal standards is that the standard will behave
identically in the extraction to the compound being analysed. An isotopically
Introduction to Environmental Analysis
labelled compound would be closer in behaviour than a chemically distinct compound.
A second benefit is that the labelled compound serves for peak identification - an important consideration when you remember the large number of peaks which may be found even in a selected ion chromatogram.
1. The peak should occur at the expected retention time for the chromatographic column. It is easy to forget that the mass spectrometer is simply a highly sophisticated detector for the chromatograph and that retention times are a good primary means of identification.
2. The peak should be monitored at two or more m/z values, corresponding to the same molecular fragment with different distributions of 35Cl and 37Cl in the molecule. The relative intensities should correspond to the expected statistical distribution. A complete mass spectrum could be used to attribute the peak to a dioxin or diobenzofuran rather than an impurity. However, the fragmentation patterns of the dioxins and furans are often too similar to allow positive identification of individual members of the two groups.