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(d) The results would be described as both inaccurate and imprecise.
The graphs have been redrawn, indicating their linear working ranges (see Figure SAQ 2.2 below). Numerically, the linear working ranges are as follows:
(a) The linear working range is between 0 and 50 mg l-1.
(b) The linear working range is between 0 and 10 mgl-1.
(a) 4035> 3°-i 25-
ts 20S 15105 0
10 20 30 40
Concentration (mg l-1)
Concentration (mg l 1)
Figure SAQ 2.2 Plots showing different linear dynamic range values.
20 40 60 80
Concentration (mg l-1)
232 Methods for Environmental Trace Analysis
(c) The linear working range is between 0 and 200 mgl-1.
(d) The linear working range is between 0 and 50 mgl-1.
The magnitude of error, E, can be calculated as follows:
y^fxj — x)2 = 466
S’2 = 466/9 = 51.8 V (variance) = 51.8/10 = 5.18
E (at the 95% confidence interval) = ±2.26(5.18)0 5 = ±5.14 ppm
(Note: the value of 2.26 was obtained from tables of critical values of Student’s t statistics at the 95% confidence interval for n — 1.)!
This infers that in taking 10 samples, an error of 5.14 ppm was tolerated, and that the concentration of lead in the sample should be expressed as 93 ± 5.14 ppm.
The number of samples required can be calculated by using equation (3.4) as follows:
(1.96)2(51.8)/22 = 49.7
Therefore, 50 samples are required.
(Note: the value of 1.96 was obtained from tables of critical values of Student’s t statistics at the 95% confidence interval for n = to.)!
In either case, you would first need to ensure that you have the necessary container for the storage of the sample and that the container has been pre-treated
! Student’s t statistics (distributions) are widely used in solving statistical problems in chemical analysis involving small numbers of samples (n < 30). For further details, see the texts listed in Section 22.214.171.124.
Responses to Self-Assessment Questions
appropriately. In addition, care will have been needed to obtain a representative sample of the water (see Chapter 3) and in an appropriate manner.
It is also good practice when sampling water to rinse out the pre-cleaned container with the sample prior to sample collection. This allows the container to be pre-conditioned with the sample prior to collection.
Total lead analysis. It is recommended that 100 ml of the water sample is introduced into a pre-cleaned and rinsed polyethylene container with a polypropylene cap, or into a glass bottle. In both cases, acidify the water to pH < 2 with nitric acid. Ensure that the container is completely full of the sample. In this situation, the sample can be held for up to six months for the analysis of total lead.
Sulfate analysis. It is recommended that 50 ml of the water sample is introduced into a pre-cleaned and rinsed polyethylene container with a polypropylene cap, or into a glass bottle. In both cases, store the sample at 4°C, either in a cool box on-site and during transportation, and then in a fridge in the laboratory. Ensure that the container is completely full of the sample. In this situation, the sample can be held for up to 28 days for the analysis of sulfate.
Dieldrin analysis. Dieldrin is an organochlorine pesticide. It is recommended that 500 ml of the water sample is introduced into a pre-cleaned and rinsed glass bottle. For preservation of the sample, two options are possible, i.e. either add 1 ml of a 10 mg ml-1 HgCl2 solution, or add the appropriate extraction solvent (or pre-concentrate the sample on a solid-phase extraction cartridge (see Chapter 8)). Ensure that the container is completely full of the sample. In this situation, the sample can be held for up to seven days if using HgCl2 solution (or 40 days, if extraction solvent is added) for the analysis of dieldrin.
Hydrofluoric acid is the reagent used for dissolving silica-based materials. The silicates are converted to a more volatile species in solution, according to the following equation:
SiO2 + 6HF = H2(SiF6) + 2H2SO4
While a different mass of sample is taken, ranging from 50-2000 mg for fish and 25-2000 mg for sediment, most methods involve some form of liquid-solid extraction, i.e. a known mass of sample is extracted with a solvent. Often, the sample has previously been acidified and the solvent is usually toluene. Then, depending upon the separation technique, some additional sample work-up
Methods for Environmental Trace Analysis
is required. Gas chromatography appears to be the most common method of separation. Often, for gas chromatography, the methylmercury requires to be converted into a volatile form. This has been most commonly camed out with either NaBH4 or NaBEt4, or by butylation with a Grignard reagent (butylmagne-sium chloride). A range of detection systems have been used for gas chromatography, including the conventional type of detector, i.e. electron-capture detection, through to atomic spectroscopy, e.g. cold-vapour atomic absorption spectroscopy.