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X-ray fluorescence (XRF) spectroscopy can be used to analyse metals in solids directly. In this technique, sample atoms are irradiated with X-rays, thus causing ejection of electrons from the inner shell. The loss of these lower-energy electrons results in outer-shell electrons filling the vacancies, and hence emitting X-rays characteristic of the sample atom. Two different instruments for XRF are available, dependent upon how the radiation is observed. In wavelength-dispersive XRF, the emitted radiation from the sample is dispersed by a crystal into its component wavelengths. Sequential scanning of the different wavelengths thus allows measurements of the different elements. The alternative approach is to use energy-dispersive XRF. In this situation, all emitted radiation is measured at
204 Methods for Environmental Trace Analysis
Figure 11.18 Typical example of an anodic stripping voltammogram, obtained for the analysis of trace metals in solution.
Figure 11.19 Typical example of an X-ray fluorescence trace, obtained for the analysis of trace metals in an environmental solid sample.
the detector and sorted electronically. A typical XRF trace of an environmental sample is shown in Figure 11.19.
Ion chromatography can be used to determine both anions and cations. However, it is its use for anion determination which is of importance in this chapter. Just as in high performance liquid chromatography (described above), separation is achieved by using a column. In this case, the column is likely to be based
Instrumental Techniques for Trace Analysis
i i i i I i i i i | i i i i | i i i r | i i i i | i i i i | i i i i | i i
0 2 4 6 8 10 12 14
Figure 11.20 Typical ion chromatogram, obtained for the separation/analysis of anions in an environmental sample.
on poly(styrene divinylbenzene), with a mobile phase of sodium hydroxide or a sodium carbonate/hydrogen carbonate buffer. Detection is with a conductivity detector, which measures the increase in conductivity of the eluent containing the sample anions. A typical chromatogram for the separation/determination of anions in a sample is shown in Figure 11.20.
The final stage in the analytical process is to measure the concentration of the environmental pollutant. This chapter has described appropriate techniques for the measurement of both metals and organic compounds. While the primary descriptions have focused on atomic spectroscopy for metals and chromatography for organic compounds, some related techniques have been discussed briefly.
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)
Recording of Information in the Laboratory and Selected Resources
• To enable relevant scientific information to be recorded in the laboratory.
• To allow correct sample pre-treatment details to be noted.
• To allow correct sample preparation details to be noted.
• To allow correct analytical technique information to be noted.
• To be aware of relevant additional resources that can be consulted, including books, journals, CD-ROMs, videos and the Internet.
12.1 Recording of Information
Prior to commencing any experimentation, you are required to complete a hazard and risk assessment of the chemicals and apparatus that you will use, i.e. a Control of Substances Hazardous to Health (COSHH) assessment (see Chapter 1).
When carrying out any laboratory work, it is important to record information in a systematic manner, relating to sample information, sample treatment and the analytical technique being used, calibration strategy and the recording of results. Below are some examples of data sheets that could be used to ensure that all information is recorded in a systematic way. These data sheets are not intended to be totally comprehensive, and so may be altered and amended by the individual worker as required.
Methods for Environmental Trace Analysis
The first data sheet (A) allows identification of any preliminary sample pretreatment that may be necessary prior to sample preparation. Data sheets (B) and (C) identify the sample preparation techniques for inorganic and organic samples, respectively, while data sheets (D) and (E) contain, respectively, the instrumental requirements for the most commonly used inorganic and organic analytical techniques.
Recording of Information and Selected Resources 12.1.2 Examples of Data Sheets
Data Sheet A: Sample Treatment
• Grinding and sieving
Grinder used (model/type)
Particle size (sieve mesh size),,
• Mixing of the sample
Manual shaking yes/no
Mechanical shaking yes/no
• Sample storage
• Chemical pre-treatment
pH adjustment yes/no
Addition of alkali (specify) or acid (specify)