Download (direct link):
Table 10.3 Comparative recoveries for the two-step K-D and EVACS approaches for a range of volatile organic compoundsa (from 200 to 1 ml) . Reprinted with permission from Ibrahim, E. A., Suffet, I. H. and Sakla, A. B., Anal. Chem., 59, 2091-2098 (1987). Copyright (1987) American Chemical Society
Compound Boiling point (°C) Initial concentration (mgr1) Two-step K-D (% recovery)6 EVACS (% recovery)6,c
Trichloroethylene 87.0 0.40 17 (±3) 24(±1)
Benzene 80.1 0.52 29(±2) 38(±1)
Perchloroethylene 121.14 0.40 53(±3) 63 (±4)
Toluene 110.6 0.29 53(±1) 68 (±6)
Chlorobenzene 132.22 0.41 55(±3) 68(±1)
aConditions: solvent, dichloromethane; evaporation rate, 2 mlmin-1 for EVACS, although uncontrollable for K-D; N2 flow rate, 1 ml s-1; number of determinations, 3.
^Figures in parentheses indicate standard deviations (SDs).
^Significantly better recovery determined statistically by analysis of variance (ANOVA) calculations, at 10.915 (theoretical significance).
technique, where vigorous boiling occurs at the beginning, followed by a cooling-down during operation, depending on the surrounding conditions, due to a lack of control of the heat supplied.
• Any volume of sample can be concentrated in the same container with continuous operation down to a 0.5-1 ml final volume.
Pre-Concentration Using Solvent Evaporation
• Sample contamination via transfer of the K-D glassware is eliminated.
• The EVACS avoids the risk of loosing samples by evaporation to dryness even if the system is left unattended for hours because of the special design of the nitrogen tube. Evaporation to dryness can happen with the K-D system during the first or second step.
• The apparatus is easy to build and operate.
• Automation makes the process very easy, but even without automation, the apparatus needs only minimal attention to maintain the same level in the concentration chamber.
• Economically, the EVACS has the advantage that solvents can be recovered for re-use for similar samples, especially when evaporating large volumes of solvents, without disturbing the process.
In addition, the EVACS procedure has been designed to avoid most of the difficulties and weaknesses associated with the conventional K-D method. Possible errors that are still associated with both of the EVACS and K-D techniques can result from the following:
• Cross-contamination can occur between samples - thus careful cleaning is required between each sample stage.
• The possibility of thermal decomposition of certain compounds, which therefore requires fine adjustment of the heat supply, especially during nitrogen stripping.
• Quantitative errors can result from the manual addition of internal standards into the final volume prior to analysis.
• Instrumental analytical errors - these can be minimized by the addition of internal standards.
In order to achieve the lowest levels of any analyte in the environment, we require the use of appropriate samples in which the analyte is homogeneously distributed. Then, by careful sample pre-treatment and selection of the most appropriate extraction/digestion procedures, we can then go ahead and carry out the final analysis. Now, assuming that we have taken due regard to minimize the risk of contamination, we should then expect the analytical instrument to record an appropriate signal (response). However, this may not always be the case. Often, the concentration levels of potential environmental contaminants are so low that an extra step is required. This chapter has introduced the approaches available to pre-concentrate such samples prior to analysis. The techniques described are based on elimination of excess solvent as a means of pre-concentration.
Methods for Environmental Trace Analysis
1. Karasek, F. W., Clement, R. E. and Sweetman, J. A., Anal. Chem., 53, 1050A-1058A (1981).
2. Gunther, F. A., Blinn, R. C., Kolbezen, M. J. and Barkley, J. H., Anal. Chem., 23, 1835-1842 (1951).
3. Ibrahim, E. A., Suffet, I. H. and Sakla, A. B., Anal. Chem., 59, 2091-2098 (1987).
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)
Instrumental Techniques for Trace Analysis
• To be able to identify the correct analytical technique for the type of inorganic or organic pollutant under investigation.
• To understand the principles of gas chromatography (GC).
• To be able to identify the instrumental requirements for GC.
• To understand the principles of high performance liquid chromatography (HPLC).
• To be able to identify the instrumental requirements for HPLC.
• To understand the relevance of infrared (IR) spectroscopy for total petroleum hydrocarbon analysis.
• To understand the principles of atomic spectroscopy.
• To be able to identify the instrumental requirements for flame atomic absorption spectroscopy (FAAS).
• To be able to identify the instrumental requirements for graphite-furnace atomic absorption spectroscopy (GFAAS).