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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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Pre-Concentration Using Solvent Evaporation
177
Figure 10.3 Schematic diagram of the automatic evaporative concentration system (EVACS): W, solvent; ?, vapour. 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.
10.5 Gas ‘Blow-Down’
In this method, a gentle stream of (high-purity) purge gas is passed over the surface of the extract (Figure 10.4). The latter may be contained inside a conical-tipped or similar vessel. In this situation, the purge gas is directed towards the side of the vessel, and not directly onto the top of the extract, in order to induce
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Methods for Environmental Trace Analysis
Figure 10.4 Schematic of a typical gas ‘blow-down’ system (Turbovap™) system used for the pre-concentration of compounds in organic solvents.
a swirling action. The extract-containing vessel may be partially immersed in a water bath to speed up the evaporation process.
DQ 10.3
How else might the evaporation rate be increased?
Answer
Alternative ways to speed up the evaporation process are to increase the flow rate of the impinging gas (NOTE - too high a rate and losses may occur), alter its position with respect to the extract surface, or increase the solvent extract surface area available for evaporation.
If carry-over of trace quantities of the aqueous sample or high-boiling point solvent has occurred, it may not be possible to evaporate them without significant
Pre-Concentration Using Solvent Evaporation
179
losses of the analyte of interest. The extract may be taken to dryness or left as a small volume (~1 ml).
A comparison between the EVACS and Kuderna-Danish (K-D) approaches for solvent concentration during environmental analysis of trace organic chemicals has been made [3]. Some selected results for a range of organic compounds, lower-boiling-point compounds, and volatile organic compounds are shown in Tables 10.1-10.3, respectively. The authors of this study [3] conclude that the evaporative concentration system (EVACS) is more efficient than the Kuderna-Danish evaporation technique for concentrating semi-volatile-volatile organic analytes (bpt 80-385°C) from a low-boiling solvent (e.g. dichloromethane, bpt 40°C). The advantages of the EVACS over the K-D approach were described as follows:
• The temperature control associated with the EVACS could allow evaporation of a wider range of solvents and avoid violent boiling (bumping) which often happens with the K-D technique.
• Fine adjustment of the temperature gives better control on compounds that are sensitive to thermal decomposition, e.g. 2,4-dichlorophenol.
• The time of operation is known for the EVACS, depending on the evaporation rate needed for a certain application. This is not possible with the K-D
Table 10.1 Comparative recoveries for two-step K-D and EVACS approaches for a range of organic compounds“ (from 200 to 1 ml) [3]. 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 Initial concentration (mgr1) Two-step K-D (% recovery)6 EVACS (% recovery)6
Acetophenone 0.241 82(±1) 93(±6)c
Isophorone 0.243 85(±3) 88(±4)
2,4-Dichlorophenol 0.238 78(±7) 99(±2)c
Quinoline 0.201 81(±12) 100(±3)c
1-Chlorodecane 0.239 93(±2) 91 (±6)c
2-Methylnaphthalene 0.174 87(±1) 98(±2)c
Biphenyl 0.226 83(±2) 92(±6)c
1-Chlorododecane 0.239 87 (±4) 96(±2)c
Diacetone-L-sorbose 0.214 — 98(±4)
Anthracene 0.159 93(±5) 97(±4)
Dioctylphthalate 0.211 71 (±4) 99(±4)c
“Conditions: solvent, dichloromethane; evaporation rate, 4 mlmin-1 for EVACS, although uncontrollable for K-D; N2 flow rate, 1 mls-1; number of determinations, 4. b Figures in parentheses indicate standard deviations (SDs).
c Significantly better recovery values determined statistically by analysis of variance (ANOVA) calculations, at ?0.975 (theoretical significance).
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Methods for Environmental Trace Analysis
Table 10.2 Comparative recoveries for the two-step K-D and EVACS approaches for a range of lower-boiling-point compounds" (from 200 to 1 ml) [3]. 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 Initial concentration (mgV1) Two-step K-D (% recovery)6 EVACS (% recovery)6
Ethyl butyrate 0.27 70(±2) 70(±3)
Ethylbenzene 0.29 72 (±2) 71 (±3)
Cyclohexanone 0.29 74(±1) 76(±4)
Anisole 0.29 72(±1) 76(±3)c
1,4-Dichlorobenzene 0.27 75 (±2) 80(±2)c
2-Ethylhexanol 0.26 76(±1) 82(±3)c
Tolunitrile 0.27 82(±2) 80(±1)
Naphthalene 0.26 88(±4) 96(±3)c
Benzothiazole 0.27 100(±1) 99(±3)
Ethyl cinnamate 0.26 90(±1) 98(±6)
aConditions: solvent, dichloromethane; evaporation rate, 4 mlmin-1 for EVACS, although uncontrollable for K-D; N2 flow rate, 1 mls-1; number of determinations, 4.
^Figures in parentheses indicate standard deviations (SDs).
^Significantly better recovery determined statistically by analysis of variance (ANOVA) calculations, at to.975 (theoretical significance).
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