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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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• To be able to carry out Soxhlet extraction in a safe and controlled manner.
• To be able to carry out shake-flask extraction in a safe and controlled manner.
• To be able to carry out ultrasonic extraction in a safe and controlled manner.
• To understand the important properties of a supercritical fluid.
• To appreciate the instrumental requirements for supercritical fluid extraction (SFE).
• To be able to carry out SFE in a safe and controlled manner.
• To appreciate the instrumental requirements for microwave-assisted extraction (MAE).
• To be able to carry out MAE in a safe and controlled manner.
• To understand the theory relating to pressurized fluid extraction (PFE).
• To appreciate the instrumental requirements for PFE.
• To be able to carry out PFE in a safe and controlled manner.
• To appreciate the requirements for matrix solid-phase dispersion (MSPD).
• To be able to carry out MSPD in a safe and controlled manner.
7.1 Introduction
Extraction of organic pollutants from solid or semi-solid matrices has been widely investigated in recent years. New instrumental approaches have led to developments in terms of speed of extraction, reduction in organic solvent consumption and the introduction of automation. However, these instrumental approaches
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Methods for Environmental Trace Analysis
Figure 7.1 Various methods of extraction of analytes from solid matrices.
come with a higher price tag than the conventional approaches. The conventional approaches are normally based on larger volumes of organic solvent, glassware and simpler forms of heating. They can also be described as very labour-intensive and relatively slow. Nevertheless, they have been available for many years and are often readily available in the average laboratory.
Essentially, extraction of environmental pollutants from solid or semi-solid matrices can be divided into several categories based on the method of extraction, mode of heating and presence or not of some type of agitation. This is demonstrated in Figure 7.1.
SAQ 7.1
From an examination of Figure 7.1, which of the techniques shown would you identify as conventional techniques and which as instrumental approaches?
7.2 Soxhlet Extraction
Soxhlet extraction is used as the benchmark against which any new extraction technique is compared. The basic Soxhlet extraction apparatus consists of a solvent reservoir, an extraction body, a heat source (e.g. an isomantle) and a water-cooled reflux condenser (Figure 7.2). A Soxhlet uses a range of organic solvents to remove organic compounds, primarily from solid matrices. The solid sample (ca. 10 g if a soil) and a similar mass of anhydrous sodium sulfate are placed in the porous thimble (cellulose), which in turn is located in the inner tube of the Soxhlet apparatus. The apparatus is then fitted to a round-bottomed flask
Solids
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Figure 7.2 The basic Soxhlet extraction system. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission.
of appropriate volume containing the organic solvent of choice, and to a reflux condenser. The solvent is then boiled gently using an isomantle - the solvent vapour passes up through the tube marked (A), is condensed by the reflux condenser, and the condensed solvent falls into the thimble and slowly fills the body of the Soxhlet apparatus. When the solvent reaches the top of the tube (B), it syphons over into the round-bottomed flask the organic solvent containing the analyte extracted from the sample in the thimble. The solvent is then said to have completed one cycle.
DQ 7.1
Based on a solvent volume of 150 ml, how long would you estimate one cycle takes?
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Answer
One cycle takes approximately 15 min, i.e. about four cycles per hour.
The whole process is repeated frequently until the pre-set extraction time is reached. As the extracted analyte will normally have a higher boiling point than the solvent, it is preferentially retained in the flask and fresh solvent recirculates. This ensures that only fresh solvent is used to extract the analyte from the sample in the thimble. A disadvantage of this approach is that the organic solvent is below its boiling point when it passes through the sample contained in the thimble. In practice, this is not necessarily a problem as Soxhlet extraction is normally carried out over long time-periods, i.e. 6, 12, 18 or 24 h.
SAQ 7.2
In terms of percentage efficiency at removing an analyte from a matrix, what would you envisage the ‘extraction efficiency-time profile’ to look like?
A typical procedure for Soxhlet extraction is shown in Figure 7.3.
Figure 7.3 Typical procedure used for the Soxhlet extraction of solids.
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Automated Soxhlet extraction, or Soxtec, utilizes a three-stage process to obtain rapid extractions. In Stage 1, a thimble containing the sample is immersed in the boiling solvent for approximately 60 min. After this, the thimble is elevated above the boiling solvent and the sample extracted as in the standard Soxhlet extraction approach (Stage 2). This is carried out for up to 60 min. The final stage (Stage 3) involves the evaporation of the solvent directly in the Soxtec apparatus (10-15 min). This approach has several advantages, including speed (it is faster than normal Soxhlet extraction, i.e. approximately 2 h per sample), the fact that it uses only 20% of the solvent volume of Soxhlet extraction, and the sample can be concentrated directly in the Soxtec apparatus.
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