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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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10.2 Rotary Evaporation
In this method, the solvent is removed under reduced pressure by mechanically rotating the flask containing the sample in a controlled-temperature water bath (Figure 10.1). The (waste) solvent is condensed and collected for disposal.
DQ 10.2
What type of problems might you encounter with this approach?
Answer
Problems can occur due to loss of volatile analytes, adsorption onto glassware, entrainment of analyte in the solvent vapour and the uncontrollable overall evaporation process.
Figure 10.1 A typical rotary evaporation system used for the pre-concentration of compounds in organic solvents.
Pre-Concentration Using Solvent Evaporation 175
10.3 Kuderna-Danish Evaporative Concentration
The Kuderna-Danish evaporative condenser was developed in the laboratories of Julius Hyman and Company, Denver, Colorada, USA [1, 2]. This consists of an evaporation flask (500 ml) connected at one end to a Snyder column and the other end to a concentrator tube (10 ml) (Figure 10.2). The sample-containing organic solvent (200-300 ml) is placed in the apparatus, together with one or two boiling chips, and heated with a water bath. The temperature of the water bath should be maintained at 15-20°C above the boiling point of the organic solvent. The positioning of the apparatus should allow partial immersion of the concentrator tube in the water bath but also allow the entire lower part of the evaporation flask to be bathed with hot vapour (steam). Solvent vapours then rise and condense within the Snyder column. Each section of this column consists of a narrow opening covered by a loose-fitting glass insert. Sufficient pressure needs to be generated by the solvent vapours to force their way through the column. Initially, a large amount of condensation of these vapours returns to the
Figure 10.2 Schematic diagram of the Kudema-Danish evaporative concentration condenser system. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission.
176
Methods for Environmental Trace Analysis
bottom of the Kuderna-Danish apparatus. In addition to continually washing the organics from the sides of the evaporation flask, the returning condensate also contacts the rising vapours and assists in the process of recondensing volatile organics. This process of solvent distillation concentrates the sample to approximately 1-3 ml in 10-20 min. Escaping solvent vapours are recovered by using a condenser and collection device. The major disadvantage of this method is that violent solvent eruptions can occur in the apparatus, thus leading to sample losses. ‘Micro-Snyder’ column systems can be used to reduce the solvent volume still further.
10.4 Automated Evaporative Concentration System
In this system, solvent from a pressure-equalized reservoir (500 ml capacity) is introduced, under controlled flow, into a concentration chamber (Figure 10.3) [3]. Glass indentations regulate the boiling of solvent so that bumping does not occur. This reservoir is surrounded by a heater. The solvent reservoir inlet is situated under the level of the heater, just above the final concentration chamber. This chamber is calibrated to 1.0 and 0.5 ml volumes. A distillation column is connected to the concentration chamber. Located near the top of the column are four rows of glass indentations which serve to increase the surface area. Attached to the top of the column is a solvent-recovery condenser with an outlet to collect, and hence recover the solvent.
To start a sample run, the apparatus is operated with 50 ml of high-purity solvent under steady uniform conditions at total reflux for 30 min to bring the system to equilibrium. Then, the sample is introduced into the large reservoir, either as a single volume or over several time-intervals. (NOTE - A boiling point difference of approximately 50°C is required between solvent and analyte for the highest recoveries.) The temperature is maintained to allow controlled evaporation. For semi-volatile analytes, this is typically at 5°C higher than the boiling point of the solvent. The distillate is withdrawn while keeping the reflux ratio as high as possible. During operation, a sensor monitors the level of liquid, thus allowing the heating to be switched off or on automatically (when liquid is present, the heat is on, and vice versa). After evaporation of the sample below the sensor level, the heating is switched off. After 10 min, the nitrogen flow is started for the final concentration stage from 10 ml to 1 ml (or less). Mild heat can be applied according to the sensitivity of the solvent and analyte to thermal decomposition. When the liquid level drops below the tube, stripping almost stops. The tube is sealed at the bottom, so that the nitrogen is dispersed above the sample and the reduction of volume becomes extremely slow. This prevents the sample from going to dryness, even if left for hours. The sample is drained and the column is rinsed with two 0.5 ml aliquots of solvent. Further concentration can take place, if required.
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