31 Centrifugal Vacuum Evaporation
Centrifugal evaporators induce solvent boiling under vacuum and so the samples are cold. Centrifugal evaporators use cold traps to recover the vaporised solvent. Centrifugation ensures that solvent boils from the sample surface downwards, thereby eliminating boiling over, foaming and solvent bumping and so preventing sample loss and cross-contamination. Solvent at the liquid surface is at the pressure of the equipment, whereas solvent below this level is at a higher pressure due to the extra weight of solvent multiplied by the g force exerted by the centrifuge rotor. Systems with very high rotor speeds generating 500g or more are proven to prevent solvent bumping. The centrifugal evaporation technique accommodates a wide range of solvents and can concentrate, dry to a fi lm or freeze dry samples. Controls on the system permit hands free operation, with the most advanced systems having automatic detection for the end of the method built in.
Results of Evaluation
The systems were tested for their suitability for use in the sample preparation process, with particular attention to cross-contamination / bumping / foaming, solvent recovery and degree of user intervention. The results are shown below in Figure 2.
Bumping / Foaming
Vortexing System
Samples foam resulting in sample loss
Blow Down No foaming or bumping
Solvent Recovery
External solvent
condenser, like rotary evaporator
None Must
continually adjust gas jet height above liquid
Centrifugal Evaporation
No foaming or bumping
Built in solvent
condenser Figure 2. Results of solvent evaporation system evaluation
None required
Low Medium Fast
Centrifugal Evaporation
Slow User Attention
None required
Risk of Cross Contamination
High
Evaporation Time
Slow Conclusions
Working with the rotary evaporator, a batch consisting of a total of 20 samples (reference solutions, samples and spikes) would require approximately 1.5 hours (evaporation time) for the fi rst evaporation step and another 2.5 – 3 hours for the second evaporation. A total of 4 – 4.5 hours spent where the operator has to keep an eye on the evaporator and change the samples every few minutes.
The centrifugal evaporator would require for the same batch up to 3 hours for both evaporation steps. In that time, the operator is free to do other work.
The centrifugal evaporator can take a total of 48 nitrofuran samples. These 48 samples would require approximately one hour more to evaporate (both steps) than a batch of 20. This would be an estimated 2 – 3 hours saved in comparison to working with a rotary evaporator.
Overall the installation of the EZ-2 Envi evaporator (Genevac Ltd, Ipswich, UK. Shown in fi gure 3) at KLZH is estimated to save the laboratory 2-3 hours per day that previously was spent on evaporation tasks. The instrument enabled the processing of larger sample series within a given day. It signifi cantly reduced the contamination (carry-over) issue. Previously, the frequent false positive fi ndings were the reason for re-analysis of affected sample or whole sample series. Very important is also the fact that the EZ-2 Envi operates unattended in a fully automatic manner.
References
Vass, Hruska & Franek. 2008. Nitrofuran Antibiotics: a review on the application, prohibition and residual analysis. Veterinarni Medicina, 53, 2008 (9), pp 269-500.
About the Authors
A. Kaufmann, K. Maden und S. Walker are working at the offi cial food control authority of the canton of Zurich (Kantonales Labor Zurich). The group focuses on the analysis of veterinary drug residues in animal based food products. Commonly uses analytical techniques are Liquid chromatography coupled to high resolution mass spectrometry and liquid chromatography coupled to tandem quadrupole mass spectrometry.
Figure 3. EZ-2 Envi Evaporator
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