I am using Labconco Vacuum Concentrator to evaporate my samples to dryness. The solvent is usually hexane, methanol, acetonitrile or water. My analytes in the solvent include PAHs, organophosphate flame retardants (tributyl phosphate, triphenyl phosphate etc.). I am afraid that my analytes may evaporate also in the vacuum concentrator. So I want to know, is there a general rule to guide me on the choosing of solvent and setting of instrument temperature so that the solvents are all evaporated and the analytes are all retained. Thanks in advance!
A first approach I think would be to find the phase diagrams of the pure components of your mixture, and choose a regime (temperature and pressure) where your solvent should be gasseous while your analyte should be liquid. However there might be a significant interaction between the components of your mixture with the result that whatever is evaporating is still a mixture of your solvent and analyte (although probably with a higher solvent:analyte ratio than you have in your liquid, so you would still get concentration at first). To be sure, maybe you can analyse the outflow from your concentrator chamber (using a GC?) to see check it for the presence of your analyte.
About volatility-based separation of trialkyl phosphates ― often used as solvating extractants ― from diluting solvents, you may find useful refer to:
http://pubs.acs.org/doi/abs/10.1021/ja01328a506 http://www.orgsyn.org/demo.aspx?prep=CV2P0109#Ref110N26
I think the best would be if you could find a binary phase diagram of your solvent and your analyte at the pressure at which your concentrator is operating, and then look for the behaviour close to 100% solvent. However I doubt that you will be able to find this data in literature. Therefore the phase diagrams of your individual components may give you a first indication for a good combination of temperature and pressure. I see no reason why this would not be the case for the low concentrations you mention. However, in this way you have no guarantee that no analyte is being evaporated, so you need to find a way to check this.
Dear Wang,
As indicated by Peter and others, you need to look at the boiling points of the various analytes under vacuum along with that of the solvent(s) in the system. A solvent keep may be needed. If you go dry, you'll loose the PAHs for sure. This paper may help. There are more on our website at spscientific.com
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