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General Description
In analyses of semivolatile and volatile analytes, headspace SPME offers several important advantages over headspace analyzers.
When you use a headspace analyzer to trap semivolatile analytes in water-based samples, you must heat the sample to 110°C or higher. Much water will be transferred with the analytes. Often the analyte-containing volume is large and you must cryogenically refocus the analytes at the column inlet. Water that is transferred with the analytes can freeze in the line.
With headspace SPME, samples usually require warming only to 40-60°C. The analytes will concentrate on the fiber with little water vapor. Then, because the fiber is desorbed rapidly in the injection port, the analytes usually desorb as a focused plug -- no cryogenics required. In addition, with headspace SPME sample volume is small, e.g., 3mL in a 4mL vial.
Even volatile organic compounds (VOCs) can be desorbed from an SPME fiber with sufficient speed to eliminate the need for cryogenics. Use a Carboxen™/polydimethylsiloxane fiber to retain extremely volatile analytes (gases at room temperature). Only ethane, methane, and some fixed gases are too small to be retained in the pores of Carboxen particles. Other SPME fibers are suitable for extracting semivolatiles by headspace SPME.
The decision to use headspace SPME or fiber immersion SPME is based almost entirely on the vapor pressure of the analytes. If there is significant vapor pressure, headspace SPME is advantageous; analyte recoveries are cleaner and faster than by immersion SPME, and the fiber will last longer. If you have a solid sample matrix, headspace SPME is mandatory.
For more information about SPME of volatile analytes, refer to Application Note 11 and Application Note 56. For more information about SPME of semivolatiles, refer to Application Note 6, Application Note 17, and Application Note 81 .
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