Headspace Solid Phase Microextraction. Dynamics and Quantitative Analysis before Reaching a Partition Equilibrium

Abstract

Solid phase microextraction (SPME) has remarkable advantages in headspace chemical analysis over conventional static headspace and purge-and-trap techniques in terms of its ease of use and superior reproducibility. A theoretical treatment of the headspace SPME process is proposed in this report that focuses on the mass transfer at the two interfaces, condensed phase/gas phase and gas phase/SPME polymer. The rate-determining step of the SPME process can be either the analyte evaporation from the condensed phase to its headspace or the analyte diffusion from the SPME polymer film surface into its inner layers. Mass transfer in the gas phase is considered a fast process. The mathematical solution provides an expression that correlates the amount of extracted analyte with its initial concentration in the condensed phase. A directly proportional relationship exists between them in all cases of headspace SPME, and this relationship indicates that SPME quantification is feasible before reaching a partition equilibrium once the SPME conditions and the sampling time are held constant. Experimental extraction−time profiles fit the theoretical predictions. Data plots of the extracted amount vs the initial concentration showed excellent linearity with a sampling time much shorter than that required to reach a partition equilibrium.