Enhanced Sorptive Extraction by TF-SPME

Miniaturized and solventless sorptive extraction techniques like stir bar sorptive extraction (SBSE) and solid phase microextraction (SPME) have become common choices for sample extraction techniques in the past few decades. These techniques are simple to perform, do not require further concentration steps, are environmentally friendly (no organic solvents necessary), only require small sample volumes, and decrease operation costs. Although SPME is a simple and widespread choice for volatile extractions, its analyte capacity is limited due to the relatively small amount of absorption material available on the fiber. SBSE devices (GERSTEL Twister®) provide higher volumes of sorbent for increased capacity, but the predominantly used polydimethylsiloxane (PDMS) phase has a low affinity for mid- and high-polarity compounds (log KO/W < 3). Thin film SPME (TF-SPME), a novel extraction technique from the inventor of SPME, overcomes these limitations by using different absorbent coatings with a greater affinity for polar compounds compared to PDMS alone.

TF-SPME utilizes a carbon mesh sheet (typically 20 x 4.7 mm) impregnated with a sorptive phase and can be used in headspace or immersion mode. Liquids are most often extracted by immersing the TF-SPME device with a stir bar to agitate the liquid, while solids are extracted in headspace mode in an agitator. Due to its unique geometry, the TF-SPME device can be used for direct sampling by placing it in contact with the surface or skin of the sample of interest. The technique is also convenient tool for on-site sampling (directly in environmental air or water) due to the structural robustness of the device and the ease of its introduction into remote locations.

The application of TF-SPME to a wide range of matrices such as industrial materials, water, food, and beverages has revealed its enhanced capability compared to SPME.  Because these techniques are based on the equilibrium-driven diffusion of analytes between the sample matrix and extraction phase, extraction efficiency can be optimized by increasing the volume of the extraction phase. The geometry of TF-SPME devices enhances the sampling rate through its thin extraction phase and large surface area, providing a high surface area-to-volume ratio (Table 1). This helps to reduce the time it takes to reach equilibrium, while still increasing the capacity of the extraction device (Emmons, Tajali, & Gionfriddo, 2019).  The extraction of several polyaromatic hydrocarbons (PAHs) was compared between a 100 µm PDMS SPME fiber (surface area = 9.4 mm2) and a 10 x 10 mm PDMS TF-SPME membrane (surface area = 200 mm2), the TF-SPME device with a larger surface area by a factor of 20. After 3 minutes of immersive extraction from a 1 L sample solution (stirred), the amounts extracted by the membrane were 7-20 times higher than those extracted by the fiber (Bruheim, Liu, & Pawliszyn, 2003).