Thin Film Solid Phase Microextraction: A Review of Benefits, Techniques, and Applications

Author: Anita Peterson, Ph.D.

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 K_O/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).

Table 1. Extraction phase surface area and volume for various sorptive extraction devices

Available Extraction Phases for TF-SPME

Like traditional SPME, TF-SPME devices are available with several different extraction phases to cover a wide range of polarity. The available phases employ PDMS loaded with either carboxen (CAR), divinylbenzene (DVB) or hydrophilic lipophilic balanced (HLB) particles. The three phases have been compared with respect to various types of beverages, demonstrating the polarity coverage of each. The CAR/PDMS-coated TF-SPME device was found to be best for very volatile organic compounds (VVOCs), characterized by log K_O/W values below 2. The DVB/PDMS phase covered volatile and semi-volatile organic compounds (VOCs and SVOCs), including compounds with relatively low log K_O/W values (<3). The HLB/PDMS phase covered the widest range and is well-suited for VVOCs, VOCs, and SVOCs, though it was most efficient for compounds with log K_O/W values above 3 (AppNote 219).

Thermal Desorption of TF-SPME with the GERSTEL Thermal Desorption Unit

The thermal desorption (TD) of sorptive extraction devices like TF-SPME membranes and Twisters involves a two-stage process via the GERSTEL Thermal Desorption Unit (TDU). The device(s) are first heated in the TDU with a high gas flow to release all volatile analytes adsorbed onto the extraction phase. Analytes are then transferred to the Cooled Injection System (CIS), which acts as a cryotrap to focus volatiles prior to rapid heating and discrimination-free transfer into the analytical column. The short sample path, the liner-in-liner design, and the absence of valves or transfer lines between the TDU and the cryo-trap (Figure 1) ensures minimal analyte loss prior to gas chromatograph (GC) injection, sharp chromatographic peaks, high reproducibility, and little to no carry over.

TF-SPME and Twister Combined Extraction

The incorporation of TF-SPME and SBSE into a single extraction workflow has proven to be a highly effective extraction technique, providing complementary coverage of compounds of different polarities. The combined extraction technique is usually performed on liquids, where stirring of the sample is provided by the Twister while the TF-SPME device is immersed in the sample. The two devices can then be conveniently desorbed together in a single TD tube (Figure 1). Compared to each technique alone, the combination of Twister and TF-SPME was found to yield the highest responses for a large group of volatile compounds covering a wide range of polarity (log K_ow from -0.26 to 4.83). The higher surface area and volume of absorption material allowed for much higher analyte capacity.

Figure 1. Workflow for Twister and TF-SPME combined extraction followed by thermal desorption by the GERSTEL Thermal Desorption Unit (TDU) and GC introduction by the Cooled Injection System (CIS)

uantification capabilities were improved by extending the linearity range and increasing the response slope and thus sensitivity for most of the tested compounds in headspace and direct immersion modes. Enhanced recoveries (> 20%) for highly polar compounds (log K_O/W < 1.0) were also observed for the combined extraction method due to the near doubling of absorption material and thus reduced competition for attachment sites (Huang et al., 2020).

TF-SPME Applications

TF-SPME has been successfully utilized in a wide range of applications including environmental, water, foods, flavors, fragrances, beverages, off-odors, and material emissions. Through improved detection limits for various volatile and semi-volatile compounds and decreased extraction times, TF-SPME can meet the rigorous demands of quality control and research and development in these industries.

Environmental/Water

The TF-SPME technique is especially well-suited for on-site/field sampling of environmental matrices, particularly water systems. Fast on-site sampling of environmental samples is often necessary to avoid the loss and degradation of analytes during collection, transportation, and storage. Analysis is usually performed with a portable GC-MS in addition to a benchtop GC-MS. TF-SPME has been successfully employed as a passive sampler for on-site water testing in different water systems to determine polycyclic aromatic hydrocarbons (Bragg et al., 2006; Qin et al., 2008) and industrially impacted water to determine contaminants like toluene, ethylbenzene, and xylene (Grandy, J. J., Boyaci, E., & Pawliszyn, J., 2016). Grandy et al. (2018) performed on-site extraction of chlorination byproducts in a private hot tub. This study demonstrated significantly improved responses for chlorinated analytes provided by the HLB/PDMS phase compared to DVB/PDMS, which was still found to extract 35-75 times more analyte than the comparative SPME fiber. The HLB/PDMS-coated TF-SPME device was also applied to produced water (PW), the primary waste by-product of hydraulic fracturing. Despite PW samples containing high amounts of dissolved solids, covering a diverse range of matrices, and possessing varied physicochemical properties (hypersaline, oily, corrosive), the HLB device was robust and enabled a broad range of compounds to be extracted without the need for filtering samples. Comprehensive and convenient extraction of PW was achieved with TF-SPME with minimal analyte loss compared to typically utilized sample preparation methods like liquid-liquid extraction (LLE), solvent dilution, conventional SPME and SBSE (Emmons et al., 2020).

The quantitation of pesticides in surface water was performed by TF-SPME (PDMS/DVB carbon mesh-supported membrane) followed by TD-GC-MS. The validation results of this method were compared to the standard LLE method described by the US EPA (Method 8270). This method is used in many analytical laboratories for routine analysis of surface water samples, requiring large sample volumes to obtain the sensitivity required to meet US EPA maximum contaminant levels (MCLs). TF-SPME achieved low limits of detection (0.01 – 0.25 µg/L) for the 23 pesticides tested, meeting the requirements reported by US EPA and for some compounds, 2 orders of magnitude lower than dictated MCLs. Many of these LODs were significantly lower than those achieved by the LLE method, while requiring much less sample (30 mL vs 800 mL for LLE). The accuracy and precision of the TF-SPME method were good and comparable to the LLE method. The TF-SPME method was validated and favored for an eco-friendlier analysis of surface water for pesticides as it requires no solvent and thus lowers the cost of analysis. Additionally, it allows for higher laboratory throughput as it is much less laborious and tedious to perform, all while achieving low LODs (Piri-Moghadam et al., 2017).

Foods, Flavors, Fragrances & Beverages

Food and beverage products are routinely monitored for quality, authenticity, and safety. Aroma and flavor profiles of these products are extremely important for customer acceptance and the identification of off-flavor notes can help pinpoint product defects. TF-SPME as well as the combined extraction with TF-SPME and Twister have been used to analyze a variety of different food and beverage matrices with improved responses for various classes of compounds compared to SPME or SBSE alone. DVB/PDMS-coated TF-SPME membranes in direct immersion mode have enabled the highly efficient extraction of important volatile aroma compounds from many beverages, including wine, coffee, and soda. A comparison between conventional SPME and TF-SPME (DVB/PDMS) for the extraction of white wine (Sauvignon Blanc/Gewurztraminer blend) showed significantly higher responses by TF-SPME for all identified compounds (Figure 2). Flavor compound extraction was particularly enhanced for early eluting, low molecular weight alcohols and ethyl esters (log K_O/W < 2) responsible for the floral and fruity flavors integral to this type of wine (AppNote 200).

Figure 2. Stacked view of total ion chromatograms and identified compounds for a Sauvignon Blanc/Gewurztraminer blend wine using SPME (top) and TF-SPME (bottom) as displayed in AppNote 200.

All three available TF-SPME phases were compared for the analysis of tea, fruit juice, beer, and nut milk. This study highlighted the ability of TF-SPME in combination with Twister to extract flavor compounds covering a wide range of log K_O/W values and polarity. High polarity VOCs (log K_O/W < 2) were extracted to a greater degree with the CAR/PDMS and HLB/PDMS phases, while mid-polarity VOCs (2 < log K_O/W < 4) were best extracted by the DVB/PDMS phase. The HLB/PDMS phase demonstrated the best coverage for late eluting, high log K_O/W compounds like long chain fatty acids. The incorporation of Twister into the workflow increased the extraction phase volume available and thus the analyte capacity for all tested phases (AppNote 219). High fat food samples have also been analyzed for the determination of aroma and flavor compounds using DVB/PDMS-coated devices in headspace mode with agitation and heating. Samples in this study included dark chocolate, blue cheese, Caesar dressing, and cream cheese (AppNote 202). A comparison of individual and combined sorptive extraction techniques (TF-SPME and SBSE) for the analysis of whiskey demonstrated the complementary coverage on important aroma compounds of different polarities. Responses for the combined extraction exceeded those obtained by each technique individually for all identified compounds (Figure 3). Extracted and identified compounds covered a wide range of polarity (log K_O/w from -0.17 to 3.80) (TF-SPME and Twister Flyer).

Off-Odors

One of the goals of analyzing materials and consumer goods is to assess and identify off-odors. Off-odors in materials can be a major problem for manufacturers as they lead to consumer complaints and brand damage. Compounds responsible for these off-odors are often present at trace levels but are still detectable to the nose as they have low odor thresholds, complicating their identification in complex matrices. Kfoury et al. (AppNote 218) overcame difficulties in the identification of off-odor compounds in paper products using an Olfactory Detection Port (ODP) coupled with GC-MS and TF-SPME for sample extraction. GC-MS Olfactometry (GC-MS/O) enables simultaneous detection of off-odors and mass spectral determination while the high capacity of TF-SPME devices provide more mass-on-column for improved compound identification. In this study, the primary off-odors were detectable at the ODP, but

Figure 3. Comparison of individual and combined extraction techniques (TF-SPME and SBSE) for the determination of aroma compounds in whiskey by TD-GC-MS.

not the MS, so TF-SPME devices were placed in direct contact with paper samples to increase analyte recovery. To further increase mass-on-column, four TF-SPME devices were employed to extract the complaint paper sample, desorbed sequentially (by multi-desorption mode), and combined into a single GC-MS run. This technique enabled the identification of trimethylamine as one of the major compounds contributing to the unpleasant fishy odor detected in the paper products.

Off-odor compounds can have a strong impact on consumer acceptance of foods and beverages. Combined extractions using TF-SPME and Twister devices have been integral in identifying these compounds in various products, including wine and beer. The increased volume of extraction phase provided by the two devices and the extended polarity coverage provided by TF-SPME enable detection and identification of such compounds, despite the extremely low concentrations at which they are typically present. TF-SPME and Twister were combined for the extraction of commercial beers at the end of their shelf life, resulting in the detection of several off-flavor compounds, including E-2-nonenal (stale), 3-methyl-2-butene-1-thiol (skunky odor of light-damaged beer) and 4-vinylguaiacol (undesirable clove odor) (Marsili and Laskonis, 2019).

Smoke-tainted wines are characterized by smoky, burnt, and medicinal odors. These undesirable odors are imparted by volatile phenols, which are introduced into grapes in the vineyard by smoke from nearby wildfires. TF-SPME has proven to be a great complement to SBSE for the analysis of smoke-tainted wines for free volatile phenols, including guaiacol, 4-methylguaiacol (creosol), 4-ethyl guaiacol, meta- and para-cresol, syringol and 4-methylsyringol. As these compounds are polar and can be present at low levels, they can be difficult to extract from wine matrices. While HS-SPME (the conventional extraction method for smoke taint compounds) lacks the extraction capacity to provide the required limits of detection for a wide range of volatile phenols, the combined extraction using TF-SPME and Twister has been found to efficiently extract these compounds so that the required detection limits were easily surpassed (Marsili, 2019).

Conclusion

The development of carbon mesh-supported TF-SPME membranes has expanded the field of solventless microextraction techniques and provided a great improvement to conventional SPME.  TF-SPME has been employed for the extraction of volatile and semi-volatile compounds from a wide range of matrices and results obtained are comparable to conventional extraction methods (such as LLE). Results of these comparisons show that the larger surface area and sorbent volume of the device as well as the wide polarity range of the sorptive phase enable fast extraction and improved analyte recovery compared to SPME and in some cases SBSE.  The robust and planar geometry of the TF-SPME device makes it amenable to environmental on-site sampling, as well as direct sampling of sample surfaces or skins. There is an abundance of ways in which TF-SPME devices can be utilized, especially when combined with the GERSTEL Twister and the use of multiple membranes to increase mass on column and analyte capacity.

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