Thin Film Microextraction (TFME)
Thin Film Microextraction (TFME) is a technology developed to address the limiting uptake rate and capacity sometimes observed with fiber microextraction. It is particularly relevant for extraction of more polar compounds from aqueous matrixes. Relative to fiber SPME, both the volume of extraction phase and the surface-to-volume ratio are significantly larger. Two configurations of the technology have been introduced. The Brush configuration is suitable for high throughput processing of aqueous samples for liquid chromatographic applications, while the Membrane configuration is appropriate for gas chromatographic applications.
Because of the slow diffusion coefficients of analytes in aqueous samples, full equilibration between a sample and an extraction phase can require a very long time, even when good sample agitation is used. This occurs because the rate of mass transfer from sample to sorbent is limited by diffusion across the liquid-phase boundary layer, which is not affected significantly by degree of agitation in the sample. However, the uptake rate per unit time is related to the total contact area between the sorbent and the sample. Thus a larger surface to volume ratio allows a higher uptake rate per unit time, which translates to better sensitivity for a given pre-equilibrium extraction time .
The higher sorbent capacity is a benefit as polar analytes tend to suffer lower extraction efficiencies by most sorbents under optimal extraction conditions than less polar analytes. This is related to the lower affinities these compounds have for sorbents relative to the sample matrix. Thus higher volumes of extraction phases are required to extract a sufficient amount for the desired sensitivity.
The two configurations of TFME are designed to work with 96-well sample plates (Brushes) for introduction to LC, and standard thermal desorption injectors (Membranes) for GC.
Thin Film Brushes
The Brush configuration of TFME utilizes sorbents typically found in SPE cartridges. These are coated in a film on the tips of an array of flattened pins using an appropriate binding agent. The result is an open bed (bedless) sorbent that may be put into direct contact with the sample. Similarly to matrix solid phase dispersion, analytes in even very complex samples such as homogenized animal or vegetable tissue will move into the sorbent according to their affinity. Recovery of the sorbent, with extracted analyte, involves simply pulling the pin out of the matrix and giving it a brief rinse with water to dislodge the remaining matrix from the surface. The pins themselves are configured in a flattened 'paddle' shape to enable a higher surface to volume ratio and less sample displacement than a cylindrical pin would have, while enable efficient mixing when placed into a deep-well 96-well plate on an orbital shaker.
Subsequent to extraction and rinsing, the Brush may be placed into a second 96-well plate containing desorption solvent. An aqueous mixture of acetonitrile or water typically provides both high desorption efficiency and a solution compatible with injection for LC-MS. For higher desorption efficiency pure solvent may be used, followed by either blow-down and reconstitution in an LC-compatible solution or dilution with water sufficient for injection.
Sample processing may be accomplished either with a manual workstation which holds the brush in position over a 96-well plate on an orbital shaker, or through the use of robotic workstation that provides automation of all pre-conditioning, extraction, washing and desorption steps. Both are available through PAS Technologies, Magdala, Germany [info(at)pas-tec.com]. The brushes are currently under product development through Supelco.
Thin film brushes references:
Bojko, B.; Vuckovic, D.; Mirnaghi, F.; Cudjoe, E.; Wasowicz, M.; Jerath, A.; Pawliszyn, J. Ther. Drug Monit. 34 (2012) 31. (Application).
Mirnaghi, F.S.; Chen, Y.; Sidisky, L.M.; Pawliszyn, J. Anal. Chem. 83 (2011) 6018. (Sorbent Development).
Cudjoe, E.; Vuckovic, D.; Hein, D.; Pawliszyn, J. Anal. Chem. 81 (2009) 4226. (Technology Development).
Vuckovic, D.; Cudjoe, E.; Hein, D.; Pawliszyn, J. Anal. Chem. 80 (2008) 6870. (Instrument Development).
Thin Film Membranes
The Thin Film Membrane is an additional sampling device that has been developed to achieve higher extraction efficiency and sensitivity, but which is primarily used for GC introduction with a thermal desorption injector. A thin film of PDMS is cut and may be mounted in a wire enclosure for support. After conditioning and baking-out, the device can be used for both active and passive sampling).
For active (spot) sampling, the membrane is put into contact with a gaseous or liquid sample for a defined period of time. Subsequently the membrane may be rolled and placed into a sealed desorption tube for transfer to the laboratory for analysis, or into a standard desorption tube which is subsequently mounted on a sealed autosampler tray. Similar to the Brush device, an intermediate brief wash may be used to dislodge surface adhered matrix prior to thermal desorption.
The membrane may also be used for monitoring volatiles emitted from a surface, by placing the membrane onto the surface in question. If desired an intermediate mesh layer may be used to avoid direct transfer of analytes from the surface to the membrane. An impervious top layer may be incorporated to prevent loss of volatiles from the membrane during extraction. In this configuration the device has been used to monitor the emission of volatiles from skin surfaces, as a non-invasive means of monitoring physiological response to disease or other stressors, or of monitoring exposure to environmental chemicals.
For passive (integrated or time-weighted-average, TWA) analysis, a longer exposure time period and larger membrane are used. Preliminary experiments are conducted to verify the maximum time of linear uptake, although integrated analysis over several hours to days is possible for monitoring pollutants in environmental waters.
Internal standard calibration by pre-loading calibrant on the membrane provides for quantitative results in either spot or TWA modes of sampling. Where GC-MS analysis is employed, deuterated internal standards provide optimal performance.
Current research involves embedding sorbent particles within the PDMS matrix to broaden the range of target analytes amenable to such analysis
Thin film membrane references:
Bruheim, X. Liu, J. Pawliszyn, Anal. Chem. 75 (2003) 1002.
L. Bragg, Z. Qin, M. Alaee, J. Pawliszyn, J. Chromatogr. Sci. 44 (2006) 317. (Passive sampling)
Z. Qin, L. Bragg, G. Ouyang, V.H. Niri, J. Pawliszyn, J. Chromatogr. A. 1216 (2009) 6979. (Active sampling).