What Is Solid-Phase Microextraction ?

Solid-Phase Microextraction or SPME is a sampling technique designed to absorb  molecules prior to analysis by techniques such as gas chromatography or high performance liquid chromatography (Zhang and Pawliszyn, 1993). It is ideal for trace compound analysis.

The technique was developed by Janusz Pawliszyn at the Department of Chemistry and Waterloo Centre for Groundwater Research, University of Waterloo, in Ontario, Canada. 

The essential requirement of the technology is to use a fiber coated with an extracting phase which might be a liquid or polymer or a solid, a sorbent. These materials extract different types of analytes which can be both volatile and non-volatile molecules. The analytes might be present in a gaseous or liquid phase.

The quantity of analyte extracted by the fibre is proportional to its concentration in the sample. The level is dependent on whether equilibrium is reached. In some methods, a short time pre-equilibrium step is used aided by agitation and convection.

The amount of flavor compounds sorbed on the SPME coating can be determined from the equation:-

n = C0V1V2K1K2/(K1K2V1 + K2V3 + V2),

where n is the mass of the flavor compound sorbed by the SPME coating; C0 is the initial concentration of the flavor compound in the liquid matrix; and V1, V2, and V3 are the volumes of the SPME coating, liquid matrix, and headspace, respectively; K1 is the partition coefficient of the flavor compound between the SPME coating and the headspace; and K2 is the partition coefficient between the headspace and the liquid matrix. Because the partition coefficient (K) is equal to K1K2, it is controlled by both the partition coefficient K1, between the SPME coating and the headspace gas phase, and the partition coefficient K2, between the headspace gas phase and liquid matrix  (Zhang and Pawliszyn, 1993).

Following extraction, the SPME fiber is transferred to an injection port of a separating instrument. The device might be a gas chromatograph coupled to a mass spectrometer. Desorption of the analyte occurs and the analysis is performed.

Benefits Of This Extraction Technology

SPME is rapid, simple and does not need any solvents. Conventional techniques using steam distillation or direct solvent extraction produces extracts with the flavour molecules from the liquid extract but not the headspace.

Sampling takes less than an hour ! The detection limits are parts per trillion (ppt) for various compounds. SPME is used for field applications where on-site sampling can be achieved without the need for GC-MS equipment at the site. The samples with proper storage are analyzed days later without loss of volatiles. 

Examples

Flavor analysis has benefited greatly from application of this extraction method. Wine flavor compounds have been identified using this technique (Wada et al., 1997; Vas et al., 1998; Rocha et al., 2001), diacetyl in wine (Hayasaka and Bartowsky, 1999)

References

Hayasaka, Y.; Bartowsky, E. J. (1999) Analysis of diacetyl in
wine using solid-phase microextraction combined with
gas chromatography-mass spectrometry. J. Agric. Food
Chem.  47, pp. 612-617

Rocha, S., Ramalheira, V., Barros, A., Delgadillo, I., & Coimbra, M. A. (2001). Headspace solid phase microextraction (SPME) analysis of flavor compounds in wines. Effect of the matrix volatile composition in the relative response factors in a wine model. Journal of Agricultural and Food Chemistry49(11), pp. 5142-5151

Vas, G.; Ga´l, L.; Harangi, J.; Dobo´, A.; Ve´key, K. (1998)
Determination of volatile aroma compounds of Bla¨ufrankisch
wines extracted by solid-phase microextraction.
J. Chromatogr. Sci. 36, pp. 505-510

Wada, K.; Shibamoto, T. (1997) Isolation and identification of
volatile compounds from a wine using solid phase
extraction, gas chromatography, and gas chromatography/mass
spectrometry. J. Agric. Food Chem. 45,
pp. 4362-4366.

Zhang, Z., Pawliszyn, J. (1993) Headspace Solid-Phase Microextraction. Anal. Chem. 65(14) pp. 1843-1852

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