Rotating Disk Sorptive Extraction

by Prof. Dr. Pablo Richter, Department of Inorganic and Analytical Chemistry, Faculty of Chemical and Pharmaceutical Sciences, University of Chile.

The modern trends in analytical chemistry promote efficiency and green technology in sample preparation. In this context, our research team developed a new technique in 2009 that is capable of extracting pollutants from liquid samples on a rotating PTFE disk with one surface coated with an extraction phase [1]. The disk has embedded a miniature magnetic rod, which allows rotation. We have termed this procedure rotating-disk sorptive extraction (RDSE).

RDSE is currently available in two configurations. In the classic version (Figure 1), the extraction phase is a polymeric film adhered to one side of the PTFE disk (configuration 1). In this configuration, polydimethylsiloxane (PDMS), nylon and octadecyl (C18) have been used as the sorptive material for the extraction of low-polarity analytes (Log Kow between 3 and 7) [1-12]. However, for more polar analytes, quantitative recoveries are not achievable with short extraction times [2].

 

Figure 1. (A) Classic extraction device used in RDSE in which a polymeric material is adhered to one side of the PTFE disk. (B) Sample processing by RDSE. (C) Detail of the vial containing the sample and the rotating disk.

The second RDSE version (configuration 2) [13,14] consists of a disk that contains a cavity that is loaded with an extraction phase and is then covered with a glass-fiber filter and sealed with a ring of Teflon (Figure 2). This configuration was successfully employed for the extraction of florfenicol from porcine plasma [14] and non steroidal anti-inflammatory drugs (NAIDs) in water using Oasis® HLB as the sorptive material [13]. The use of the disk with cavity allows the incorporation of commercial or synthesized SPE extraction phases and maximizes the sorptive capacity by recirculating the sample through the phase using the rotating disk technology. Molecularly imprinted polymers (commercial or synthesized in the laboratory) can also be employed as an alternative to HLB, thereby enabling improved selectivity, and extraction efficiency.

 

Figure 2. Rotating disk with a cavity in which an extraction phase is loaded and is then covered with a glass-fiber filter and sealed with a ring of PTFE.

From a theoretical point of view, thermodynamic and kinetic parameters are critical factors in the efficiency of a given microextraction technique. Both kinds of parameters must be considered because the main objective of these techniques is to achieve efficient analyte extraction in a reasonable time [15, 16].  Previous studies of the partitioning of organic compounds between PDMS and water suggested that the overall mass transfer of low-polarity organic compounds is not limited by internal diffusion in the PDMS but rather by diffusion in the aqueous boundary layer [7, 17-19]. Thus, the rate-determining step in the equilibrium extraction time is the diffusion of the analytes through the water boundary layer. Efficient stirring of the sample contacting the sorptive phase is necessary to achieve the partition equilibrium as rapidly as possible, because the thickness of the boundary layer is reduced when the rotating velocity is increased [15,20]. Further, initial extraction rate is roughly proportional to the surface area of the extraction phase [15], consequently the equilibration time can be greatly shortened when the extraction device provide a larger surface area/volume ratio.

The extraction device used in RDSE exhibits an extraction phase with a high surface-area-to-volume ratio and can be stirred at much higher velocities than the stir bar used in SBSE without damaging the phase, because the extraction phase is only in contact with the liquid sample. Thus, higher rotating velocities facilitate analyte mass transfer to the sorptive surface [1,10-13].

These two configurations of the extraction device provide to RDSE a highly versatility, since any sorptive material used in both SPE and SBSE could be immobilized on the rotating disk. In addition, RDSE provides some advantages over SPE, especially by allowing the recirculation of the sample through the extraction phase and, thus, maximizing its sorptive capacity (in SPE, the sorption occurs while the sample passes unidirectionally through the solid support). Furthermore, in RDSE, the interface is continuously renewed during the extraction process, which minimizes the involved cleanup steps for complex samples that are required with SPE.

Other important characteristics of RDSE are related with the geometry of the extraction device, which allows an easier automation of the extraction process [8], a direct spectroscopic measurement in the extraction phase [4-6], and the feasibility of the use in bioavailability studies [12].

Recently a novel automatic sorptive microextraction approach combining sequential injection-based programmable flow with rotating disk sorptive extraction (RDSE) was proposed for the clean-up and concentration of low polarity organic species in urine samples [8]. Compared to its batch counterpart [13], the main advantage of the proposed dynamic method is its improved sample throughput with sample preparation times decreasing from 90 min to merely 15 min when the SI-RDSE is used.

Most of the microextraction techniques have primarily been used with gas or liquid chromatography. However in RDSE the analyte can also be directly evaluated using solid phase spectroscopy in the solid phase because of its geometry. In this context, RDSE methods have been described associated to spectrophotometry [4-6] and to excitation-emission fluorescence spectroscopy [9].

Table 1 shows the applications of RDSE by using both disk configurations. Complex samples such as wastewater, plasma, urine can also be analyzed because large molecules such as proteins and lipids cannot pass rapidly across the water boundary layer.

Table 1. Applications of RDSE in different samples associated to various analytical techniques.

Analyte	Disk configuration/ sorbent	Sample	Technique	Reference
Alkylphenols	1/PDMS	River water	GC-MS	1
Pesticides	1/PDMS	River water	GC-MS	2
PAHs	1/PDMS	Waters	GC-MS	3
Cristal violet	1/PDMS	Fish- farming water	Spectrophotometry	4
Malachite green	1/PDMS	Fish- farming water	Spectrophotometry	5
Copper	1/PDMS	Drinking water	Spectrophotometry	6
Hexachlorobenzene	1/C18	Waters	GC-ECD	7
Diclofenac and ibuprofen	1/C18	urine	SI-HPLC-DAD	8
PAHs	1/Nylon	Waters	Fluorescence	9
Triclosan and methyl-triclosan	1/PDMS	Wastewater Soil leachates	GC-MS	10-12
NSAIDs	2/HLB	Wastewater	GC-MS	13
Florfenicol	2/HLB	Plasma	HPLC-DAD	14

Acknowledgements

Fondecyt, Chile (Projects 1140716 and 1100085) is gratefully acknowledged for financial support.

References

[1]     P. Richter, C. Choque, A. Giordano, B. Sepúlveda, J. Chromatogr. A 1216 (2009) 8598–8602.

[2]     A. Giordano, P. Richter, I. Ahumada, Talanta 85 (2011) 2425–2429.

[3]     Y. Corrotea, K. Sánchez, M. A. Rubio, P. Richter, J. Chil. Chem. Soc., 49 (2014) 2477-2480.

[4]     V. Manzo, O. Navarro, L. Honda, K. Sánchez, M. I. Toral, P. Richter, Talanta 106 (2013) 305–308.

[5]     P. Richter, A. Cañas, C. Muñoz, C. Leiva, I. Ahumada, Anal. Chim. Acta, 695 (2011) 73–76.

[6]     C. Muñoz, M. I. Toral, I. Ahumada, P. Richter, Anal. Sci., 30 (2014) 613-617.

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[8]     V. Manzo; M. Miró; P. Richter, J. Chromatogr. A 1368 (2014) 64–69.

[9]     A. Cañas, P. Richter, G. M. Escandar, Anal. Chim. Acta 852 (2014) 105–111

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About the author

Pablo Richter is Professor of Analytical Chemistry at the Faculty of Chemical and Pharmaceutical Sciences of the University of Chile. He was born in Santiago (Chile) and received from the University of Chile both his graduate degree and doctoral degree in chemistry in 1985 and 1991, respectively. Afterwards, he worked as postdoctoral fellow (1991-1992) at the University of Córdoba, Spain with Prof. M. Valcárcel and Prof. M.D. Luque de Castro and (1996) at the Oklahoma State University with Prof. H. Mottola. He has published more than 110 research papers.