2. Dos formas en que un enunciado puede ser informativo
2.3 Comparación informativa de dos frases respecto a un mismo contexto
SBSE was invented in 1999 by Baltussen et al. [22,23]. SBSE is another sorptive technique, where a magnetic sir bar encapsulated in a glass sleeve and coated with PDMS (Figure 3.5.) is used for extraction. SBSE follows the same principle as SPME except that the former method uses larger amounts of stationary phase (PDMS) ranging from 55 to 219 μl (50 to 250 times greater than SPME), which provides higher sample capacity and improved sensitivity [40].
PDMS Coating Metal core Glass jacket
PDMS Coating Metal core Glass jacket
Figure 3.5. Diagram representing a PDMS coated stir bar. (Adapted from [24]).
SBSE can be performed either in the headspace mode (Figure 3.6.) or by directly immersing the PDMS coated stir bar into the aqueous matrix. In the former mode the extraction is performed by suspending the PDMS coated stir bar in the gas phase of the sample and a magnetic stirrer is used to agitate the sample in order to enhance the migration of volatile and semi-volatile compounds into the headspace where they can partition into the PDMS phase. In the direct SBSE mode of extraction, the stir bar is directly immersed into the aqueous sample and used as a stirrer while trapping the organic compounds. In spite of increasing equilibrium times compared to direct immersion SBSE, headspace extraction offers benefits in reducing the risk of
contamination and increasing the lifetime of the PDMS phase (especially for very dirty or complex samples) [41-45]. After sampling, the stir bar is removed, rinsed using small amounts of deionized water, and dried gently with lint free tissue to avoid droplets of moisture and remove chemicals like proteins or sugars that can degrade the stationary phase, especially when used in the direct mode. Analytes are normally desorbed thermally. The stir bar is placed in a glass tube and positioned in the thermal desorption unit. Analytes released thermally from the polymer are typically trapped in a GC inlet at a very low temperature, commonly ranging between -100 to -50 °C.
Once desorption and re-trapping process is completed, analysis and data recording will begin using a GC system.
Magnetic stirrer
(open at the bottom)
Magnetic stirrer
(open at the bottom)
Figure 3.6. Schematic representation of headspace SBSE. (Adapted from [46]).
Recovery of a compound from a water sample by SBSE can be calculated from the sample volume, the volume of PDMS phase and the analyte’s octanol-water distribution coefficient (Ko/w). In principle, SBSE follows similar thermodynamics as SPME. It is assumed that the partitioning coefficient between PDMS and water (KPDMS/W) is roughly similar to the octanol-water partition coefficient (KO/W), hence:
W
Sample preparation
48 where CSBSE and CW correspond to the analyte concentration in the PDMS coated stir bar and water phase, respectively. mSBSE and mW are the mass of analyte in the PDMS coated stir bar and water phase, respectively. VSBSE and VW correspond to the volume of the stir bar coating and the water phase, respectively, and β is the phase ratio, which is equal to VW/VSBSE. Using simple mathematical rearrangement, equation 3.2.
can be re-written as:
SBSE
where mo is the total mass of the analyte originally present in the water sample. In sorptive extraction techniques, the extraction efficiency or recovery is expressed as the ratio between the extracted amount of analyte in the stationary phase (mPDMS) and the initial amount of analyte originally present in the water (mo = mw + mPDMS).
Therefore, equation 3.3 can be rearranged for calculating recovery as:
From the above equation, the main parameter that determines the recovery of an analyte from the sample is the ratio of the partitioning constant and the phase ratio.
This implies that when the KO/W/β = 1, the recovery is 0.5 (50%). As the KO/W/β decreases, the recovery becomes closer to KO/W/β and at values of KO/W/β more than 5, extraction is essentially quantitative (Figure 3.7.) [22].
As outlined above, the recovery of an analyte from a water sample during SBSE extraction depends on the sample volume, the volume of the stationary phase (PDMS) and the analyte’s octanol–water distribution coefficient (Ko/w). In SPME the maximum volume of PDMS in the fibre is 0.5 μl for a 100 μm thick film. This shows that a sample volume of 10 ml will have a phase ratio of 2 × 104 which indicates that quantitative extraction is only achieved for analytes with KO/W larger than 105. A very limited number of compounds reveal such high value of KO/W. On the other hand, a stir bar coated with a 100 μl PDMS used to extract from 10 ml of water sample provides a phase ratio (β) of 100. This indicates that analytes with a KO/W of > 500 are
extracted quantitatively (Figure 3.7.). This provides simple quantification and at the same time an increase in sensitivity for compounds with KO/W below 105 [8,22,23,40,47,48]. It has also been reported that the film thickness has a more pronounced effect on the recovery compared to the length of the stir bar [49].
0 50 100
1 10 100 1000 10000 100000
% Recovery SBSE SPME
KO/W 0
50 100
1 10 100 1000 10000 100000
% Recovery SBSE SPME
KO/W
Figure 3.7. Theoretical recovery of a compound using a volume of 100 and 0.5 μl PDMS in SBSE and SPME extraction methods, respectively, from a 10 ml of water sample as a function of their octanol–water partition coefficient (Ko/w). (Adapted from [22]).
In spite of the advantages of SBSE outlined compared to SPME, the method suffers from lack of variety of stationary phases. Currently PDMS is the only commercially available phase. It is therefore difficult to extract polar compounds from the sample, which leads to low recovery. However, with some adjustments to the sample including its pH, it is possible to improve the recovery of polar analytes like acids [50,51]. Due to the availability of different stationary phases (including polar ones) as outlined above, SPME is a better choice for polar compounds. Dual-phase stir bars, the integration of PDMS with other polymers, have showed better performance but are still not available on the market [52]. Likewise, the need for a thermal desorption unit for SBSE makes this approach more costly. Liquid desorption (LD) can be an option, but only with very highly concentrated samples. Since its development, SBSE techniques have been applied extensively for the analysis of volatile compounds responsible for the aroma and flavor of wine [45,50,53-58].
Despite advancements in science and technology as well as increase the global
Sample preparation
50 wine aroma and flavor. In an exceedingly competitive market, wine producers are investing in technology to increasing production as well as improve the quality of their product. As a result, the South African wine industry has in 2006 launched a project aimed at studying the characteristic nature of South African wines based on their chemical constituents, including their volatile content. Chapters 5 – 9 will include reports of the work done as part of this project.
3.4. References
[1] R. Castro, R. Natera, P. Benitez, C.G. Barroso, Anal. Chim. Acta 513 (2004) 141.
[2] S. Selli, A. Canbas, V. Varlet, H. Kelebek, C. Prost, T. Serot, J. Agric. Food Chem. 56 (2008) 227.
[3] Y. Fang, M. Qian, Flavour Fragr. J. 20 (2005) 22.
[4] A. Calleja, E. Falqué, Food Chem. 90 (2005) 357.
[5] D. Komes, D. Ulrich, T. Lovric, Eur. Food Res. Technol. 222 (2006) 1.
[6] S. Pedersen-Bjergaard, K.E. Rasmussen, T.G. Halvorsen, J. Chromatogr. A 902 (2000) 91.
[7] D. Hernanz, V. Gallo, Á.F. Recamales, A.J. Meléndez-Martínez, F.J. Heredia, Talanta 76 (2008) 929.
[8] A. Tredoux, Stir bar sorptive extraction for the analysis of beverages and foodstuffs. PhD Thesis, University of Stellenbosch, South Africa 2008.
[9] T. Hyötyläinen, M. Riekkola, Anal. Chim. Acta 614 (2008) 27.
[10] C.F. Poole, S.K. Poole, Chromatography Today, Elsevier, Amsterdam, The Netherlands, 1991.
[11] D.W. Brousmiche, J.E. O'Gara, D.P. Walsh, P.J. Lee, P.C. Iraneta, B.C.
Trammell, Y. Xu, C.R. Mallet, J. Chromatogr. A 1191 (2008) 108.
[12] Waters,
http://www.waters.com/waters/nav.htm?locale=en_US&cid=10048919.
[13] L.D. Preston, D.E. Block, H. Heymann, G. Soleas, A.C. Noble, S.E. Ebeler, Am. J. Enol. Vitic. 59 (2008) 137.
[14] L. Mateo-Vivaracho, J. Cacho, V. Ferreira, J. Chromatogr. A 1185 (2008) 9.
[15] E. Campo, J. Cacho, V. Ferreira, J. Agric. Food Chem. 56 (2008) 2477.
[16] V. Ferreira, N. Ortín, J.F. Cacho, J. Chromatogr. A 1143 (2007) 190.
[17] R. Castro, R. Natera, E. Durán, C. García-Barroso, Eur. Food Res. Technol.
228 (2008) 1.
[18] E. Campo, J. Cacho, V. Ferreira, J. Chromatogr. A 1140 (2007) 180.
[19] C. Bicchi, A. D'Amato, F. David, P. Sandra, J. High Resolut. Chromatogr. 12 (1989) 316.
[20] C.L. Arthur, J. Pawliszyn, Anal. Chem. 62 (1990) 2145.
Sample preparation
52 [21] J. Pawliszyn, Solid Phase Microextraction—Theory and Practice, Wiley VHC,
Inc., NY, USA, 1997.
[22] E. Baltussen, P. Sandra, F. David, C.A. Cramers, J. Microcol. Sep. 11 (1999) 737.
[23] H.A. Baltussen, New Concepts in Sorption Based Sample Preparation for Chromatography. PhD Thesis, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2000.
[24] B.T. Weldegergis, Analysis of Organochloro-Pesticides in Eritrean Water and Sediment Samples. MSc Thesis, University of Stellenbosch, South Africa, 2004.
[25] H. Lord, J. Pawliszyn, J. Chromatogr. A 885 (2000) 153.
[26] H. Kataoka, Anal. Bioanal. Chem. 373 (2002) 31.
[27] M. Liu, Z. Zeng, Y. Tian, Anal. Chim. Acta 540 (2005) 341.
[28] G. Vas, K. Vékey, J. Mass Spectrom. 39 (2004) 233.
[29] H. Kataoka, H.L. Lord, J. Pawliszyn, J. Chromatogr. A 880 (2000) 35.
[30] T. Górecki, X. Yu, J. Pawliszyn, Analyst 124 (1999) 643.
[31] F. Rodrigues, M. Caldeira, J.S. Câmara, Anal. Chim. Acta 609 (2008) 82.
[32] R. Godelmann, S. Limmert, T. Kuballa, Eur. Food Res. Technol. 227 (2008) 449.
[33] T.L. Galvan, S. Kells, W.D. Hutchison, J. Agric. Food Chem. 56 (2008) 1065.
[34] L. Setkova, S. Risticevic, J. Pawliszyn, J. Chromatogr. A 1147 (2007) 213.
[35] B. Fedrizzi, F. Magno, D. Badocco, G. Nicolini, G. Versini, J. Agric. Food Chem. 55 (2007) 10880.
[36] J. Bosch-Fusté, M. Riu-Aumatell, J.M. Guadayol, J. Caixach, E. López-Tamames, S. Buxaderas, Food Chem. 105 (2007) 428.
[37] A.M. Jordão, J.M. Ricardo-da-Silva, O. Laureano, A. Adams, J.
Demyttenaere, R. Verhé, N. De Kimpe, J. Wood Sci. 52 (2006) 514.
[38] O. Gürbüz, J.M. Rouseff, R.L. Rouseff, J. Agric. Food Chem. 54 (2006) 3990.
[39] R.M. Peña, J. Barciela, C. Herrero, S. García-Martín, J. Sci. Food Agric. 85 (2005) 1227.
[40] R.E. Majors, F. David, B. Tienpont, P. Sandra, LCGC North America 21 (2003) 1.
[41] J.J. Rodríguez-Bencomo, J.E. Conde, F. García-Montelongo, J.P. Pérez-Trujillo, J. Chromatogr. A 991 (2003) 13.
[42] B. Tienpont, F. David, C. Bicchi, P. Sandra, J. Microcol. Sep. 12 (2000) 577.
[43] C. Bicchi, C. Cordero, C. Iori, P. Rubiolo, J. High Resolut. Chromatogr. 23 (2000) 539.
[44] J.F. Cavalli, X. Fernandez, L. Lizzani-Cuvelier, A.M. Loiseau, J. Agric. Food Chem. 51 (2003) 7709.
[45] A. Tredoux, A. de Villiers, P. Májek, F. Lynen, A. Crouch, P. Sandra, J.
Agric. Food Chem. 56 (2008) 4286.
[46] B.T. Weldegergis, A. de Villiers, S. Seethapathy, C. McNeish, T. Górecki, A.M. Crouch., Characterization of South African Pinotage wines using GC x GC-TOFMS. 31st Conference of the South African Society for Enology &
Viticulture (SASEV), 11-14 November 2008, Somerset West, South Africa.
[47] E. Baltussen, C. Cramers, P. Sandra, Anal. Bioanal. Chem. 373 (2002) 3.
[48] F. David, P. Sandra, J. Chromatogr. A 1152 (2007) 54.
[49] P. Sandra, B. Tienpont, F. David, R.I.C. ApplNote 2 (2003) 1.
[50] B.T. Weldegergis, A.G.J. Tredoux, A.M. Crouch, J. Agric. Food Chem. 55 (2007) 8696.
[51] E.A. Pfannkoch, J.A. Whitecavage, V.R. Kinton, Gerstel, ApplNote 5 (2003) 1.
[52] C. Bicchi, C. Cordero, E. Liberto, P. Rubiolo, B. Sgorbini, F. David, P.
Sandra, J. Chromatogr. A 1094 (2005) 9.
[53] B.T. Weldegergis, A.M. Crouch, J. Agric. Food Chem. 56 (2008) 10225.
[54] E. Coelho, R. Perestrelo, N.R. Neng, J.S. Câmara, M.A. Coimbra, J.M.F.
Nogueira, S.M. Rocha, Anal. Chim. Acta 624 (2008) 79.
[55] J. Marín, A. Zalacain, C. De Miguel, G.L. Alonso, M.R. Salinas, J.
Chromatogr. A 1098 (2005) 1.
[56] R.F. Alves, A.M.D. Nascimento, J.M.F. Nogueira, Anal. Chim. Acta 546 (2005) 11.
[57] J. Díez, C. Domínguez, D.A. Guillén, R. Veas, C.G. Barroso, J. Chromatogr.
A 1025 (2004) 263.
[58] Y. Hayasaka, K. MacNamara, G.A. Baldock, R.L. Taylor, A.P. Pollnitz, Anal.
Bioanal. Chem. 375 (2003) 948.