Desorption, SBSE

PLE pressurized liquid extraction, SPE solid phase extraction, UE ultrasonic extraction, DSPE dispersive solid phase extraction, SBSE stir bar sorptive extraction, TD-GC-MS thermal desorption-gas chromatography-mass spectrometry, LAS linear alkylbenzene sulfonates, CDEAs coconut diethanol amides, NPEOs nonylphenol ethoxylates, DP degradation products, SPC sulphenyl carboxylates, PCDD dibenzo-p-dioxins (PCDD), PCDF dibenzofurans, PCBs biphenyls... 

Stir-bar sorptive extraction (SBSE) is carried out using a commercially available glass stir bar (Twister, from Gerstel GmbH) coated with polydimethylsiloxane (PDMS). A special thermal desorption unit is necessary to introduce the extract into a GC. It can be applied to headspace extraction, but is intended for stirring liquid samples for extraction. The same coatings used for SPME can be used for SBSE, and thus similar selectivity should be observed. 

The TDU equipment (commercially available from Gerstel GmbH, Miilheim an der Ruhr, Germany) is fully automated and connected on-line to a GC equipped with a programmable temperature vaporizer (PTV) injector for simultaneous cryotrapping of the analytes before injection. Another approach for analyte desorption is to place the stir bar in a small volume of a conventional HPLC liquid (or mobile phase) for HPLC analysis. The SBSE stir bars are trademarked as Twisters they can also be purchased from Gerstel. For more detailed information on SBSE technology, the reader is referred to two recent review articles.44 45... 

The mode of operation in SBSE resembles that in SPME but with a few distinctive differences. For instance, after carrying out an SBSE procedure, two Twisters (dual SBSE extraction) can be simultaneously desorbed in one thermal desorption tube in order to further enhance the sensitivity. In the so-called multishot mode, up to five Twisters were desorbed in one desorption tube with in situ derivatization for quantification of estrogens in river water.46... 

Roy, G., R. Vuillemin, and J. Guyomarch. 2005. On-site determination of polynuclear aromatic hydrocarbons in sea water by stir bar extraction (SBSE) and thermal desorption (GC-MS). Talanta 66 540-546. 

The powerful potentialities of SBSE followed by thermal desorption and GC-qMS methodology to characterize Madeira wine was also explored by Perestrelo et al. (2009). This methodology provided higher ability for profiling traces and ultratraces of compounds in Madeira wines, including esters (80.7-89.7%), higher alcohols (3.5-8.2%), Ci3 nor-isoprenoids (1.7-6.5%), carboxylic acids (1.6H-.2%), aldehydes (0.9-3.7%) pyrans (0.2-1.7%), lactones (0.3-2.7%), and mono (0.1-1.4%), and sesqui-terpenoids (0.1-0.8%). The authors reported that the concentration of some of them is above their odor threshold, and therefore can probably play a remarkable impact on the aroma complexity of the corresponding wines. 

SBSE can be successfully used in the analysis of environmental samples [93-97] and for food analysis [98, 99]. PDMS is the most commonly used polymer, primarily because of its thermal stability and durability. SBSE has been modified by application of derivatization with different reagents (acetic anhydride, BSTFA, etc) [100-104]. This approach is suitable for the extraction of compounds requiring derivatization. The use of multistep derivatization with several extraction elements (each reaction is performed on a different stir bar) allows efficient extraction, desorption, and chromatographic analysis of compounds with different functional groups (e.g., phenols, steroids, amines, thiazoles, ketones). Acetic anhydride (ester formation), ethyl chloroformate (reaction of acids and amines), tetraethyloborane, and sodium bis-trimethylotrifluoroacetamide have been used for extraction and simultaneous derivatization [105]. 

Figure 9.5. The El fragmentation spectra of dicarboximide fungicides vinclozolin, iprodione, procymidone, and of (3,5-dichlorophenyl)hydantoin (the iprodione degradation product) recorded by stir bar sorptive extraction and thermal desorption-GC/ MS analysis (SBSE-TD-GC/MS). (Reprinted from Journal of Chromatography A, 928, Sandra et al., Stir bar sorptive extraction applied to the determination of dicarboximide fungicides in wine. p. 121, Copyright 2001, with permission from Elsevier.)...

Vinclozolin, procymidone, iprodione, and 3,5-dichlorophenyl hydan-toin (the iprodione degradation product) in wine were determined by SBSE and negative-ion liquid desorption-APCI (fragmentor voltage 70V, capillary voltage 4000V, mass range m/z 200-350) (Sandra et al.,... 

SBSE has many similarities to SPME, as it is also a solventless sample preparation technique and it uses similar sorbents (based on PDMS). SBSE was first described by Baltussen and co-workers in 1999 (78). In SBSE, an aqueous sample is extracted by stirring for a certain time with a PDMS-coated stir-bar. The stir-bar is therea r removed from the sample and the absorbed compounds are then either thermally desorbed and analyzed by GC-MS, or desorbed by means of a liquid for interfacing to a LC system. Heat-desorption gives higher sensitivity while liquid desorption provides higher selectivity. 

FIGU RE 2.15 Schematic representation of the thermal desorption unit in SBSE. (Reprinted from Vercauteren, J., Peres, C., Devos, C., Sandra, P., Vanhaecke, F., and Moens, L., Anal. Chem., 73, 1509-1514, 2001. With permission from American Chemical Society.)... 

Ref. [41] describes a procedure developed for the determination of eight organophosphoms insecticides in natural waters using SBSE combined with thermal desorption-GC-atomic emission detection (AED). Optimization of the extraction and thermal desorption conditions showed that an extraction time of 50 min and a desorption time of 6 min were sufficient. Addition of salt and adjustment of the pH were not necessary. Recoveries of seven of the compounds studied between 62 and 88%. For fenamiphos, which is highly water-soluble, recovery was only 15%. The very low detection limits, between 0.8 ng/1 (ethion) and 15.4 ng/1 (fenamiphos), indicate that the SBSE-GC-AED procedure is suitable for sensitive detection of OPPs in natural waters. 

All the eight flavor samples were also analyzed using a 6890 GC coupled to a 5973 MSD, both from Agilent Technoligies. This instrument was equipped with a thermal desorption autosampler. The samples were diluted 10,000 fold and extracted for one hour with Twister stir bar sorptive extraction (SBSE). Both the thermal desorption autosampler and the Twister are from Gerstel. The autosampler was operated in the standard mode, splitless. The transfer temperature was 275 °C with an initial temp of 20 °C held for 0.4 min and then ramped at 60 °C/min to 250 °C and held for 5 min. The volatiles from the SBSE were then trapped on a CIS 4 operating in the split mode. The initial temp was -120 °C held for 0.2 min and then ramped at 12 °C/min to 280 C which was held for 3 min. The GC oven was initially at 40 °C for 2 min and then ramped to 280 °C at 10 °C/min and held for 5 min. The MSD was scanned from 35 to 350 m/z with a 70 eV ionization energy. 

Differences between different formulations were detected using a mass spectral based chemical sensor with static headspace introduction. Results were validated using traditional GC/MS with SBSE-thermal desorption introduction. Advantages of using the chemical sensor include fast sample throughput and easy to interpret results. For this particular application, flavor formula 1 was found to be very similar to flavor formula 4. This result was also confirmed by the flavor supplier. 

SBSE presents a series of advantages over the rest of extraction techniques is solvent-free (environmental friendly) could be completely automated don t requires pre-treatment of samples (reduces analytical errors) and presents greater sensitivity than SPME, reaching lower detection and quantitation limits. However, it presents two clear disadvantages compared with the other extraction techniques PDMS is the only phase commercially available to date, limiting the extraction of polar substances and a specific thermal desorption unit is required for optimize the process (Castro et al., 2008). The increase of the extraction yields for the recovery of polar compoimds could be carry out by in-site derivatization. Recently, other phases under development were referred, namely those based on the sol-gel technology, restricted access materials and molecular imprinted polymers (Prieto et al., 2010). 

SBSE Adsorption of analytes from liquid samples (solutions) on a coated stir bar followed by their thermal desorption for GC analysis Glass-linked magnetic stir bar coated with a sorbent thermodesorption unit... 

Stir bar has to be removed, introduced into a glass tube, and transferred to thermal desorption instrument. After desorption and cryofocusing within a cooled programmed temperature vaporization (PTV) injector, the volatiles were transferred onto the analytical GC column. Comparison of SPME and the above-mentioned stir bar sorptive extraction (SBSE) technique using identical phases for both techniques exhibited striking differences in the recoveries, which has been attributed to ca. 100 times higher phase ratio in SBSE than in SPME. A comprehensive treatment of SBSE, discussion of the principle, the extraction procedure, and numerous applications was recently been published by David and Sandra (2007). 

The rapid rise in popularity of SPME had led to an increased interest in all types of sorbent-based extractions. Stirbar sorptive extraction (SBSE) is an outgrowth of SPME that employs a coated stirbar as the organic phase. Finally, there are a number of flowthrough extraction configurations that are combined with thermal desorption. These techniques offer interesting alternatives to classical solvent and headspace extractions. 

FIGURE 11.23 Instrumentation for SBSE (a) stirbar is coated with polydimethylsilox-ane (b) stirbar is placed into a tonpraature-programmed GC inlet or thermal desorption apparatus for desorption of analytes and injection into the GC [reprinted with permission from J. Vercauteren et al.. Anal Chem. 73, 1509-1514 (2001), Figures 1 and 2 copyright 2001, American Chemical Society]. 

Sorbtive extraction methods involve the extraction of compounds into a solid absorbent (fibre) and—for volatile compounds—a subsequent thermal desorption in a GC injector. The two main techniques are solid phase micro-extraction (SPME) (Pawliszyn, 1997) and stir-bar sorbtive extraction (SBSE) (Hanaoka et al, 2000). Both methods are good candidates for the automation required in the context of quality control, as commercially available robots are capable of controlling the sampling and injecting the extracted analytes into the GC. 

Recently, a novel technique for isolation and concentration from aqueous samples, namely Stir Bar Sorptive Extraction (SBSE), was developed by E. Baltussen et al. (109). SBSE has already proven to be a very versatile technique for analytical chemistry. To illustrate the potential of SBSE for volatile analytes, this new technique in combination with thermal desorption and capillary gas chromatography coupled to a mass spectrometer was used for enrichment of lemon-flavored beverages (110). 

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