April 25, 2014

Effervescence-assisted dispersive micro-solid phase extraction

In 2011 a new sample pre-treatment technique, called Effervescence-assisted dispersive micro-solid phase extraction, was proposed. The technique was based on the use of an effervescent reaction (reaction between a proton donor and a carbonate source releasing gaseous CO2) for the efficient dispersion of a sorbent. For this purpose a lab-made effervescent tablet, containing all the reagents necessaries to perform the dispersive extraction (NaH2PO4) as proton donor, Na2CO3 as carbonate source and the appropriate sorbent) is directly added to the sample. The tablet, which is 250 mg in weight and 102 mm in diameter, is produced by the simple blending of the precursors and their final compression in a hydraulic manual press. The final tablets are stable enough if they are stored under inert atmosphere to preserve them from the environmental humidity, which reduces the CO2 releasing potential and dispersion efficiency.

The tablet composition is optimized considering the effervescent precursors in one hand and the sorbent in the other. The effervescent precursors must be stoichiometrically adjusted to produce a complete reaction with a minimum pH variation within the sample. The nature of these compounds determines also the hygroscopicity of the tablet. The selected precursors produce a pH variation of 0.2 pH units during the extraction time (considered as the time required to complete dissolution of the tablet, in this case 4.5 min c.a.) and a hygroscopicity (measured as weight variation in non-inert storage conditions) of 0.55 % in 192 h.

The sorbents employed in the two published investigation are a commercial polymeric sorbent (OASIS HLB from Waters Corp.) and multiwalled carbon nanotubes (MWCNTs). The first approach employs the tablet inside a 10 mL syringe in which sorbent dispersion occurs immediately after sample aspiration. The sorbent with the extracted analytes is retained using a 0.2 μm in-syringe filter and eluted prior to the UPLC-UV analysis. This method was used to the analysis of nitroaromatic compounds in environmental water samples with great values in terms of sensitivity and reproducibility as shown in table 1. 

Table 1. Analytical figures of merit of the presented alternatives

Analytes
Sample volume (mL)
LOD (μg/L)
RSD
(%)
EF
dSPME





OASIS HLB
Nitroaromatic comp.

10
1.8-7
1.7-8.6
13-17
MWCNTs
Triazines
100
0.15-0.40
3.9-9.3
480-755
dLPME






1-Octanol/MNPs
Triazines
20
0.02-0.06
7.8-11.7
21-185
Legend: LOD, limit of detection; RSD, relative standard deviation; EF, enrichment factor

The second option developed introduces a nanostructured sorbent (MWCNTs) in the effervescent tablet. One of the main problems associated to the use of raw carbon nanotubes is the aggregation tendency, especially in aqueous samples. With the effervescent dispersion this problem is avoided. The dispersion generated during extraction, without the use of any organic solvent or surfactant is stable enough to interact with the analytes in the sample, in this case a 100 mL aqueous sample contained in a glass beaker. Due to the sample volume employed the sorbent with extracted analytes is recovered by vacuum filtration using a commercial PTFE tape as filter. To evaluate the dispersion process a simple experiment using different dispersion alternatives is performed. For this purpose different dispersion are prepared (Fig 1); (A) uncompressed effervescent tablet powder, (B) effervescent tablet containing the sorbent, (C) MWCNTs directly added to an aqueous sample and (D) MWCNTs added to a water sample in which an effervescent tablet without sorbent had been dissolved. The vials containing these mixtures are mechanically agitated and leave at rest for two minutes before taking the pictures. As can be seen the effervescent tablet is the responsible of the efficient dispersion during extraction and not the ionic strength derived from the effervescent precursors. 
Figure 1. Different dispersions obtained for MWCNTs (for details, read text) 


This alternative was evaluated by the extraction of triazine herbicides from environmental water samples by GC-MS with good values of sensitivity and reproducibility as can be seen in table 1.

Effervescence allows also the dispersion of liquid in a new liquid-liquid micro extraction approach

The most recent effervescent-approach is based on the dispersion of a very low volume (20 μL) of an organic solvent (1-octanol) in aqueous samples, using Fe3O4 magnetic nanoparticles co-dispersed with the extractant phase to achieve the extractant phase separation in a very simple way. The dispersion is based in the effervescent reaction between a carbonated aqueous sample and a liquid mixture containing 20 μL of organic solvent and 10 mg of magnetic nanoparticles in acetic acid media. A volume of extractant mixture (250 μL) is injected in the bottom sample and the effervescent reaction disperses both solvent and magnetic nanoparticles. The subsequent application of an external magnet permits the solvent recovery by the interaction of the alcoholic group of the solvent and the hydroxide residues of the MNPs surface. This alternative is evaluated by the extraction of selected triazine herbicides by GC-MS with great values of sensitivity and reproducibility.

Conclusions

The employ of effervescence as dispersion method for solid or liquids reduces the use of organic solvent or apparatus (like vortex or ultrasonic baths) to perform the extraction. Furthermore, the reagents employed are non-toxic and cheaper than other alternatives. The simplicity of the inclusion in a single device (in this case a tablet) permits the employ of these alternatives for the on-site analysis of environmental waters. In the case of the dLLME, the very low volume of extractant used permits high pre-concentration factors. The simplicity of the extractant phase recovery process, using an external magnet avoids the use of complex lab-ware (such a separation funnel) or centrifuges, which probably affects the reproducibility of the method at this very low volume.

References:

(1) Effervescence-assisted dispersive micro-solid phase extraction. Link to the article
(2) Effervescence-assisted carbon nanotubes dispersion for the micro-solid-phase extraction of triazine herbicides from environmental waters. Link to the article
(3) Effervescence assisted dispersive liquid–liquid microextraction with extractant removal by magnetic nanoparticles. Link to the article

Guillermo Lasarte-Aragones studied Biochemistry in the University of Córdoba (Spain) until 2007, getting later on a Master degree in "Molecular, cellular an genetic biotechnology". In his early research, Guillermo worked at mitochondrial redoxins and molecular defenses against oxidative stress using Saccharomyces cerevisiae as model organism. Nowadays he is developing his PhD Thesis under the supervision of Prof. Valcárcel, Cárdenas and Lucena at the same University. His work is focused on the innovative uses of carbon dioxide on the development of novel microextraction techniques.

Guillermo on twitter: https://twitter.com/LasarteG 

Research group: http://www.uco.es/grupos/FQM-215/



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