Direct immersion extraction

SPME can either be performed by head-space extraction (HS-SPME) by placing the fiber in the vapour above a gaseous, liquid or solid sample, or by direct immersion extraction (DI-SPME), by immersing the fiber in a liquid sample. After a certain extraction time, the SPME needle is removed from the septum and inserted into the injection port of the GC or into the desorption chamber of the SPME-HPLC interface. The desorption is performed by heating the fiber in the GC inlet, or by pumping a solvent through the desorption chamber of the SPME-HPLC interface. The main advantages of SPME compared to LLE and solid phase extraction (SPE) are that no or little solvent is... 

Koster et al. [140] conducted on-fiber derivatization for SPME to increase the detectability and extractability of drugs in biological samples. Amphetamine was used as a model compound. The extraction was performed by direct immersion of a 100-pm polydimethylsiloxane-coated fiber into buffered human urine. On-fiber derivatization was performed with pentafluorobenzoyl chloride either after or simultaneously with extraction. 

Chlorophyll concentrations were estimated from 40 ml subsamples following Suzuki and Ishimaru (1990). Samples were filtered on Whatman GF/F glass-fibre filters (pore size 0.45 pm). Chlorophyllous pigments were extracted by direct immersion of the filters in 5 ml of N,N-dimethylformamide, and actual extractions were made in the dark at 20°C. Concentrations of chlorophyll a in the extracts were determined following Strickland and Parsons (1972) using a Turner 450 fluorometer previously calibrated with chlorophyll a extracted from Anacystis nidulans (Sigma Chemicals, St Louis). 

Zeng, D., Chen, B., Yao, S. et al. (2006). Determination of tetramethylenedisulfotetramine in human urine with gas chromatograph-flame thermionic detection coupling with direct immersed solid-phase micro-extraction. Forensic Sci. Int. 159 168-74. 

SPME is a multiphase equilibrium technique and, therefore, the analytes are not completely extracted from the matrix. Nevertheless, the method is useful for quantitative work and excellent precision and Unearity have been demonstrated. An extraction is complete when the concentration of analytes has reached distribution equilibrium between the sample and coating. This means that once the equihbrium is achieved, the amount extracted is independent of further increase in extraction time. If extraction is terminated before the equihbrium is reached, good precision and reproducibihty is still obtained if incubation temperature, sample agitation, sample pH and ionic strength, sample and headspace volume, extraction and desorption time are kept constant. The theory of the thermodynamic, kinetic and mass transfer processes underlying direct immersion and HS-SPME has been extensively discussed by Pawhszyn [2]. The sensitivity and time required to reach adsorption equilibriiun depends on the partition coefficients between the fiber and the analytes, and the thickness of the phase. Limits of detection and quantitation are often below 1 ppb. 

FIGURE 9.2 Total ion chromatogram of a spiked water solution (10 ixglX) extracted for 30 min with direct immersion of a carbowax divinylbenzene 65 fim fiber (1) toluidine (2) / -chloroanniline (3) 2,4 and 2,5-dichloroanniline (4) 3,5-dichloroanniline and (5) 3,4-dichloroanniline. (Reproduced with permission from Muller, L., Fattore, E., and Benfenati, E. J. Chromatogr. A, 791, 221-230, 1997. Copyright 1997, Elsevier Science.)... 

Ultimately, molecular selectivity is desirable. Calix-[4]-arenes improved the extraction of propranolol. Antibodies can be used for affinity separations. Li et al." have demonstrated selectivity for barbiturates using a specially designed receptor molecule. The need for more selectivity is made clear by the following common observation. Recall that SPME extractions can take a long time to reach equilibrium. Is it worthwhile to wait for equilibrium It can be as mentioned above, it should be more reproducible. But interestingly, for many samples extracted by direct immersion of the probe into the sample there is no detection limit advantage to longer extractions because interferences... 

Reference standards, on the other hand, require a more simple preparation regime. The standard of interest is usually prepared by direct immersion in a suitable solvent. It should be noted, however, that it is recommended that at least one reference standard solution be subjected to the same extraction processes that are applied to the sample. This standard solution will act as a quality control (QC) sample in order to check the extraction processes. 

Immersed MBRs have been developed out of a need to simplify the use of these systems and to operate more cost-effectively than the external loops with respect to both energy consumption and cleaning requirements. In these configurations, the membranes are directly immersed in the tanks containing the biological sludge, and the treated permeate is extracted. 

More than one hundred papers reporting diverse applications to analyse wines were published until know. The majority of the referred methodologies use the headspace mode (HS-SPME) instead of the direct immersion mode (DI-SPME). In terms of performance, SPME showed comparable results to LLE or SPE. However, SPME is simpler and solvent-free, and uses smaller volumes of sample nevertheless, on the other hand, LLE had the possibility of carrying out simultaneously the extraction of several samples (Bohlscheid et al., 2006 Castro et al, 2008). When the interest is to obtain the maximum information about the volatile fraction of a wine, the coating DVB/CAR/PDMS seem to be the most suitable (Tat et al., 2005). On the other hand, for specific applications, the choice of a suitable solid-phase, depends on the class of compounds be analyzed, e.g. CAR/PDMS for volatile sulphides and disulphides (Mestres et al., 1999), on-fibre derivatization (PA) for the determination of haloanisoles and halophenols (Pizarro et al, 2007). 

Pena, R.M. Barciela, J. Herrero, C. Garda-Martin, S. (2005). Comparison of ultrasound-assisted extraction and direct immersion solid-phase microextraction methods for the analysis of monoterpenoids in wine. Talanta 67,129-135 Perestrelo, R. Fernandes, A Albuquerque, F.F. Marques, J.C. Camara, J.S. (2006). 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

Extraction of analyte from aqueous samples can be performed either by direct immersion of the fiber into the liquid sample [6,14 16,18-20] or by headspace sampling [17]. Adsorbed analytes are then thermally desorbed in the injection port of a GC and quantified using an appropriate detector. 

The mass of analyte extracted into the fiber is independent of the fiber location, whether directly immersed in the aqueous phase or in the headspace above it, assuming that the system is fully brought to equilibrium, and as long as the phase volumes are constant. 

The main issues in choosing an extraction mode and an agitation method involve the nature of the sample and matrix volatility and affinity to the matrix. From the point of view of fiber lifetime, headspace extraction is preferred, but samples must be at least somewhat volatile and not very strongly bound to the matrix. For samples with lower volatility, direct immersion is preferred, although fiber lifetime is a consideration if the matrix is especially dirty. The second consideration in choosing the extraction mode is equilibrium versus exhaustive extraction. In SPME, equilibrium extraction is much simpler to perform, so it is generally... 

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