Extraction techniques restricted-access materials

An evaluation of the scientific literature reveals that over 500 papers on veterinary drug residue analysis were published in the 5-year period of 2005-2009. Liquid extraction (LE) and liquid-solid extraction (LSE) were found to be very popular sample treatment techniques that were used in 30% and 60% of the reported studies, respectively. Here, LE includes all liquid-based approaches such as liquid-liquid extraction (LLE), extrelut liquid-liquid extraction, liquid-liquid micro-extraction, and pressurized liquid extraction (PLE). LSE includes solid phase extraction (SPE) and all other sorbent-based extraction procedures, such as solid phase micro-extraction (SPME), stir bar sorptive extraction (SBSE), restricted-access materials (RAM), turbulent-flow chromatography (TEC), dispersive SPE (dSPE), and matrix solid phase dispersion... 

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). 

Today, there is strong interest in the development of online sample treatment techniques that allow the handling of untreated biological samples. Thus, in online SPE-LC, deproteination of plasma and serum is required before extraction, especially if the same cartridge is used for repeated analysis. For this purpose, restricted-access materials (RAMs) have been developed, which combine size-exclusion and reversed phase mechanisms, allowing extraction and cleanup of samples in the same step. RAMs have became quite popular for the direct injection of biological fluids, since they prevent the access of matrix components (e.g., proteins) while retaining the analytes in the interior of the sorbent. 

SPE is an exhaustive and almost solvent-free sample preparation technique. Typically, a tube is filled with a sorbent, which can be porous particles or a polymerized monolith. Various interactions are used to extract analytes from complex samples. Many of the commercially available SPE systems are for single use, but some, like RAM (restricted access materials) and MIP (molecular imprinted polymers), are typically obtained as reusable extraction devices. As will be discussed in detail, the extraction sorbents mainly function as normal phase, reversed phase, cation exchange, anion exchange, or a combination of these. 

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