Sample cleanup sorbent selection

MIP-SPE sorbents are stable in both aqueous and organic solvents and are very selective for the analyte of interest. Increased selectivity relative to other sorbents produces increased sensitivity because larger sample volumes can be extracted. Also, increased selectivity results in efficient sample cleanup of the analyte in the presence of complex biological or environmental matrix... 

There are two strategies for sample cleanup using this approach. The first is to select a sample solvent that allows substances of interest to be totally retained on the extraction column sorbent while eluting substances that would interfere with the chromatographic assay. The analytes of interest are then eluted with a small volume of a solvent that will displace the analytes from the sorbent. This strategy is useful when the analyte of interest is present in a low concentration. The alternative approach is to retain the matrix interferences while eluting the desired analyte. 

Many HPLC techniques have had only limited application because they lacked selectivity and sensitivity. In various European interlaboratory studies with several food matrices, the use of sample cleanup was considered essential to maintain high reliability (7). Several sample preparation methods have been designed to overcome these problems. In particular, cartridges packed with modified silica sorbents have provided a simple alternative to traditional sample preparative procedures, such as ion exchange resins (76). 

An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

A recent approach to producing highly selective sorbents for SPE is based on molecular recognition technology and utilizes antibodies immobilized by covalent reaction onto solid supports such as silica (Figure 2.30). Preparation of immunoaffinity sorbents for SPE was reviewed by Stevenson [101] and Stevenson et al. [102], Using immunosorbents, efficient cleanup is achieved from complex biological and environmental samples. 

In most cases, the extracts obtained after LLE or SPE were analyzed without including a cleanup step. Laborious cleanup steps are necessary when analysis is performed by GC-EID or GC-ECD, but nowadays the most common determination technique is GC-MSD and, due to its selectivity, there is no need for a cleanup step for surface water samples. So, the cleanup step is not part of the routine methods for phthalate water analysis. It is only necessary if high background levels or high amounts of polar substances are found in the samples, as could be the case in extracts from landfill leachates of wastewaters.Cleanup is generally performed using a polar sorbent, such as activated alumina or florisil. Lopez-Avila et al. ° have compared florisil and alumina columns to perform the cleanup step. Alumina was preferred over florisil mainly because it allows recovery of all target compounds in the elution step, while three of the 16 phthalates included in the study could not be recovered with florisil clean up. 

Immunoaffinity sorbents (ISs) are more selective compared to ODS sorbents. The first commercial ISs were introduced for the cleanup of samples for the determination of aflatox-ins (Groopman and Donahue, 1988). ISs have been synthesized for a limited number of pesticides and pesticide classes. Immunosorbents arc formed by covalently bonding antibodies to an appropriate sorbent. Immunosorbent is packed into a solid phase extraction cartridge or precolumn as a classical extraction sorbent (Bouzige and Pichon, 1998). There is much interest in developing ISs for single analytes, which are particularly difficult to analyze at the trace level because of a lack of available extraction methods,. such as acephate and methamidophs. 

Ahmadi et al. used MlPs as a selective sorbent for extraction of 3,4-methylene dioxymethamphetamine [MDMA], a strong central nervous system stimulant from plasma samples [Ahmadi et al., 2011]. Ab initio calculations were used to measure AE of template-monomer complexes on Gaussian 03 software utilizing B3LYP/6-31-G[d,p] basis set. The MDMA interacted more strongly with methacrylic acid [MAA] in comparison to other functional monomers and the MAA-based MlPs, was used for the cleanup of MDMA from human plasma with the sufficient accuracy and precision. 

Eurther improvements can be expected on each of the different steps of isolation, cleanup, chromatography, and detection systems for determination of VK in biological samples. TLC and direct-phase HPLC as chromatographic methods will be reduced to a minimum. Development of new materials for solid phase extraction (e.g., Oasis, a new polymeric SPE sorbent) and improved materials for reversed-phase HPLC, including smaller, narrow-bore or capillary columns, should bring advantage to RP-HPLC, especially in connection with MS detection, but also with fluorescence and electrochemical detection and help to obtain smaller sample volumes and/or shorter analysis times. However, the greatest advance, in our opinion, will be in the field of detection instruments—in particular, in mass selective detection instruments. 

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