Analytical Limbo: How Low Can You Go? - - Chromatography Online
Analytical Limbo: How Low Can You Go?

LCGC North America
Volume 24, Issue 9

Today, classical liquid–liquid extraction (LLE) increasingly is replaced by elegant sorbent-based extraction methods such as solid-phase extraction (SPE), solid-phase microextraction (SPME), and stir-bar sorptive extraction (SBSE). The beauty of these sorbent-based extraction methods lies in the simplicity of their experimental setup, perceived fast extraction rates, significant reduction in hazardous solvent consumptions, versatility in the wide choice of sorbent materials available (which permits selectivity toward the target analytes), and their general good reproducibility. Most extraction sorbents operate by physical-chemical distribution processes (for example, adsorption–absorption and ionic bonding), although the use of antibody–antigen recognition to effect specific isolation and concentration of analytes also has been realized in recent years with immunoaffinity sorbents. These immunoaffinity sorbents are made from antibodies immobilized onto silica sorbent supports and the high association equilibrium constants (as much as 1012 M-1 ) of these antibody–antigen complexes ensure high probability of such complex formations (even when only one specie of the complex is in very low concentration) for highly effective concentration of the target from sample solution. Immunoaffinity SPE columns are available commercially for specific applications (for example, LSD ImmunoElute for extraction of lysergic acid diethylamide [LSD]) and have been applied successfully to extract analytes such as clenbutarol (9), as well as morphine and metabolites from biological matrices (10,11). Recently, immunoaffinity sorbent for SPME applications for the extraction of theophylline from serum also has been developed by Yuan and colleagues (12). Further details on immunoaffinity SPE are available (13,14).

Other new generation sorbents for SPE include molecularly imprinted polymers (MIPs) and restricted-access materials (RAMs). The former are synthetic alternatives to immunoaffinity sorbents and are much less labor intensive and cheaper to manufacture. The use of MIPs in SPE and SPME formats has been reported for drugs (15–17) and pesticides (18). While RAMs can provide opportunities for simplifying sample preparation from biological matrices by directly partitioning the sample into the protein and analyte portions, their use does not necessarily lead to greater sensitivity in the extraction process. The incorporation of RAMs in both SPE and SPME has been reported, albeit mainly for high performance liquid chromatography (HPLC) applications.

New SPE configurations in the form of pipette-tip designs and multiwell filtration plates are aimed at accelerating sample preparation turnaround times in routine laboratories faced with high throughput workload (19,20). Batch-to-batch and manufacturer-to-manufacturer variations of SPE columns are the main concerns to the current and prospective users of SPE. Continual improvements in quality control protocols and innovation in production technology are expected to address the issue of product variability, which will be a major determinant for the outlook of the SPE manufacturing industry (21). Trends toward miniaturization and higher flow systems (for example, monolithic columns) might well be in store for the new generation SPE columns.

The popularity of SPME, which integrates sampling, extraction, and concentration, has revolutionized the analytical approach in many laboratories. This is evident from the explosion of literature ranging from theory to application notes on the technique since its first introduction by Arthur and Pawliszyn (22). Unlike SPE or LLE, almost all of the extracted analytes on the SPME fiber are desorbed thermally into the analytical system, thus, ensuring high sensitivity in the analysis. Headspace SPME can be considered to be the most widely used implementation mode, as this mode almost can eliminate matrix interferences (from the extraction process), prolong the life of the fiber coating, and completely eradicate the use of hazardous solvents. Headspace SPME was found to be 20 times more sensitive than the heated headspace method for the extraction of amphetamine from urine analyzed by chemical ionization (CI) GC–MS with selected ion monitoring (SIM) (23). To date, analytes with a wide diversity in volatile characteristics have been extracted successfully with headspace SPME, including flavors and fragrances, pesticides, and drugs. Many developments in sorbent materials for SPME also mirror those for SPE (that is, immuno-affinity, MIPs, and RAMs), which already have been described. On-fiber derivatization, with derivatization agent applied before or after the extraction step, extends SPME to more polar analytes. For future SPME designs, the incorporation of different derivatization agents onto the sorbent coatings for exclusive use in simultaneous sampling and on-fiber derivatization can provide another convenient option for the analytical chemist. Certainly, the process in which the derivatization agent is incorporated (for example, absorption versus adsorption), chemical and physical properties of the sorbent support, as well as stability and dynamics of the derivatization chemistry, are among the factors that must be considered for the success of this idea. In general, chemical derivatization of the analytes can enhance the sensitivity of detection significantly, regardless of the type of extraction method used.


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