SPE sorbents

A programmed-temperature vaporizer (PTV) has also been used as an interface for introducing the LC fraction to the GC unit (84,96) and to desorb the analytes retained in the SPE sorbent contained in the PTV liner. Water samples can then be injected directly in to the PTV injector. 

A technique that attempts to combine the extraction and SPE into a single step is matrix solid-phase dispersion (MSPD). In this technique, a nonpolar (such as Cig) SPE sorbent is blended directly into tissue matrix, the mixture is packaged into an SPE cartridge, and the cartridge is eluted like a typical SPE cartridge. The advantage of MSPD is reduced sample size and increased efficiency due to a reduced number of steps. 

Another subset of SPE is immunoaffinity extraction, in which an antibody specific to the analyte is incorporated into the SPE sorbent. This technique is very selective to the analyte and would be very effective in separating the marker residue from tissue-related matrix components. Disadvantages of immunoaffinity extraction are the need to develop a specific antibody-based SPE for each analyte. This approach holds promise for the future as the development of antibody-based methods becomes more commonplace. 

Table 2 Common SPE sorbents used in analytical methods for pesticides...

For each sample, condition Cig SPE (6-cc/l-g) cartridges with 6mL of methanol and 3 x 6mL of 0.005% aqueous H3PO4 in 10% MeOH-HaO. Stop Flow as the solution reaches the top of the sorbent (the SPE sorbent must not be allowed to go... 

The first reversed-phase SPE sorbents were based on silica gel particles, similar to the particles used in HPLC. A number of phases are available ranging from C8 to C18 to anion- and cation-exchange functionalities. Recent advances in particle technology have included polymeric materials that combine the benefits of a water-wettable particle to retain polar analytes with a reversed-phase, hydrophobic moiety to... 

Desorption experiments indicated that the pigments are very strongly bonded to SPE sorbents X and XI, while they can be easily eluted from the octadecylsilica SPE cartridge. Because of their high sample capacity and high preconcentration factor, ODS SPE cartridges were employed in further experiments. RP-HPLC-DAD analyses were carried out in an ODS column (250 X 4mm i.d. particle size 5/an) at ambient temperature. Solvents A and B were... 

SPE SORBENTS USED FOR THE PRECONCENTRATION OF PIGMENTS OF RED WINES AND THEIR SAMPLE CAPACITY... 

Study, SPE sampling of >6>C-associated residues cannot be ruled out. Also, no data are available on the potential of the surficial retention of colloids or DOC by the SPE sorbent. These factors would lead to an overestimation of dissolved concentrations. Other sampling equipment may also yield inaccurate measurements of dissolved concentrations. Pre-equilibration of laboratory exposure systems is a separate issue related to attainment of steady-state water concentrations in the exposure system prior to initiation of the test. Clearly, the issue of DOC or colloidally sorbed contaminants deserves additional discussion. 

A second approach to on-line SPE is to use an SPE extraction column that can be used for hundreds of samples. In the simplest of systems, two pumps (either HPLC systems or stand-alone pumps) are connected to an extraction column and an analytical column via 6 or 10 ports, and these are further linked to an MS system. The pump that is connected in-line with the autosampler loads the sample under high flow rate (3 to 5mL/min). The large molecules from the matrix are not retained by the SPE sorbent and are diverted to waste. The analytes of interest are retained by the sorbent. The valve then switches so that the second pump with the elution solvent is now in-line with the SPE column and elutes the analytes onto the analytical column for HPLC/MS analysis. This type of system has proved useful for the analysis of small molecules in a variety of sample matrices such as plasma and urine. While it is relatively straightforward to plumb this type of system with components already in the laboratory, commercial systems are available from such companies... 

Traditional (i.e., non-MIP) SPE sorbents are similar to HPLC stationary phases. The advantages of many of these materials are that they are widely available, well characterized, have high binding capacity, and show linear adsorption behavior. One may observe that just a few types are used in the majority of sample preparations, i.e., these materials are quite generic, and it is the wash and elution step which is varied according to the application. The generic nature of these materials is also a drawback because it reflects their limited selectivity. 

Considering the expected wider use of MIPs as SPE sorbents, we discuss below some general rules of thumb for developing and using MISPE methods. 

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