Cartridges with different SPE mechanisms (reversed-phase or ion exchange), binding strength (degree of hydrophobicity), and binding capacities are available for selection. The introduction of polymer-based SPE sorbents greatly increased the choices beyond conventional silica-based materials (Hennion 1999; Hsieh 2004). When choosing SPE cartridges, loading capacity along with the need to balance retention of and adsorption of all analytes are factors to consider.
An ideal SPE cartridge should have enough capacity and retain sufficient analytes to achieve good recovery while providing good adsorption so that chromatographic separation is not compromised. Other modifications such as online dilution may be needed to offset certain disadvantages.
Brostallicin, a synthetic DNA minor groove binding agent and anticancer drug candidate, was measured in human plasma using a Prospekt-2 system (Calderoli et al. 2003). With a pKa of 12, the compound was found not to retain well on C2, C8, C8-end capped, and C18 cartridges under neutral loading (water). A HySphere resin SH SPE cartridge (10 x 2 mm inner diameter, 15 to 25 mM, Spark) with a strong hydrophobic resin phase (modified polystyrene divinylbenzene) was used. Plasma samples (200 mL) were injected after the addition of an internal standard, then loaded onto a SPE cartridge with 500 mL of water (1 min/min) and washed with water (1 mL/min). The analytes were backflushed onto a precolumn and eluted from the analytical column (Platinum Cyano, 100 x 4.6 mm inner diameter, 3.6 mm, Alltech, Italy) with a 70:30 v/v acetonitrile-ammonium formate buffer (pH 3.5, 20mM) at a flow rate of 1 mL/min. The run time was 8 min and recovery was 40.2 to 57.9% for QC samples. The method was validated at a calibration range of 0.1 to 500 ng/mL with low carry-over (0.04%).
Rodriguez-Mozaz et al. (2004) measured eight estrogens and metabolites in natural and treated water. Four SPE cartridges were tested: PLRP-s (cross-linked styrene divinylbenzene polymer), two Hysphere resin GP (polydivinylbenzene, 10 to 12 mm and 8 mm) units, and a Hysphere C18 EC (end capped octadecyl-bonded silica cartridge, 10 x 2 mm, Spark). While other cartridges did not retain the polar compounds well (<70%), PLRP-s yielded good recovery (>74%) and chromatographic resolution with sample volumes as large as 250 mL. The LLOQ was 0.02 to 1.02 ng/L.
Wissiack et al. (2000) measured 12 phenols in surface water. Five polymer-based SPE materials were tested alone with a single silica-based compound: Hysphere SH (polydivinybenzene 15 to 25 mm, Spark), Hysphere GP (polydivinylbenzene, 5 to 15 mm, Spark), PRP-1 (cross-linked styrene divinylbenzene, 12 to 20 mm, Hamilton), PLRP-s (cross-linked styrene divinylbenzene, 15 to 25 mm, Polymer), Hysphere C18 HD (end-capped, C18 phase with high density of octadecyl chains, Spark), and Oasis (macroporous polydivinylbenzene-N-vinylpyrrolidone copolymer, Waters). Samples and SPE sorbents were acidified (sulfuric acid, pH 2.5) to achieve better retention. Hysphere GP and Waters Oasis cartridges yielded excellent recovery (>94%) for water samples up to 10 to 20 mL. The polymeric SPE cartridge retained analytes better than the analytical column. The mismatch was overcome by adapting an analytical column of larger dimension and stronger retention
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