Over-Drying of Silica-Based SPE Cartridges Can Lead to Poor Sample Recovery

For most reversed-phase SPE applications using a silica-based SPE media (e.g. those using Strata™ C18-E), cartridges (or wells in the case of 96-well plates) are initially conditioned using an organic solvent such as methanol, followed by rinsing with water or a buffer. If silica-based SPE media is allowed to dry out for extended periods of time during these conditioning steps, they may exhibit poor retention for target analytes, especially if the target compounds are polar in nature. It is therefore essential that SPE users pay close attention to their methods during this crucial conditioning step in order to obtain accurate and reproducible results.

The figure below shows two Strata C18-T cartridges (1 gm/6 mL) that were loaded with 4 mL of a blue dye (CuCl2). The cartridge on the left (A) was first conditioned using 12 mL of methanol, then was dried for ten minutes under high vacuum (10” of Hg), followed by 12 mL of water. As you can see from the figure, the blue dye is found uniformly distributed throughout the sorbent bed. In contrast, the cartridge on the right hand side (B) was conditioned with methanol, followed immediately by water without any drying. Under these conditions, the blue dye is visible as a tight, focused band at the head of the sorbent bed. After elution, recovery from cartridge B was far greater than was the recovery from cartridge A (the dried cartridge). This difference in recovery can be attributed primarily to the poor retention of the dye compound due to drying of the sorbent bed.

Tips to Prevent Accidental over-drying to prevent loss of Analyte

• Don’t allow your bed to dry out at any step (unless it is specifically indicated in the method).
• Use gravity flow as an alternative to vacuum to prevent over-drying.
• Centrifuge at low velocities (e.g. 500 RPM) or use positive pressure.
• Switch to an equivalent polymeric sorbent, such as Strata-X, which do not display shifts in retention and recovery due to periodic drying.

A. Silica C18 sorbent dried after conditioning with methanol
B. Silica C18 sorbent conditioned without drying