In addition to many demonstrated applications of ADS with C-4, C-8, and C-18 bonded-silica reverse-phase materials within the inner pores of the particles, a mixed-mode column packing is also available that relies on affinity and size-exclusion principles. The CAT-PBATM column (Iris Technologies, Lawrence, Kansas) is filled with a porous copolymer that contains specially modified phenylboronic acid (PBA) as an affinity ligand. Low molecular-weight analytes such as catecholamines (e.g., epinephrine, norepinephrine, and dopamine) reach the inner pore surface. Under slightly alkaline load conditions (pH 8.7), these catecholamines are extracted onto the PBA; selective retention occurs due to the formation of a cyclic ester between the boronic acid bound to the stationary phase and the 1,2-dihydric alcohol (diol) functional group on the catecholamines. Following a wash step to remove residual components, the pH is changed to acidic, which causes the cyclic boronic acid ester to hydrolyze, and the catecholamines are eluted onto an analytical column via a switching valve. Separation is achieved under reverse-phase C-18 conditions . As an LC separation with electrochemical detection is taking place, the CAT-PBA SPE column is automatically regenerated and prepared for another extraction. The lifetime of the CAT-PBA SPE column has been reported by the manufacturer to exceed 2000 untreated urine samples (100 yL each) and 1000 untreated plasma samples (500 yL each).
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