Visible light initiated polymerization of styrenic monolithic stationary phases using 470 nm light emitting diode arrays

Abstract

Poly(styrene-co-divinylbenzene) monolithic stationary phases have been synthesized for the first time by photoinitiated polymerization. An initiator composed of (+)-(S)-camphorquinone/ethyl-4-dimethylaminobenzoate/N-methoxy-4-phenylpyridinium tetrafluoroborate was activated using a 470 nm light emitting diode array as the light source. Spatially controlled polymerization of styrenic monoliths has been achieved within specific sections of a 100 microm id polytetrafluoroethylene-coated fused-silica capillary using simple photo masking. The sharpness of the edges was confirmed by optical microscopy, while SEM was used to verify a typical porous, globular morphology. Flow resistance data were used to assess the permeability of the monoliths and they were found to have good flow through properties with a flow resistance of 0.725 MPa/cm at 1 microL/min (water, 20 degrees C). Conductivity profiling along the length of the capillary was used to assess their lateral homogeneity. Monoliths which were axially rotated during polymerization were found to be homogeneous along the whole length of the capillary. The monolithic stationary phases were applied to the RP gradient separation of a mixture of proteins. Column fabrication showed excellent reproducibility with the retention factor (k) having a RSD value of 2.6% for the batch and less than 1.73% on individual columns.

Fig.1

Absorbance spectra of styrene (0.51 mol l⁻¹, 20% of conc. in prepolymer solution), divinylbenzene (0.41 mol l⁻¹, 20% of conc. in pre-polymer solution) and emission spectra of 254 nm, 365 nm and 470 nm LEDs. Monomers are dissolved in a solution of acetonitrile/1-propanol/1-decanol.

Fig.2

Effect of the addition of components of the initiator complex on the absorbance of the dye sensitiser and the rate of the reaction. The lines represented on the legend by CQ (0 min, 120 min) are the spectra of a solution containing 1% CQ only, the lines represented by CQE (0 min, 120 min) have 5% EDAB added to the original CQ solution and the lines represented by CQEM (0 min, 120 min) have 5% EDAB and 5% MPPB added to the original solution.

Fig.3

Optical micrograph of the edges of the poly(styrene-co-divinylbenzene) monolith within a poly(tetrafluoroethylene) coated fused silica capillary.

Fig.4

Scanning electron micrograph of a 100 µm id PTFE coated fused silica capillary filled with a poly(styrene-co-divinylbenzene) monolith.

Fig.5

Flow resistance poly(styrene-co-divinylbenzene) monoliths in capillary.

Fig.6

Capactively coupled contactless conductivity detection profile of 4 monolithic stationary phases which have been kept stationary (Col 1) or rotated (Col 8, Col 9, Col 10) during polymerisation. Scanning electron micrographs of poly(styrene-co-divinylbenzene) monoliths with approx. 60% pore volume which have been left stationary (A) and rotated (B), during polymerisation under the optimum conditions are shown on the right hand side.

Fig.7

Example of a separation of a mixture of proteins using poly(styrene-co-divinylbenzene) monolith. Peaks (order of elution) are ribonuclease A, cytochrome C, myoglobin and ovalbumin. Separation was carried out at a flow rate of 1 µl min⁻¹, gradient of 0–60% 0.1% formic acid in ACN in 10 min, UV detection at 210 nm.