RP 18 or RP 18 Plus? (Part 1 of 2)
Core-shell phases – RP 18 or RP 18plus?
Over the last few years, modern core-shell technology has developed some highly efficient HPLC stationary phases. These new stationary phases meet the demand for high resolution separations with short analysis times at moderate pressures. Reversed Phase (RP) core-shell silicas with other modifications modifications and different particle sizes are offered.
Core-shell phases consist of a solid non-porous core of silicon dioxide coated with a porous shell of silica, which is built up in a sol-gel process. The diffusion path of the mobile phase through the porous layer of the stationary phase is shortened considerably compared to the diffusion through fully porous particles of equal particle size. The short diffusion paths in the core-shell particles reduce the dwell time of the analyte molecules in the stationary phase, so that even at high flow rates of the mobile phase, optimal separation results can be obtained. Use of core-shell phases allows better resolution combined with lower back pressure and shorter analysis time compared with silica phases based on fully porous particles.
RP 18 or RP 18plus?
Manufacturers also offer core-shell phases with different C18 modifications just as is already the case with based on fully porous C18 silica phases. For column selection the question for the user is which of these modifications is best suited for their specific separation task.
MACHEREY-NAGEL has expanded the product range of core-shell phases with NUCLEOSHELL® RP 18plus. Beside the hydrophobic RP 18 phase, this phase provides a further selectivity.
As the combined Tanaka plots of both phases show, the retention of nonpolar analytes is stronger for the RP 18 phase (higher capacity). In contrast, the ion exchange capacity at pH 2.7 and the hydrogen bonding capacity is more distinct for RP 18plus. Due to an optimized endcapping the RP 18Plus phase shows a lower ion exchange capacity at pH 7.6.
The hydrophobic RP 18Plus phase with a distinct polar selectivity is recommended for the analysis of polar substances such as the determination of vancomycin.
NUCLEOSHELL® RP 18 features a stronger steric selectivity, i.e. the possibility to separate substances with similar molecule structure but different stereochemical properties such as planarity. Due to the increased number of chains on the particle surface as well as polymeric alignment of the C18 chains, planar molecules such as triphenylene can penetrate deeper into the interstices than non-planar molecules like o-terphenyl. Thus, planar molecules have higher retention.
For further successful applications see the SORBTECH application database. Here you can also find application notes of the NUCLEOSHELL® phases Phenyl-Hexyl, PFP and HILIC with alternative selectivities compared to C18 phases.