RP 18 or RP 18 Plus? (Part 2 of 2)

Particle size 2.7 or 5 µm?

The phases NUCLEOSHELL® RP 18 and RP 18plus are both offered with particle sizes 2.7 µm and 5 µm.

RP 18 or RP 18 plus 5The 2.7 µm core-shell particle has a solid core (silicium dioxide) with a diameter of 1.7 µm and a porous shell (silica gel) with a layer thickness of 0.5 µm. The 5 µm particle has a core diameter of 3.8 µm and a layer thickness of the shell of 0.6 µm.

As only the porous shell of a core-shell particle is chromatographically active.  The surface area per gram sorbent is smaller than for fully porous particles with the same particle size. Thus, NUCLEOSHELL® RP 18 and RP 18plus with 5 µm particles have a smaller surface (90 m2/g) than the fully porous silica phase NUCLEODUR® C18 Gravity (330 m2/g) with same particle size of 5 µm. The surface of the NUCLEOSHELL® phases with 2.7 µm (130 m2/g) is larger than the surface of the respective 5 µm phase with 90 m2/g. Also, the carbon load is higher for the smaller 2.7 µm phases.

Physical data of NUCLEOSHELL® vs. NUCfLEODUR®

Phase Particle size
Pore size
Carbon load
NUCLEOSHELL® RP 18plus 2.7 90 130 5.7
5 90 90 4.4
NUCLEOSHELL® RP 18 2.7 90 130 7.8
5 90 90 6.1
NUCLEODUR® C18 Gravity 5 110 330 18


In relation to the theoretical plates N and the resolution Rs, 2.7 µm phases show the highest values.

Theoretical column efficiency (optimal conditions)
Silica Particle size dp
Column length
Theoretical plates N Resolution Rs
Analysis time
NUCLEOSHELL® 2.7 100 25000 112 40
5 150 23000 115 60
NUCLEODUR® 1.8 100 22000 105 40
3 150 20000 100 60
5 250 20000 100 100


For a transfer of pharmacopeia methods with fully porous 5 µm C18 phases under consideration of the allowed variability (particle size: isocratic method up to -50 %, gradient method 0 %), 2.7 µm core-shell phases (-46 %) are suitable for an isocratic method and 5 µm core-shell phases (0 %) for a gradient method. The following application note shows an example for the transfer of an analysis according to the European Pharmacopeia (Ph. Eur.).

Isocratic transfer of the analysis of amlopidine besilate acc. to Ph. Eur.

RP 18 or RP 18 plus 6

Isocratic transfer of the analysis of amlopidine besilate acc. to Ph. Eur.

Another transfer of an isocratic separation method from porous to core-shell stationary phase NUCLEOSHELL® RP 18plus, 2.7 µm is presented in application no. 126860.  Comparison with a fully porous silica phase in application no. 126630 shows a distinct advantage of the core-shell method.

The determination of azithromycin according to Ph. Eur. with a gradient method on NUCLEOSHELL® RP 18plus, 5 µm is shown with the application no. 126880. Further determinations according to the European Pharmacopeia can be found in the MN application database.


The core-shell phases NUCLEOSHELL® RP 18 and RP 18plus are hydrophobic phases with different selectivities, which can be optimally applied for any separation task regarding polar and stereochemical properties of the analytes.

With the availability of phases with particle sizes of 2.7 and 5 µm pharmacopeia methods with porous silica phases can be reliably transferred within the limits of allowed variability. Thus the advantages of high resolution and short analysis time at moderate pressure can be used.

Kinetex® – Phenomenex Inc. (USA)



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