| 03/11 |
| Topic |
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Core-shell phases – highest efficiency for all HPLC instruments |
In the past years manufacturers of HPLC columns and instruments have made different efforts to develop products with high separation efficiency combined with shorter analysis times and higher sample throughput.
Modern core-shell technology also meets this demand for high resolution and short analysis at moderate pressures.
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While a reduction in particle size of totally porous silicas down to sub-2 µm particles results in a distinct shortening of analysis time, the column back pressure sometimes may increase up to 1000 bar, necessitating costly acquisition of ultra high pressure LC instrumentation. While monolithic phases enable the required decrease in analysis times on conventional instruments at low pressures, they do not reach the separation efficiency of sub-2 µm particles.
Since the requirements of HPLC separations with respect to highest efficiency have not yet been met, further optimizations were necessary.
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Optimization of HPLC particles |
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In order to increase the efficiency of HPLC columns with respect to higher resolution and shorter analysis times, a higher plate number N, i.e. a lower plate height h at high linear velocity u is required.
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Van Deemter equation
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However, in agreement with the van Deemter equation, the plate height h, after reaching a minimum (maximum plate number), increases with further increase of the linear velocity u. As a consequence, with 5 and 3 µm particles a higher flow rate will result in a shorter analysis time, however, under loss of resolution.
Lower plate heights at high flow rates are achieved with sub-2 µm particles. Terms A and C of the van Deemter equation are functions of the particle diameter. Thus for smaller particles the plate height will be lower. However, the resulting high column back pressures up to 1000 bar place high requirements on LC instrumentation.
Conventional HPLC systems are not designed for such pressures. Very high linear velocities at pressures of only 100 bar are obtained with monolithic silica columns. However, their plate height is approximately comparable with 3 µm porous silicas, only. In addition they show disadvantages with respect to loading capacity.
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Consequently, the aim of further optimization of HPLC particles is to reconcile the advantages of high flow rates with moderate pressure and high resolution. State-of-the-art core-shell technology meets this demand by using a silica, which is not fully porous.
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Core-shell technology |
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In core-shell technology a sol-gel process is used to build a shell of porous silica around a core of solid silicon dioxide. Depending on the manufacturer, a 2.7 (or 2.6) µm core-shell particle has a core of 1,7 (or 1,9) µm and a porous shell of 0.5 (or 0.35) µm, respectively. The diffusion path of the mobile phase through the porous layer (mean diffusion path about 0.5 µm) is shortened considerably compared to fully porous 3 µm particles (mean diffusion path about 1.5 µm).
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Shorter diffusion paths allow a rapid mass transfer of the analyte between the stationary phase (core-shell particle) and the mobile phase (eluent). In this case term C of the van Deemter equation is a function of the “effective particle size“ which is determined by the porous shell and thus smaller than for totally porous particles. A small term C enables the required low plate height h even at high velocities u. Short diffusion paths also allow high flow velocities for rapid analyses without peak broadening.
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Diffusion paths
Core-shell vs. totally porous particles
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Term A, the so-called Eddy diffusion, is a function of the particle size, too. It is discussed, whether the smaller size of the core-shell particles and a narrow particle size distribution decrease the Eddy diffusion, resulting in a smaller peak width. Anyway, a narrow particle size distribution results in a homogeneous and thus stable packing.
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Van Deemter plots
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The van Deemter plots show that the minimum plate height h of the 3 µm totally porous phase is relatively high and at a low flow velocity. As expected, the sub-2 µm phase (1,8 µm) shows low plate heights also for higher velocities. Most of the core-shell phases investigated exhibit the benefit of even lower h values at comparably high velocities. The respective minima of effective plate height are at relatively high linear velocities.
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Pressure drop
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Considering the pressure drop, the benefits of core-shell phases become even more apparent. Even for high flow velocities pressures are only slightly higher than for the 3 µm phase, while the pressure increases considerably for the sub-2 µm phase (1.8 µm).
Thus, use of core-shell phases allows better resolution combined with lower back pressure and shorter analysis time.
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Resolution Rs
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NUCLEOSHELL core-shell phases |
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The novel NUCLEOSHELL phases from MACHEREY-NAGEL are based on the beneficial core-shell technology. They consist of a solid core of silicon dioxide of 1.7 µm diameter and a homogeneous 0.5 µm shell of porous silica resulting in a particle size of 2.7 µm. A comparison of the theoretical column efficiency with totally porous silica phases shows, that resolution is increased and analysis time is reduced considerably.
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Theoretical column efficiency
(optimal conditions)
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Two modifications are available based on core-shell technology: an octadecyl modification (RP 18) and an ammonium – sulfonic acid modification (HILIC).
NUCLEOSHELL RP 18 combines the advantages of rapid and efficient HPLC with the wide applicability of a RP 18 phase. The MN application database contains numerous separation examples. Due to the state-of-the-art base deactivation, suitability for LC/MS and the high pH stability (1–11) almost all analytical RP 18 methods from porous silica columns can be transferred.
Especially for the analysis of highly polar analytes (e.g., nucleobases, nucleotides, melamine, amino acids, catecholamines) NUCLEOSHELL HILIC is recommended. This phase too can be used with LC/MS detection.
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Conclusion |
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Core-shell technology enables HPLC phases with highest efficiency concerning resolution and analysis time at moderate pressures.
The new NUCLEOSHELL RP 18 utilizes this advantage for analytical RP 18 applications on all HPLC instruments; NUCLEOSHELL HILIC is recommended for polar analytes.
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