As we all know, core-shell chromatography columns are known to have the advantages of high column efficiency, low back pressure, and high sensitivity. However, how these advantages of core-shell chromatographic columns come, many people may have a little understanding. Here is an explanation of the characteristics of the core-shell column.

To understand the characteristics of core-shell columns, we must first understand the classical Van Deemter equation.

When we study chromatographic theory, we all study rate theory to explain the cause of chromatographic peak broadening. This is the classic Van Deemter equation,

As follows:

H Theoretical plate height

A Eddy current diffusion term

B/μ Vertical Diffusion Term

Cμ mass transfer resistance term

Or a slightly more detailed equation is as follows:

In the first term, the eddy current diffusion term, λ is the filling non-uniformity factor; d_{p} is the average particle size of the filling stationary phase.

In the second molecular diffusion term, G is the bending factor between the packings in the column (≈0.6); Dm is the diffusion coefficient of the solute in the liquid mobile phase, Dm≈10⁻⁵cm²/s. μ is the liquid mobile phase in the packed column Average linear velocity in , cm/s.

In the third term, the mass transfer resistance term, df is the diffusion coefficient of the solute in the stationary liquid, cm²/s.W is the packing factor of the chromatographic column, and the value of W is small for short and thick columns.

Compared with fully porous chromatographic columns, the high column efficiency and high analytical efficiency of core-shell chromatographic columns can be explained from the Van Deemter equation (shown in Figure ).

The packing of core-shell chromatographic column (Fused-Core) is generally composed of internal solid spheres (the material and formation method are related to the manufacturer, and different manufacturers have different technologies) and porous silica gel or mixed materials wrapped on the solid sphere. It is composed of granulated silica gel.

The first is that the core of the core-shell chromatography packing is a solid sphere, which leads to a smaller axial diffusion effect of this type of packing, which is reflected in the Van Deemter equation and mainly affects the second parameter Dm, which reduces the axial expansion of the chromatographic peak.

Secondly, due to the existence of the solid sphere, the temperature transfer in the radial direction is accelerated, making the temperature distribution more uniform and accelerating the mass transfer rate; in addition, the mass transfer path of the core-shell filler particles is much shorter than that of the fully porous filler, making the mass transfer rate Faster than fully porous packing

Last but not least, due to the difference in the preparation process (the preparation of core-shell chromatographic column packing is to prepare solid spheres first, and then “coat” the surface of fully porous silica gel; the fully porous chromatographic column packing is mostly “one-time molding” ”), the particle size distribution of core-shell chromatographic column packing particles is more uniform and continuous than that of fully porous chromatographic column packing particles. It is reflected in the Van Deemter equation and mainly affects the first parameter dp, which is the eddy current diffusion term.

The more uniform the particle size distribution of the packing, the smaller the eddy diffusion term, the smaller the contribution to the height of the theoretical plate, and the more the number of theoretical plates of the chromatographic column. As shown in the figure below, the eddy current diffusion term is the main factor affecting the diffusion in the chromatographic peak column. Model B has a more uniform and continuous particle size distribution than Figure A, so it has a smaller eddy current diffusion effect and a higher column efficiency , which is shown in the chromatogram that its peak width is smaller and the resolution of adjacent chromatographic peaks is greater.

For the above three reasons, the height of the theoretical plate is greatly reduced, so the core-shell chromatographic column has higher column efficiency and a wider optimal flow rate range than the fully porous chromatographic column, so it has a higher efficiency. High analysis speed and lower system back pressure.

## Welch’s Boltimate® Core-Shell Columns

○ It has the advantages of ultra-high resolution, high resolution and high column efficiency of sub-2μm chromatographic column, while the column back pressure is less than 50% of that of sub-2μm column.

○ Compared with traditional 3μm and 5μm analytical columns, the column efficiency, speed, resolution and sensitivity are greatly improved, the diffusion path is reduced, and the column efficiency is improved.

○ Narrower particle size distribution, the chromatographic column uses a 2μm frit, which avoids the disadvantage that the column pressure of the sub-2μm column is easy to rise, and still has stable and reliable high performance for samples with complex matrices, and is more durable.

○ Perfectly compatible with any existing HPLC/UHPLC liquid phase equipment.

○ Support the use of high pressure conditions of 600bar.

### The characteristics of each bonded phase filler in Boltimate^{R}

Bonded phase | Feature Description | Particle size | Solid core diameter | Porous layer thickness | Aperture | Specific surface area | Carbon load | Capped | pH stability | USP number |

C 18 | Universal C18 has suitable retention ability for acids, bases and neutral compounds with high resolution | 2.7 | 1.7 | 0.5 | 90 | 120 | 9 | Double capped | 2-8.5 | L1 |

LP-C18 | Using unique bonding technology, it can be used under high temperature and low pH conditions; and the chromatographic column packing is not end-capped, which avoids the selectivity change caused by the hydrolysis of end-capping reagents under acidic conditions | 2.7 | 1.7 | 0.5 | 90 | 120 | 7 | Uncapped | 1-8.5 | L1 |

EXT C18 | The porous layer that has undergone organic-inorganic hybridization greatly enhances the pH tolerance of silica gel. Universal C18 | 2.7 | 1.7 | 0.5 | 90 | 120 | 8 | Double capped | 1.5-12 | L1 |

Phenyl-Hexyl | Phenyl-hexyl bonded core-shell column with unique retention of aromatic compounds | 2.7 | 1.7 | 0.5 | 90 | 120 | 7 | Double capped | 2-8.5 | L11 |

EXT-PFP | The porous layer that has undergone organic-inorganic hybridization greatly enhances the pH tolerance of the silica gel, and is bonded to a fluorophenylpropyl functional group, with unique -T and polarity selectivity | 2.7 | 1.7 | 0.5 | 90 | 120 | 5 | Double capped | 1.5-10 | L43 |

HILIC | Unmodified silica bare spheres, silanols have good retention of polar compounds in HILIC mode, complementary to reversed-phase column selectivity | 2.7 | 1.7 | 0.5 | 90 | 120 | – | – | 2-8.5 | L3 |

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