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Gritti F, Hlushkou D, Tallarek U. Multiple-open-tubular column enabling transverse diffusion. Part 1: Band broadening, Journal of Chromatography A, Vol. 1625, August 16, 2020, 461325



  • Multiple-open-tubular columns enabling transverse diffusion (MOTTD) are studied.
  • Model of band broadening in MOTTD is derived from a random-walk approach.
  • Model is calibrated and validated from random-walk particle-tracking simulations.
  • Difference between modeled and simulated plate heights always smaller than 10%.
  • MOTTD columns are intrinsically superior to current packed and monolithic columns.



We derive a model of band broadening in multiple-open-tubular columns enabling transverse diffusion (MOTTD). In MOTTD columns, the flow channels are straight, parallel, cylindrical tubes arranged in a hexagonal compact array. A mesoporous material or stationary phase (130 Å bridged-ethyl hybrid (BEH) silica support) is filling the volume between the flow channels. The model is based on Giddings’ random-walk theory of non-equilibrium chromatography. It is calibrated for the unknown configuration factor, qs, related to the specific geometry of the stationary phase in MOTTD columns. qs values are found based on the best fit of the model to simulated dispersion data obtained by the lattice-Boltzmann method for modelling fluid flow and a random-walk particle-tracking technique to address advective–diffusive transport of the analytes. For the model calibration, simulations are performed for different ratios, ρ, of the average inner diameter of the flow channels to their closest center-to-center distance under retained and non-retained conditions.


The model is successfully validated (average relative errors below 10%) under both retained and non-retained conditions. For the same column format (4.6 mm i.d.  ×  150 mm), external porosity, zone retention factor, and relative standard deviation of the distribution of the inner diameters of the flow channels, the derived model reveals the intrinsic advantage of MOTTD columns (center-to-center distance between flow channels of 5 µm and ρ = 0.62) over a conventional column packed with 5 µm 130 Å BEH silica particles and the same multiple porous-layer open-tubular column (MPLOT) disabling transverse dispersion. MOTTD columns are weakly affected by the polydispersity of the inner diameter of the flow channels. Provided MOTTD columns could be prepared at a small feature size of 5 µm or less, they are expected to deliver a significant improvement in column technology relative to current particulate and silica monolithic columns.



Professor Fabrice Gritti, principal consulting scientist for Waters Corporation, the leading U.S. chromatography company, and Professors Dzmitry Hlushkou and Ulrich Tallarek, from the Philips University in Marburg, Germany, have recently published an article in one of the foremost scientific journals in the discipline, the Journal of Chromatography A[1], that examines the potential of SEPARATIVE’s proprietary universal multicapillary diffusional-bridging (UMDB) columns – called multiple-open-tubular columns enabling transverse diffusion (MOTTD) in the article – as an alternative to the poorly efficient multiple porous-layer open tubular (MPLOT) column technologies currently in use, and presents a model of band broadening along MOTTD columns in order to evaluate their separation time-pressure performance.


Prof. Gritti’s work is based on the discovery in 2014 of François Parmentier, CEO of the Lyon-based startup SEPARATIVE, that multi-capillary chromatography is possible if it is carried out in a block of porous material. This discovery of diffusional bridging led Parmentier to register five patents which protect SEPARATIVE’s rights to this technology for twenty years and cover all aspects of the technology’s future use, from the chromatography process and its optimization to products and materials. “This technology consists of a bundle of parallel, straight, and cylindrical flow-through channels (mobile phase) separated by a mesoporous material (stationary phase) enabling transverse diffusion of the analyte molecules across the entire column diameter,” features that are covered by two patents.


The chromatography columns whose efficacy is compared in the article are divided into three groups:

  • Conventional packed columns filled with fine particles which have very low permeability and require high operating pressures that are very expensive;
  • Conventional glass-tube monolithic multicapillary MPLOT columns which are highly permeable and require very economical low operating pressures. These are, however, very inefficient since their performance is severely affected by the polydispersity of the inner diameter of the capillaries;
  • Finally, SEPARATIVE’s UMDB columns which are highly permeable and require very economical low operating pressures. These columns which are not yet commercialized enable the analytes to be exchanged between the open channels by transverse diffusion.


The article by Gritti establishes a mathematical model for the separation time-pressure performances of the three column technologies. Using this model, the authors show that the SEPARATIVE’s technology presents the best compromise to obtain high performance at lower cost. “Regardless of the operating conditions (flow rate and retention),” they write, “the MPLOT column is by far the least efficient column among all.” This is due to the fact that “performance of MPLOT columns is far too sensitive to the polydispersity of the channel [inner diameter] and to the increase of the retention factor.” SEPARATIVE’s columns overcome this limitation by enabling molecular exchange between the multiple flow-through channels, i.e. diffusional bridging. In addition, when comparing flow rate and retention, SEPARATIVE’s column outperforms the traditional packed column.


The best way to compare columns is to measure what is called the “separation impedance”. The lower this number, the more efficient the column. The article states: “At flow rates close to the optimum flow rate, the impedance of the MOTTD column is always at least one order of magnitude lower than that of the packed column. The minimum column impedance Emin of the MOTTD column increases from about 50 to 100, 170, and to a maximum of 220 with increasing k1 from 0.8 to 2, 5, and to 100, respectively. No other column technology available today can provide such a low column impedance. Emin is around 3000 for particulate columns and the minimum impedance of the first generation of silica monolithic columns (2 µm through-pore size and 1.3 µm skeleton thickness) is typically around 1000.” SEPARATIVE’s technology allows for an efficiency gain of a factor of 30 compared to conventional columns, which is a revolution.


The authors conclude their study by saying: “This work should encourage chemists, material scientists, and experts in the growing field of 3D-printing technologies to prepare such a novel silica monolithic architecture. The main practical challenge is that the channel i.d. should be close to 3 µm, the distance between the channels around 5 µm, and their length longer than 5 cm in order to deliver a MOTTD efficiency similar or better than those of conventional columns packed with sub-2 µm particles. The preparation of such structures may rely first on the preparation of organic elongated fibers with cross-sectional area of a few µm2. They will then serve in a second step as template materials immersed in a sol–gel phase producing the mesoporous stationary phase around them. In a second step, the organic fibers could be removed by hydrothermal treatment and final calcination at high temperatures.”


This is exactly the process used by SEPARATIVE in its UMDB columns, resulting from six years of research and development, that was presented by François Parmentier at the 17th International Symposium on Preparative and Industrial Chromatography and Allied Techniques (SPICA) in 2018.[2]


60µm monolith channel


What are the practical implications of this new technology for the existing equipment currently installed in laboratories? Chromatographic processes are set to become 10 times faster and more efficient. Furthermore, the SEPARATIVE devices will be far less expensive than existing technologies, making it far more accessible for many laboratories. SEPARATIVE is aiming for a high-performance liquid chromatography (HPLC) device at 10,000 €, similar to the cost of a flame ionization detector (FID) that is frequently used in gas chromatography. Process engineers in all areas of process engineering will thus have at their disposal a new energy-saving, economical and highly productive separation tool that will allow all kinds of industrial mixtures to be separated using the full range of known chemical effects, instead of relying only on heat.


What will this mean for the common man? This technology will make it feasible to develop new drugs that were impossible before and lead to more powerful medical analysis tools for the early detection of disease. According to François Parmentier, this will open the way for new means of energy storage and should accelerate research in “green” chemistry leading to new and more economical biomaterials and biofuels. “Up to ten-percent savings in global CO2 emissions can be achieved by replacing separation methods based on distillation with SEPARATIVE technology,” he comments. “In the future, we will be able to exploit very low-grade metal deposits such as seawater and thereby ensure unlimited development of electrical energy and the independence of all nations in raw materials and energy.”



[1] Gritti F, Hlushkou D, Tallarek U. Multiple-open-tubular column enabling transverse diffusion. Part 1: Band broadening, Journal of Chromatography A, Vol. 1625, August 16, 2020, 461325

[2] Francois PARMENTIER, OC09 – Improvement of Multicapillary Packing Efficiency by Diffusional Bridging: A Theoretical and Dynamic Simulation Study, SPICA 2018, Darmstadt, Germany, October 7-10, 2018.