Chromatography


Reducing HPLC/UHPLC System Baseline Noise and Increasing Sensitivity Using Novel 3D Printed, High Performance Static Mixers


James K. Steele, PhD, R&D Scientist, Christopher J. Martino, Chemical Engineer, and Kenneth L. Rubow, PhD, VP Filtration/Advanced Media Science Mott Corporation, 84 Spring Lane, Farmington, CT 06032. Correspondence: skane@mottcorp.com


A revolutionary new inline static mixer has been developed and specifi cally tailored to meet the exacting demands of high performance and ultrahigh performance liquid chromatography (HPLC and UHPLC) systems. Poor mixing of two or more mobile phases results in higher signal to noise ratio and, thus, decreased sensitivity. The homogenous static mixing of two or more fl uids, while utilising the minimal internal volume and physical size of the static mixer represents the ultimate criteria for the ideal static mixer. The new static mixer accomplishes this goal via use of a novel 3D printing technology to create a unique 3D structure that achieves improved hydrodynamic static mixing with the highest percentage reduction in baseline sine wave per unit of internal mixture volume. Up to 98% reduction in baseline sine wave was achieved using 1/3 the internal volume of commonly available mixers. This mixer consists of interconnected 3D fl ow passageways that have varying cross- sectional areas and varying path lengths as the fl uid transverses across and through complex 3D geometric shapes. The mixing in the multitude of tortuous fl ow paths is coupled with localised turbulent fl ow and eddies to create mixing on the micro-, meso-, and macro-scale. Computational fl uid dynamic (CFD) modelling was employed in the design of this unique mixer. The test data presented demonstrates that superior mixing is achieved while minimising the internal volume.


Liquid chromatography has been the work horse instrument for many industries such as pharmaceuticals, pesticides, environmental, forensics, and chemical analysis for 30+ years. The ability to measure down to the part per million levels and lower is crucial to the development of technologies for each industry. Low mixing effi ciency, resulting in poor signal to noise ratios, has plagued the chromatography world when it comes to limits of detection and sensitivity. When combining two solvents for HPLC testing, it is sometimes necessary to induce mixing by external means to homogenise the two solvents as some solvents do not mix easily. If complete mixing of the solvents is not performed, degradation of the HPLC chromatogram may occur as observed by excessive baseline noise and/or poor peak shapes. If poor mixing is present, baseline noise will appear as a sine wave (rise and fall) of the detector signal versus time. At the same time, poor mixing will both broaden and create asymmetrical peaks leading to reduced analytical effi ciency, peak shape and peak resolution. The industry has recognised that inline and tee type static mixers are a means to improve on these limitations and allow the user to achieve lower limits of detection (sensitivity). The ideal static mixer will combine the advantages of high mixing effi ciency, low dead volume and low pressure drop, while minimising the volume and maximising the throughput of the system. Furthermore, as analyses become more challenging, analysts are having to use more polar and diffi cult to mix solvents on a regular basis. This means that better mixing is a necessity for future testing, thereby further driving the need for superior mixer designs and performance.


Mott Static Mixer Mott has recently developed a new line of patent-pending PerfectPeakTM


pending internal fl ow path of the mixer shaded in blue. The varying cross sectional areas of the internal fl ow path and directional fl ow changes within the internal fl ow volume create regions of turbulent and laminar fl ow that create mixing on the micro-, meso-, and macro-scales. Computational fl uid dynamic (CFD) modelling was employed in the design of this unique mixer to analyse fl ow patterns and to improve designs prior to fabrication of prototypes for internal analytical testing and customer beta site evaluations.


Additive manufacturing is a process where 3D geometrical components can be printed directly from a CAD drawing without conventional machining (mills, lathes, etc.). These new static mixers were designed to be manufactured utilising this process where the mixer housing was generated from a CAD drawing and the part(s) manufactured (printed) one layer at a time utilising additive manufacturing. Here, a layer of metal powder is laid down roughly 20 µm thick and a computer controlled laser selectively melts and fuses the powder into a solid form. Another layer is applied over that layer and laser sintering is applied. This process is repeated until the part(s) are fully fabricated. The powder is then removed from the part where no laser bonding had occurred leaving behind a 3D printed part matching the original CAD drawing. The fi nal products are somewhat comparable to micro-fl uidic processes, with the main difference being that micro-fl uidic components are generally 2D (planar) while when using additive manufacturing, one can create complex fl ow patterns in 3D geometries. The mixers can currently be fabricated as 3D printed parts in 316L stainless steel and in titanium. Most metallic alloys, polymers, and some ceramics can be used to fabricate components using this method and their use will be considered for future developments/products.


inline static


mixers with three internal volumes: 30 µL, 60 µL, and 90 µL. These sizes cover the range of volumes and mixing performance needed for the majority of HPLC testing where enhanced mixing with low dispersion is required. All three models are 0.5 inches in diameter and have Industry leading performance in a compact engineered design. They are fabricated in 316L stainless steel which is passivated for inertness but could also be available in Titanium and other corrosion resistant and chemically inert metal alloys. The maximum operating pressure for these mixers is up to 20,000 psi.


Presented in Figure 1a is a photograph of the Mott 60 µL static mixer developed for maximum mixing effi ciency while utilising a smaller internal volume comparable to standard mixers in this category. This new static mixer design utilises a novel additive manufacturing technology to create a unique 3D structure that achieves static mixing using less internal fl ow volume than any mixer currently used in the chromatography industry. This mixer consists of interconnected three-dimensional fl ow passageways that have varying cross-sectional areas and varying path lengths as the fl uid transverses through and across internal complex geometric obstacles. Shown in Figure 1b is a schematic representation of this new mixer utilising industry standard 10-32 threaded HPLC compression fi ttings for the inlet and outlet, with the boundary of the patent


(a)


(b)


Figure 1. Photograph of a Mott 90 µL static mixer (a) and a schematic representation (b) showing a cross-section view with the mixer fl uid fl ow path shaded in blue.


CFD Modelling


Computational Fluid Dynamics (CFD) simulations of the static mixer performance were performed during the design stage to assist in the development of effi cient designs and to reduce trial and error experimentation, which can be time consuming and expensive. CFD modelling of the static mixer and standard tubing (to simulate no mixer) was performed using COMSOL Multiphysics package. Modelling was performed using pressure-driven laminar fl ow fl uid mechanics to understand the fl uid velocity and pressure within the


INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2017


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