Flow rate dependent extra-column variance from injection in capillary liquid chromatography

Minimize Precolumn Band Broadening with Immiscible Solvent Sandwich Injection

We investigated the possibility of reducing the effect of precolumn band broadening (PreCBB) by sandwiching the sample between two small plugs of an immiscible liquid. It has been found that in cases of severe PreCBB, improvements in peak efficiency can amount up to 20 times for the early-eluting compounds. For smaller degrees of PreCBB, the gain on the efficiency of early-eluting compounds is smaller (order of 50%), yet it is still significant. It has been verified that the presence of the immiscible fluid sandwich does not affect the repeatability of the analysis nor the linearity of the calibration curves used for analyte quantitation. It is also shown that the main effect of the two sandwich plugs is the minimization of the dispersion in the precolumn transfer tubing itself, which makes the method fundamentally different from pure on-column focusing methods such as the performance optimizing injection sequence (POISe) method. It is further demonstrated that both halves of the sandwich are needed, since the beneficial effect is clearly much smaller when only one plug is present. A drawback of the method is that some of the late-eluting peaks are slightly adversely affected by the presence of the sandwich liquid in the case where 127 μm i.d. tubing was used. The mechanism for this peak deterioration effect is at present still unclear but only occurs under gradient conditions and is clearly linked to the size of the sandwich plugs (the smaller the plugs, the smaller the adverse effect) and the internal diameter of the tubing used between the injection valve and the column.

Computational Fluid Dynamics Study of the Dispersion Caused by Capillary Misconnection in Nano-Flow Liquid Chromatography

It is well known that high-speed/high-efficiency separations in nano-flow liquid chromatography (LC) are very sensitive to the quality of the connections between the column and the rest of the instrument. In the present study, two types of connection errors (capillary misalignment and the occurrence of an inter-capillary gap) have been investigated using computational fluid dynamics. Interestingly, it has been found that large degrees of capillary misalignment (assuming an otherwise perfect contact between the capillary end-faces) can be afforded without introducing any significant dispersion over the entire range of investigated relative misalignment errors (0 ≤ ε/dcap ≤ 75%), even at the largest flow rates considered in nano-LC. On the other hand, when an inter-capillary gap is present, the dispersion very rapidly increases with the radial width Dc of this gap (extra variance ∼Dcn with n even reaching values above 4). The dependency on the gap length Lc is however much smaller. Results show that, when Dc ≤ 30 μm and Lc ≤ 200 μm, dispersion losses can be limited to the order of 1 nL2 at a flow of 1.5 μL/min, which is generally very small compared to the dispersion in the capillaries (20 μm i.d.) themselves. This result also reconfirms that zero-dead volume connectors with a sufficiently narrow bore can in theory be used without compromising peak dispersion in nano-LC, at least when the capillaries can be matched perfectly to the connector in- and outlet faces. The results are also indicative of the extra dispersion occurring inside microfluidic chips or in the connections between a microfluidic chip and the outer world.

[Research advances in nano liquid chromatography instrumentation]

The miniaturization of liquid chromatography equipment is among the most important focus areas in chromatographic technology. It involves the miniaturization of the physical dimensions of the instrument, size of the separation material, and inner diameter of the column. The advantages of a reduced inner diameter of the column have been investigated for several decades, and can be summarized as follows. First, the sample consumption is lower, which is particularly beneficial when a limited amount of sample is available, as is the case with natural products, and in biochemistry and biomedicine. Second, the consumption of the mobile phase is reduced, making the process environmentally friendly and facilitating green chemistry. This allows the addition of more expensive solvent additives, such as chiral additives or isotopic reagents, while maintaining a low analysis cost. Moreover, the degree of band dilution in the column is lower than that with conventional liquid chromatography under the same sample injection conditions. Thus, enhanced mass sensitivity is achieved. Other benefits of a reduced inner diameter of the column include temperature control due to effective heat transfer through the columns and easier coupling to mass detectors, which is particularly advantageous for analyzing complex samples. Typically, the term “nano liquid chromatography” is associated with liquid chromatography, which employs capillary columns of inner diameters less than 100 μm and flow rates in the range of tens to hundreds of nanoliters per minute. Because of the extremely low flow rates and small column volume, the extra-column effect becomes more prominent. Thus, the requirements for every component of liquid chromatographs are augmented toward improving their performance and optimizing the extra-column band broadening of the entire system. The solvent delivery equipment should be able to pump mobile phases accurately and steadily at nanoliter-level flow rates. A gradient mode is required to achieve this, which implies that the lowest flow rate for a single pump unit should reach a few nanoliters per minute. A certain operating pressure is also necessary to employ columns with different inner diameters and particle sizes. A precise and repeatable sample injection procedure is essential for nano liquid chromatography. The injection volume and mode should be suitable for capillary columns, without inducing a significant extra-column effect. A higher-sensitivity detector should be employed, and sample dispersion should be limited. The improved tubing and connection method in nano liquid chromatography should offer stability, reliability, and ease of operation. The extra-column volume should also be restricted to suit nanoliter-level flow rates. Considering that most nano liquid chromatographic instruments have been coupled with a mass detector, this review mainly focused on nanoliter solvent delivery modules, sample injection modules, and tubing and connection modules. By searching and summarizing research articles, technical patents, and brochures of instrument manufacturers, technical routes and research progress on these modules were described in detail. The pump designs can be classified into four types. Pneumatic amplifying pumps have been used in ultra-high-pressure applications. The flow-splitting delivery system, though easy to realize, may lead to a large amount of solvent wastage. Splitless pumps, which are classified based on two main principles, are widely used. Some pumps based on other physical phenomena have been suggested; however, they lacked stability and robustness. Two types of injection modes have been utilized in nano liquid chromatography. The direct nanoliter injection mode typically takes advantage of the groove on the rotor of a switching valve. The trapping injection mode uses trap columns to enable the introduction of large sample volumes. As for the tubing and connection, a few appropriate designs can be acquired from commercial suppliers. The robustness has been improved using some patented technologies. The optimization principles and research progress on optical absorption detection are briefly introduced. Finally, commercial nano liquid chromatographic systems are compared by considering the pumps and injectors.

Liquid chromatography above 20,000 PSI

Continued improvements in HPLC have led to faster and more efficient separations than previously possible. One important aspect of these improvements has been the increase in instrument operating pressure and the advent of ultrahigh pressure LC (UHPLC). Commercial instrumentation is now capable of up to ~20 kpsi, allowing fast and efficient separations with 5-15 cm columns packed with sub-2 μm particles. Home-built instruments have demonstrated the benefits of even further increases in instrument pressure. The focus of this review is on recent advancements and applications in liquid chromatography above 20 kpsi. We outline the theory and advantages of higher pressure and discuss instrument hardware and design capable of withstanding 20 kpsi or greater. We also overview column packing procedures and stationary phase considerations for HPLC above 20 kpsi, and lastly highlight a few recent applicatioob pressure instruments for the analysis of complex mixtures.

Miniaturised electrically actuated high pressure injection valve for portable capillary liquid chromatography

A miniaturised high pressure 6-port injection valve has been designed and evaluated for its performance in order to facilitate the development of portable capillary high performance liquid chromatography (HPLC). The electrically actuated valve features a very small size (65 × 19 × 19mm) and light weight (33g), and therefore can be easily integrated in a miniaturised modular capillary LC system suited for portable field analysis. The internal volume of the injection valve was determined as 98 nL. The novel conical shape of the stator and rotor and the spring-loaded rotor performed well up to 32MPa (4641psi), the maximum operating pressure investigated. Suitability for application was demonstrated using a miniaturised capillary LC system applied to the chromatographic separation of a mixture of biogenic amines and common cations. The RSD (relative standard deviation) values of retention times and peak areas of 6 successive runs were 0.5-0.7% and 1.8-2.8% for the separation of biogenic amines, respectively, and 0.1-0.2% and 2.1-3.0% for the separation of cations, respectively. This performance was comparable with bench-top HPLC systems thus demonstrating the applicability of the valve for use in portable and miniaturised capillary HPLC systems.

Recent advances in capillary ultrahigh pressure liquid chromatography

In the twenty years since its initial demonstration, capillary ultrahigh pressure liquid chromatography (UHPLC) has proven to be one of most powerful separation techniques for the analysis of complex mixtures. This review focuses on the most recent advances made since 2010 towards increasing the performance of such separations. Improvements in capillary column preparation techniques that have led to columns with unprecedented performance are described. New stationary phases and phase supports that have been reported over the past decade are detailed, with a focus on their use in capillary formats. A discussion on the instrument developments that have been required to ensure that extra-column effects do not diminish the intrinsic efficiency of these columns during analysis is also included. Finally, the impact of these capillary UHPLC topics on the field of proteomics and ways in which capillary UHPLC may continue to be applied to the separation of complex samples are addressed.

Improvement in Liquid Chromatographic Performance of Organic Polymer Monolithic Capillary Columns with Controlled Free-Radical Polymerization

Capillary columns containing butyl or lauryl methacrylate monoliths were prepared using two different free-radical polymerization methods: conventional free-radical polymerization and controlled/living free-radical polymerization, both initiated thermally, and these methods were compared for the first time. Both monolith morphology and chromatographic efficiency were compared for the synthesized stationary phases using scanning electronic microscopy (SEM) and capillary liquid chromatography, respectively. Columns prepared using controlled method gave better chromatographic performance for both monomers tested. The lauryl-based monolith showed 7-fold improvement in chromatographic efficiency with a plate count of 42,000 plates/m (corrected for dead volume) for a non-retained compound. Columns fabricated using controlled polymerization appeared more homogenous radially with fused small globular morphologies, evaluated by SEM, and lower column permeability. The columns were compared with respect to resolving power of a series of alkylbenzenes under isocratic and gradient elution conditions.

Hand-portable liquid chromatographic instrumentation

Over the last four decades, liquid chromatography (LC) has experienced an evolution to smaller columns and particles, new stationary phases and low flow rate instrumentation. However, the development of person-portable LC has not followed, mainly due to difficulties encountered in miniaturizing pumps and detectors, and in reducing solvent consumption. The recent introduction of small, non-splitting pumping systems and UV-absorption detectors for use with capillary columns has finally provided miniaturized instrumentation suitable for high-performance hand-portable LC. Fully integrated microfabricated LC still remains a significant challenge. Ion chromatography (IC) has been successfully miniaturized and applied for field analysis; however, applications are mostly limited to inorganic and small organic ions. This review covers advancements that make possible more rapid expansion of portable forms of LC and IC.