Investigation and optimization of the extra-column band broadening in micro-flow capillary liquid chromatography

We report on the examination of extra-column band broadening (ECBB) effects as they can be observed when using commercially available HPLC products for micro-LC, with the aim of proposing some general rules for a rational design of the instrument set-up. For this purpose, we systematically assessed the ECBB contribution of the different LC instrument parts under fixed isocratic measurement conditions, using coumarin compounds with a retention factor (k) of respectively ∼1.2 and ∼2.6 at a flow rate (F) of 2.0 μL/min and a commercial LC column with an inner diameter (i.d.) of 0.2 mm. To avoid that the measurement itself would affect the ECBB, detection was carried out using an on-capillary LED induced fluorescence detector. With this approach, the ECBB effect of (1) the flow-channel tubing i.d., (2) the tubing union, (3) the connection fitting, and (4) the injection valve was quantified in terms of its volumetric peak variance. Results show that the ECBB of a standard instrument set-up can be reduced with hundreds of nL2 per optimised extra-column instrument part. For instance, the use of the commercially available tubing with unified ferrule-nut structure, which has become very popular because of its user- friendliness, causes an additional peak variance (Δσv2) of ∼300 nL2 compared to that of carefully manually-prepared tubing connections (with the same 20-25 μm i.d.) using conventional ferrules, nuts, and sleeves. To emphasize the importance of a proper ECBB control in practical LC analysis, we also investigated the impact of the post-column tubing i.d. for the gradient separation of peptides (cytochrome c digest). The ECBB effect of the post-column tubing i.d. was found to be larger than in the isocratic, small molecule case, because the combination of the well-compressed peaks in the LC column and the stronger ECBB effects caused by the slower diffusivity (Dm) of peptides compared to that of small-sized molecules (σv2 ∝ 1/Dm) makes such separations very vulnerable to ECBB performance losses.

Comparison of experimental and simulated separation performance in capillary tube-in-manifold devices

A metal tube-in-manifold packed bed capillary column device, designed to overcome common limitations associated with capillary LC separations, is described. Experimental results of initial packing tests with sub-3 μm core-shell particles demonstrated efficiencies greater than 47,000 plates/m for a separation performed using the column device. Computational fluid dynamics (CFD) modeling of the multicomponent separation used for this work was validated against experimental LC results and the optimized model was able to effectively predict component peak retention times. However, the accuracy of predicted efficiencies requires further refinement. The tube-in-manifold design demonstrates that packed capillary columns with cylindrical cross-sectional channel geometry and ultrahigh pressure, low dead volume fluidic connections are achievable.

Considerations for Method Development and Method Translation in Capillary Liquid Chromatography: A Tutorial

HPLC continues to be one of the most widely used measurement techniques for chemical analysis. Capillary LC, which utilizes narrow diameter columns operated at lower flow rates than analytical-scale LC, continues to gain adoption based on its reduced mobile phase consumption and increased sensitivity when coupled to MS detection. This tutorial offers practical insights into the most critical aspects of translating analytical-scale separations to the capillary scale. The selection of pumping systems, detectors, and the potential for performance loss due to extra-column effects are examined within the context of separations using columns with inner diameters ≤ 0.3 mm. Column choices within this diameter range are also detailed, both in terms of stationary phase support options and general commercial availability. The impact of these various factors on the effective development/translation of LC methods down to flow rates under 10 μL/min is described to provide readers with a basis for implementing these strategies within their own analytical workflows.

Miniaturized liquid chromatography in environmental analysis. A review

The greater and more widespread use of chemicals, either from industry or daily use, is leading to an increase in the discharge of these substances into the environment. Some of these are known to be hazardous to humans and the environment and are regulated, but there is a large and increasing number of substances which pose a potential risk even at low concentration and are not controlled. In this context, new techniques and methodologies are being developed to deal with this concern. Miniaturized liquid chromatography (LC) emerges as a greener and more sensitive alternative to conventional LC. Furthermore, advances in instrument miniaturization have made possible the development of portable LC instrumentation which may become a promising tool for in-situ monitoring. This work reviews the environmental applications of miniaturized LC over the last 15 years and discusses the different instrumentation, including off- and on-line pretreatment techniques, chromatographic conditions, and contributions to the environmental knowledge.

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.

Fundamental investigation of the dispersion caused by a change in diameter in nano liquid chromatography capillary tubing

We report on a Computational Fluid Dynamics (CFD) study of the extra dispersion caused by the change in diameter when coupling two pieces of capillary tubing with different diameter. In this first investigation into the problem, the focus is on the typical flow rates (0.25≤F≤2μL/min) and diameters (d≤40μm) used in nano-LC, considering both the case of either a doubling or halving of the diameter. The CFD simulations allow to study the problem from a fundamental point of view, i.e., under otherwise perfect conditions (perfect alignment, zero dead-volume). Flow rates, capillary diameters, diffusion coefficients and liquid viscosities have been varied over a range relevant for nano-LC (Reynolds-numbers Re ≤ 1), with also an excursion made towards high-temperature nano-LC conditions (Re ≥ 10 and more). The extra dispersion caused by the change in diameter has been quantified via a volumetric variance σ²_conn, defined in such a way that the overall dispersion across the entire capillary system can be easily reconstructed from the known analytical solutions in the individual segments. When the two capillaries are longer than their diffusion entry length, covering most of the practical cases, σ²_conn converges to a limiting value σ²_conn,∞ which varies to a close approximation with the square of flow rate. Under the investigated nano-LC conditions, the σ²_conn,∞-values are surprisingly small (e.g., on the order of 0.01 to 0.15 nL² in a 20 to 40μm connection) compared to the dispersion occurring in the remainder of the capillaries.

The 2022 Winners of the Lifetime Achievement and Emerging Leader in Chromatography Awards

Barry L. Karger and James P. Grinias are the winners of the 15th annual LCGC Lifetime Achievement and Emerging Leader in Chromatography Awards, respectively. Here, we review their achievements.

The within- and between-laboratories reproducibility and predictive capacity of Amino acid Derivative Reactivity Assay using 4 mM test chemical solution: Results of ring study implemented at five participating laboratories

Amino acid derivative reactivity assay (ADRA) for skin sensitization was adopted as an alternative method in the 2019 OECD Guideline for the Testing of Chemicals (OECD TG 442C). The molar ratio of the nucleophilic reagent to the test chemicals in the reaction solution was set to 1:50. Imamura et al. reported that changing this molar ratio from 1:50 to 1:200 reduced in false negatives and improved prediction accuracy. Hence, a ring study using ADRA with 4 mM of a test chemical solution (ADRA, 4 mM) was conducted at five different laboratories to verify within- and between-laboratory reproducibilities (WLR and BLR, respectively). In this study, we investigated the WLR and BLR using 14 test chemicals grouped into three classes: (1) eight proficiency substances, (2) four test chemicals that showed false negatives in the ADRA with 1 mM test chemical solution (ADRA, 1 mM), but correctly positive in ADRA (4 mM), and (3) current positive control (phenylacetaldehyde) and a new additional positive control (squaric acid diethyl ester). The results showed 100% reproducibility and 100% accuracy for skin sensitization. Hence, it is clear that the ADRA (4 mM) is an excellent test method in contrast to the currently used ADRA (1 mM). We plan to resubmit the ADRA (4 mM) test method to the OECD Test Guideline Group in the near future so that OECD TG 442C could be revised for the convenience and benefit of many ADRA users.

[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.

Adding Size Exclusion Chromatography (SEC) and Light Scattering (LS) Devices to Obtain High-Quality Small Angle X-Ray Scattering (SAXS) Data

We describe the updated size-exclusion chromatography small angle X-ray scattering (SEC-SAXS) set-up used at the P12 bioSAXS beam line of the European Molecular Biology Laboratory (EMBL) at the PETRAIII synchrotron, DESY Hamburg (Germany). The addition of size exclusion chromatography (SEC) directly on-line to the SAXS capillary has become a well-established approach to reduce the effects of the sample heterogeneity on the SAXS measurements. The additional use of multi-angle laser light scattering (MALLS), UV absorption spectroscopy, refractive index (RI), and quasi-elastic light scattering (QELS) in parallel to the SAXS measurements enables independent molecular weight validation and hydrodynamic radius estimates. This allows one to address sample monodispersity as well as conformational heterogeneity. The benefits of the current SEC-SAXS set-up are demonstrated on a set of selected standard proteins. The processed SEC-SAXS data and models are provided in the Small Angle Scattering Biological Data Bank (SASBDB) and are labeled as “bench-marked” datasets that include the unsubtracted data frames spanning the respective SEC elution profiles and corresponding MALLS-UV-RI-QELS data. These entries provide method developers with datasets suitable for testing purposes, in addition to an educational resource for SAS data analysis and modeling.

Retention and effective diffusion of model metabolites on porous graphitic carbon

The study of metabolites in biological samples is of high interest for a wide range of biological and pharmaceutical applications. Reversed phase liquid chromatography is a common technique used for the separation of metabolites, but it provides little retention for polar metabolites. An alternative to C18 bonded phases, porous graphitic carbon has the ability to provide significant retention for both non-polar and polar analytes. The goal of this work is to study the retention and effective diffusion properties of porous graphitic carbon, to see if it is suitable for the wide injection bands and long run times associated with long, packed capillary-scale separations. The retention of a set of standard metabolites was studied for both stationary phases over a wide range of mobile phase conditions. This data showed that porous graphitic carbon benefits from significantly increased retention (often >100 fold) under initial gradient conditions for these metabolites, suggesting much improved ability to focus a wide injection band at the column inlet. The effective diffusion properties of these columns were studied using peak-parking experiments with the standard metabolites under a wide range of retention conditions. Under the high retention conditions, which can be associated with retention after injection loading for gradient separations, Deff/Dm∼0.1 for both the C18-bonded and porous graphitic carbon columns. As C18 bonded particles are widely, and successfully utilized for long gradient separations without issue of increasing peak width from longitudinal diffusion, this suggests that porous graphitic carbon should be amenable for long runtime gradient separations as well.

Chromatographic performance of microfluidic liquid chromatography devices: Experimental evaluation of straight versus serpentine packed channels

We prepared a series of planar titanium microfluidic (μLC) columns, each 100 mm long, with 0.15, 0.3 and 0.5 mm i.d.'s. The microfluidic columns were packed with 1.8 μm C18 sorbent and tested under isocratic and gradient conditions. The efficiency and peak capacity of these devices were monitored using a micro LC instrument with minimal extra column dispersion. Columns with serpentine channels were shown to perform worse than those with straight channels. The loss of efficiency and peak capacity was more prominent for wider i.d. columns, presumably due to on-column band broadening imparted by the so-called "race-track" effect. The loss of chromatographic performance was partially mitigated by tapering the turns (reduction in i.d. through the curved region). While good performance was obtained for 0.15 mm i.d. devices even without turn tapering, the performance of 0.3 mm i.d. columns could be brought on par with capillary LC devices by tapering down to 2/3 of the nominal channel width in the turn regions. The loss of performance was not fully compensated for in 0.5 mm devices even when tapering was employed; 30% loss in efficiency and 10% loss in peak capacity was observed. The experimental data for various devices were compared using the expected theoretical relationship between peak capacity P_c and efficiency N; (P_c-1) = N^0.5 × const. While straight μLC columns showed the expected behavior, the devices with serpentine channels did not adhere to the plot. The results suggest that the loss of efficiency due to the turns is more pronounced than the corresponding loss of peak capacity.

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.