Numerical investigation of band spreading generated by flow-through needle and fixed loop sample injectors

Hyphenation of microflow chromatography with electrospray ionization mass spectrometry for bioanalytical applications focusing on low molecular weight compounds: A tutorial review

Benefits of miniaturized chromatography with various detection modes, such as increased sensitivity, chromatographic efficiency, and speed, were recognized nearly 50 years ago. Over the past two decades, this approach has experienced rapid growth, driven by the emergence of mass spectrometry applications serving -omics sciences and the need for analyzing minute volumes of precious samples with ever higher sensitivity. While nanoscale liquid chromatography (flow rates <1 μL/min) has gained widespread recognition in proteomics, the adoption of microscale setups (flow rates ranging from 1 to 100 μL/min) for low molecular weight compound applications, including metabolomics, has been surprisingly slow, despite the inherent advantages of the approach. Highly heterogeneous matrices and chemical structures accompanied by a relative lack of options for both selective sample preparation and user-friendly equipment are usually reported as major hindrances. To facilitate the wider implementation of microscale analyses, we present here a comprehensive tutorial encompassing important theoretical and practical considerations. We provide fundamental principles in micro-chromatography and guide the reader through the main elements of a microflow workflow, from LC pumps to ionization devices. Finally, based on both our literature overview and experience, illustrated by some in-house data, we highlight the critical importance of the ionization source design and its careful optimization to achieve significant sensitivity improvement.

Detailed comparison of in-house developed and commercially available heart-cutting and selective comprehensive two-dimensional liquid chromatography systems

Recently, two-dimensional liquid chromatography (2D-LC) has become a popular approach to analyze complex samples. This is partly due to the introduction of commercial 2D-LC systems. In the past, 2D-LC was carried out on in-house developed setups, typically consisting of several switching valves and sample loops as the interface between the two dimensions. Commercial systems usually offer different 2D-LC modes in combination with specialized software to operate the instrument and analyze the data. This makes them highly user-friendly, however, at an increased cost compared to in-house developed setups. This study aims to make a comparison between an in-house developed 2D-LC setup and a commercially available 2D-LC instrument. The comparison is made based on experimental differences, in addition to more general differences, including cost price, flexibility, and ease of operation. Special attention is also paid to the different strategies to deal with the mobile phase incompatibility between the highly orthogonal separation mechanisms considered in this work: hydrophilic interaction liquid chromatography (HILIC) and reversed-phase LC (RPLC). For the commercial 2D-LC instrument, this is done using active solvent modulation (ASM), a valve-based approach allowing the on-line dilution of the effluent eluting from the first dimension column before transfer to the second dimension (2D) column. For the in-house developed setup, a combination of restriction capillaries and a trap column is used. Using a sample of 28 compounds with a large polarity range, peak shapes and recoveries of the 2D-chromatograms are compared for both setups. For early eluting compounds, the selective comprehensive approach, currently only possible on the commercial 2D-LC instrument, results in the best peak shapes and recoveries, however, at the cost of an increased analysis time. In general, depending on the analytical goal (single heart-cut versus full-comprehensive 2D-LC), an in-house developed system can be satisfactory for the analysis of specific target compounds/samples. For more complex problems, it can be interesting to use a more specialized commercial 2D-LC instrument. Overall, this comparison study provides advice for analytical scientists, who are considering to use 2D-LC, on the type of equipment to consider, depending on the needs of their particular applications.

Temperature Responsive × Fast Chiral Comprehensive Liquid Chromatography: a New 2D-LC Platform for Resolving Mixtures of Chiral Isomers

Chiral resolution of solutes occurring in mixtures of unrelated species is of relevance in life sciences and in pharmaceutical analysis. While this is conceptually achievable by comprehensive two-dimensional liquid chromatography (LC × LC), few approaches exist whereby the second dimension comprises the chiral separation. The latter is preferable in combination with a conventional reversed phase type of separation in the first dimension as it offers an extension of a conventional achiral analysis. The implementation of such rapid chiral analyses in the second dimension was, thus far, limited by the challenging transfer of the first dimension mobile phase to the second dimension while still achieving chiral separation. In this study, the combination of temperature-responsive and reversed-phase chiral liquid chromatography is assessed in terms of enantioselective separation of a broad range of pharmaceutical compounds. Applying temperature-responsive liquid chromatography (TRLC) in the first dimension allows for analyses to be performed under purely aqueous conditions, which then allows for complete and more generic refocusing of (organic) solutes prior to the second dimension. This offers an enhanced ability to employ fast and broad compositional gradients over the chiral dimension, which broadens the applicability of the technique. In the proposed platform, seven chiral columns (superficially porous and fully porous columns (comprising both polysaccharide and macrocyclic antibiotic phases)) and four mobile phase gradients were screened on a pharmaceutical test mixture. The platform was shown to be able to offer the necessary resolving power for the molecules at hand and offers a new approach for chiral screening of mixtures of unrelated compounds.

Optimizing transfer and dilution processes when using active solvent modulation in on-line two-dimensional liquid chromatography

Two-dimensional liquid chromatography (2D-LC) is becoming increasingly popular for the analysis of complex samples, which is partly due to the recent introduction of commercial 2D-LC systems. To deal with the mobile phase incompatibility between highly orthogonal retention mechanisms, such as hydrophilic interaction liquid chromatography (HILIC) and reversed-phase LC (RPLC), several strategies have been introduced over the years. One of these strategies is active solvent modulation (ASM), a valve-based approach allowing the on-line dilution of the effluent eluting from the first dimension before transfer to the second dimension. This strategy has gained a lot of attention and holds great potential, however, no clear guidelines are currently in place for its use. Therefore, this study aims to investigate how the ASM process can be optimized when using highly incompatible LC combinations, such as HILIC and RPLC, in a simplified selective comprehensive 2D-LC set-up (sHILIC x RPLC) to suggest guidelines for future users. Using a representative sample, the dilution factor (DF), the duration of the ASM phase, the filling percentage of the sample loops, and their unloading configuration are investigated and optimized. It is observed that a DF of 10 with an optimal ASM phase duration, a sample loop filling of maximum 25%, and an unloading configuration in backflush mode, result in the best peak shapes, intensities, and recoveries for early eluting compounds, while keeping the total analysis time minimal. Based on these results, some general recommendations are made that could also be applied in other 2D-LC modes, such as comprehensive 2D-LC (LC x LC), heart-cutting 2D-LC (LC-LC), and other chromatographic combinations with mobile phase incompatibility issues.

Packed modulation loops to reduce band broadening in two-dimensional liquid chromatography

Modulation interfaces employing sample loops are applied in many hyphenated separations such as two-dimensional liquid chromatography (2D-LC). When the first-dimension effluent in 2D-LC is eluted from the modulation loop, dispersion effects occur due to differences in the laminar flow velocity of the filling and emptying flow. These effects were recently studied by Moussa et al. whom recommended the use of coiled loops to promote radial diffusion and reduce this effect. In the 1980s, Coq et al. investigated the use of packed loops, which also promote radial diffusion, in large volume injection 1D-LC. Unfortunately, this concept was never investigated in the context of 2D-LC modulation. Our work evaluates use of packed loops in 2D-LC modulation and compares them to unpacked coiled and uncoiled modulation loops. The effect of the solvents, loop volume, differences in filling and emptying rates, and loop elution direction on the elution profile was investigated. Statistical moments were used as a pragmatic tool to quantify elution profile characteristics. Decreased dispersion was observed in all cases for the packed loops compared to unpacked loops and unpacked coiled loops. In particular for larger loop volumes the dispersion was reduced significantly. Furthermore, countercurrent elution resulted in narrower elution profiles in all cases compared to concurrent elution. We found that packed modulation loops are of high interested when analytes are not refocussed in the second-dimension separation (e.g. for size-exclusion chromatography). Moreover, our work suggests that the use of packed loops may aid in prevention of loop overfilling.

Modelling of analyte profiles and band broadening generated by interface loops used in multi-dimensional liquid chromatography

Currently, the shape and variance of the analyte band entering the second dimension column when injected from an open loop interface in two-dimensional liquid chromatography is not fully understood. This is however important as it is connected to several other variables encountered when developing 2D-LC methods, including the first dimension flow rate, the sampling (modulation) time and the loop volume. Both numerical simulation methods and experimental measurements were used to understand and quantify the dispersion occurring in open tubular interface loops. Variables included are the analyte diffusion coefficient (D_mol), loop filling and emptying rates (F_fill & F_empty), loop inner diameter or radius (R_loop) and loop volume (V_loop). For a straight loop capillary, we find that the concentration profile (as measured at the loop outlet) depends only on a single dimensionless parameter t_empty^*=V_loopF_empty·D_molR_loop² and the ratio of the filling and emptying flow rates F_empty/F_fill. A model depending only on these two parameters was developed to predict of the peak variance resulting from the filling and emptying of a straight capillary operated in the first-in-last-out (FILO) modulation mode. Comparison of the concentration profiles and the corresponding variances obtained by either numerical simulation or experiments with straight capillaries shows the results generally agree very well. When the straight capillary is replaced by a tightly coiled loop, significantly smaller (20-40%) peak variances are observed compared to straight capillaries. The magnitude of these decreases is not predicted as well by simulations, however the simulation results are still useful in this case, because they represent an upper boundary (i.e., worst-case scenario) on the predicted variance.

Column-in-valve designs to minimize extra-column volumes

We report on the design and performance of in-house built column cartridges that can be directly screwed into the ports of a commercial rotor-stator valve to minimize extra-column band broadening and pressure-drop losses when pursuing ultra-fast separations such as those needed in 2D and 3D-LC separations. Two basic designs were evaluated and were compared with the results obtained with a commercial screw-in column cartridge. The system produces an extra-column band broadening as low as 0.05 to 0.1 μL² for the employed UV-detector set-up. Despite these very low values, the obtained separation efficiency of the in-house fabricated cartridge columns was very low, corresponding to a reduced minimal plate height around h=7 at the very best, which, for the 1.7 μm particle and 26.4 mm long columns corresponds to a number of theoretical plates of N=2200 under isocratic conditions. A similar poor performance was obtained with a commercial column cartridge with similar dimensions using the same set-up. One possible explanation of the observed performance could be found in the inner diameter of the column cartridges (i.d. =0.75 mm and 1 mm) which, for the employed sub 2-μm particles, falls into a region of column diameters that, according to literature models, is most likely to suffer from inherent packing problems.

Pharmaceutical impurity analysis by comprehensive two-dimensional temperature responsive × reversed phase liquid chromatography

In this study, the possibilities of temperature responsive × reversed phase liquid chromatography (TRLC × RPLC) are assessed in terms of pharmaceutical impurity analysis. Due to the increased peak capacity per unit time they offer, two-dimensional LC approaches are gaining relevance for the analysis of complex drug formulations. Because the latter depicts a larger predisposition for the occurrence of an increased number of impurities, current 1D-HPLC approaches often prove insufficient. Since many LC × LC methods are limited by modulation, solvent compatibility, orthogonality, and sensitivity issues, the combination of TRLC × RPLC is explored in this work for pharmaceutical impurity analysis. As this combination of a purely aqueous separation with RPLC allows for systematic and optimization-free refocusing in the second dimension, it opens possibilities for generic LC × LC requiring minimal to no method development, in this way overcoming a major perceived contemporary hurdle of LC × LC. The approach is demonstrated with a representative mixture of 17 solutes comprising 11 corticosteroids and 6 progestogens. Orthogonality and peak capacities were assessed on three RP core-shell column selectivities (Poroshell EC-C18, phenyl-hexyl and PFP). Although the TRLC × EC-C18 combination offered somewhat better orthogonality, the combination with the PFP column proved the best for the separation at hand. Depending on the composition of the mixture, the use of full, shifted, or segmented gradients allowed facile optimization of the separation. The developed platform allowed detection of the impurities at the 0.05% level compared to a selected main compound, while also opening up possibilities for analysis of formulations comprising two active ingredients.

Effect of the feed injection method on band broadening in analytical supercritical fluid chromatography

The behavior of a novel type of SFC injector, the feed injector, was investigated. In SFC, the sample compounds are usually diluted in a solvent which has a higher elution strength than the mobile phase, which leads to solvent mismatch upon injection and evidently band broadening. The feed injector differs from standard injectors as the sample, contained in the sample needle or loop, is not switched in line with the mobile phase flow, but directly injected/added to the mobile phase flow (F). The subsequent mixing of sample and mobile phase flows inherently results in a dilution of the sample, thus reducing the solvent mismatch. However, for a given injection/feed flow rate Ffeed, the total volume in which the sample is contained increases with a factor (Ffeed + F)/Ffeed. In addition, to ensure that all of the loaded sample is injected on the column, an additional overfeed volume (Vov) needs to be injected after the sample plug. To better understand the effect of these operating parameters, a wide range of injection conditions was investigated by varying the Ffeed/F-ratio, Vov, overfeed solvent etc. under SFC conditions. It was found that an optimal Ffeed/F exists which is independent of F and decreases with increasing solvent strength dependency of the sample compound. Decreasing Vov has a beneficial effect on peak dispersion but can only be varied over a certain range to ensure the full injection of the loaded sample. On the other hand, it was found that a much larger gain could be made by switching the overfeed solvent to one more compatible with the CO2-based mobile phase. Further reduction of the band broadening could be achieved by applying partial sample injections.