Enhancing the Possibilities of Comprehensive Two-Dimensional Liquid Chromatography through Hyphenation of Purely Aqueous Temperature-Responsive and Reversed-Phase Liquid Chromatography

Feed injection in liquid chromatography: Reducing the effect of large-volume injections from purely organic diluents in reversed-phase liquid chromatography

In liquid chromatography (LC), discrepancies in liquid properties such as elution strength and viscosity lead to a mismatch between the sample diluent and mobile phase. This mismatch can result in peak deformation, including peak splitting or even breakthrough, particularly when large sample volumes are injected. The formation of a T-junction between sample solution and mobile phase flow stream, a technique previously used in supercritical fluid chromatography, is the key enabler of feed injection in LC. This T-junction allows the injection needle to infuse the sample directly into the mobile phase. It ensures that the diluent is continuously mixed with the mobile phase before introduced onto the column, thereby reducing the initial solvent mismatch. The degree of dilution depends on the ratio between mobile phase flow rate (Qmp) and feed rate (Qfeed) at which the sample is infused. Our study examined the effect of several parameters on the feed injection of large sample volumes from purely organic diluents in reversed-phase LC. These parameters included the type of diluent, compound retention factor (k), injected sample volume (Vinj), and Qmp. With varied Qfeed, all compounds revealed a similar range of optimal values for Qr = (Qmp-Qfeed)/Qfeed between 2 and 5, a range unaffected by Vinj and Qmp. For Qr > 5, the slope of the plate height curves (H vs. Qr) decreases with increasing k, potentially extending the range of optimal Qr-values. However, the best Qr-value for a separation is determined by the compound with the smallest k, simplifying optimization. Using feed injection, we were able to reduce plate heights by up to a factor of 8 compared to classic flow-through injection of large sample volumes.

High-Efficiency Effect-Directed Analysis Leveraging Five High Level Advancements: A Critical Review

Organic contaminants are ubiquitous in the environment, with mounting evidence unequivocally connecting them to aquatic toxicity, illness, and increased mortality, underscoring their substantial impacts on ecological security and environmental health. The intricate composition of sample mixtures and uncertain physicochemical features of potential toxic substances pose challenges to identify key toxicants in environmental samples. Effect-directed analysis (EDA), establishing a connection between key toxicants found in environmental samples and associated hazards, enables the identification of toxicants that can streamline research efforts and inform management action. Nevertheless, the advancement of EDA is constrained by the following factors: inadequate extraction and fractionation of environmental samples, limited bioassay endpoints and unknown linkage to higher order impacts, limited coverage of chemical analysis (i.e., high-resolution mass spectrometry, HRMS), and lacking effective linkage between bioassays and chemical analysis. This review proposes five key advancements to enhance the efficiency of EDA in addressing these challenges: (1) multiple adsorbents for comprehensive coverage of chemical extraction, (2) high-resolution microfractionation and multidimensional fractionation for refined fractionation, (3) robust in vivo/vitro bioassays and omics, (4) high-performance configurations for HRMS analysis, and (5) chemical-, data-, and knowledge-driven approaches for streamlined toxicant identification and validation. We envision that future EDA will integrate big data and artificial intelligence based on the development of quantitative omics, cutting-edge multidimensional microfractionation, and ultraperformance MS to identify environmental hazard factors, serving for broader environmental governance.

Temperature-responsive comprehensive two-dimensional liquid chromatography coupled to high resolution mass spectrometry for the elucidation of the oxidative degradation processes of chemicals of environmental concern

This study explores the possibilities offered by temperature-responsive liquid chromatography (TRLC) based comprehensive 2-dimensional liquid chromatography in combination with reversed-phase liquid chromatography (RPLC) for the analysis of degradation products formed upon oxidative treatment of persistent organic pollutants, in this case exemplified through carbamazepine (CBZ). The TRLC×RPLC combination offers the possibility to overcome peak overlap and incomplete separation encountered in 1D approaches, while the transfer of the purely aqueous mobile phase leads to refocusing of all analytes on the second dimension column. Consequently, this allows for about method-development free and hence, easier LC×LC. The study focuses on the oxidative degradation of CBZ, a compound of environmental concern due to its persistence in water bodies. The TRLC×RPLC combination effectively separates and identifies CBZ and its degradation products, while offering improved selectivity over the individual TRLC or RPLC separations. This allows gathering more understanding of the degradation cascade and allows real-time monitoring of the appearance and disappearance of various degradation products. The compatibility with high-resolution mass spectrometry is last shown, enabling identification of 21 CBZ-related products, nine of which were not previously reported in CBZ degradation studies. The approach's simplicity, optimization-free aspects, and ease of use make it a promising tool for the analysis of degradation pathways in environmental contaminants.

On-column modification for the creation of temperature-responsive stationary phases

Temperature-responsive liquid chromatography (TRLC) offers an alternative for retention and selectivity optimisation in HPLC. This approach thereby exploits temperature gradients on stimuli-responsive stationary phases and forfeits the necessity for solvent gradients, allowing analyses to be performed using aqueous mobile phases. Consequently, it can be employed as a green alternative to reversed-phase separations. However, current production to obtain temperature-responsive columns inherently require dedicated column packing processes with polymer-modified particles. To facilitate the development of temperature-responsive phases, a flow-through modification procedure was developed allowing on-column modification of aminopropyl silica columns. Three columns were manufactured using this novel flow-through approach, which achieved identical column efficiencies compared to existing TRLC column. Demonstrating the possibility of bypassing the dedicated packing processes without losing efficiency. Additionally, it was observed that flow-through produced columns yielded higher retention at elevated temperatures despite their reduced carbon load. Further investigation of the carbon load revealed the presence of stationary phase gradients, whose influence was studied via novel developed retention experiments, which revealed a negligible change reduction in retention upon a change of polymer modification from 19.8% to 14.5%. However, further decrease from 14.5% to 12.3% resulted in a larger change. Interestingly, a further enhancement in apparent plate numbers was observed when operating the column under a reversed flow, yielding an increasing stationary phase gradient. This on-column modification procedure demonstrates the potential for modification of existing (commercial) packed columns to achieve temperature-responsive phases without loss of efficiency or retention. Thus, not only facilitating accessibility to temperature-responsive phases, but also aiding with development of further generations of temperature-responsive phases by removing the need for packing optimisation. Additionally, a novel experiment was set up to easily investigate the effect of inhomogeneous stationary phases retention.

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.

Enhancing the compatibility of normal-phase chromatography x reversed-phase chromatography by combination of low-temperature sensitive aqueous-phase compatible normal- phase chromatography and at-column dilution modulation

The nearly opposite retention mechanism in the two-dimensional liquid chromatography (2D-LC), which combines normal phase liquid chromatography (NPLC) and reversed phase liquid chromatography (RPLC), shows extremely high orthogonality and theoretical peak capacity. However, peak breakthrough and peak distortion caused by the highly incompatible 2D mobile phases counteracts the advantages offered by high orthogonality. To address this difficulty, this study proposes a comprehensive two-dimensional NPLC × RPLC integrating temperature-sensitive aqueous-phase compatible normal-phase chromatography (TSACNPLC) and at-column dilution modulation (ACDM). The proposed 2D-LC system uses an aqueous-miscible acetonitrile/methanol eluent in the 1st D NPLC, instead of an aqueous-phase immiscible eluent, such as n-hexane/methanol, to increase the miscibility with the RP mobile phase system. Additionally, the system exploits temperature-sensitive retention behavior to enhance the retention ability of aqueous-phase compatible NPLC. To verify the feasibility of the proposed 2D-LC, this study selected three multi-component samples with mid-to-low polarity, including ethoxylated (n ≈ 6) bisphenol A (BPA-6EO), ethoxylated (n ≈ 6) tristearylphenol (TSP-6EO), and safflower methanol extract. Next, the effectiveness of the constructed 2D-LC was systematically investigated, including low temperature-induced retention enhancement of NPLC, overcoming solvent incompatibility by ACDM, and optimization of 2 D separation conditions, was systematically investigated.

Perspectives in Hydrophobic Interaction Temperature- Responsive Liquid Chromatography (TRLC)

Temperature-responsive liquid chromatography (TRLC) is an emerging green high performance liquid chromatography (HPLC) mode allowing reversed phase-type separations while necessitating only water as the mobile phase. The columns therein are typically packed with silica particles to which stimuli-responsive polymers are anchored. In hydrophobic interaction TRLC, such polymers depict a loss of water solubility when increasing the temperature above a characteristic conversion temperature, causing large changes in retention over quite narrow and mild temperature ranges (~5–55 °C). TRLC circumvents the concerns about analyte or column degradation that can occur when implementing high temperatures (>80 °C) on conventional reversed- phase columns. It allows for high performance liquid chromatography (HPLC) using only water often spiked with the additives typically used in reversed-phase LC. Therefore, this separation mode allows for greener, cheaper, and isocratic analyses under non-denaturing conditions. The absence of compositional solvent gradients also allows for the exploitation of temperature gradients in combination with refractive index detection. Purely aqueous hydrophobic interaction TRLC is mostly applicable for solutes depicting a 1 < LogP < 5, yet these ranges can be expanded through implementation of combined aqueous or organic mobiles phases, while preserving the temperature-responsive effects. In this first TRLC installment, our recent developments, new possibilities, and current limitations of the use of 1-D TRLC are discussed, while the column performance is described with respect to the fundamentals of HPLC.

Comprehensive two-dimensional temperature-responsive × reversed phase liquid chromatography for the analysis of wine phenolics

Phenolic compounds are an interesting class of natural products because of their proposed contribution to health benefits of foods and beverages and as a bio-source of organic (aromatic) building blocks. Phenolic extracts from natural products are often highly complex and contain compounds covering a broad range in molecular properties. While many 1D-LC and mass spectrometric approaches have been proposed for the analysis of phenolics, this complexity inevitably leads to challenging identification and purification. New insights into the composition of phenolic extracts can be obtained through online comprehensive two-dimensional liquid chromatography (LC × LC) coupled to photodiode array and mass spectrometric detection. However, several practical hurdles must be overcome to achieve high peak capacities and to obtain robust methods with this technique. In many LC × LC configurations, refocusing of analytes at the head of the ²D column is hindered by the high eluotropic strength of the solvent transferred from the ¹D to the ²D, leading to peak breakthrough or broadening. LC × LC combinations whereby a purely aqueous mobile phase is used in the ¹D and RPLC is used in the ²D are unaffected by these phenomena, leading to more robust methods. In this contribution, the combination of temperature-responsive liquid chromatography (TRLC) with RPLC is used for the first time for the analysis of phenolic extracts of natural origin to illustrate the potential of this alternative combination for natural product analyses. The possibilities of the combination are investigated through analysis of wine extracts by TRLC × RPLC-DAD and TRLC × RPLC-ESI-MS.

Investigation of the potential of mixed solvent mobile phases in temperature-responsive liquid chromatography (TRLC)

Temperature-responsive liquid chromatography (TRLC) allows for extensive retention and selectivity tuning through temperature in HPLC. This is mainly achieved through the use of a stationary phases comprising of a temperature-responsive polymer which undergoes a reversible change from hydrophilic to hydrophobic behaviour upon increasing the temperature. The approach can allow for reversed phase type separations to be achieved with purely aqueous mobile phases, whereby the retention is controlled through temperature instead of mobile phase composition. Despite the promising nature of such form of retention control under isocratic mobile phase conditions, TRLC can suffer from excessive retention of highly apolar solutes even at lower column temperatures whereby the polymer is considered hydrophilic. This is related both to a residual apolarity of the polymer chain and due to the high log P's and low water solubility of higly apolar compounds. While it was known that elution in TRLC doesn't necessarily has to be performed under purely aqueous conditions and that the use of organic co-solvents to the water is possible, the impact thereof on the temperature responsive behaviour itself had not yet been investigated in a systematic way. Therefore in this work the advantages and drawbacks of the use of the organic co-solvents methanol and acetonitrile in TRLC is assessed on two types of temperature reponsive phases: poly-N-N-propylacrylamide (PNNPAAm) and poly-N-isopropylacrylamide (PNIPAAm). The influence of organic co-solvents is investigated with two representative test mixtures (comprising 4 parabens and 5 apolar steroids).

Implementations of temperature gradients in temperature-responsive liquid chromatography

Temperature Responsive Liquid Chromatography (TRLC) offers an alternative and environmentally friendly way to perform reversed-phase like separations. Its use of temperature responsive polymers to control retention based on column temperature, instead of the fraction of organic modifier in the mobile phase mobile, eliminates the need for solvent composition gradients and allows, for example, for purely aqueous separations. In principle this temperature induced retention should allow for gradient elutions to be performed using downward temperature gradients to control retention and refocus the analyte peaks. Yet, the unavailability of dedicated commercial temperature controlling systems allowing suitable temperature control in TRLC limits implementations thereof often to isothermal or step gradient applications. In this work we study the potential of 1) a simple yet programmable water bath and of 2) a modified HPLC system allowing column temperature programming through controlled mixing of a warm and cold mobile phase streams. The performance of both systems was evaluated under both isocratic and gradient applications, resulting in a more thorough understanding of the influence of temperature gradients in TRLC. This knowledge is then applied to a sample of phenolic solutes, illustrating that, although both systems have some flaws, both are able to impose temperature gradients in TRLC resulting in significantly reduced retention and enhanced refocusing of the analyte peak.

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.

Exploration of the Selectivity and Retention Behavior of Alternative Polyacrylamides in Temperature Responsive Liquid Chromatography

Temperature responsive liquid chromatography (TRLC) allows for separation of organic solutes in purely aqueous mobile phases whereby retention is controlled through temperature. The vast majority of the work has thus far been performed on poly[N-isopropylacrylamide] (PNIPAAm)-based columns, while the performance of other temperature responsive polymers has rarely been compared under identical conditions. Therefore, in this work, two novel TRLC phases based on poly[N-n-propylacrylamide] (PNNPAAm) and poly[N,N-diethylacrylamide] (PDEAAm) are reported and compared to the state of the art PNIPAAm based column. Optimal comparison is thereby obtained by the use of controlled radical polymerizations, identical molecular weights, and by maximizing carbon loads on the silica supporting material. Analysis of identical test mixtures of homologue series and pharmaceutical samples revealed that PNNPAAm performs in a similar way as PNIPAAm while offering enhanced retention and a shift of the useable temperature range toward lower temperatures. PDEAAm offers a range of novel possibilities as it depicts a different selectivity, allowing for enhanced resolution in TRLC in, for example, coupled column systems. Reduced plate heights of 3 could be obtained on the homemade columns, offering the promise for reasonable column efficiencies in TRLC despite the use of bulky polymers as stationary phases in HPLC.

Understanding the effect of monomer structure of oligoethylene glycol acrylate copolymers on their thermoresponsive behavior for the development of polymeric sensors

Oligoethylene glycol acrylate (OEGA) polymers are a class of thermoresponsive polymers. Three new OEGA monomer combinations were investigated, which revealed three different types of thermoresponsive behavior as a function of copolymer composition.