Assessment of Two-Dimensional Separative Systems Using Nearest-Neighbor Distances Approach. Part 1: Orthogonality Aspects

Computer-driven optimization of complex gradients in comprehensive two-dimensional liquid chromatography

Method development in comprehensive two-dimensional liquid chromatography (LC × LC) is a complicated endeavor. The dependency between the two dimensions and the possibility of incorporating complex gradient profiles, such as multi-segmented gradients or shifting gradients, renders method development by "trial-and-error" time-consuming and highly dependent on user experience. In this work, an open-source algorithm for the automated and interpretive method development of complex gradients in LC × LC-mass spectrometry (MS) was developed. A workflow was designed to operate within a closed-loop that allowed direct interaction between the LC × LC-MS system and a data-processing computer which ran in an unsupervised and automated fashion. Obtaining accurate retention models in LC × LC is difficult due to the challenges associated with the exact determination of retention times, curve fitting because of the use of gradient elution, and gradient deformation. Thus, retention models were compared in terms of repeatability of determination. Additionally, the design of shifting gradients in the second dimension and the prediction of peak widths were investigated. The algorithm was tested on separations of a tryptic digest of a monoclonal antibody using an objective function that included the sum of resolutions and analysis time as quality descriptors. The algorithm was able to improve the separation relative to a generic starting method using these complex gradient profiles after only four method-development iterations (i.e., sets of chromatographic conditions). Further iterations improved retention time and peak width predictions and thus the accuracy in the separations predicted by the algorithm.

Polybutylene terephthalate-based stationary phase for ion-pair-free reversed-phase liquid chromatography of small interfering RNA. Part 2: Use for selective comprehensive two-dimensional liquid chromatography

With the increasing numbers of nucleic acid-based pharmaceuticals like antisense oligonucleotides (ASO), small interfering ribonucleic acid (siRNA) entering the market, research facilities, pharmaceutical industries and also regulatory authorities have been looking for efficient analytical methods for these synthetic oligonucleotides (ON). Besides of conventional one-dimensional (1D) reversed-phase liquid chromatography with or without ion-pairing (IP-RP-LC, RP-LC), hydrophilic liquid chromatography (HILIC) and mixed-mode chromatography (MMC), two-dimensional (2D) approaches combining two orthogonal chromatographic techniques also become more relevant due to the high structural complexity of oligonucleotides. Recently, we tested a polybutylene terephthalate(PBT)-based stationary phase under ion-pairing free RP mode for the liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) analysis of siRNA (Patisiran). In this study, retention profile and chromatographic orthogonality, respectively, were compared to other LC-modes like HILIC, IP-RPLC, another ion-pair free cholesterol-bonded RPLC and MMC considering their normalized retention times. Finally, because of higher orthogonality, the ion-pairing free PBT-bonded RPLC as first dimension (¹D) was hyphenated with HILIC in the second dimension (²D) in a selective comprehensive 2D-LC setup leading to an enhanced resolution for peak purity evaluation of the main ON entities.

Off-line two-dimensional separation involving supercritical fluid chromatography for the characterization of the wastewater from algae hydrothermal liquefaction

An off-line multidimensional method involving liquid chromatography combined with supercritical fluid chromatography was developed for the characterization of the wastewater of hydrothermal liquefaction of microalgae Chlorella sorokiniana. The first dimension consisted of a phenyl hexyl column operated in reversed-phase mode, whereas the second dimension was performed on a diol stationary phase. Optimization of the kinetic parameters of the first and second dimensions were performed, taking into account the fraction collection system. The beneficial effect of working at high flow rate in both dimensions, as well as the need to work with short columns (50 mm) in the second dimension was evidenced. Injection volume was also optimized in both dimensions. The first dimension benefited from on-column focusing, while in the second dimension, untreated water-rich fractions could be injected without peak deformation. The performances of offline LCxSFC were compared to LC-HRMS, SFC-HRMS and LCxLC-HRMS for the analysis of the wastewater. Despite a long analysis time of 3.3h, the off-line separation coupled to high-resolution mass spectrometry exhibited a very large orthogonality with 75 % occupation rate of the separation space, reaching an effective peak capacity of 1050. While other evaluated techniques were faster, one-dimensional techniques failed to separate the numerous isomers while LCxLC exhibited lower orthogonality (45% occupation rate).

Methodology for dyes purification by preparative bidimensional offline reversed-phase high-performance liquid chromatography separation based on mobile phase pH change: Case study of Justicia spicigera

The extensive characterization of the natural dyes involves the purification of their colored compounds for their structural analysis and stability studies. As most natural compounds being ionizable, herein is presented the optimization of an easy and affordable preparative bidimensional offline reversed-phase high-performance liquid chromatography purification based on mobile phase pH change. At the analytical scale, several combinations of stationary phases and mobile phases at different pH values were investigated first. The orthogonality between the dimensions was quantitatively evaluated using the nearest-neighbor distance approach. The optimized separation method was transferred to the preparative scale for the successful isolation of two colored compounds from Justicia spicigera, a traditional dye plant from Central America. They were identified as perisbivalvine B (2-amino-8-hydroxy-7-methoxy-3H-phenoxazin-3-one) and one of its derivatives, also found in another tinctorial plant species, Peristrophe bivalvis growing in Asia.

Systematic Evaluation of Liquid Chromatography (LC) Column Combinations for Application in Two-Dimensional LC Metabolomic Studies

In comparison to proteomics, the application of two-dimensional liquid chromatography (2D LC) in the field of metabolomics is still premature. One reason might be the elevated chemical complexity and the associated challenge of selecting proper separation conditions in each dimension. As orthogonality of dimensions is a major issue, the present study aimed for the identification of successful stationary phase combinations. To determine the degree of orthogonality, first, six different metrics, namely, Pearson's correlation coefficient (1 - |R|), the nearest-neighbor distances (H̅_NND), the "asterisk equations" (A_O), and surface coverage by bins (SC_G), convex hulls (SC_CH), and α-convex hulls (SC_αH), were critically assessed by 15 artificial 2D data sets, and a systematic parameter optimization of α-convex hulls was conducted. SG_G, SC_αH with α = 0.1, and H̅_NND generated valid results with sensitivity toward space utilization and data distribution and, therefore, were applied to pairs of experimental retention time sets obtained for >350 metabolites, selected to represent the chemical space of human urine. Normalized retention data were obtained for 23 chromatographic setups, comprising reversed-phase (RP), hydrophilic interaction liquid chromatography (HILIC), and mixed-mode separation systems with an ion exchange (IEX) contribution. As expected, no single LC setting provided separation of all considered analytes, but while conventional RP×HILIC combinations appeared rather complementary than orthogonal, the incorporation of IEX properties into the RP dimension substantially increased the 2D potential. Eventually, one of the most promising column combinations was implemented for an offline 2D LC time-of-flight mass spectrometry analysis of a lyophilized urine sample. Targeted screening resulted in a total of 164 detected metabolites and confirmed the outstanding coverage of the 2D retention space.

Off-line two-dimensional liquid chromatography separation for the quality control of saponins samples from Quillaja Saponaria

Quil-A is a purified extract of saponins with strong immunoadjuvant activity. While specific molecules have been identified and tested in clinical trials, Quil-A is mostly used as a totum of the Quillaja Saponaria bark extract. Quality control of the extract stability is usually based on the monitoring of specific saponins, whereas the comparison of samples with an initial chromatogram seems more appropriate. A reference fingerprint based on comprehensive two-dimensional liquid chromatography offers a rapid detection of nonconform samples. To fulfill quality control constraints, off-line configuration using basic instrumentation was promoted. Hence, reversed-phase liquid chromatography × reversed-phase liquid chromatography and hydrophilic interaction chromatography × reversed-phase liquid chromatography methods with ultraviolet and single-quadrupole mass spectrometry detection were kinetically optimized. The reversed-phase liquid chromatography × reversed-phase liquid chromatography method used a pH switch between dimensions to maximize orthogonality. Despite diagonalization, it led to a high peak capacity of 831 in 2 h. On the other hand, the combination of hydrophilic interaction chromatography and reversed-phase liquid chromatography offered a larger orthogonality but a lower, yet satisfactory peak capacity of 673. The advantages of both methods were illustrated on degraded samples, where the reversed-phase liquid chromatography × reversed-phase liquid chromatography contour plot highlighted the loss of fatty acid chains, while the hydrophilic interaction chromatography × reversed-phase liquid chromatography method was found useful to evidence enzymatic loss of sugar moieties.

Orthogonality in the selection of biphasic solvent systems for off-line two-dimensional countercurrent chromatography from Polygonum cuspidatum Sieb. et Zucc

Off-line two-dimensional countercurrent chromatography has been widely applied to the isolation of complex samples, but little research on the investigation of orthogonality in the selection of biphasic solvent systems is available. In the present work, the orthogonality in the selection of a biphasic solvent system for liquid-liquid chromatographic separation of aqueous extract and ether extract from the traditional Chinese medicinal plant Polygonum cuspidatum Sieb. et Zucc was evaluated by the correlation coefficient and space occupancy rate. In total, 25 different biphasic solvent systems were tested, and 313 system combinations were analysed. A convex hull methodology was used to determine the separation space and to optimize separation conditions. The correlation coefficient matrix was transformed into dendrograms and a colour map to visualize the dissimilarity between, and orthogonality for, all solvent systems. The aqueous extracts from Polygonum cuspidatum were separated using selected biphasic solvent systems with high orthogonality: ethyl acetate-ethanol-water (70:1:70, v/v) and petroleum ether-ethyl acetate-water (1:5:5, v/v). The ether extracts from Polygonum cuspidatum were also separated using selected biphasic solvent systems with high orthogonality: petroleum-ethyl acetate-methanol-aqueous 0.25 M NH₃•H₂O (5:5:5:5, v/v) and petroleum-ethyl acetate-methanol-water (5:5:5:5, v/v). Thirteen compounds were successfully obtained. The experimental results demonstrated that the evaluation of orthogonality provided an alternative strategy to select an applicable solvent system for the separation of complex samples using off-line two-dimensional countercurrent chromatography.

On sample preparation methods for fermented beverage VOCs profiling by GCxGC-TOFMS

INTRODUCTION

Aromas and tastes have crucial influences on the quality of fermented beverages. The determination of aromatic compounds requires global non-targeted profiling of the volatile organic compounds (VOCs) in the beverages. However, experimental VOC profiling result depends on the chosen VOC collection method.

OBJECTIVES

This study aims to observe the impact of using different sample preparation techniques [dynamic headspace (DHS), vortex-assisted liquid-liquid microextraction (VALLME), multiple stir bar sorptive extraction (mSBSE), solid phase extraction (SPE), and solid phase micro-extraction (SPME)] to figure out the most suitable sample preparation protocol for profiling the VOCs from fermented beverages.

METHODS

Five common sample preparation methods were studied with beer, cider, red wine, and white wine samples. After the sample preparation, collected VOCs were analyzed by two-dimensional gas chromatography coupled with time of flight mass spectrometry (GCxGC-TOFMS).

RESULTS

GCxGC oven parameters can be optimized with the Box-Behnken surface response model and response measure on peak dispersion. Due to the unavoidable column and detector saturation during metabolomic analysis, errors may happen during mass spectrum construction. Profiling results obtained with different sample preparation methods show considerable variance. Common findings occupy a small fraction of total annotated VOCs. For known fermentative aromas, best coverage can be reached by using SPME together with SPE for beer, and VALLME for wine and cider.

CONCLUSIONS

GCxGC-TOFMS is a promising tool for non-targeted profiling on VOCs from fermented beverages. However, a proper data processing protocol is lacking for metabolomic analysis. Each sample preparation method has a specific profiling spectrum on VOC profiling. The coverage of the VOC metabolome can be improved by combining complementary methods.

Recent applications of chemometrics in one- and two-dimensional chromatography

The proliferation of increasingly more sophisticated analytical separation systems, often incorporating increasingly more powerful detection techniques, such as high-resolution mass spectrometry, causes an urgent need for highly efficient data-analysis and optimization strategies. This is especially true for comprehensive two-dimensional chromatography applied to the separation of very complex samples. In this contribution, the requirement for chemometric tools is explained and the latest developments in approaches for (pre-)processing and analyzing data arising from one- and two-dimensional chromatography systems are reviewed. The final part of this review focuses on the application of chemometrics for method development and optimization.

Selective and comprehensive characterization of the quinochalcone C-glycoside homologs in Carthamus tinctorius L. by offline comprehensive two-dimensional liquid chromatography/LTQ-Orbitrap MS coupled with versatile data mining strategies

An offline 2D LC/LTQ-Orbitrap MS approach and versatile data mining techniques were developed to characterize new QCGs from C. tinctorius.