Fast, comprehensive two-dimensional liquid chromatography

Guidelines for bioanalytical 2D chromatography method development and implementation

2D chromatography is a rapidly evolving, very powerful tool for bioanalysis. Advances in the theory of 2D separations, instrument technology and data analysis strategies continue to complement each other and advance the state of the art. Separations of complex mixtures of biomolecules yielding several hundred peaks in practical analysis times (tens of minutes to several hours) are relatively common. However, this level of performance largely remains the domain of expert researchers and several practical limitations stand in the way of more widespread use of 2D separations among practitioners. While off-the-shelf instruments are increasing in number, the most effective 2D instruments are often home-built, and analysis of the extremely rich datasets resulting from these separations continues to be a serious bottleneck in the overall workflow. This review summarizes some of the most serious challenges in method development and describes best practices to help guide users in designing effective 2D separations for bioanalysis.

Mass-spectrometry-based metabolomics: limitations and recommendations for future progress with particular focus on nutrition research

Mass spectrometry (MS) techniques, because of their sensitivity and selectivity, have become methods of choice to characterize the human metabolome and MS-based metabolomics is increasingly used to characterize the complex metabolic effects of nutrients or foods. However progress is still hampered by many unsolved problems and most notably the lack of well established and standardized methods or procedures, and the difficulties still met in the identification of the metabolites influenced by a given nutritional intervention. The purpose of this paper is to review the main obstacles limiting progress and to make recommendations to overcome them. Propositions are made to improve the mode of collection and preparation of biological samples, the coverage and quality of mass spectrometry analyses, the extraction and exploitation of the raw data, the identification of the metabolites and the biological interpretation of the results.

A visual approach to stationary phase selectivity classification based on the Snyder-Dolan Hydrophobic-Subtraction Model

A novel type of stationary phase selectivity classification "triangle" has been developed based on the Snyder-Dolan (S-D) Hydrophobic-Subtraction Model, wherein the apices of a set of four triangles represent the relative contributions of steric hindrance (chi(S)), hydrogen-bonding acidity (chi(A)), hydrogen-bonding basicity (chi(B)), cation-exchange capacity (chi(C)) to selectivity. We found that "effective selectivity" of a stationary phase is mathematically given by the ratio of system dependent interaction coefficients but not their absolute values. Thus by normalizing the S*, A, B and C terms of the S-D model by H, we were able to obtain four parameters which fully define the chromatographic selectivity of the stationary phases. By examining the parameters in groups of three, we can represent all the result in a set of four "selectivity triangles". The distinctive feature of this approach compared to the S-D phase classification scheme is that it allows the visualization of column selectivity by plotting three-dimensional data in a two-dimensional space. Moreover, it very clearly shows that the RPLC columns thus far characterized cover only a small fraction of separation selectivity space leaving a great deal of room for researchers to develop novel RPC materials. Various applications of these "selectivity triangles" will be discussed in this paper.

Ultraperformance liquid chromatography with electrospray ionization ion trap mass spectrometry for chondroitin disaccharide analysis

Chondroitin sulfate (CS) has an important role in cell division, in the central nervous system, and in joint-related pathologies such as osteoarthritis. Due to the complex chemical structure and biological importance of CS, simple, sensitive, high resolution, and robust analytical methods are needed for the analysis of CS disaccharides and oligosaccharides. An ion-pairing, reversed-phase, ultraperformance liquid chromatography (IPRP-UPLC) separation, coupled to electrospray ionization mass spectrometry with an ion trap mass analyzer, was applied for the analyses of CS-derived disaccharides. UPLC separation technology uses small particle diameter, short column length, and elevated column temperature to obtain high resolution and sensitivity. Hexylamine (15 mM) was selected as the optimal ion-pairing reagent.

Application of Snyder-Dolan classification scheme to the selection of "orthogonal" columns for fast screening of illicit drugs and impurity profiling of pharmaceuticals--I. Isocratic elution

Fourteen judiciously selected reversed phase columns were tested with 18 cationic drug solutes under the isocratic elution conditions advised in the Snyder-Dolan (S-D) hydrophobic subtraction method of column classification. The standard errors (S.E.) of the least squares regressions of logk' vs. logk'(REF) were obtained for a given column against a reference column and used to compare and classify columns based on their selectivity. The results are consistent with those obtained with a study of the 16 test solutes recommended by Snyder and Dolan. To the extent these drugs are representative, these results show that the S-D classification scheme is also generally applicable to pharmaceuticals under isocratic conditions. That is, those columns judged to be similar based on the 16 S-D solutes were similar based on the 18 drugs; furthermore those columns judged to have significantly different selectivities based on the 16 S-D probes appeared to be quite different for the drugs as well. Given that the S-D method has been used to classify more than 400 different types of reversed phases the extension to cationic drugs is a significant finding.

Quantitative characterization of solid epoxy resins using comprehensive two dimensional liquid chromatography coupled with electrospray ionization-time of flight mass spectrometry

A comprehensive multidimensional liquid chromatography system coupled to Electrospray Ionization-Mass Spectrometry (LCxLC-ESI-MS) was developed for detailed characterization and quantitation of solid epoxy resin components. The two orthogonal modes of separation selected were size exclusion chromatography (SEC) in the first dimension and liquid chromatography at critical conditions (LCCC) in the second dimension. Different components present in the solid epoxy resins were separated and quantitated for the first time based on the functional groups and molecular weight heterogeneity. Coupling LCxLC separations with mass spectrometry enabled the identification of components resolved in the two-dimensional space. Several different functional group families of compounds were separated and identified, including epoxy-epoxy and epoxy-alpha-glycol functional oligomers, and their individual molecular weight ranges were determined. Repeatability obtained ranged from 0.5% for the main product to 21% for oligomers at the 0.4% concentration level.

Metabolomics in evaluation of glucose disorders

PURPOSE OF REVIEW

Recent advances in metabolomic tools now permit to characterize dysregulated metabolic pathways in various diseases associated with the identification of sensitive and specific early responding biomarkers that are critical both for the diagnosis of the type of insult as well as for the selection and evaluation of therapy.

RECENT FINDINGS

This short review describes progresses made in analytical science and their applications in the field of glucose disorders. Recent studies focused mainly on type 2 diabetes both in human and animal models in order to validate early biomarkers and effects of drugs on disease progression. The potential of using the metabolomic approach was also demonstrated for diagnosing diabetic complications such as diabetic nephropathy.

SUMMARY

In addition to its application in the discovery of disease biomarkers, metabolomics can contribute to the elucidation of pathophysiological mechanisms.

Off-line comprehensive 2-dimensional hydrophilic interaction x reversed phase liquid chromatography analysis of procyanidins

The development of an off-line comprehensive 2-dimensional liquid chromatography (2-D-LC) method for the analysis of procyanidins is reported. In the first dimension, oligomeric procyanidins were separated according to molecular weight by hydrophilic interaction chromatography (HILIC), while reversed phase LC was employed in the second dimension to separate oligomers based on hydrophobicity. Fluorescence, UV and electrospray ionisation mass spectrometry (ESI-MS) were employed for identification purposes. The combination of these orthogonal separation methods is shown to represent a significant improvement compared to 1-dimensional methods for the analysis of complex high molecular weight procyanidin fractions, by simultaneously providing isomeric and molecular weight information. The low correlation (r(2)<0.2100) between the two LC modes afforded a practical peak capacity in excess of 2300 for the optimal off-line method. The applicability of the method is demonstrated for the analysis of phenolic extracts of apple and cocoa.

Optimal gradient operation in comprehensive liquid chromatography x liquid chromatography systems with limited orthogonality

A novel strategy is described for designing optimal second dimension (2D) gradient conditions for a comprehensive two-dimensional liquid chromatography system where the two dimensions are not fully orthogonal. Using the approach developed here, the initial and final organic modifier content values resulting in the highest coverage of separation space can be derived for each 2D gradient run. Theory indicates that these values can be determined by adapting 2D gradient operation to the degree of orthogonality. The new method is tested on a comprehensive two-dimensional liquid chromatography system that uses reversed phase (RP) columns showing different selectivities in the two dimensions. A comparison between analyses carried out using normal and optimized 2D gradients showed that the latter allow a more efficient use of analysis time. This can result either in an improved peak capacity or in decreasing total analysis time, depending on the final goal of the experiment. In the latter scenario, the number of separated peaks is comparable to that obtained using gradients spanning a wide range of organic modifier but, now, in half the time. As test samples complex mixtures of peptides were analyzed.

Equation for peak capacity estimation in two-dimensional liquid chromatography

Two-dimensional liquid chromatography (2DLC) is a very powerful way to greatly increase the resolving power and overall peak capacity of liquid chromatography. The traditional "product rule" for peak capacity usually overestimates the true resolving power due to neglect of the often quite severe under-sampling effect and thus provides poor guidance for optimizing the separation and biases comparisons to optimized one-dimensional gradient liquid chromatography. Here we derive a simple yet accurate equation for the effective two-dimensional peak capacity that incorporates a correction for under-sampling of the first dimension. The results show that not only is the speed of the second dimension separation important for reducing the overall analysis time, but it plays a vital role in determining the overall peak capacity when the first dimension is under-sampled. A surprising subsidiary finding is that for relatively short 2DLC separations (much less than a couple of hours), the first dimension peak capacity is far less important than is commonly believed and need not be highly optimized, for example, through use of long columns or very small particles.

Optimization strategies for off-line two-dimensional liquid chromatography

A step by step strategy of optimization of comprehensive off-line two-dimensional liquid chromatography (2D-LC) separations is proposed. The goal of an optimization process in the separation sciences is either to achieve a given resolution (a target peak capacity in 2D-LC) within as short a time as possible or to reach the highest possible resolution in a given analysis time. The proposed method takes into account the characteristics of the columns used in the first and the second dimension and the number of fractions of the first dimension eluent that should be collected. The effect of the time spent during the analysis on the second dimension column to carry out necessary tasks that are not the separation itself (called the additional time) on the maximum peak capacity that is achievable was carefully investigated. It was shown that (1) an increase in the peak capacity of the first dimension column combined with the collection of larger volume fractions permits a significant reduction of the time needed to achieve the desired peak capacity; and (2) there is an optimum fraction collection ratio (or number of collected fractions per peak) which yields the target peak capacity in the minimum time. The proposed strategy was used for the optimization of the separation of samples of BSA tryptic digest by an off-line 2D-LC using an SCXmultiply sign in circleRP-HPLC method. As a result of this optimization, a peak capacity of 4000 could be achieved in about 5h with the two columns available. The time needed for the optimized analysis was less than two thirds of the analysis time that would have been needed had the conventional rule of thumb of sample collection in comprehensive on-line 2D-LC (4 samples/peak) been followed.

Approaches to comprehensive multidimensional liquid chromatography systems

This work compares the performance of the three different schemes implementing two-dimensional liquid chromatography (2D-LC) in terms of the peak capacity that they can generate and of the time that they need to complete a two-dimensional analysis. We discuss in detail how time is spent in these two-dimensional liquid chromatography x liquid chromatography (LC x LC) schemes and how to compare them. Keeping constant the characteristics of the first-dimension separation, we systematically varied those of the second-dimension separation and of its coupling to the first-dimension. In the process, five systems were created, based on the principles of the three known implementations of comprehensive 2D-LC. This work demonstrates an original method for the selection of the best comprehensive 2D-LC approach, depending on the desired peak capacity and on time constraints. The decision to use a 2D-LC method arises from the need to achieve a given resolution (i.e., a target peak capacity) within as short a time as possible or to reach the highest possible resolution in a given analysis time. Using the most appropriate schemes, we suggest how it is realistically possible to generate peak capacities ranging from 266 in just over 20 min or about 2800 in 2.3 h. When the time available for a two-dimensional separation is very short and the desired peak capacity cannot be achieved in 1D-LC, an on-line 2D-LC approach is unquestionably best. However, if a longer analysis time is acceptable, a 10-fold increase in the peak capacity can be obtained at the cost of a mere 7-fold increase in total analysis time.

Smart templates for peak pattern matching with comprehensive two-dimensional liquid chromatography

Comprehensive two-dimensional liquid chromatography (LCxLC) generates information-rich but complex peak patterns that require automated processing for rapid chemical identification and classification. This paper describes a powerful approach and specific methods for peak pattern matching to identify and classify constituent peaks in data from LCxLC and other multidimensional chemical separations. The approach records a prototypical pattern of peaks with retention times and associated metadata, such as chemical identities and classes, in a template. Then, the template pattern is matched to the detected peaks in subsequent data and the metadata are copied from the template to identify and classify the matched peaks. Smart Templates employ rule-based constraints (e.g., multispectral matching) to increase matching accuracy. Experimental results demonstrate Smart Templates, with the combination of retention-time pattern matching and multispectral constraints, are accurate and robust with respect to changes in peak patterns associated with variable chromatographic conditions.

Stationary phases for hydrophilic interaction chromatography, their characterization and implementation into multidimensional chromatography concepts

Hydrophilic interaction chromatography (HILIC) is becoming increasingly popular for separation of polar samples on polar columns in aqueous-organic mobile phases rich in organic solvents (usually ACN). Silica gel with decreased surface concentration of silanol groups, or with chemically bonded amino-, amido-, cyano-, carbamate-, diol-, polyol-, or zwitterionic sulfobetaine ligands are used as the stationary phases for HILIC separations, in addition to the original poly(2-sulphoethyl aspartamide) strong cation-exchange HILIC material. The type of the stationary and the composition of the mobile phase play important roles in the mixed-mode HILIC retention mechanism and can be flexibly tuned to suit specific separation problems. Because of excellent mobile phase compatibility and complementary selectivity to RP chromatography, HILIC is ideally suited for highly orthogonal 2-D LC-LC separations of complex samples containing polar compounds, such as peptides, proteins, oligosaccharides, drugs, metabolites and natural compounds. This review attempts to present an overview of the HILIC separation systems, possibilities for their characterization and emerging HILIC applications in 2-D off-line and on-line LC-LC separations of various samples, in combination with RP and other separation modes.