Comprehensive liquid chromatography: Fundamental aspects and practical considerations—A review

Analysis of circulating lipid-phase micronutrients in humans by HPLC: review and overview of new developments

Retinol, tocopherols, coenzyme Q10, carotenoids, and vitamin D are lipophilic compounds shown to function as important health-protective agents by mitigating the damaging effects of oxidative and other injury. Scientific interest in evaluating these compounds has resurfaced in recent years, particularly in the nutritional, clinical and epidemiologic fields, and has precipitated the development of a multitude of new analytical techniques. This review considers recent developments in HPLC-based assays since 2007 for the simultaneous determination of these lipid-phase compounds utilizing exclusively serum or plasma as these matrices are mostly used in clinical and epidemiological investigations. We also provide an overview of blood measurements for selected carotenoids, tocopherols, coenzyme Q10 and retinol from the last 15years of healthy umbilical cord blood, children, and adults.

Direct bioanalytical sample injection with 2D LC–MS

New analytical platforms have been developed in response to the need for attaining increased peak capacity for multicomponent complex analysis with higher sensitivity and characterization of the analytes, and high-throughput capabilities. This review outlines the fundamental principles of target and comprehensive 2D LC method development and encompasses applications of LC–LC and LC × LC coupled to MS in bioanalysis using a variety of online analytical procedures. It also provides a rationale for the usage of the most employed mass analyzers and ionization sources on these platforms.

High electric field strength two-dimensional peptide separations using a microfluidic device

New instrumentation has been developed to improve the resolution, efficiency, and speed of microfluidic 2D separations using MEKC coupled to high field strength CE. Previously published 2D separation instrumentation [Ramsey, J. D. et al., Anal. Chem. 2003, 75, 3758-3764] from our group was limited to a maximum potential difference of 8.4 kV, resulting in an electric field strength of only approximately 200 V/cm in the first dimension. The circuit described in this report has been designed to couple a higher voltage supply with a rapidly switching, lower voltage supply to utilize the best features of each. Voltages applied in excess of 20 kV lead to high electric field strength separations in both dimensions, increasing the separation resolution, efficiency, and peak capacity while reducing the required analysis time. Detection rates as high as six peptides per second (based on total analysis time) were observed for a model protein tryptic digest separation. Additionally, higher applied voltages used in conjunction with microfluidic chips with longer length channels maintained higher electric field strengths and produced peak capacities of over 4000 for some separations. Total separation time in these longer channel devices was comparable to that obtained in short channels at low field strength; however, resolving power improved approximately threefold.

Mass spectrometry detection in comprehensive liquid chromatography: basic concepts, instrumental aspects, applications and trends

The review, as can be deduced from the title, focuses on both theoretical and practical aspects of the use of mass spectrometry as a third, added dimension to a comprehensive LC (LC × LC) system, generating the most powerful analytical tool today for non-volatile analytes. The first part deals with the technical requirements for linkage of an LC × LC system to an MS one, including the choice of the mobile phase (buffer and salts), flow rate (splitting), type of ionization (interface); advantages and disadvantages of off-line and on-line methods are discussed, as well. A discussion of the various aspects of instrumentation is provided, both from a chromatographic and mass spectrometry standpoint, with particular emphasis directed to the choice of column sets, spatial resolution, mass resolving power, mass accuracy, and tandem-MS capabilities. The extent to which mass spectrometry may be of aid in unraveling column-outlet multicompound bands is highlighted, along with its effectiveness as a chromatographic detector of excellent sensitivity, universality yet with potential in terms of selectivity and amenability to quantitative analysis over a wide dynamic range. The following section of the review contains significant applications of comprehensive two-dimensional LC coupled to MS in different areas of research, with details on interfaces, column stationary phases, modulation and MS parameters. It is not the intention of the authors to provide a comprehensive description of the techniques, but merely to discuss only those aspects which are essential for successful applications of the LC-MS combination. The reader will be acquainted with the enormous potential of this hyphenated technique, and the factors and instrumental developments that have concurred to make it emerge to a central role in specialized fields, such as proteomics.

Mass Spectrometry-based Proteomics and Peptidomics for Systems Biology and Biomarker Discovery

The scientific community has shown great interest in the field of mass spectrometry-based proteomics and peptidomics for its applications in biology. Proteomics technologies have evolved to produce large datasets of proteins or peptides involved in various biological and disease progression processes producing testable hypothesis for complex biological questions. This review provides an introduction and insight to relevant topics in proteomics and peptidomics including biological material selection, sample preparation, separation techniques, peptide fragmentation, post-translation modifications, quantification, bioinformatics, and biomarker discovery and validation. In addition, current literature and remaining challenges and emerging technologies for proteomics and peptidomics are presented.

Resolving the chemical heterogeneity of natural organic matter: new insights from comprehensive two-dimensional liquid chromatography

For the purpose of resolving the chemical heterogeneity of natural organic matter (NOM), comprehensive two-dimensional liquid chromatography (LC×LC) was employed for the first time to map the hydrophobicity versus molecular weight (MW) distribution of two well-known complex organic mixtures: Suwannee River Fulvic Acids (SR-FA) and Pony Lake Fulvic Acids (PL-FA). Two methods have been developed using either a conventional reversed-phase (RP) silica column or a mixed-mode hydrophilic interaction column operating under aqueous RP mode in the first dimension, and a size-exclusion column in the second dimension. The LC×LC fractions were screened on-line by UV at 254 nm, molecular fluorescence at excitation/emission wavelengths (λ(Exc)/λ(Em)) of 240/450 nm, and by evaporative light scattering. The MW distributions of these two NOM samples were further characterized by number (Mn) and weight (Mw) average MW, and by polydispersity (Mw/Mn). Findings suggest that the combination of two independent separation mechanisms is promising in extend the range of NOM separation. For the cases where NOM separation was accomplished, smaller Mw group fractions seem to be related to a more hydrophobic nature. Regardless of the detection method, the complete range of MW distribution provided by both comprehensive LC×LC methods was found to be lower than those reported in the literature.

Comprehensive two-dimensional liquid chromatography — practical impacts of theoretical considerations. A review

A theory of comprehensive two-dimensional separations by liquid chromatographic techniques is overviewed. It includes heart-cutting and comprehensive two-dimensional separation modes, with attention to basic concepts of two-dimensional separations: resolution, peak capacity, efficiency, orthogonality and selectivity. Particular attention is paid to the effects of sample structure on the retention and advantages of a multi-dimensional HPLC for separation of complex samples according to structural correlations. Optimization of 2D separation systems, including correct selection of columns, flow-rate, fraction volumes and mobile phase, is discussed. Benefits of simultaneous programmed elution in both dimensions of LCxLC comprehensive separations are shown.

Experimental setup, modulation of the fraction collection and transfer from the first to the second dimension, compatibility of mobile phases in comprehensive LCxLC, 2D asymmetry and shifts in retention under changing second-dimension elution conditions, are addressed. Illustrative practical examples of comprehensive LCxLC separations are shown.

An overview of recent advances in HPLC instrumentation

This review highlights the fundamentals and the most prominent advances in the field of HPLC instrumentation over the last decades. Fundamental aspects and practical considerations of column switching, conventional (heart-cut) and comprehensive two-dimensional LC are presented. Different aspects of microcolumn- and nanoliquid-chromatography are reviewed. Recent progress in column technology and the demands and developments in instrumentation and accessories for miniaturized LC are also discussed. In the field of miniaturization, particularly in chip-based nano-LC systems, some aspects on micro-fluidic chip fabrication, using particle-packed HPLC microchips or polymer-based monoliths, are addressed. An introduction to ultra performance LC (UPLC) is also presented.

Recent advances on multidimensional liquid chromatography-mass spectrometry for proteomics: from qualitative to quantitative analysis--a review

With the acceleration of proteome research, increasing attention has been paid to multidimensional liquid chromatography-mass spectrometry (MDLC-MS) due to its high peak capacity and separation efficiency. Recently, many efforts have been put to improve MDLC-based strategies including "top-down" and "bottom-up" to enable highly sensitive qualitative and quantitative analysis of proteins, as well as accelerate the whole analytical procedure. Integrated platforms with combination of sample pretreatment, multidimensional separations and identification were also developed to achieve high throughput and sensitive detection of proteomes, facilitating highly accurate and reproducible quantification. This review summarized the recent advances of such techniques and their applications in qualitative and quantitative analysis of proteomes.

Hydrophilic interaction liquid chromatography (HILIC)--a powerful separation technique

Hydrophilic interaction liquid chromatography (HILIC) provides an alternative approach to effectively separate small polar compounds on polar stationary phases. The purpose of this work was to review the options for the characterization of HILIC stationary phases and their applications for separations of polar compounds in complex matrices. The characteristics of the hydrophilic stationary phase may affect and in some cases limit the choices of mobile phase composition, ion strength or buffer pH value available, since mechanisms other than hydrophilic partitioning could potentially occur. Enhancing our understanding of retention behavior in HILIC increases the scope of possible applications of liquid chromatography. One interesting option may also be to use HILIC in orthogonal and/or two-dimensional separations. Bioapplications of HILIC systems are also presented.

Sample preparation techniques for the untargeted LC-MS-based discovery of peptides in complex biological matrices

Although big progress has been made in sample pretreatment over the last years, there are still considerable limitations when it comes to overcoming complexity and dynamic range problems associated with peptide analyses from biological matrices. Being the little brother of proteomics, peptidomics is a relatively new field of research aiming at the direct analysis of the small proteins, called peptides, many of which are not amenable for typical trypsin-based analytics. In this paper, we present an overview of different techniques and methods currently used for reducing a sample's complexity and for concentrating low abundant compounds to enable successful peptidome analysis. We focus on techniques which can be employed prior to liquid chromatography coupled to mass spectrometry for peptide detection and identification and indicate their advantages as well as their shortcomings when it comes to the untargeted analysis of native peptides from complex biological matrices.

Comprehensive hydrophilic interaction and ion-pair reversed-phase liquid chromatography for analysis of di- to deca-oligonucleotides

A comprehensive two-dimensional HPLC approach with a high degree of orthogonality was developed for analysis of di- to deca-oligonucleotides (ONs). Hydrophilic interaction liquid chromatography (HILIC) was used in the first dimension, and ion-pair reversed-phase liquid chromatography (IP-RPLC) was employed in the second dimension. The two dimensions were connected via a ten-port valve interface equipped with octadecyl silica (ODS) traps to immobilize and focus the ONs eluting from the first dimension prior to IP-RPLC separation. An aqueous make-up flow was used for effective trapping. The comprehensive two-dimensional HPLC system was optimized with a mixture consisting of 27 oligonucleotide standards. An overall chromatographic peak capacity of 500 was obtained. The use of the volatile buffer triethylamine acetate in the second dimension allowed straightforward coupling to electrospray ionization mass spectrometry (ESI-MS) and detection of each ON in the negative ionization mode.

Separation and identification of flavonoids from complex samples using off-line two-dimensional liquid chromatography tandem mass spectrometry

In this study, a LC-based identification strategy was proposed and off-line two-dimensional liquid chromatography was developed for the separation and identification of flavonoids from complex samples. The highly orthogonal separation system was composed of a CD column, an OEG column and an XTerra C(18) column. The CD column was employed for the first dimensional separation and the OEG column and XTerra C(18) column were used for the second dimensional separation in parallel. A mixture of six traditional Chinese medicines was used as a complex sample in testing this method. Unknown peaks were identified by matching their retention times and accurate mass molecular weights with those of references. The identification efficacy of parallel unidimensional liquid chromatography and off-line two-dimensional liquid chromatography was compared, and the latter was demonstrated to be more efficient for the identification of target compounds-especially co-eluted compounds or minor compounds from complex samples.

Investigation of interpolation techniques for the reconstruction of the first dimension of comprehensive two-dimensional liquid chromatography-diode array detector data

Simulated and experimental data were used to measure the effectiveness of common interpolation techniques during chromatographic alignment of comprehensive two-dimensional liquid chromatography-diode array detector (LC×LC-DAD) data. Interpolation was used to generate a sufficient number of data points in the sampled first chromatographic dimension to allow for alignment of retention times from different injections. Five different interpolation methods, linear interpolation followed by cross correlation, piecewise cubic Hermite interpolating polynomial, cubic spline, Fourier zero-filling, and Gaussian fitting, were investigated. The fully aligned chromatograms, in both the first and second chromatographic dimensions, were analyzed by parallel factor analysis to determine the relative area for each peak in each injection. A calibration curve was generated for the simulated data set. The standard error of prediction and percent relative standard deviation were calculated for the simulated peak for each technique. The Gaussian fitting interpolation technique resulted in the lowest standard error of prediction and average relative standard deviation for the simulated data. However, upon applying the interpolation techniques to the experimental data, most of the interpolation methods were not found to produce statistically different relative peak areas from each other. While most of the techniques were not statistically different, the performance was improved relative to the PARAFAC results obtained when analyzing the unaligned data.