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

Comprehensive on-line two-dimensional liquid chromatography × supercritical fluid chromatography with trapping column-assisted modulation for depolymerised lignin analysis

Lignin depolymerisation produces a large variety of low molecular weight phenolic compounds that can be upgraded to value-added chemicals. Detailed analysis of these complex depolymerisation mixtures is, however, hampered by the lack of resolving power of traditional analysis techniques. In this study, a novel online comprehensive two-dimensional reversed-phase liquid chromatography (RPLC) × supercritical fluid chromatography (SFC) method with trapping column interface was developed for the separation of phenolic compounds in depolymerised lignin samples. The trapping capacities of different trapping columns were evaluated. The influence of large volume water-containing injection on SFC performance was studied. The relation between peak capacity and first dimension flow rate and gradient was investigated. The optimized method was applied for the analysis of a depolymerised lignin sample. The RPLC × SFC system exhibited high degree of orthogonality. Compared with traditional loop based interface, trapping column interface can significantly shorten the analysis time and offer higher detectability, with the disadvantage of more severe undersampling in the first dimension.

LC coupled to ESI, MALDI and ICP MS - A multiple hyphenation for metalloproteomic studies

A new multiple detection arrangement for liquid chromatography (LC) that supplements conventional electrospray ionization (ESI) mass spectrometry (MS) detection with two complementary detection techniques, matrix-assisted laser desorption/ionization (MALDI) MS and substrate-assisted laser desorption inductively coupled plasma (SALD ICP) MS has been developed. The combination of the molecular and elemental detectors in a single separation run is accomplished by utilizing a commercial MALDI target made of conductive plastic. The proposed platform provides a number of benefits in today's metalloproteomic applications, which are demonstrated by analysis of a metallothionein mixture. To maintain metallothionein complexes, separation is carried out at a neutral pH. The effluent is split; a major portion is directed to ESI MS while the remaining 1.8% fraction is deposited onto a plastic MALDI target. Dried droplets are overlaid with MALDI matrix and analysed consecutively by MALDI MS and SALD ICP MS. In the ESI MS spectra, the MT isoform complexes with metals and their stoichiometry are determined; the apoforms are revealed in the MALDI MS spectra. Quantitative determination of metallothionein isoforms is performed via determination of metals in the complexes of the individual protein isoforms using SALD ICP MS.

Comparison of multivariate curve resolution strategies in quantitative LCxLC: Application to the quantification of furanocoumarins in apiaceous vegetables

Comprehensive two-dimensional liquid chromatography (LC × LC) has been gaining popularity for the analysis of complex samples in a wide range of fields including metabolomics, environmental analysis, and food analysis. While LC × LC can provide greater chromatographic resolution than one-dimensional LC (1D-LC), overlapping peaks are often still present in separations of complex samples, a problem that can be alleviated by chemometric curve resolution techniques such as multivariate curve resolution-alternating least squares (MCR-ALS). MCR-ALS has also been previously shown to assist in the quantitative analysis of LC x LC data by isolating pure analyte signals from background signals which are often present at higher levels in LC x LC compared to 1D-LC. In this work we present the analysis of a dataset from the LC × LC analyses of parsley, parsnip and celery samples for the presence and concentrations of 14 furanocoumarins. Several MCR-ALS implementations are compared for the analysis of LC × LC data. These implementations include analyzing the LC x LC chromatogram alone, analyzing the one-dimensional chromatogram alone, as well as two hybrid approaches that make use of both the first and second dimension chromatograms. Furthermore, we compared manual integration of resolved chromatograms versus a simple summation approach, using the resolved chromatographic peaks in both cases. It is found that manual integration of the resolved LC × LC chromatograms provides the best quantification as measured by the consistency between replicate injections. If the summation approach is desired for automation, the choice of MCR-ALS implementation has a large effect on the precision of the analysis. Based on these results, the concentrations of the 14 furanocoumarins are determined in the three apiaceous vegetable types by analyzing the LC × LC chromatograms with MCR-ALS and manual integration for peak area determination. The concentrations of the analytes are found to vary greatly between samples, even within a single vegetable type.

Development of small-volume, microfluidic chaotic mixers for future application in two-dimensional liquid chromatography

We report a microfluidic chaotic micromixer with staggered herringbone grooves having a geometry optimized for fast mobile-phase modification at the interface of a two-dimensional liquid chromatography system.

Hydrophilic interaction and reversed phase mixed-mode liquid chromatography coupled to high resolution tandem mass spectrometry for polar lipids analysis

A hydrophilic interaction liquid chromatography (HILIC) fused-core column (150×2.1mm ID, 2.7μm particle size) and a short reversed-phase liquid chromatography (RPLC) column (20mm×2.1mm ID, 1.9μm) were serially coupled to perform mixed-mode chromatography (MMC) on complex mixtures of phospholipids (PL). Mobile phase composition and gradient elution program were, preliminarily, optimized using a mixture of phosphatidylcholines (PC), phosphatidylethanolamines (PE), their corresponding lyso-forms (LPC and LPE), and sphingomyelins (SM). Thus a mixture of PC extracted from soybean was characterized by MMC coupled to electrospray ionization (ESI) high-resolution Fourier-transform mass spectrometry (FTMS) using an orbital trap analyzer. Several previously undiscovered PC, including positional isomers (i.e. 16:0/19:1 and 19:1/16:0) of PC 35:1 and skeletal isomers (i.e. 18:1/18:2 and 18:0/18:3) of PC 36:3 were identified. Therefore, high-resolution MS/MS spectra unveiled the occurrence of isomers for several overall side chain compositions. The proposed MMC-ESI-FTMS/MS approach revealed an unprecedented capability in disclosing complexity of an actual lipid extract, thus representing a very promising approach to lipidomics.

Reducing Dilution and Analysis Time in Online Comprehensive Two-Dimensional Liquid Chromatography by Active Modulation

Online comprehensive two-dimensional liquid chromatography (LC × LC) offers ways to achieve high-performance separations in terms of peak capacity (exceeding 1000) and additional selectivity to realize applications that cannot be addressed with one-dimensional chromatography (1D-LC). However, the greater resolving power of LC × LC comes at the price of higher dilutions (thus, reduced sensitivity) and, often, long analysis times (>100 min). The need to preserve the separation attained in the first dimension ((1)D) causes greater dilution for LC × LC, in comparison with 1D-LC, and long analysis times to sample the (1)D with an adequate number of second dimension separations. A way to significantly reduce these downsides is to introduce a concentration step between the two chromatographic dimensions. In this work we present a possible active-modulation approach to concentrate the fractions of (1)D effluent. A typical LC × LC system is used with the addition of a dilution flow to decrease the strength of the (1)D effluent and a modulation unit that uses trap columns. The potential of this approach is demonstrated for the separation of tristyrylphenol ethoxylate phosphate surfactants, using a combination of hydrophilic interaction and reversed-phase liquid chromatography. The modified LC × LC system enabled us to halve the analysis time necessary to obtain a similar degree of separation efficiency with respect to UHPLC based LC × LC and of 5 times with respect to HPLC instrumentation (40 compared with 80 and 200 min, respectively), while at the same time reducing dilution (DF of 142, 299, and 1529, respectively) and solvent consumption per analysis (78, 120, and 800 mL, respectively).

A validated LC-MS/MS assay for quantification of 24(S)-hydroxycholesterol in plasma and cerebrospinal fluid

24(S)-hydroxycholesterol [24(S)-HC] is a cholesterol metabolite that is formed almost exclusively in the brain. The concentrations of 24(S)-HC in cerebrospinal fluid (CSF) and/or plasma might be a sensitive marker of altered cholesterol metabolism in the CNS. A highly sensitive 2D-LC-MS/MS assay was developed for the quantification of 24(S)-HC in human plasma and CSF. In the development of an assay for 24(S)-HC in CSF, significant nonspecific binding of 24(S)-HC was observed and resolved with the addition of 2.5% 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) into CSF samples. The sample preparation consists of liquid-liquid extraction with methyl-tert-butyl ether and derivatization with nicotinic acid. Good linearity was observed in a range from 1 to 200 ng/ml and from 0.025 to 5 ng/ml, for plasma and CSF, respectively. Acceptable precision and accuracy were obtained for concentrations over the calibration curve ranges. Stability of 24(S)-HC was reported under a variety of storage conditions. This method has been successfully applied to support a National Institutes of Health-sponsored clinical trial of HP-β-CD in Niemann-Pick type C1 patients, in which 24(S)-HC is used as a pharmacodynamic biomarker.

Metabolome analysis via comprehensive two-dimensional liquid chromatography: identification of modified nucleosides from RNA metabolism

Modified nucleosides derived from the RNA metabolism constitute an important chemical class, which are discussed as potential biomarkers in the detection of mammalian breast cancer. Not only the variability of modifications, but also the complexity of biological matrices such as urinary samples poses challenges in the analysis of modified nucleosides. In the present work, a comprehensive two-dimensional liquid chromatography mass spectrometry (2D-LC-MS) approach for the analysis of modified nucleosides in biological samples was established. For prepurification of urinary samples and cell culture supernatants, we performed a cis-diol specific affinity chromatography using boronate-derivatized polyacrylamide gel. In order to establish a 2D-LC method, we tested numerous column combinations and chromatographic conditions. In order to determine the target compounds, we coupled the 2D-LC setup to a triple quadrupole mass spectrometer performing full scans, neutral loss scans, and multiple reaction monitoring (MRM). The combination of a Zorbax Eclipse Plus C18 column with a Zorbax Bonus-RP column was found to deliver a high degree of orthogonality and adequate separation. By application of 2D-LC-MS approaches, we were able to detect 28 target compounds from RNA metabolism and crosslinked pathways in urinary samples and 26 target compounds in cell culture supernatants, respectively. This is the first demonstration of the applicability and benefit of 2D-LC-MS for the targeted metabolome analysis of modified nucleosides and compounds from crosslinked pathways in different biological matrices.

Multi-dimensional separations of polymers

Synthetic polymers and comprehensive two-dimensional liquid chromatography (LC × LC) are a synergistic combination. LC × LC provides unique insights in mutually dependent molecular distributions. Synthetic polymers offer clear demonstrations of the value of LC × LC.

Deconvolution of overlapping spectral polymer signals in size exclusion separation-diode array detection separations by implementing a multivariate curve resolution method optimized by alternating least square

Peaks eluting from a size exclusion separation (SEC) are often not completely baseline-separated due to the inherent dispersity of the polymer. Lowering the flow rate is sometimes a solution to obtain a better physical separation, but results in a longer retention time, which is often not desirable. The chemometrical deconvolution method discussed in this work provides the possibility of calculating the contribution of each peak separately in the total chromatogram of overlapping peaks. An in-house-developed MATLAB script differentiates between compounds based on their difference in UV-spectrum and retention time, using the entire 3D retention time UV-spectrum. Consequently, the output of the script offers the calculated chromatograms of the separate compounds as well as their respective UV-spectrum, of which the latter can be used for peak identification. This approach is of interest to quantitate contributions of different polymer types with overlapping UV-spectra and retention times, as is often the case in, for example, copolymer or polymer blend analysis. The applicability has been proven on mixtures of different polymer types: polystyrene, poly(methyl methacrylate) and poly(ethoxyethyl acrylate). This paper demonstrates that both qualitative and quantitative analyses are possible after deconvolution and that alternating concentrations of adjacent peaks do not significantly influence the obtained accuracy.

Combining liquid chromatography with multiplexed capillary gel electrophoresis for offline comprehensive analysis of complex oligonucleotide samples

Offline two dimensional liquid chromatography (LC)×capillary gel electrophoresis (CGE) and LC×(24) multiplexed-CGE methodologies were developed for the separation of oligonucleotides of therapeutic size. Both ion-pair chromatography (IPC) and ion-exchange chromatography (IEC) were studied as methods for the first dimension and single and multiplexed capillary electrophoresis methods in entangled polymer solutions were used for the second dimension separations. Electrokinetic and pressure injection were evaluated for the analysis of the collected LC fractions. The comprehensive separation was optimized with standard mixtures of poly adenosine, thymidine, cytosine and uracil homodeoxyoligonucleotides up to 35 bases long. Highly orthogonal methodologies and overall peak capacities of 6435 and 6993 for IPC×CGE and IEC×CGE, respectively, were obtained within a few hours analysis time.