Optimizing separations in online comprehensive two-dimensional liquid chromatography

Online comprehensive two-dimensional liquid chromatography has become an attractive option for the analysis of complex nonvolatile samples found in various fields (e.g. environmental studies, food, life, and polymer sciences). Two-dimensional liquid chromatography complements the highly popular hyphenated systems that combine liquid chromatography with mass spectrometry. Two-dimensional liquid chromatography is also applied to the analysis of samples that are not compatible with mass spectrometry (e.g. high-molecular-weight polymers), providing important information on the distribution of the sample components along chemical dimensions (molecular weight, charge, lipophilicity, stereochemistry, etc.). Also, in comparison with conventional one-dimensional liquid chromatography, two-dimensional liquid chromatography provides a greater separation power (peak capacity). Because of the additional selectivity and higher peak capacity, the combination of two-dimensional liquid chromatography with mass spectrometry allows for simpler mixtures of compounds to be introduced in the ion source at any given time, improving quantitative analysis by reducing matrix effects. In this review, we summarize the rationale and principles of two-dimensional liquid chromatography experiments, describe advantages and disadvantages of combining different selectivities and discuss strategies to improve the quality of two-dimensional liquid chromatography separations.

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.

Quantification in comprehensive two-dimensional liquid chromatography

Although the use of comprehensive two-dimensional liquid chromatography (LCxLC) as a powerful separation technique is continuously increasing, its employment in quantification experiments is rather limited. The present research is focused on the quantification of a series of standards, as well as real-world sample compounds, by using dedicated laboratory-constructed LCxLC software, developed through a novel approach. Moreover, the difficulties encountered during software operation, in various elution conditions, are described and discussed. The results attained were compared with those observed in conventional LC, and no statistically significant differences were observed in the determination of aurapten in grapefruit oil. However, a loss in sensitivity was observed when using LCxLC (limit of detection = 0.10 ppm) compared to conventional LC (limit of detection = 0.05 ppm) as a consequence of the sample dilution in comprehensive two-dimensional liquid chromatography.

Comprehensive multidimensional liquid chromatography: Theory and applications

Comprehensive two-dimensional (2D) liquid chromatographic (LC x LC) techniques can be considered innovative methods only recently developed and adopted in many configurations. The revolutionary aspect of comprehensive two-dimensional techniques, with respect to classical multidimensional (MD) chromatography, is that the entire sample is subjected to the 2D advantage. The major benefit is that the separation capacities of each dimension are multiplied, offering a high peak capacity to resolve samples of great complexity. The first part of the present review briefly describes the theoretical and practical aspects related to the development of a multidimensional comprehensive liquid chromatographic method. Applicational experiences in comprehensive liquid chromatography are then described, divided into four groups, according to the HPLC modes used in the two dimensions and to the nature of the samples analyzed.

Two-dimensional liquid chromatography normal-phase and reversed-phase separation of (co)oligomers

Many samples contain compounds with various numbers of two or more regular structural groups. Such "multidimensional" samples (according to the Giddings' notation) are best separated in orthogonal chromatographic systems with different selectivities for the individual repeat structural groups, described by separation factors. Correlations between the repeat group selectivities characterize the degree of orthogonality and suitability of chromatographic systems for two-dimensional (2D) separations of two-dimensional samples. The range of the structural units in that can be resolved in a given time can be predicted on the basis of a model describing the repeat group selectivity in the first- and second-dimension systems. Two-dimensional liquid chromatographic system combining reversed-phase (RP) mode in the first dimension and normal-phase (NP) mode in the second dimension were studied with respect to the possibilities of in-line fraction transfer between the two modes. Hydrophilic interaction liquid chromatography (HILIC) with an aminopropyl silica column (APS) is more resistant than classical non-aqueous NP systems against adsorbent desactivation with aqueous solvents transferred in the fractions from the first, RP dimension to the second dimension. Hence, HILIC is useful as a second-dimension separation system for comprehensive RP-NP LCxLC. A comprehensive 2D RP-NP HPLC method was developed for comprehensive 2D separation of ethylene oxide-propylene oxide (EO-PO) (co)oligomers. The first-dimension RP system employed a 120 min gradient of acetonitrile in water on a C18 microbore column at the flow-rate of 10 microL/min. In the second dimension, isocratic HILIC NP with ethanol-dichloromethane-water mobile phase on an aminopropyl silica column at 0.5 mL/min was used. Ten microliter fractions were transferred from the RP to the HILIC NP system at 1 min switching valve cycle frequency.