Optimization of the separation performance of fluid-based separation techniques, and especially of liquid chromatography (LC), is essential for analysis of the complex samples encountered in today’s research (omics, food, environmental, natural product, etc. samples).
Around a decade ago, tremendous improvements were made in LC column technology (sub-2 µm porous particles, core-shell particles) and in instrumentation (pressures up to 1200 bar) opening new possibilities in terms of speed and resolution to LC practitioners for tackling complex samples. A now well-accepted metric of separation power in gradient elution, mandatory for samples of high complexity, is the peak capacity (nc). nc is the maximum number of peaks that can fit side-by-side between the first and last peak of interest with resolution 1. With state-of-the-art LC, peak capacities of ca. 600 (in ca. 1 h) in conventional one-dimensional LC (1D-LC) can routinely be obtained.
It is, however, wishful thinking that such peak capacities are sufficient to separate very complex mixtures. Peak capacity should significantly outstrip the number of components in a sample; the statistical theory of peak overlap (developed by Giddings) indicates to resolve 98% of randomly distributed sample components, nc should exceed the number of components by a factor 100 ! This means that a nc value of 10.000 corresponding to ca. 1x108 theoretical is needed to “chromatographically” resolve a sample containing 100 components.
One straightforward approach to increasing nc is multidimensional LC, especially two-dimensional LC or 2D-LC. In 2D-LC the sample is subjected to two different separation mechanisms and this in an off-line or on-line way. On-line 2D-LC is divided in heart-cutting LC (LC-LC), multiple heart-cutting or parking LC (mLC-LC) and comprehensive LC (LC×LC). LC-LC and mLC-LC are applied to better resolve components in a selected retention time window or windows, while in LC×LC, the entire sample is subjected to two separation mechanisms.
The recent developments in 2D-LC hardware design, software approaches and column configurations will be presented. The figures of merit of LCxLC will be discussed and illustrated with analysis of several complex matrices including some of QA/QC.
Frantisek Svec, Xin Wang, Adeela Saeed, Fernando Maya, Alexandros Lamprou, Hongxia Wang
Typically, we controlled chemistry and through it selectivity of our monolithic columns by using: (i) direct copolymerization of functional monomers, (ii) preparation of parent monolith with reactive functionalities and its post-polymerization functionalization, (iii) thermally or photoinitiated grafting of functional monomers on the pore surface, and (iv) attachment of gold and silver nanoparticles to the pore surface followed by reaction with functional thiols. Recently, a new approach was introduced that enables functionalization of pore surface of monoliths and involves application of metal-organic frameworks (MOFs). These frameworks are compounds consisting of metal ions or clusters coordinated to rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. We used two implementations: (i) admixing preformed MOF to the polymerization mixture followed by the thermally initiated free radical polymerization and (ii) forming the MOF within the pores applying layer-by-layer approach. The former technique will be demonstrated with the preparation of monolithic column designed for enantioseparation while the latter led to column for selective preconcentration of phosphopetides and to column for the separation of native fatty acids according to their length and degree of unsaturation.
This presentation provides an overview of the use of the solvation parameter model for characterizing the separation properties of chromatographic and liquid-liquid distribution systems from a knowledge of the contribution of intermolecular interactions to two-phase distributions. The same process used for chromatographic systems can be applied to study the fate, transport and distribution of compounds in environmental systems providing a bridge between the two processes. Although we have developed tools for screening chromatographic databases to find models that emulate environmental processes this has proven to be equivalent to searching for a needle in a haystack. An alternative, and more successful approach, has been to build models of the environmental process directly, allowing the prediction of properties for other compounds with known descriptor values. The central role chromatographic an liquid-liquid partition methods occupy in the experimental determination of the six descriptors used in the solvation parameter model will described and illustrated for recent studies of polycyclic aromatic compounds, plasticizers, and organosiloxanes of environmental interest [1, 2]. The general approach is shown to be suitable for devising conditions for analysis (sample preparation and chromatographic separations), and the prediction of environmental properties without additional experimental effort.
(1) C. F. Poole, T. C. Ariyasena and N. Lenca. Estimation of the Environmental Properties of Compounds from Chromatographic Measurements and the Solvation Parameter Model. J. Chromatogr. A 1317 (2013) 85-104.
(2) T.C. Ariyasena and C. F. Poole. Determination of Descriptors for Polycyclic Aromatic Hydrocarbons and Related Compounds by Chromatographic Methods and Liquid-Liquid Partition in Totally Organic Biphasic Systems. J. Chromatogr. A 1361 (2014) 240-254.