Stationary-phase-assisted modulation is used to overcome one of the limitations of contemporary comprehensive two-dimensional liquid chromatography, which arises from the combination of a first-dimension column that is typically narrow and long and a second-dimension column that is wide and short. Shallow gradients at low flow rates are applied in the first dimension, whereas fast analyses (at high flow rates) are required in the second dimension. Limitations of this approach include a low sample capacity of the first-dimension column and a high dilution of the sample in the complete system. Moreover, the relatively high flow rates used for the second dimension make direct (splitless) hyphenation to mass spectrometry difficult. In the present study we demonstrate that stationary-phase-assisted modulation can be implemented in an online comprehensive two-dimensional LC (LC × LC) setup to shift this paradigm. The proposed active modulation makes it possible to choose virtually any combination of first- and second-dimension column diameters without loss in system performance. In the current setup, a 0.30 mm internal diameter first-dimension column with a relatively high loadability is coupled to a 0.075 mm internal diameter second-dimension column. This actively modulated system is coupled to a nanoelectrospray high-resolution mass spectrometer and applied for the separation of the tryptic peptides of a six-protein mixture and for the proteome-wide analyses of yeast from Saccharomyces cerevisiae. In the latter application, about 20000 MS/MS spectra are generated within 24 h analysis time, resulting in the identification of 701 proteins.

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Post-polymerization photografting is a versatile tool to alter the surface chemistry of organic-based monoliths so as to obtain desired stationary phase properties. In this study, 2-acrylamido-2-methyl-1-propanesulfonic acid was grafted to a hydrophobic poly(butyl methacrylate-co-ethylene glycol dimethacrylate) monolith to create a strong cation exchange stationary phase. Both single-step and two-step photografting were addressed, and the effects of grafting conditions were assessed. An experimental design has been applied in an attempt to optimize three of the key parameters of the two-step photografting chemistry, i.e. the grafting time of the initiator, the monomer concentration and the monomer irradiation time. The photografted columns were implemented in a comprehensive two-dimensional column liquid chromatography (tLC × tLC) workflow and applied for the separation of intact proteins and peptides. A baseline separation of 11 intact proteins was obtained within 20 min by implementing a gradient across a limited RP composition window in the second dimension. tLC × tLC with UV detection was used for the separation of cytochrome c digest, bovine serum insulin digest and a digest of a complex protein mixture. A semi-quantitative estimation of the occupation of separation space, the orthogonality, of the tLC × tLC system yielded 75 %. The tLC × tLC setup was hyphenated to a high-resolution Fourier transform ion cyclotron resonance mass spectrometer instrument to identify the bovine serum insulin tryptic peptides and to demonstrate the compatibility with MS analysis.Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-015-8615-4) contains supplementary material, which is available to authorized users.

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