Liquid flow in capillary (electro)chromatography: Generation and control of micro- and nanoliter volumes

Evolution in miniaturized column liquid chromatography instrumentation and applications: An overview

The purpose of this article is to underline the miniaturized LC instrumental system and describe the evolution of commercially available systems by discussing their advantages and drawbacks. Nowadays, there are already many miniaturized LC systems available with a great variety of pump design, interface and detectors as well as efficient columns technologies and reduced connections devices. The solvent delivery systems are able to drive the mobile phase without flow splitters and promote gradient elution using either dual piston reciprocating or syringe-type pumps. The mass spectrometry as detection system is the most widely used detection system; among many alternative ionization sources direct-EI LC-MS is a promising alternative to APCI. In addition, capillary columns are now available showing many possibilities of stationary phases, inner diameters and hardware materials. This review provides a discussion about miniaturized LC demonstrating fundamentals and instrumentals' aspects of the commercially available miniaturized LC instrumental system mainly nano and micro LC formats. This review also covers the recent developments and trends in instrumentation, capillary and nano columns, and several applications of this very important and promising field.

Capillary liquid chromatography fraction collection and postcolumn reaction using segmented flow microfluidics

A challenge for capillary LC (cLC) is fraction collection and the manipulation of fractions from microscale columns. An emerging approach is the use of segmented flow or droplet technology to perform such tasks. In this work, a fraction collection and postcolumn reaction system based on segmented flow was developed for the gradient cLC of proteins. In the system, column effluent and immiscible oil are pumped into separate arms of a tee resulting in regular fractions of effluent segmented by oil. Fractions were generated at 1 Hz corresponding to 5 nL volumes. The fraction collection rate was high enough to generate over 30 fractions per peak and preserve chromatographic resolution achieved for a five-protein test mixture. The resulting fractions could be stored and subsequently derivatized for fluorescence detection by pumping them into a second tee where naphthalene dicarboxyaldehyde, a fluorogenic reagent, was pumped into a second arm and added to each fraction. Proteins were derivatized within the droplets enabling postcolumn fluorescence detection of the proteins. The experiments demonstrate that fraction collection from cLC by segmented flow can be extended to proteins. Further, they illustrate a potential workflow for protein analysis based on postcolumn derivatization for fluorescence detection.

A theoretical and experimental study of the electrophoretic extraction of ions from a pressure driven flow in a microfluidic device

The electrophoretic extraction of ions from a hydrodynamic flow stream is investigated at an intersection between two microfluidic channels. A pressure gradient is used to drive samples through the main channel, while ions are electrophoretically extracted into the side channels. Hydrodynamic restrictors and a neutral coating are used to suppress bulk flow through the side channels. A theoretical model that assumes Poiseuille flow in the main channel and neglects molecular diffusion is used to calculate the extraction efficiency, eta, as a function of the ratio, R, of the average hydrodynamic velocity to the electrophoretic velocity. The model predicts complete extraction of ions (eta=1) for R<2/3 and a monotonic decrease in eta as R becomes greater than 2/3, which agrees well with the experimental results. Additionally, the model predicts that the aspect ratio of the microfluidic channel has little effect on the extraction efficiency. It is anticipated that this device can be used for on-line process monitoring, sample injection, and 2D separations for proteomics and other fields.

Atmospheric pressure free liquid infrared MALDI mass spectrometry: toward a combined ESI/ MALDI-liquid chromatography interface

A new atmospheric pressure (AP)-MALDI-type interface has been developed based on a free liquid (FL) microbeam/microdroplets and a mid-infrared optical parametric oscillator (mid-IR OPO). The device is integrated into a standard on-line nanoESI interface. The generation of molecular ions in the gas phase is believed to be the result of a fast (explosive) laser-induced evaporative dispersion(not desorption) of the microbeam into statistically charged nanodroplets. Only the lowest charge states appear insignificant abundance in this type of experiment. Mass spectra of some common peptides have been acquired in positive ion mode, and the limit-of-detection of this first prototype (liquid microbeam setup) was evaluated to be 17 fmol per second. To improve the duty cycle and to reduce the sample consumption, a droplet-on-demand system was implemented (generating 100 pL droplets).With this setup, about 20 attomole of bradykinin were sufficient to achieve a signal-to-noise ratio better than five.This setup can be operated at flow rates down to 100 nL/min and represents a liquid MALDI alternative to the nanoESI. Our particular interest was the application of the developed ion source for on-line coupling of liquid chromatography with mass spectrometry. The flow rates(>100 microL/min), required for stable operation of the ion source in continuous liquid microbeam mode, matches perfectly the flow rate range of micro HPLC. Therefore, online LC/MS experiments have been realized, employing a microbore C18 reversed-phase column to separate an artificial peptide mixture and tryptic peptides of bovine serum albumin (performing a peptide mass fingerprint). In the latter case, sequence coverage of more than 90%has been achieved.

Effects of inner diameter of monolithic column on separation of proteins in capillary-liquid chromatography

Polymer monolithic columns with I.D. between 100 and 320 microm were prepared by in-situ polymerization of styrene and divinylbenzene in fused silica capillaries. The effects of monolithic column I.D. on the separation of proteins in reversed-phase capillary-liquid chromatography under gradient elution were systemically studied. The loading capacity was positively proportional to the volume of the stationary phase. It was found that the smaller diameter columns showed better performance for protein separation. The minimum plate height decreases from 34.99 microm (320 microm I.D. column) to 5.39 microm (100 microm I.D. column) for a retained protein. After studying the three parameters of the Van Deemter equation, it was interpreted that the smaller diameter can provide less flow resistance and the better performance may also be improved by the increasing of the effective diffusion. This conclusion was also supported by the data of separation permeability and breakthrough curves.

Neuropeptidomics: expanding proteomics downwards

Biological function is mainly carried out by a dynamic population of proteins and peptides which may be used as markers for disease diagnosis, prognosis and as a guide for effective treatment. The study of proteins is called proteomics and it is generally performed by two-dimensional gel electrophoresis and mass spectrometric methods. However, gel-based proteomics is methodologically restricted from the low mass region, which includes important endogenous peptides. The study of endogenous peptides, peptidomics, is complicated by protein fragments produced post-mortem during conventional sample handling. Nanoflow liquid chromatography and MS, together with improved methods for sample preparation, have been used to semi-quantitatively monitor endogenous peptides in brain tissue. When rapidly heat-denatured brain tissue was analysed, these methods enabled simultaneous detection of hundreds of peptides and the identification of several endogenous peptides not previously described in the literature. In an application of the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model for Parkinson's disease, the expression of the small protein PEP-19 was compared with controls. The levels were found to be significantly decreased in the striatum of MPTP-treated animals.

Sample injection in capillary electrochromatography by heart-cut technique

The splitter working in heart-cut regime was used for sample injection in capillary electrochromatography. The principle was implemented in an automated microgradient system allowing to inject from microlitre down to nanolitre volumes with high repeatability and minimal extra-column band broadening. The apparatus is able to deliver discrete volumes of liquids at a preset volumetric flow rate and to stop and restore the flow at any moment. This brings a high degree of liquid manipulation flexibility. An extremely low split ratio is sufficient during the analysis, which saves mobile phase consumption substantially. The key parameters influencing the function of the heart-cut splitter were characterised. The function of the apparatus was demonstrated under isocratic, preconcentration and gradient capillary electrochromatography separation conditions. In all cases the statistic evaluation of the main parameters was performed, showing that high repeatability of retention times, peak heights and areas was achieved.

Monolithic columns with a gradient of functionalities prepared via photoinitiated grafting for separations using capillary electrochromatography

Stationary phases for capillary electrochromatography with a longitudinal gradient of functionalities have been prepared via photoinitiated grafting of polymer chains onto the pore surface of a porous polymer monolith. In order to achieve the desired retention and electroosmotic flow, the hydrophobic poly(butyl methacrylate-co-ethylene dimethacrylate) monolith with optimized porous properties was grafted with a layer of ionizable polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid). A moving shutter and a neutral density filter were used to control the dose of UV light received at different locations along the monolith in order to create the longitudinal gradient of functionalities. Formation of the desired gradients was confirmed using electron probe microanalysis of different locations along the column. The preparation technique significantly affects performance in the CEC mode as demonstrated on the separations of a model mixture using columns both with homogeneous distribution of grafts and with a gradient of functionality. Columns grafted with the gradient of functionalities were found superior to those functionalized uniformly. A comparison of the performance of the gradient column with another containing evenly distributed functionalities showed the performance benefits of the "gradient" column.