C18 silica packed capillary columns with monolithic frits prepared with UV light emitting diode: Usefulness in nano-liquid chromatography and capillary electrochromatography

Nano-liquid chromatography-mass spectrometry and recent applications in omics investigations

Liquid chromatography coupled to mass spectrometry (LC-MS) is one of the most powerful tools in identifying and quantitating molecular species. Decreasing column diameter from the millimeter to micrometer scale is now a well-developed method which allows for sample limited analysis. Specific fabrication of capillary columns is required for proper implementation and optimization when working in the nanoflow regime. Coupling the capillary column to the mass spectrometer for electrospray ionization (ESI) requires reduction of the subsequent emitter tip. Reduction of column diameter to capillary scale can produce improved chromatographic efficiency and the reduction of emitter tip size increased sensitivity of the electrospray process. This improved sensitivity and ionization efficiency is valuable in analysis of precious biological samples where analytes vary in size, ion affinity, and concentration. In this review we will discuss common approaches and challenges in implementing nLC-MS methods and how the advantages can be leveraged to investigate a wide range of biomolecules.

Integrated Microstructured Photonic Fiber as a Bifunctional Robust Frit and Efficient Electrospray Emitter of a Packed Column for Capillary Liquid Chromatography-Tandem Mass Spectrometry Analysis of Complex Biological Samples

Although capillary liquid chromatography married with tandem mass spectrometry (cLC-MS/MS) has become a powerful technique for proteomics and metabolomics research, it is still a great challenge to fabricate durable capillary-based analytical columns coupling continuous nanoflow (<1 000 nL/min) electrospray ionization (ESI) with MS, owing to the issue of clogging and fragile of emitters. Here, we proposed a simple approach to integrate microstructured photonic fibers (MPFs) into wide bore capillaries with 150 μm i.d., serving as an integral bifunctional frit or/and ESI emitter of packed columns. Two kinds of MPFs containing 126 homogeneous microchannels with different inner diameter, 3.2 μm for MPF-1 and 2.6 μm for MPF-2, were explored for preparation. The octadecylsilicate (ODS) silica-packed column using MPF-1 as a frit exhibited the lowest plate heights of 14.2-19.7 μm for five alkylbenzenes at the velocity of 1.5 mm/s, which were slightly lower than those of packed column with porous polymer monolith (PPM)-based frit by cLC coupling with ultraviolet (UV) detection. Additionally, the packed columns with integral MPF frit-emitters were further applied in analysis of a complex biological sample of digest of Hela cells by cLC-MS. An average of 7109 unique peptides could be identified in a single analysis by using MPF-1 emitter, and 7110 unique peptides were identified by using the MPF-2 emitter, which were superior to the identified result of packed column with an integral tapered tip emitter (6894 peptides). It is obvious that this novel integral MPF-based frit-emitter does not easily suffer from the issues of cracking owing to the silica cladding around independent microchannels (>100), which always encumbers both independent and integral tapered tip emitters for cLC-MS.

Rapid Formation of Polymer Frits in Fused Silica Capillaries Using Spatially defined Thermal Free-Radical Initiated Polymerization

Column preparation in capillary chromatography commonly relies upon the generation of on-column porous frits. Here, we report a simple, robust and low-cost approach for preparing polymer frits on-column, in a rapid and spatially controlled manner using thermal free-radical initiated polymerization. In this approach, a simple, temperature-controlled heating apparatus is positioned adjacent to a 100 μm i.d. fused-silica capillary for a defined duration. Frits were synthesized in 3-(trimethoxysilyl)propyl methacrylate modified capillaries using a monomer solution of 2,2-azobisisobutyronitrile, glycidyl methacrylate, ethylene glycol dimethacrylate, and decanol. Frit length and stability were investigated as a function of polymerization time and temperature. Frit length was easily controlled via a combination of polymerization time and temperature and position was readily controlled using a simple mechanical placement jig. Thermal initiated frits were stable throughout column packing and did not require removal of the capillary polyimide coating. The thermal initiation approach offers higher throughput, with polymerization times of < 2 min compared to ≥ 30 min for UV-initiated polymerization and significantly reduces the cost, enabling broader access to on-column frit technology for a variety of capillary separation applications.

Capillary electrochromatography-mass spectrometry for the determination of 5-nitroimidazole antibiotics in urine samples

The separation of eight antibiotics belonging to 5-nitroimidazole family was carried out by means of CEC coupled with MS. Preliminary experiments were carried out with ultraviolet detection in order to select the proper stationary and mobile phase. Among the different stationary phases studied (namely Lichrospher C18, 5 μm particle size; Cogent(TM) Bidentate C18, 4.2 μm; Pinnacle II™ Phenyl, 3 μm; Pinnacle II™ Cyano, 3 μm), Cogent™ Bidentate C18 (4.2 μm) gave the best performance. For CEC-MS coupling, a laboratory assembled liquid-junction-nano-spray interface was used. In order to achieve a good sensitivity, special attention was paid to both optimization of the sheath liquid composition as well as selection of the injection mode. Under optimized CEC-ESI-MS conditions, the separation was accomplished within 22 min by using a column packed with a mixture of Bidentate C18:Lichrospher Silica-60 (5 μm) 3:1 w/w, an inlet pressure of 11 bar, a voltage of 15 kV, and a mobile phase composed by 45:10:45 v/v/v ACN/MeOH/water containing ammonium acetate (5 mM pH 5). A combined hydrodynamic and electrokinetic injection of 8 bar, 15 kV, and 96 s was adopted. The method was validated in terms of repeatability and intermediate precision of retention times and peak areas, linearity, and LODs and LOQs. RSDs values were <2.9% for retention times and <16.1% for peak areas in both intraday and interday experiments. LOQ values were between 0.09 and 0.42 μg/mL for all compounds. Finally, the method was applied to the determination of three most employed 5-nitroimidazole antibiotics (metronidazole, secnidazole, and ternidazole) in spiked urine samples, subjected to a SPE procedure. Recovery values in the 67-103% range were obtained. Furthermore, for the selected antibiotics, CEC-MS(2) spectra were obtained providing the unambiguous confirmation of these drugs in urine samples.

High-performance liquid chromatography on glass chips using precisely defined porous polymer monoliths as particle retaining elements

A stable and permanent integration of miniature packed bed separation columns into microfluidic systems is a major issue in nano liquid chromatography. Various approaches like differently shaped retaining elements or the use of key stone effect have been investigated. We show a flexible integration of miniature packed bed separation columns into microfluidic chips utilising common HPLC material achieved by laser-assisted generation of narrow, photopolymerised frits. The generated retaining elements serve as an in- and outlet frits for the columns. An optimised pre-polymeric solution, consisting of butyl acrylates and a porogen, allows a precise fabrication of frit-type structures with lengths of less than 100 m and the capability to withstand common slurry packing pressures of more than 250 bar. The separation of seven polycyclic aromatic hydrocarbons by pressure-driven, reversed-phase chromatography proves the high quality of the created chromatographic column inside a glass chip. Plate heights down to 2.9 were achieved and extremely fast separations with sub-second peak widths were performed in isocratic and gradient elution modes on very short columns (≤ 25 mm).

Recent advances on capillary columns, detectors, and two-dimensional separations in capillary electrochromatography

As a typical miniaturized analytical technique, CEC has attracted much attention because of its low sample and solvent consumption, high efficiency, high selectivity, high resolution, and fast speed. In this review, we mainly cover the development of capillary columns and detection techniques in the CEC since 2009. Herein, three types of capillary columns, namely, open-tubular capillary columns, monolithic columns and packed columns, and several types of detectors are reviewed in detail. Moreover, a 2D separation system based on CEC is also reported.

Silica-particle-supported zwitterionic polymer monolith for microcolumn liquid chromatography

A silica-particle-supported zwitterionic polymeric monolithic column, shortened as supported column (S-column), was prepared by the in situ polymerization of methacrylic acid, ethylene dimethacrylate, and 2-(dimethylamino)ethyl methacrylate in the presence of a ternary porogenic solvent containing water, methanol, and cyclohexanol in a 250 μm id fused-silica capillary prepacked with 5 μm bare silica particles. In the S-column, a thin layer of the polymers was formed around the silica particles in the form of nanoglobules, leaving the interstitial spaces between the particles free for liquid flow. The effects of the preparation conditions on the morphology of the monolith were investigated by scanning electron microscopy and backpressure measurements. The selected volumetric ratio of porogens, monomer concentration, polymerization time, and temperature are 1:1:8 (water/methanol/cyclohexanol), 25% v/v, 5 h, and 60°C, respectively. The S-column was evaluated by comparison with its conventional organic counterpart in terms of morphology, mechanical stability, permeability, swelling-shrinking behavior, capacity, and efficiency. The results demonstrate that the S-column is superior to its counterpart in all the terms with the exception of permeability. The above merits and zwitterionic property of the S-column were further confirmed by separate separations of four inorganic anions and three organic cations.

Comparison of different types of outlet frits in slurry-packed capillary columns

Fused-silica capillary columns for high-performance liquid chromatography with 320 and 250 μm inner diameter were prepared by slurry packing with 5 and 3 μm Nucleosil C18 stationary phase. Different types of mechanical and monolithic outlet frits were used and their influence on the resulting column performance was evaluated. Columns with quartz wool exhibited symmetrical peaks and low theoretical plate height, and the preparation time was short. The performance of monolithic frits varied based on type of monolith, length of the frit, and silanization procedure. The best frit performed similarly to the quartz wool ones, but the preparation took several hours. Their main advantage lies in the possibility of on-column detection, because the detection window can be burnt immediately behind the frit.