Inner wall coating of micro-LC columns. Wall effects in LC

An elastic inner wall coating in the fused silica capillaries used for Micro-LC (LC on packed fused silica capillary columns) stabilizes the packed bed and thus increases column efficiency and life expectancy. Probably the particles of packing material are partly forced into the elastic polymer layer which thus holds the packing in position. Bonded polymers of very different chemical polarity can be used with equally good results. Variation of the coating layer thickness shows that there is an optimum value around 0.3 μm. A discussion of various wall effects in LC columns is presented. The i.d. of the columns is a most important parameter in this respect.     

烷基化和硅烷化β-环糊精手性固定相的研究

合成了2,6－O－二戊基－3－O－三甲基硅烷基－β－环糊精,采用超动态法,成功地将其涂渍到弹性石英毛细管柱,经测试,柱性能稳定、寿命长、置于空气中一年半仍具有较高的柱效和手性拆分能力,利用该柱,分离了醇、酮、烯烃等一些对映体,考察了不同稀释剂配比所得固定相的立体选择性。     

A cold silanization method for preparation of medium polarity capillary columns

Silane-coupling agents, commonly used for fiberglass reinforced plastics (FRP), were applied for deactivation of silanol sites in glass capillary columns prior to coating with mediumpolarity phases such as Carbowax and Superox. The columns, prepared in a two-stage process in the case of glass (acidic leaching and drying, dynamic cold silanization followed by static coating with the phase) or a one stage process in the case of fused silica, gave the best results when deactivation and hence wettability were induced by glycidoxypropyltrimethoxy silane.     

Parts per quadrillion level ultra-trace determination of polar and nonpolar compounds via solvent-free capillary microextraction on surface-bonded sol-gel polytetrahydrofuran coating and gas chromatography-flame ionization detection

Sol-gel polytetrahydrofuran (poly-THF) coating was developed for high-sensitivity sample preconcentration by capillary microextraction (CME). Parts per quadrillion (ppq) level detection limits were achieved for both polar and nonpolar analytes through sample preconcentration on sol-gel poly-THF coated microextraction capillaries followed by gas chromatography (GC) analysis of the extracted compounds using a flame ionization detector (FID). The sol-gel coating was in situ created on the inner walls of a fused silica capillary using a sol solution containing poly-THF as an organic component, methyltrimethoxysilane (MTMOS) as a sol-gel precursor, trifluoroacetic acid (TFA, 5% water) as a sol-gel catalyst, and hexamethyldisilazane (HMDS) as a deactivating reagent. The sol solution was introduced into a hydrothermally-treated fused silica capillary and the sol-gel reactions were allowed to take place inside the capillary for 60 min. A wall-bonded coating was formed due to the condensation of silanol groups residing on the capillary inner surface with those on the sol-gel network fragments evolving in close vicinity of the capillary walls. Poly-THF is a medium polarity polymer, and was found to be effective in carrying out simultaneous extraction of both polar and nonpolar analytes. Efficient extraction of a wide range of trace analytes from aqueous samples was accomplished using sol-gel poly-THF coated fused silica capillaries for further analysis by GC. The test analytes included polycyclic aromatic hydrocarbons (PAHs), aldehydes, ketones, chlorophenols, and alcohols. To our knowledge, this is the first report on the use of a poly-THF based sol-gel material in analytical microextraction. Sol-gel poly-THF coated CME capillaries showed excellent solvent and thermal stability (>320 degrees C).     

Approaches for the coating of capillary columns with highly phenylated stationary phases for high-temperature GC

Two highly phenylated tetramethyl-p-silphenylene-diphenylsiloxane copolymers were coated on fused silica capillary columns and used as stationary phases in GC. The copolymers offered new insights into the coating process and column preparation due to their physicochemical properties. The fused silica capillary surface had to be pretreated in various ways to achieve a homogeneous film and a well deactivated surface: etching with ammonium bifluoride; leaching with sodium hydroxide and hydrochloric acid; silylation with tetraphenyldimethyldisilazane and triphenylsilylamine. Droplet formation was observed on tetraphenyldimethyldisilazane silylated surfaces leading to capillary columns with low separation efficiency. The topology of inhomogeneous films was investigated by light microscopy, scanning electron microscopy, and Auger electron spectroscopy. It became apparent that the stationary phase did not form droplets but islands, which are connected by a wetting layer according to the Stranski-Krastanov growth mode. Both copolymers are potential stationary phases for high-temperature GC with promising properties. They offer a higher overall polarity than 75% phenyl, 25% methyl-polysiloxanes in combination with increased thermal stability and reduced bleed levels.     

Preparation,evaluation, and comparison of wide bore (320 μm) and narrow bore (50 μm) cyanosilicone-coated capillary columns for gas chromatography

The preparation of wide bore (320 μm) and narrow bore (50 μm) fused silica capillary columns is described for immobilized cyanopropyl substituted silicones containing 60 and 88% substitution. The effect of high temperature deactivation with cyanopropylcyclosiloxanes was studied with a special test mixture. Curing was achieved with dicumyl peroxide or azo-tert-butane. The columns were evaluated and compared in terms of efficiency, activity, polarity, and temperature stability. Different coating methods were compared for the narrow bore columns. The activity of the 60% cyanopropyl columns that had been immobilized with dicumyl peroxide was significantly larger than for azo-tert-butane immobilized columns. The polarity of polar columns appeared to depend greatly on column temperature and is completely different for wide and narrow bore columns.     

Characterization,evaluation and in-situ cross-linking of new polar phases in capillary gas chromatography

Summary The possibilities of OV-1701 and RSL-310, two new stationary phases, have been evaluated for capillary gas chromatography in fused silica columns. OV-1701 is a cyanopropylphenyldimethyl polysiloxane of moderate polarity possessing excellent chromatographic characteristics. The phase exhibits high coating and chromatographic efficiencies, high temperature stability and is suitable for cross-linking. RSL-310 is a polar liquid stationary phase yet to be permanently bonded in a capillary column. The selectivity of both phases extends the applicability of fused silica columns.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Coupling miniaturized liquid chromatography to capillary gas chromatography (micro-LC-CGC): Possibilities of reversed phase LC

Retention gaps with different polarity treatments were evaluated for reversed phase solvents. Aminopropyl- and cyanopropyl-deactivated retention gaps showed the best results for methanol-water mixtures. A reversed phase packed fused silica capillary LC column is connected on-line with a capillary gas chromatography column. The combination was used for the analysis of diazepam in urine. Volume overloading on packed fused silica columns without loss of too much efficiency was demonstrated for propranolol.     

A cationic β‐cyclodextrin as a dynamic coating for the separation of proteins in capillary electrophoresis

A cationic cyclodextrin was used as dynamic coating for the capillary electrophoresis of a model mixture of proteins (i.e., ubiquitin, α‐lactoglobulin, cytochrome‐c, and myoglobin) as positively charged species in a fused silica capillary. An interesting feature of the coating is that by simple adjustment of the concentration of cyclodextrin added into the background electrolyte, a neutral or positively charged surface, which was beneficial in preventing protein adsorption at the inner capillary wall surface, was obtained. This is the first demonstration of a dynamic coating that yielded a neutral surface for protein separations in capillary electrophoresis. Based on electro‐osmotic flow measurements, addition of 0.05 to 0.10 mg/mL quaternary β‐cyclodextrin in a low pH electrolyte resulted in a neutral or positive surface (undetectable to very slow anodic electro‐osmotic flow). The coating approach afforded the electrophoretic separation of the mixture of proteins at positive polarity with good repeatability and separation performance.     

Testing the polarity and adsorptivity of nondeactivated GC capillary surfaces

Split injection of 1-pentanol headspace on to raw or treated fused silica tubing yields information on the characteristics of its surface (silanol concentration, polarity, adsorptivity). Fused silica tubing from three sources was compared. The effect of various leaching and etching procedures used for column preparation was studied, as was the stability of uncoated fused silica precolumns towards water and some organic solvents.     

A fully automated linear polyacrylamide coating and regeneration method for capillary electrophoresis of proteins

Surface modification of the inner capillary wall in CE of proteins is frequently required to alter EOF and to prevent protein adsorption. Manual protocols for such coating techniques are cumbersome. In this paper, an automated covalent linear polyacrylamide coating and regeneration process is described to support long‐term stability of fused‐silica capillaries for protein analysis. The stability of the resulting capillary coatings was evaluated by a large number of separations using a three‐protein test mixture in pH 6 and 3 buffer systems. The results were compared to that obtained with the use of bare fused‐silica capillaries. If necessary, the fully automated capillary coating process was easily applied to regenerate the capillary to extend its useful life‐time.     

Purification of OV-17 by supercritical fluid fractionation for fused silica capillary gas chromatography

Commercially available OV-17 was partitioned into a low and a higher molecular weight fraction by supercritical fluid extraction with carbon dioxide. This higher molecular weight fraction is more thermally stable than the original OV-17 due to its more viscous and gum-like nature. Based on the growing evidence in the literature that a deactivating agent for fused silica open tubular columns should be compatible with the stationary phase, a precoat of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane was applied prior to the static coating of high molecular weight OV-17. Fused silica OV-17 columns prepared in this fashion are stable up to 300°C.     

Polyethyleneimine-bonded phases in the separation of proteins by capillary electrophoresis

A hydrophilic, positively charged, durable coating has been developed for capillary electrophoresis of macromolecules. Polyethyleneimine is adsorbed to the inner wall of fused silica capillaries and the adsorbed coating cross-linked into a stable layer. Capillaries of polyethyleneimine-coated silica gave unique separations owing to the reversal of electro-osmotic flow caused by the positively charged coating. The resulting coating was stable from pH 2-12 and could be used over a wide pH range without substantial change in electro-osmotic flow. High-molecular-weight polymers were needed to give thick coatings which mask silanol groups on the wall. Proteins were resolved quickly and efficiently with good recovery using capillaries of 50 cm in length.     

Separation of somatropin charge variants by multiple‐injection CZE with Polybrene/chondroitin sulfate A double‐coated capillaries

The performance of dynamic double‐coated fused‐silica capillaries with Polybrene and chondroitin sulfate A has been compared with uncoated fused‐silica capillaries for the determination of recombinant human growth factor (somatropin) charge variants. The separations were carried out under the same electrophoretic conditions as described in the European Pharmacopoeia, i.e. at pH 6.0 and 30°C. The coating significantly reduced the interactions between the proteins and the surface of the fused‐silica capillary. The first five separations performed in a new bare fused‐silica capillary were discarded because of very poor separation performance as a result of protein–surface interactions. There was an approximate twofold increase in the interday migration time precision (%RSD ≤ 6.5%) in the double‐coated capillaries. The method was successfully transferred to a multiple CZE mode where two samples were analyzed in a single electrophoretic run. The average purity of somatropin certified reference standard was 98.0% (%RSD ≤ 0.3%) determined by using uncoated and coated capillaries.     

Electrophoretic separation of aniline derivatives using fused silica capillaries coated with acid treated single-walled carbon nanotubes

This paper reports on a new strategy for coating fused silica capillaries based on the ionic adsorption of acid treated single-walled carbon nanotubes (SWCNTs) on a poly(diallydimethylammonium chloride)-modified fused silica surface. The coated capillaries were used to demonstrate their performance for baseline separation of a mixture of seven nitrogen-containing aromatic compounds compared to capillary zone electrophoresis. This combined layer formed a coating material that could be useful for improvement of the selectivity of the solutes in an electrical field. We reasoned that the interaction of the solutes and the modified capillary wall occurred mainly via ionic interactions with the charged moieties of CNTs. The single-walled CNT modified capillaries were very stable and could be used for over 200 repeated analyses without compromising its analytical performance.     

Behaviour of fused silica capillaries subjected to gamma radiation. Part 2: Physical aspects

Five commercial varieties of uncoated fused silica capillary tubing used in high resolution gas chromatography were subjected to cobalt-60 gamma radiation. The advantages and superior performance of fused silica open tubular columns coated with OV-1 and subsequently immobilized or crosslinked in situ by radiation is reported in a previous paper in this Journal; however, in this investigation a loss in flexibility of irradiated capillaries was also noted. Quantitative measurements on flexibility were performed on specimens of irradiated fused silica capillaries, indicating that changes do indeed occur upon irradiation. Scanning electron micrographs also illustrate slight deterioration of the outer protective polyimide coating. Most surprisingly, it was also established that a variation does exist in the flexibility of the commercially available raw material studied.     

Preparation of On-Column Frits in Packed Fused Silica Capillaries by Sol-Gel Technology

Existing methods for preparing frits in packed fused silica capillaries as used for electrochromatography and micro HPLC are not applicable to all silica based packing materials and involve a high thermal stress for both the stationary phase and the fused silica tubing including the polyimide coating. A new procedure for the production of such on-column frits under mild conditions by a sol-gel type reaction of polydimethoxysiloxane (PDMOS) is described in this paper. Reaction conditions were established for optimum mechanical stability and high permeability of the frits. Frits produced in this manner showed no noticeable effect on the overall efficiency.     

High temperature gas chromatography on narrow bore capillary columns

The use of high-temperature-stable, medium polarity glass capillary columns coated with immobilized PS-090 (a 20 % diphenyl-substituted, CH₃O-terminated polydimethylsiloxane) has made it possible to analyze routinely, and with good separation efficiency, high molecular weight compounds such as triglycerides and free base porphyrins. Cold on-column injection was used throughout this work to avoid discrimination against involatile compounds, and disposable (fused silica) retention gaps were used to protect the column against contamination with involatile material. On-column injection into narrow bore glass columns was achieved by using glass-to-silica connections to attach wider bore (0.2 mm i.d.) deactivated fused silica tubing to the columns.     

Preparation of thermally stable phenylpolysiloxane fused silica capillary columns. Optimization and evaluation of the deactivation by capillary GC and solid state 29Si NMR

The deactivation of fused silica capillary columns with a laboratory-made poly-diphenylvinylmethylhydrosiloxane copolymer has been investigated. The deactivation obtained at different temperatures and reaction times is characterized with a dual column capillary GC system [1]. In parallel, the effect of the silylation temperatures and reaction times on the nature, the structure, and the chemical properties of the deactivation layer has also been studied by solid-state ²⁹Si NMR spoctroscopy. A fumed silica, Cab-O-Sil M5, was used as a model substrate for these spectroscopic studies. The deactivated fused silica capillaries show an excellent thermal stability (up to 400°C), a high resistance to solvolysis, and a minimal interaction to various critical test components. A good wettability of the fused silica capillary columns deactivated with this reagent was confirmed by successful subsequent coating with polysiloxanes with different phenyl contents.     

Atomic force microscopy of coated glasses

Atomic force microscopy has been used to investigate the topology of alkoxide gel dip coatings on different substrates. Results of SiO(2) - TiO(2) - ZrO(2) (STZ) coatings are presented on float glass, on polished fused silica, on commercially coated insulating flat glass, and on PtRh. Consolidated STZ coatings display the so-called glass pattern with ripples equal or less than 2 nm high. The same pattern is seen on partially dense STZ coatings, as soon as the surface is stiff enough for scanning, and also on the bottom of a 50 nm deep sputtering crater in the consolidated coating. The vitreous STZ coating on the fire side of the float glass is as flat as the float glass itself. It has the same tendency to contamination. 100 nm wide and 50 nm deep polishing grooves on fused silica have been filled up with the 80 nm thick coating, only dips of a few nm remain. The trenches between the SnO(2) crystallites on the insulating flat glass were filled up and the roughness of the substrate was partially reduced. PtRh sheet remained rough even after the coating. On the partially densified STZ coating, sputtering generates a grained surface.