Free release static coating of glass capillary columns; Static coating at elevated pressure — Theoretical considerations and practice

Summary Static coating of glass capillary columns has hitherto solely been carried out by evaporation of the stationary phase solvents under vacuum conditions. However, since a solvent vapour pressure higher than the external pressure is the only requirement for the vapour to flow out of the column, evacuation should not be necessary. Several important factors in the static coating procedure, such as mass (solvent vapour) — and heat transfer, heat of vaporisation of the solvent and viscosity of the stationary phase solution at elevated temperature and outlet pressure are discussed, principally to rationalise an improved static coating procedure. The alternative, so-called free-release static coating procedure, was evaluated practically by coating several columns with OV-101 and SE-30. Coating speed was found to be rapid and relatively constant whereas coating efficiency was between 80 and 100%.     

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.     

Effect of solvent composition on free release static coating of capillary columns

Summary Three aspects with respect to the selection of solvents for static coating of capillary columns, i.e. coating speed, occurrence of bumping and solubility of stationary phases are discussed. Hypotheses are proposed in an attempt to explain the observed facts that mixed solvents result in much higher coating speeds than those obtained from pure solvents, that a proper choice of solvents together with a good and uniform deactivitation of the column inner wall is needed to prevent bumping. Stationary phase solubility vs. solvent composition is also briefly discussed.     

Influence of the diameter of monolithic capillary columns on their gas chromatography characteristics

Divinylbenzene polymer monolithic capillary columns were prepared on the basis of capillaries 0.01 to 0.53 mm in diameter. Separation properties of the columns were investigated with the use of a test mixture of light hydrocarbons. The permeability and C parameter in the Van Deemter equation were determined for all the columns. For the most part, the columns had similar characteristics: permeability was in the range (2.2 ± 0.2) × 10^?9 cm², with parameter C in the range (0.7 ± 0.2) × 10^?3 s (with n-butane as a sorbate). It was thus established that capillary diameter has only a slight effect on the efficiency of monolithic capillary columns (unlike packed capillary columns and microcolumns, whose properties, according to the literature data, depend strongly on the column diameter). The difference in properties between the narrowest monolithic column (capillary diameter 0.01 mm) and the others is explained by column overloading.     

Acceleration of free release static coating of capillary columns by the presence of polar solvents in the coating solution – a theoretical study

The cause of an observed acceleration in the free release static coating process of a capillary when more polar solvents are added to alkanes as stationary phase solvents has been studied. By using a computer program, 9 of the relevant physical parameers were calculated in relation to the composition of the solvent hixtures. Curves of these parameters vs composition of the mixures were drawn. From the results it can be deduced that the change of these parameters with the increase of polar solvents in the solution, is probably insufficient to explain the drastic increase of the free release static coating speed. A physical model is proposed to calculate the depth of the meniscus around which sufficient heat can be transferred into the evaporating surface per unit time, to evaporate the solvent at a rate that the coating speed indicates. It is demonstrated that when pentane-acetone mixtures are used to coat a capillary, the depth of the meniscus indeed is greater than when pure pentane is used. The proposed model may lead to a better understanding of phenomena involved in the static coating process.     

Device for filling of small diameter capillaries with stationary phase solutions in liquefied gases

This paper describes a high pressure device for filling small diameter capillaries with stationary phase solutions is described. A liquid is forced into the capillary column with the help of high pressure syringe whose needle (provided with a side opening) is tightened in a PTFE seal. The device allows use of liquefied gas as solvent. A detailed procedure is given for filling the capillary with stationary phase solution. The performance of the device was evaluated by filling 12 m × 15 μm i. d. glass capillary with 6.5 % (w/v) SE-54.     

Preparation of highly selective zeolite ZSM-5 membranes by a post-synthetic coking treatment

Zeolite ZSM-5 membranes with high n-butane:isobutane selectivities, e.g., 322 at 185°C, are obtained by a selective deposition of coke into non-zeolitic pores. The zeolite membranes are prepared by in situ crystallization on either bare porous α-Al₂O₃ support disks or disks that are pretreated to include a diffusion barrier. The post-synthetic coking treatment is accomplished by impregnating these membranes with liquid 1,3,5-triisopropylbenzene (TIPB) for 24 h at room temperature and then calcining them in air at 500°C for 2 h. Calcination at 500°C for up to 30 h does not destroy the high n-butane:isobutane selectivity. Thermogravimetric analysis (TGA) experiments on two model pore systems ZSM-5 (5.5 Å) and Vycor glass (40–50 Å) suggest that micro-defects are selectively eliminated by the TIPB coking treatment while the intracrystalline pore space of the ZSM-5 is not affected. The elimination of non-zeolitic pores results in a large increase of n-butane:isobutane pure gas flux ratio (45 vs. 320 at 185°C) accompanied by a fourfold reduction of the n-butane flux. The permeation experiments reveal that the n-butane flux increases nonlinearly with the partial pressure in the feed while the n-butane:isobutane pure gas flux ratio remains relatively unchanged.     

Preparation and evaluation of 3 m open tubular capillary columns with a zwitterionic polymeric porous layer for liquid chromatography

A 3 m zwitterionic polymeric porous layer open tubular column (3 m × 25 μm id × 375 μm od) with a polymeric porous layer thickness of 4 μm was fabricated by the copolymerization of [2‐(methacryloyloxy)ethyl] dimethyl‐(3‐sulfopropyl) ammonium hydroxide and N,N’‐methylenebis(acrylamide). The effects of the diameter of the capillary, reaction temperature, and polymerization time on the preparation of the open tubular column were investigated. Characterized by scanning electron microscopy, the zwitterionic layer was observed to be rough and throughout the fused‐silica capillary homogenously, which increased the phase ratio. The separation of neutral, basic, and acidic compounds demonstrates the strong hydrophilicity of the poly[2‐(methacryloyloxy)ethyl] dimethyl‐(3‐sulfopropyl) ammonium hydroxide coating. In addition, the poly[2‐(methacryloyloxy) ethyl] dimethyl‐(3‐sulfopropyl) ammonium hydroxide porous layer open tubular column was applied for the analysis of flavonoids from the rootstalk of licorice, revealing the potential in separating complex samples. The relative standard deviation of retention time for run‐to‐run (n = 5), day‐to‐day (n = 3), and column‐to‐column (n = 3) of toluene, N,N‐dimethylformamide, formamide, and thiourea were below 1.2%, exhibiting good repeatability.     

Automated at-column injection into narrow bore capillary GC columns

An injector designed for automatic direct liquid injection into narrow bore capillary GC columns has been constructed and evaluated. The tip of the syringe needle is aligned with, and positioned close to, the column entrance in a small, pressurized cavity: when the sample is dispensed it is immediately forced into the column by the action of the surrounding carrier gas. A standard autosampler equipped with a standard stainless steel syringe needle was utilized for at-column sample transfer into 100 μm i.d. columns. RSD values for n-alkanes were between 0.1 and 0.3% for relative area counts and approximately 1% for absolute area counts.     

Coupled LC-GC: Evaporation rates for partially concurrent eluent evaporation using an early solvent vapor exit

Partially concurrent eluent evaporation presupposes an eluent evaporation rate in the GC pre-column that approaches the LC flow rate. Discharging the vapors through the whole GC column, evaporation rates reach 10–30 μl/min, i.e. are suitable just for LC flow rates typical for packed capillary LC columns. With an early vapor exit, evaporation rates are increased to 100–200 μl/min (under extreme conditions to some 800 μl/min), thus fitting the LC flow rates of 2 mm i.d. columns. Evaporation rates were measured for a standard set of pre-columns and conditions. The dependence of the evaporation rate on temperature, inlet pressure, carrier gas, and internal diameter of the retaining pre-column are discussed in order to allow the design of a GC system producing a desired evaporation rate.     

Semimicro Size Exclusion Chromatography For Oligomers. I. Column Packing Procedure

Abstract

Polystyrene gels of a particle diameter 10 ± 2 μm for the use in oligomer separation were packed into 1.5 mm i.d. × 25 cm length columns by the balanced density slurry-packing technique under a constant flow rate of 500 μL/min. The slurry solvent was a mixture of toluene and chloroform (50.5/49.5, v/v). The example of the number of theoretical plates (N) of these columns was 8600 plates/25 cm (HETP = 29.1 μm) at flow rate of 40 μL/min by injecting 1 μL of 0.5% benzene solution. Sixteen columns were connected and the overall value of N was 103000 plates/4 m. A typical example of oligomer separation was demonstrated. A constant-flow technique is preferable to a constant-pressure technique. When two or three column blanks were packed together, the columns located at the outlet of the packer-column assembly had higher values of N. Optimum flow rate of the slurry solvent when three column blanks were packed together lay between 400 and 500 μL/min. The packing efficiency, that is, the probability of getting valid columns was about 60%. Viscous slurry solvents were not effective to get efficient columns. To pack gels in the less swollen state gave sometimes efficient columns. Pressure monitoring in progress of packing was very effective to foresee the column efficiency.     

A novel sealing method in static coating fused-silica capillary column

Summary In this paper a new sealing method for the static coating of capillary columns is described. When one end of the fully filled capillary was immersed into liquid nitrogen in a Dewar flask, the coating solution at this end would be frozen and became a temporary seal, and an air-free solvent/seal interface was obtained. No bumping has ever been found at the interface, even when butane was used as solvent. By applying this sealing method, several capillary columns, including some narrow bore capillary, had been successfully coated.     

High‐resolution peptide separations using nano‐LC at ultra‐high pressure

We report on the optimization of nano‐LC gradient separations of proteomic samples that vary in complexity. The gradient performance limits were visualized by kinetic plots depicting the gradient time needed to achieve a certain peak capacity, while using the maximum system pressure of 80 MPa. The selection of the optimal particle size/column length combination and corresponding gradient steepness was based on scouting the performance of 75 μm id capillary columns packed with 2, 3, and 5 μm fully porous silica C18 particles. At optimal gradient conditions, peak capacities up to 500 can be obtained within a 120 min gradient using 2 μm particle‐packed capillary columns. Separations of proteomic samples including a cytochrome c tryptic digest, a bovine serum albumin tryptic digest, a six protein mix digest, and an Escherichia coli digest are demonstrated while operating at the kinetic‐performance limit, i.e. using 2‐μm packed columns, adjusting the column length and scaling the gradient steepness according to sample complexity. Finally, good run‐to‐run retention time stability (RSD values below 0.18%) was demonstrated applying ultra‐high pressure conditions.     

Measurement of intracrystalline diffusion in NaX zeolite by capillary column gas chromatography

Chromatographic HETP (height equivalent to a theoretical plate) measurements are reported for a series of hydrocarbon sorbates (n-butane, n-hexane, cyclohexane, n-octane, benzene and decalin) in a capillary column (150 μm diameter) packed with 100-μm crystals of NaX zeolite. Such a column is superior to a conventional-packed column in terms of both heat transfer and external mass transfer characteristics. Dimensionless plots of reduced HETP versus reciprocal Peclet numbers show clearly that, for the more strongly adsorbed species, the HETP can be quantitatively accounted for by external mass transfer resistance and axial dispersion so that only lower limits for intracrystalline diffusivities can be obtained. The HETP data for butane, cyclohexane and decalin show clearly the intrusion of internal (micropore) diffusional resistance so, for these species, reasonably reliable intracrystalline diffusivities can be derived. The diffusivities so obtained (for butane and cyclohexane) are smaller than previously reported nuclear magnetic resonance (NMR) self-diffusivities but larger than the zero-length column (ZLC) values.     

Effect of high temperature in determination of the mathematical dead time and the constants of the n-alkane retention time curve in gas chromatography

In gas liquid chromatography, the column dead time and the constants of the n-alkane retention time curve are calculated by the multiparametric least-squares regression iterative method at high temperature between 180 and 270°C. The method was applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87), while the second includes five glass capillary columns (four methylsilicone with different film thicknesses and Apolane-87). The calculated tM and b values were compared with those obtained by Guardino's, Grobler's, and Kaiser's methods. The influences of coating thickness and temperature thereon were investigated.     

Coating properties of a novel water stationary phase in capillary supercritical fluid chromatography

The coating properties of a novel water stationary phase used in capillary supercritical fluid chromatography were investigated. The findings confirm that increasing the length or internal diameter of the type 316 stainless‐steel column used provides a linear increase in the volume of stationary phase present. Under normal operating conditions, results indicate that about 4.9 ± 0.5 μL/m of water phase is deposited uniformly inside of a typical 250 μm internal diameter 316 stainless‐steel column, which translates to an area coverage of about 6.3 ± 0.5 nL/mm² regardless of dimension. Efforts to increase the stationary phase volume present showed that etching the stainless‐steel capillary wall using hydrofluoric acid was very effective for this. For instance, after five etching cycles, this volume doubled inside of both the type 304 and the type 316 stainless‐steel columns examined. This in turn doubled analyte retention, while maintaining good peak shape and column efficiency. Overall, 316 stainless‐steel columns were more resistant to etching than 304 stainless‐steel columns. Results indicate that this approach could be useful to employ as a means of controlling the volume of water stationary phase that can be established inside of the stainless‐steel columns used with this supercritical fluid chromatography technique.     

Concentration and temperature dependences of the liquid-vapor separation factor for ammonia with impurities of hydrocarbons and permanent gases

The phase equilibria for systems of ammonia-impurity (oxygen, nitrogen, methane, propane, ethane, n-butane, isobutane, propylene, carbon monoxide) were examined and concentration and temperature dependences of a separation factor, which are required for deep purification of ammonia used in the production of energy-efficient LEDs, were determined between liquid and vapor for the systems in a region of low concentrations of an impurity at an elevated pressure.     

An electroosmotic pump for packed capillary liquid chromatography

An electroosmotic pump (EOP) capable of generating pressure above 3 MPa and μl/min flow rate with reverse phase mobile phases of HPLC was constructed and evaluated. The pump consisted of three parallel connected fused silica capillary columns (25 cm×320 μm I.D.) packed with 2 μm silica materials, hollow electrodes, a high voltage DC power supply, and a liquid pressure transducer. The EOP was applied in a capillary liquid chromatographic system for mobile phase delivery instead of a mechanical pump. Standard samples containing thiourea, naphthalene, anthracene, phenanthrene and acetonitrile were separated on a 15 cm×320 μm I.D. 5 μm Chromasil C₁₈ packed capillary column with acetonitrile/water as mobile phase.     

A novel open-tubular capillary electrochromatography with magnetic nanoparticle coating as stationary phase

A novel open‐tubular capillary electrochromatography (OT‐CEC) with modified core/shell magnetic nanoparticles coating as stationary phase was introduced using external magnetic force to fix magnetic nanoparticles. The magnetic nanoparticles coating inside the capillary columns could be easily regenerated by removing and re‐applying the external magnetic field. Magnetic field intensity, concentration and flow rate of nanoparticles suspension were investigated to achieve simple and stable preparation. Mixture of five organic acids was used as the marker sample to evaluate the OT‐CEC system, and the relative column efficiency of anthranilic acid reaches 220 000 plates/m. The excellent within‐column and between‐column repeatability has been testified with the RSDs of retention time of less than 1.51 and 5.29%, respectively. The aqueous extract of rhizoma gastrodiae was analyzed by the OT‐CEC system, and 23 peaks were eluted in 30 min. Compared with conventional open‐tubular capillary column, this new system shows faster separation speed and higher column efficiency from the larger surface area of nanoparticles. It has great potential in the method development for the analysis of complex samples, since magnetic coating can effectively prolong the column life by expediently replacing stationary phase to eliminate the pollution or irreversible adsorption.     

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.