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     

CGC/AED and CGC/ECD/NPD comparison for the determination of acaricides in honey after hexane/acetone extraction

Summary The performance of a gas chromatographic system coupled to an atomic emission detector (GC/AED) is tested by comparison with a two-dimensional gas chromatographic system equipped with capillary columns of different polarity and simultaneous electron capture and nitrogen-phosphorus detection (ECD/NPD), for the determination of the acaricides chlordimeform, bromopropylate, amitraz and coumaphos on spiked honey samples. The acaricides were extracted with ann-hexane/acetone mixture (8020, v/v) with a further clean-up step on an octadecylsilane cartridge. The 193 nm carbon emission line is the best choice in terms of sensitivity whereas the ECD/NPD system gives better selectivity.     

Wide-bore glass capillary columns for gas chromatography. Their preparation and application

Summary Methods for the preparation of wide-bore glass capillary columns for gas chromatography are presented. The pretreatment of the columns (etching, carbonization, deactivation and/or surface-coating with porous materials), and the coating of them with polar or apolar phases (preferably by the static method) is described. The performance of the columns prepared is evaluated and a number of applications are given, such as analysis of volatile compounds, pesticides, lipids and GC/MS analysis. In some respects, the wide-bore glass capillaries have advantages over the narrow-bore types. They can be installed very easily in GC-instruments, and permit the analysis of larger samples. A system of collecting separated sub-g fractions in glass capillary traps, which makes direc re-injection from these traps possible, completes the methods presented.     

Glass capillary columns for separation of free organic acids

Glass capillary columns have been prepared without acidic additive in the stationary phase, from which free organic acids elute as sharp and symmetrical peaks. The required surface in the borosilicate glass capillary was generated by a combination of leaching with aqueous HCl and deposition of colloidal silica particles; it can be coated with stationary phases have a broad range of polarity. Aqueous samples containing free organic acids can also be analyzed in such columns in an isothermal mode.     

Evaluation of New Stationary Phases for the Separation of Fatty Acid Methyl Esters

In the analysis of fatty acids, one of the most commonly used tools is a GC separation of the fatty acid methyl esters (FAME). Many researchers perform this separation using a non-polar phase such the ubiquitous 5% phenyl / 95% methyl capillary columns found in most every chromatography laboratory. Numerous laboratories have also turned recently to polar phases such as 70% cyanopropyl columns, as this type of chemistry provides increased selectivity for unsaturated compounds, and thus improved separation of cis/trans and ω³/ω⁶ FAME isomers. Here, a series of columns nominally having 60, 70, 80, and 90% bis-cyanopropyl content have been tested for the separation of FAME isomers. Trends in retention and the influence of increasing phase polarity on effective and fractional chain lengths are highlighted to provide the FAME chromatographer with insight into which of these novel stationary phases might be best suited to their particular application. In addition, the elution temperatures (T_e) of the FAME and linear alkane standards are presented, as this information will be of value to comprehensive two-dimensional multidimensional GC (GC × GC) users who wish to use these columns in the primary dimension separation.     

Effect of the carrier gas pressure on the dynamic and separation characteristics of divinylbenzene-based monolithic capillary columns for gas chromatography

The efficiency and dynamic characteristics of divinylbenzene-based monolithic capillary columns for gas chromatography were analyzed using a test mixture composed of five light hydrocarbons. The chromatographic properties of these columns were evaluated within the framework of two varieties of the van Deemter equation, the classical one and that proposed by Giddings (with consideration given to the pressure drop across the column). An analysis of the van Deemter curves demonstrated that the main contribution to peak smearing comes from the diffusion processes in the mobile phase. The contribution from the resistance to mass transfer between the mobile and stationary phases is less important. Negative values obtained for A in the van Deemter equation and for C _s in the Giddings model, parameters that characterize the stationary phase structure and mass transfer kinetics in the stationary phase, have no physical meaning, a result calling for further studies of this type of monolithic capillary columns since the classical theory supposed these parameters to be strictly positive. Under optimal conditions, the HETP of the monolithic columns was found to be 3 to 4 times smaller than that typical of open capillary columns of the same diameter.     

Instrumental developments in capillary electrochromatography

Summary An instrumental device for capillary electrochromatography (CEC) is described that permits conditioning of packed columns at pressures up to 100 bar, in or on-column fluorescence detection and electrokinetic sample injection. Capillaries with varying inner diameters (75, 100, 150 m) packed with octadecyl silica gel (3 m) are compared for efficiency and detection sensitivity. The influence of counter pressure on electroosmotic velocity and plate number is discussed.     

Crosslinked methylphenylsilicones as stationary phases for capillary gas chromatography

Summary Methyl(phenyl)silicones offer useful selectivities when used as stationary phases in gas chromatography (GC). Such phases have, however, hitherto been of restricted importance in capillary GC due to the lack of phases having a viscosity high enough to ensure stationary phase film stability. Further, to utilize fully the possibilities of a methyl(phenyl)silicone capillary column, it must also possess high efficiency and a high degree of deactivation.In this work, the preparation of soda-glass capillary columns coated within situ cured methyl(phenyl) and methyl(tolyl)-silicones is presented. Vulcanization was made possible by the introduction of some vinyl substitution in the gum to be cured: tolyl-containing gums could be cured without the presence of vinyl groups. In addition, fused silica capillary columns coated with OV-1701 were prepared.The columns show a coating efficiency of above 80%, a thermal stability up to 320 °C and a high degree of deactivation. Their utility is demonstrated by the separation of samples containing polynuclear aromatics, antidepressants and some potent mutagens.Presented at the 14th International Symposium on Chromatography London, September, 1982     

New materials and packing techniques for micro-HPLC packed capillary columns

Summary Plastic tubing of polyetheretherketone (PEEK) is used for the preparation of HPLC packed capillary columns. The polymer is rigid but not fragile, has great resistance to chemical and physical agents and comes in standard 1/16″ O.D. These features can extend the column lifetime and facilitate the overall employment of HPLC capillary columns. Serial connection of short columns is also possible with no increase in dead volume. A new, rapid and effective packing method has been developed to be compatible with the characteristics of the material. Several C8 and C18 columns have been prepared and tested and some applications are shown.     

Packing and performance of microbore columns for adsorption and partition chromatography

Summary Microbore columns of 1 mm i.d. have turned out to be very suitable for the achievement of efficient columns.The packing procedure for stainless steel 1 mm i.d. columns from 5 to 100 cm in length was studied. Stationary phases used were: pure silica gel, octyl, octadecyl and amino bonded silicas. The main parameters (slurry composition, packing system, choice of materials) are discussed.Short columns packed with 3 or 5m particles allow high speed separations. A separation in 18 seconds is described.Very high plate numbers can be obtained with long columns. With 7–8m particles, a 1 m column can produce 50,000 plates (h=3). Columns can be joined without loss of efficiency. 270 000 theoretical plates were obtained on a 6 m adsorption column with a test mixture. In reversed-phase chromatography, bile acid sodium salts can be separated on a 1 m column. In adsorption chromatography, details are given of the separation of a polystyrene oligomer sample, as well as a light and a heavy petroleum distillate samples on a 2 m column with refractive index detection in the last-case.     

Glass capillary column with nematogenic liquid crystals as stationary phases for separating polycyclic aromatic hydrocarbons

Summary Use of the homologues and analogues of nematogenic liquid crystals of the type N,N-bis(p-methoxylbenzylidene)-,-bi-p-toluidine (BMBT) as stationary phases in glass capillary columns is described. These stationary phases are very useful for resolving closely related, rigid solute isomers of three-, four- and five-ring PAH. With these columns it is possible to achieve faster and more complete separation of PAH compared with packed columns with a liquid crystalline stationary phase.     

Glass capillary gas chromatography of highly polar solutes like diols,acids and amines deactivation of fused silica and pretreated alkali glass support surfaces

Summary The method of support surface deactivation by PSD (alkylpolysiloxane degradation) at temperature between 300 and 450°C previously described was used to deactivate both fused silica and alkali glass surfaces of capillary columns. The latter surfaces had to be pretreated before deactivation with aqueous HCl leaching or by the dealkalisation method using flowing HCl gas at 450°C and subsequent rinsing with water for alkali removal. Excellent alkylpolysiloxane columns with regard to tailing and irreversible adsorption of highly polar solutes have been obtained on both fused silica and the pretreated alkali glass. Fused silica does not require pretreatment before deactivation by the PSD-method, however. Good polyethyleneglycol (Carbowax 20 M) columns can also be obtained by coating the two types of surfaces when no deactivation is necessary. Deactivation by the PSD method cannot be applied in this case because polar stationary liquids do not adhere to alkylpolysiloxane deactivated surfaces. Sample capacity problems arising when separating highly polar solutes with non-polar stationary phases have also been investigated.     

Gas chromatography of fatty acids.

Lipids in foods contain a wide variety of fatty acids differing in chain length, degree of unsaturation, position and configuration of double bonds and the presence of special functional groups. Modern capillary gas chromatography offers excellent separation of fatty acids. Fused-silica capillary columns with stationary phases of medium polarity and non-polar methylsilicone stationary phases successfully separate most of the natural fatty acids. Special applications, such as the separation of complex cis-trans fatty acid mixtures and cyclic fatty acids, required particular chromatographic conditions, including the use of very long capillary columns or more polar stationary phases. The derivatization methods for the preparation of fatty acid esters also need to be optimized to obtain accurate quantitative results. This paper reviews the derivatization techniques, capillary columns and stationary phases commonly used in the gas chromatography of fatty acids in food.     

Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase

Summary New polyacrylate liquid crystalline compounds were coated onto glass or fused-silica capillary columns as stationary phases and applied to supercritical fluid chromatography. These stationary phases, were very stable: no bleeding was observed at 200°C and up to 200kg/cm² pressures of carbon dioxide mobile phase. The wide working range of the capillary column was extended below the g-n transition temperature. Isomeric compounds such as - and -methoxynaphthalene, anthracene and phenanthrene and several phenolic compounds were separated.     

Optimization of the porous structure and polarity of polymethacrylate-based monolithic capillary columns for the LC-MS separation of enzymatic digests

The porous structure as well as the polarity of methacrylate ester-based monolithic stationary phases has been optimized to achieve the separation of various peptides originating from enzymatic digestion. The porous structure, determined by the size of both pores and microglobules, was varied through changes in the composition of porogenic solvents in the polymerization mixture, while the polarity was controlled through the incorporation of butyl, lauryl, or octadecyl methacrylate in the polymer backbone. Both the morphology and the chemistry of the monoliths had a significant effect on the retention and efficiency of the capillary columns. The best resolution of peptidic fragments obtained by digestion of Cytochrome c with trypsin in solution was obtained in a gradient LC-MS mode using a monolithic capillary column of poly(lauryl methacrylate-co-ethylene dimethacrylate) featuring small pores and small microglobules. Raising the temperature from 25 to 60 degrees C enabled separations to be carried out at 40% higher flow rates. Separations carried out at 60 degrees C with a steeper gradient proceeded without loss of performance in half the time required for a comparable separation at room temperature. Our preparation technique affords monolithic columns with excellent column-to-column and run-to-run repeatability of retention times and pressure drops.     

Non-particulate (continuous bed or monolithic) restricted-access reversed-phase media for sample clean-up and separation by capillary-format liquid chromatography

Restricted-access reversed-phase non-particulate (continuous bed or monolithic) stationary phases of different hydrophobicity synthesized in 100 m i.d. fused silica capillaries have been evaluated. A specific property of restricted-access media (RAM) is that they interact with small analytes and exclude big molecules, e.g. proteins, from access to the active sites and adsorption on the surface. This dual property facilitates direct injection of biological fluids for drug or drug-metabolite analysis. Different RAM and RAM-precursor capillary columns were tested to assess the influence of chromatographic bed morphology on loadability. Inverse size-exclusion chromatography was used for investigation of pore structural properties of the capillary-format continuous beds. The data obtained were used to discuss the mechanism of separation of the biological samples using capillary columns and to propose a model for the topochemical architecture of the RAM investigated. Different morphology of the non-particulate reversed-phase precursors resulted in two types of RAM material shielded with hydrophilic polymer, classified as homogeneous or heterogeneous topochemistry stationary phases. Capillary columns were applied for chromatography of biological fluids. High resolution was obtained, without the need for column switching, when capillary columns operated in gradient conditions. Extensive evaluation of the chromatographic properties (hydrophobicity, efficiency, separation impedance, and loadability) of the non-particulate reversed-phase materials was performed before and after shielding with hydrophilic polymer to generate restricted-access properties. Minor changes of hydrophobicity, efficiency, or separation impedance were observed after the shielding.     

Determination of thermodynamic characteristics from retention data in GC

Summary Retention data have been measured for model substances of different polarities under isothermal conditions at several temperatures using capillary columns coated with Carbowax-20M and SP-2100 stationary phases. By using the k capacity ratios obtained for various temperatures the thermodynamic characteristics of the partition process were determined. For homologous series linear relationships can be established between thermodynamic characteristics and carbon number. Thermodynamic characteristics can be used both to compare and evaluate column quality and to predict retention indices for different temperatures and linear programmed temperature conditions.Dedicated to Professor J. F. K. Huber on the occasion of his 60th birthday.     

In-situ Preparation of Integrated Polymeric Pora-U PLOT Columns and their Applications in Gas Chromatography

Summary A new method is described to prepare polymeric porous-layer open-tubular (PLOT) columns by using a two-step in-situ polymerization technology. The integrated method involves a straightforward in-situ polymerization of the monomer. By using -(trimethoxysilyl)propyl methacrylate as a bridge, the porous polymer is bonded to the columns inner wall, and this polymer is crosslinked so the whole polymer looks like a single moleculer. The new column avoids the defects of traditional polymer PLOT columns where particles are easily released and swept through the column, causing blockage or a spiking detection signal. The new type of PLOT column is coated with a divinylbenzene and ethylene glycol dimethyl acrylate copolymer and has an increased polarity when compared to a conventional polymer Q-type PLOT column. The retention characteristics of the new column were evaluated and found to be comparable with the commercially available HP PLOT-U column. The inertness of the porous polymer allows the elution of a range of apolar and polar compounds: even most active and polar compounds such as H₂O and H₂S eluted symmetrically. The new column possesses high stability and can withstand temperatures up to 210 °C. With this new type of capillary columns, significantly better mechanical stability, temperature endurance, reproducibility, strong separation power and good inertness are obtained in combination with short analysis times.     

Preparation of Cationic Mixed-Mode Acrylamide-Based Monolithic Stationary Phases for Capillary Electrochromatography

A series of amphiphilic macroporous mixed-mode acrylamide-based continuous beds bearing positively charged quaternary ammonium groups is synthesized for capillary electrochromatography (CEC) under variation of the concentration of the cationic monomer in the polymerization mixture. Positively charged mixed-mode monolithic stationary phases are synthesized in pre-treated fused silica capillaries of 100 µm I.D via single step free radical copolymerization of cyclodextrin-solubilized N-tert-butylacrylamide, a hydrophilic crosslinker (piperazine diacrylamide), a hydrophilic neutral monomer (methacrylamide), and a positively charged monomer ([2-(methacryloyloxy)ethyl]trimethyl ammonium methyl sulfate) in aqueous solution containing the lyotropic salt ammonium sulfate as a pore-forming agent. The synthesized monolithic stationary phases contain hydrophobic, hydrophilic, and charged functionalities. They can be employed for the CEC separations of different classes of neutral and charged solutes (with varied polarity) in the reversed-phase mode, in the normal-phase mode, in the ion-exchange mode, in a mixed-mode, or in the hydrophilic interaction liquid chromatography (HILIC) mode. The influence of the concentration of the cationic monomer in the polymerization mixture on retention factor, electroosmotic mobility, and methylene selectivity (α_meth) is studied under isocratic conditions for alkylphenones in the reversed-phase mode by capillary electrochromatography (CEC). Scanning electron microscopy (SEM) micrographs demonstrate that the morphology of the synthesized monoliths (i.e., the domain size) is strongly influenced by the variation of the concentration of the cationic monomer in the polymerization mixture.     

Gas chromatographic behaviour of isomeric tricyclo[10.4.0.04,9]hexadecanes

Summary Using achiral stationary phases of different polarity in capillary gas chromatography it is demonstrated that the hydration of 1,2–5,6-dibenzocycloocta-1,5-diene yields five isomers of the tricyclo[10.4.0.0^4,9]hexadecanes. With highly polar stationary phases all isomers of the tricyclic hexadecanes and the semihydrated products in the mixture were also separated from each other. The chiral isomers were separated on cyclodextrin stationary phase. Moreover, with columns of achiral and chiral phases coupled in series, in a double oven gas chromatograph, it was possible to estimate the configuration of the isomers. The separation of diastereomers on chiral stationary phases is a useful means for structural assignment of isomeric compounds.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.