István Halász

Preparation and properties of reversed phases

Abstract The preparation of monomeric reversed phases (RPs) with C1-C18 alkyl groups are described. Methods for checking the quality of RPs are discussed. The retentions of polar and non-polar samples were found to be influenced by the eluent, the length of the organic “bristle”, the carbon content of the RP, the pore size distribution of the silica support and the total porosity of the stationary phase. The efficiency is a function of (among other factors) the composition of the eluent, the capacity ratio, the length of the bristle, the sieve fraction and the packing density. The maximum sample size is 10 times greater than that with silica and increases with increasing carbon content of the RP. The speed of analysis increases with decreasing length of the bristle. The mechanism of sorption procedures should be discussed with extreme caution after the quality of the RP has been defined.

Ultimate limits in high-pressure liquid chromatography☆

Owing to the adiabatic heat of friction, the temperature of an eluent may increase by up to 35° if Δp = 500 atm. The capacity ratio of a sample decreases with increasing Δp. Temperature (and consequently viscosity, Dm, etc.) gradients exist in axial and radial directions inside the column. Therefore, because of the equipment limitations, Δpmax is usually less than 500 atm. The value of umin (i.e., the velocity at the minimum of the h versus u curve) increases with decreasing practicle size and with decreasing viscosity of the eluent. With dp dp > 3 μm is proposed, except for trace analysis where columns packed with particles of smaller size could be preferable. When dp > 5 μm, the air classification of the support and the column packing become sophisticated. The influence of extra-column effects is not negligible.

Optimization of reversed-phase separations☆

Abstract The influence of the eluent composition on the retentions of samples achieved with reversed phases (RP), including tailing, is discussed. An “eluotropic series” for RP is proposed. Analogies between the retention orders in liquid chromatography with RP and in gas chromatography with graphitized carbon black are pointed out. The separations of ten mixtures important in routine work are described.

Particle size, pressure and analysis time in routine high-performance liquid chromatography☆

Abstract The maximum number of theoretical plates that can be generated in a column can be calculated from the empirical equations umindp = 10 and hmin = 4 dp (where the minimum linear velocity of the eluent, umin, is given in mm/sec and the average size, dp, in μm), if the average particle size dp is based entirely on chromatographic particle measurements. It was demonstrated experimentally that 30-cm silica or reversed-phase columns having over 7000 plates could be packed with dp = 10 μm. Such columns can resolve, with baseline peak separation, mixtures that possess relative retentions greater than 1.12. In routine work, the relative retentions are usually greater and it is advantageous to use only 7.5-cm long columns. Analysis times of 10 min or less can be achieved even with velocities around umin. Consequently, most routine separations require column pressures of less than 50 atm. For research studies and for the optimization of conditions for routine work, columns packed with 5-μm particles are often necessary. The reproducibility of column packing with dp

Mass transfer in ideal and geometrically deformed open tubes. : I. Ideal and coiled tubes with circular cross-section☆☆☆

Abstract A summary of the hydrodynamics relevant to open tubes is presented. The band broadening of unretained samples in ideal and coiled tubes with circular cross-sections (0.25–4 mm I.D.) was measured with n-heptane and water as eluents. The lack of any appreciable interference from instrumental factors is substantiated by the experimentally determined diffusion coefficients and critical Reynolds numbers. It is shown that carefully prepared copper tubes behave as ideally as the corresponding glass tubes. With decreasing tube diameter, wall roughness leads to greater radial mass transfer. This effect is evident in 0.25-mm I.D. copper tubes and 0.5-mm (or even 0.75-mm) I.D. stainless-steel tubes, and can be used to advantage in constructing heat exchangers, connecting tubes or reaction detectors in liquid chromatography. It is shown for the first time that the onset of turbulence can be detected with considerably higher sensitivity by means of h versus u curves than by the K versus u curves used previously. It is shown that at the onset of turbulence the specific permeability decreases by a factor of about 3. Band broadening in the turbulent region was found to be ten times greater than that calculated on the basis of the friction theory of Taylor, apparently because laminar conditions prevail in a substantial portion of the tube cross-section. If an ideal tube is coiled, its specific permeability decreases with increasing velocity. Furthermore, coiling stabilizes “laminar” flow and the beginning of the turbulent region is shifted to higher Reynolds number (i.e., higher velocities). It is shown that, as the inner diameter of the open tube decreases, higher linear velocities are required in order to decrease band broadening substantially (e.g., by a factor of 5) for coiled tubes than for ideal tubes. For example, a linear velocity of 250 cm/sec is necessary to achieve this factor of 5 with 0.25-mm I.D. tubing, even though the presence of secondary flow is indicated by the K versus u curve at u > 10 cm/sec. For liquid chromatographic separations tubes narrower than 0.25 mm I.D. are essential. Therfore, at acceptable linear velocities (

Eigenschaften kommerziell erwerblicher Silikagele in der schnellen Flüssigkeits-Chromatographie

SummaryThe particle sizes, specific surface areas, pore size distributions and pore volumes of 18 commercially available silicas, of both spherical and irregular geometry, have been measured, using various methods. The silicas were packed into columns whose packing densities and permeabilities were measured. The properties of all these silicas are rather similar. It has been shown that the ratio of the chromatographically determined particle sized and the particle size measured using a Coulter Counterdc is independent of the particle geometry (d≅1.35dc). The particle size quoted by the manufacturesdp lies betweend anddc. The specific surface areas of the silicas are more alike if they are normalized with respect to the packed column volume which is the chromatographically important value, instead of with respect to the weight of silica. The mean pore diameters were about 90 Å, which is optimal for routine chromatography. The pore volumes were 38±6% of the column volume.ZusammenfassungFür 18 kommerziell erwerbliche, runde sowie gebrochene Kieselgele wurden mit verschiedenen Methoden Teilchengröße, spezifische Oberflächen, Porenverteilungen und Porenvolumina gemessen. Mit diesen Kieselgelen wurden Säulen gepackt und Packungsdichten sowie Porositäten bestimmt. Die Eigenschaften all dieser Kieselgele weichen nicht weit voneinander ab. Es kann festgestellt werden, daß das Verhältnis von chromatographisch bestimmter Teilchengrößed zu der mit dem Coulter Counter gemessenendc unabhängig von der geometrischen Form 1,35 beträgt. Die von den Herstellern angegebenen Wertedp liegen zwischend unddc. Die spezifischen Oberflächen der Kieselgele sind einheitlicher, wenn sie auf das Volumen der gepackten Säulen — dies ist die chromatographisch relevante Größe-anstatt auf die Masse Kieselgel normiert werden. Die mittleren Porendurchmesser liegen um 90 Å und sind damit für die üblichen chromatographischen Anwendungen optimal. Die Porenvolumina liegen bei 38±6% des Säulenvolumens.

Stationäre phasen mit SiN bindung für die flügkeitschromatographie

Abstract SiN bonded stationary phases for liquid chromatography The preparation of SiN bonded stationary phases for high-pressure liquid chromatography on the surface of silica with a specific surface area of 370 m2/g is described. The organic part of the stationary phase has a monomeric structure. In order to “modify” the selectivity to a required value, one or more functional groups, such as −COOH, −SO3H, −NH2, −CN or −NO2, are introduced into the aliphatic chain. The spatial requirement of the organic “bristles” varies between 55 and 400 A2. Peak broadening, capacity ratios and mass transfer terms were determined in the linear velocity range 0.5–10 cm/sec with n-heptane, methanol and water as eluents. The change in the relative retention as a function of the eluent and of the functional groups is discussed.

Mass transfer in ideal and geometrically deformed open tubes : III. Deformed metal and plastic tubes

Abstract Dispersions ( h values) in deformed tubes are much better than those in coiled ideal tubes,even at lower velocities ( u h decreases almost linearly from 2.6 to 1 cm as the linear velocity is increased from 3 to 130 cm/sec, i.e., comparable h values are obtained at low velocities and in the turbulent region. Up to 70 theoretical plates per second can be generated for an inert sample. If this column were 100 m long, then with n -heptane as the eluent more than 7000 theoretical plates would be generated for an inert sample at u = 100 cm/sec at the cost of a pressure drop of only 7 bar. At high velocities the loss in permeability is only about 50% of that of an ideal tube. Efficient wavy tubes of large diameter (1–4 mm) can be produced, which would have advantages as LC columns and as reaction detectors, because their loadability would be high. Because of the low pressure drop and large peak volume, simple equipment could be used.

Determination of the pore size distribution, by exclusion chromatography, of ion-exchange polymers which swell in water

Abstract It is shown experimentally that the partial exclusion of sugars and dextrans from the pores of a cation-exchange resin with a given counter ion is, if the eluent is 0.2 M sodium sulphate in deionized water, a consequence of the geometry of these standard samples alone. A method is described for determining the pore size distribution of polystyrene-based cation exchangers in aqueous systems by exclusion chromatography. A given standard sample i can penetrate pores of a solid with diameters smaller than ∅i (“exclusion value”). Some ∅-values are tabulated for standard samples as determined experimentally. Pore size distributions of cation exchangers, as measured by the exclusion chromatographic method, are described. In some applications the available pore volume of a swellable cation exchanger is of interest. This is of course, among others, a function of the counter ion, because the available pore volume for sugars and dextrans is codetermined by the solvent shell of the counter ion (Gibbs-Donnan equation). With the method described above this influence can be determined by experiments in aqueous systems. It is demonstrated that an interaction takes place between poly(ethylene glycol)s and polystyrene-based cation-exchange resins with water as the eluent. High-performance liquid chromatographic separations of amino acids and similar compounds on silica and on polystyrene-based cation exchangers are discussed.

Probleme der schnellen Flüssigkeits-Chromatographie mit hohen Eingangsdrucken

SummaryProblems of speedy liquid chromatography with inlet pressures up to 300 at are discussed. An approximation formula is given for the estimation of the permeability of packed columns with particle sizes between 30–200 μm. Advantages and disadvantages of different types of high pressure pumps are summarized. Simple controlling equipments for smoothing the pulsation of high pressure pumps, a device for pressure program and a sample injection system are described. A gas-bubble free flow was achieved without degassing the liquid. Peak broadening effects inside and outside the column are calculated. The detectors are classified due to the kind of their recording and that of their sensitivity.ZusammenfassungProbleme der schnellen Flüssigkeits-Chromatographie mit Eingangsdrucken bis 300 at werden diskutiert. Eine Näherungsformel für die Abschätzung der Permeabilität gepackter Säulen (Teilchengröße 30 bis 200 μm) wird vorgeschlagen. Vor- und Nachteile verschiedenartiger Hochdruckpumpen werden zusammengefaßt. Eine einfache Regelanordnung zur Glättung der Pumpenpulsation, ein Regelsystem für Druckprogrammierung und ein System für das Einspritzen der Proben unter hohem Druck werden beschrieben. Eine blasenfreie Förderung der Flüssigkeit ohne Entgasen derselben konnte erreicht werden. Die Bandenverbreiterung innerhalb und außerhalb der Kolonne wird besprochen. Die Detektoren werden nach der Art der Anzeige und der Empfindlichkeit klassifiziert.