Claude Eon

Study of the pertinency of pressure in liquid chromatography : II. Problems in equipment design

Abstract The contributions of different parts of the chromatograph to peak broadening (injection, connection tubing, detector cell volume and detector response time) are discussed in terms of the recent trend in liquid chromatography to use finer particles, shorter columns and lower pressures. It is shown that although they tend to become more and more severe, the specifications are still not too drastic and that presently available chromatographs, when handled with care, are sufficient (in some cases only just sufficient) to allow the separation in a few minutes of compounds that require several thousand plates. Injection should be made on-column or just on the top of the column, very short connection tubing should be used and preference should be given to small cell detectors. The most critical aspect at present is the detector time constant, which is too great in many instances and should not exceed 1 sec.

Reversed-phase liquid-solid chromatography on modified carbon black

Abstract The preparation of carbon adsorbents suitable for reversed-phase high-performance liquid chromatography is described. The effect of the amount of carbon deposited by benzene pyrolysis on thermal carbon blacks on the hardness and specific surface area of the particles obtained is discussed. Optimum performances are obtained with 20% of pyrocarbon. The chromatographic performance of the columns is discussed in terms of coefficients of the Knox and Giddings equations and a scale of solvent eluotropic strengths is established. Efficiencies corresponding to HETP values of 3 and 7 times the particle diameters are obtained for unretained (k′ = 0) and retained (k′ = 1.1) compounds. The polarity of the solvent has little influence on the retention but affects the solubility in the mobile phase. The weakest solvents are low-molecular-weight, polar molecules such as water, methanol and acetonitrile, and the strongest solvents are heavy and/or aromatic molecules, such as chloroform, benzene and xylene. Homologous compounds are easy to separate, as well as geometrical isomers. Some examples of separations are described and discussed.

Modified carbon black in high-performance liquid chromatography

Abstract The potential use of modified carbon blacks as adsorbent for high-performance liquid chromatography is discussed. The technique of particle hardening through benzene pyrolysis is described. It is shown that the consolidation must be carefully controlled, as too little hardening gives mechanically unstable packings and too much gives a heterogeneous adsorbent of small specific surface area. The advantages of graphitization after hardening are discussed in terms of efficiency and column capacity. The problems of specific surface area, small particle size and efficiency of the adsorbent at relatively large retention are pointed out. Some separations are given in order to illustrate the performance of the adsorbent.

Study of the pertinency of pressure in liquid chromatography: I. theoretical analysis

Abstract Probable future liquid chromatographs are discussed, with special attention to the question whether there is any other limitation than todays technology to the use of finer particles and higher pressures. This discussion is based on new equation, relating pressure and column length at constant analysis performance parameters (i.e. constant retention time and resolution). The main conclusions seem to be that although pressure is always required and a minimum pressure is necessary to ensure any given analysis, the necessary pressure is much lower than the pressures currently used. This pressure optimization allows either to “save” pressure or to employ it optimally in analysis for which extreme performance is needed. With respect to the particle diameter there seems to be a technological limit, or at least a barrier, not far below 4–5 μm. Practical implications of the results obtained are discussed, emphasizing detector requirements and column design.

The use of syringe-type pumps in liquid chromatography in order to achieve a constant flow-rate

Abstract The variation of the flow-rate of mobile phase pumped through a chromatographic column by a screw-driven syringe-type pump is discussed. It is shown that because of the compressibility of liquids, a steady-state flow is achieved only after a long period, which depends on the characteristics of the pump, the flow resistance of the column and the nature of the liquid. This period is normally between 15 and 60 min if liquid pumping starts from zero pressure at a constant piston speed. The retention times observed for compounds injected during the transitory period can be up to several times greater than those observed for the same compounds under steady-state conditions. All retention data and efficiencies measured during the transitory period are not reproducible and are meaningless. Quantitative results also are not reproducible. Instructions are given for the better use of this type of pump, which, in spite of some well-known advantages, has been found to be very difficult to use for any measurements.

Study of the pertinency of pressure in liquid chromatography III. A practical method for choosing the experimental conditions in liquid chromatography

Abstract A simple method is derived for selecting the experimental conditions under which a given analysis should be carried out. This method allows the choice of a compromise between speed of analysis, resolution and pressure drop and rules are given that permit the best possible use of the column packings available to be made. Further calculations show that a large increase in pressure drop is necessary in order to reduce markedly the analysis time below that achieved when working at maximum column efficiency: a 10-fold increase in pressure reduces the analysis time by a factor of 4 at constant resolution, because the column length should be increased in order to keep the resolution constant. Finally, it is shown that the present state of the art makes available columns with peak capacities between 200 and 300, thus allowing the analysis of fairly complicated mixtures.

Theoretical and experimental study of recycling in high-performance liquid chromatography

Abstract Optimal recycling would demand zero extra-column band broadening connections, but it can be shown that under realistic conditions, even far from the optimal, resolution is always increased if the cycle number is sufficiently large. However, this cycle number is limited by the condition that the first eluted peak should not re-mix with the last one. For this reason, recycling of complex mixtures without fraction collection is very limited, if not impossible, but the potential of recycling a mixture of two closely eluted compounds is excellent. In such a case, recycling shows well known advantages in pressure and column length over one-cycle systems giving identical resolution and analysis time. It also displays interesting advantages in analysis time and dilution over one-cycle systems working at the same pressure. Experimental characteristics of recycling under analytical conditions with a Water M6000 pump and under preparative conditions with an Orlita MS 15-7-7 pump are given and compared with calculated results.

Surface activity coefficients as studied by gas chromatography comparison with bulk properties

Equations allowing the calculation of gas-liquid interface and bulk liquid activity coefficients from gas-liquid chromatography data have been derived. The method appears to be sensitive and quite reliable in the range of infinite dilution. Practical results seem to show that the surface activity coefficients are independent of the bulk properties for polar systems leading to a small solubility of the solute in the stationary phase.

Correlations between retention volumes and the interfacial tension of the two phases in liquid—liquid chromatography

Abstract Is is shown that the interfacial tension between the mobile and the stationary phases reflects the partition properties of the two phases, and this can be taken as a criteria of choice of the systems used in liquid—liquid chromatography. A semi-theoretical treatment shows that the Gibbs free energy of partition of solute between the two phases is quasi-proportional to the surface tension, provided that the solubility of each phase in the other is small. Such behaviour is observed with organic solvent—water systems for solvents that give a surface tension greater than 20–25 dynes/cm. As mixtures of mobile phases behave in the same way, the retention times in programmed gradient analysis can be predicted.

Rétention de chélates métalliques en chromatographie gaz-liquide : Importance relative des phénomènes de dissolution et d'adsorption

Abstract Retention of metal chelates in gas—liquid chroamtography. Relative importance of dissolution and adsorption phenomena It has been shown that the retention of metal chelates on various stationary phases is mainly due to the absorption at the gas—liquid interface. However, the classical equation which describes the retention when mixed mecanisisms occur is inappropriate to represent the behavior of such systems. The reason for this failure can be traced to the fact that both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase.