L. S. Ettre

Quantitative analysis of hydrocarbons by structural group type in gasolines and distillates : II. Liquid chromatography

Abstract After summarizing the literature on the use of liquid chromatography for the analysis of petroleum samples and distillates, results are presented on the structural group type separation of gasolines by high-performance liquid chromatography (HPLC) using a perfluorocarbon mobile phase and refractive index detection. The problem of calibration, due to the difference in response factors, and the uncertainty in the establishment of the baseline are detailed. Finally, investigations on the possibility of using infrared detection for structural group type analysis by HPLC are reported.

Considerations of the retention index concept : I. Retention index and column temperature

Abstract The influence of column temperature on the retention index values is discused in general terms. It was found that on squalane liquid phase and for hydrocarbon samples, a linear relationship can be assumed between retention index and temperature while on polar liquid phases, these plots seem to show some curvature. Retention index vs . temperature plots permit the better utilization of published data, reveal some general rules in the temperature dependency of different sdubstances and help in the choice of the proper analysis temperature and in the identification of unknown mixtures.

Very-high-speed liquid column chromatography the system and selected applications

SummaryInstrumentation needed to achieve very high performance (plate/s) liquid chromatography is discussed. With recently available systems using relatively short (100–125 mm) columns of conventional internal diameter (4–6 mm) and packed with 3–5 μm particles, analysis in very short times (usually 1–3 min) is possible. Coupling such columns in series provide very high performance in a time still significantly less than previously encountered. Performance of such liquid chromatographic systems is illustrated with a number of practical examples including analgesic tablets, cosmetics, soft drinks, antioxidants and polyaromatic hydrocarbons.

Selectivity tuning of serially connected open-tubular (capillary) columns in gas chromatography. Part I: Fundamental relationships

SummaryThe evolution of the use of mixed phases or serial column combinations is outlined, leading to systems with fixed-length columns to be used in selectivity tuning (“multi-chromatography”). The difference between multi-chromatography and multi-dimensional gas chromatography is outlined.After discussing the system used in subsequent work, the fundamental relationships of multi-chromatography are detailed. Among the basic relationships the additivity of retention data, the relationship between the individual and composite capacity factors and the relative retentivity serving as the fundamental parameter of a multi-chromatography system are discussed. The relative retentivity is derived as a function of gas holdup times, pressures, and average velocities or flow rates. Finally, the relationships between individual vs. composite relative retention, efficiency, and resolution values are deduced.

Viscosity of gases used as the mobile phase in gas chromatography

The vicosity-temperature relationship of the three gases (helium, nitrogen and hydrogen) most frequenly used as the carrier gas in gas chromatography is studied. Based on available data, equaitions are derived to describe this relationship. Using these equations viscosity data are tabulated for the chromatographically important temperature range.

Selectivity tuning of serially connected open-tubular (capillary) columns in gas chromatography. Part II. Implementation

SummaryAs a follow-up to our previous publication (see ref. [1]) dealing with the experimental system and the fundamental relationships of multi-chromatography („polarity tuning”), the present paper deals with the operation, characteristics and performance of the experimental set-up, and presents experimental verification on the validity of the basic relationships. The possibilities of influencing and adjusting the range in which the capacity factor is changed by the proper selection of column parameters are outlined and the observed changes in the retention index demonstrated. Ways to optimize the relative retentivity to achieve the best possible resolution of multi-component mixtures is illustrated. Finally, the possibility of combining temperature-programmed operation with selectivity tuning in a multi-chromatography system is demonstrated.

Performance of open tubular columns as a function of tube diameter and liquid phase film thickness

SummaryWall-coated, open-tubular (capillary) columns prepared from different diameter tubing, with different liquid phase film thickness, are compared with each other and with packed and support-coated open-tubular columns. The comparison is based on the variation of the phase ratio and the capacity factor, and includes column efficiency (HETP, theoretical plate number), resolution, retention time, and sample capacity. Problems associated with the evaluation of the sample capacity are outlined. The influence of temperature on column performance is discussed in detail. Finally, the possibilities of short, wide-bore open-tubular columns prepared with a thick liquid-phase film are demonstrated.

Generalized equations to evaluate the gas hold-up time of chromatographic systems

SummaryThe two approaches used for the calculation of the gas hold-up time and thus, the adjusted retention time are outlined, first for the restricted case when the carbon numbers of the three homologues used for the calculation are evenly spaced (c3−c2=c2−c1) and then, for the general case when (c3−c2) differs from (c2−c1). The basic difference in the philosophy of the two approaches is shown.

Open tubular columns prepared with very thick liquid phase film I. Theoretical basis

The influence of the liquid phase film thickness in open-tubular (capillary) columns on the individual resistance-to-mass-transfer terms and on the overall column efficiency is examined from the theoretical point of view. Methods for the calculation of the diffusion coefficients are discussed.

Open tubular columns prepared with very thick liquid phase film II. Investigations on column efficiency

The performance of a column prepared with an immobilized (chemically bonded) methylsilicone phase having a film thickness of 5μm is investigated in detail. Due to their high sample capacity, such columns are particularly useful when directly coupling gas chromatography with spectroscopic techniques such as Fourier-transform infrared spectroscopy. Thick-film columns can even be used with thermal conductivity detectors.