Robert L. Grob

Simplified description of high-performance liquid chromatographic separation under overload conditions, based on the Craig distribution model: II. Effect of isotherm type, and experimental verification of computer simulations for a single band

Abstract The preparative high-performance liquid chromatography (HPLC) model described in the preceding paper has been tested for nine different solute-column combinations under reversed-phase overload conditions. Following certain empirical adjustments of the model, good agreement between experimental and predicted elution curves was observed. One such adjustment relates to the value of the column saturation capacity w s . This value can be obtained either from isotherm data or from separations under overload conditions. In some cases these two w s values agree, but in other cases they differ by factors of almost 100. The latter situation appears due to strong retention of solute molecules on silanol sites, rather than on the non-polar bonded phase. Use of chromatographically derived values, of w s leads to accurate predictions of the elution band as a function of sample size. Minor adjustments were also required in the shapes of the k ′/ k o vs. w xk and N / N o vs. w xN plots predicted by Craig simulations. Finally, it was found that in every case “real” columns are more quickly overloaded with respect to N than is predicted by Craig simulation. The value of w s that can be inferred from k ′ overloading must be decreased by a factor of about 2.5 to account for the dependence of N on sample size. With these modifications of the model, it appears possible to accurately predict the elution curves for most HPLC systems as a function of sample size.

Band-spacing in reversed-phase high-performance liquid chromatography as a function of solvent strength: A simple and fast alternative to solvent optimization for method development

Abstract Numerous reports have described the use of solvent optimization for isocratic reversed-phase high-performance liquid chromatography method development. Solvent optimization involves the use of different solvents (usually methanol, acetonitrile and tetrahydrofuran) to control band-spacing for maximum resolution of the sample. Here, we examine an alternative approach, based on variation of the concentration of organic solvent in the mobile phase (solvent strength). This procedure is less powerful than classical solvent optimization, but it nevertheless possesses a significant ability to effect changes in band-spacing. It is also much more easily carried out. Many samples do not require solvent optimization, and in these cases, a change in solvent strength may be the more practical approach. The retention data required for solvent-strength optimization are most conveniently collected by using two initial gradient runs. The application of gradient retention data for developing a final isocratic separation is facilitated by the use of commercial software. The advantages and limitations of gradient-retention data for this purpose are examined.

Determination of phenols in water and wastewater by post-column reaction detection high-performance liquid chromatography

Abstract The analysis of phenols using high-performance liquid chromatography and post-column reaction detection has been studied. The system design utilizes the reaction of ten of the phenols on the U.S. Environmental Priority Pollutant List with 4-aminoantipyrine and potassium ferricyanide. The chromophores that are produced can be quantified at either 509 nm or 470 nm depending upon the structure of the eluting phenol. The separation procedures uses a linear gradient consisting of 0.2% phosphoric acid and methanol. The phosphoric acid concentration is varied from 90% to 20% in 20 min. providing for resolution of each of the ten phenols. The concentration of each of the reagents and the pH used for the post-column reaction have been studied in order to optimize the post-column quantification. In addition two methods for concentrating the phenols in water and waste-water have been examined. The recovery and repeatability of these are presented. A series of influent and effluent samples from a wastewater treatment plant have been assayed using the method presented. The results of this study are discussed.

Simplified description of high-performance liquid chromatographic separation under overload conditions, based on the craig distribution model. IV: Comparison of the model with experimental data for samples containing two or more solutes

Abstract The preceding paper presented a quantitative description of high-performance liquid chromatography (HPLC) separation under mass-overload conditions for samples that contain two or more solutes. Specifically it was suggested that such separations differ from corresponding single-solute separations in terms of a so-called “blockage” effect. The present paper describes the experimental verification of this model for samples containing two or more solutes. Examples of blockage are shown, and these are predicted quantitatively by means of our previous model. Mixtures of β-hydroxyethyl- and 7-β-hydroxypropyltheophylline were separated by reversed-phase HPLC on a 15 x 0.46 cm C 8 column. Total sample mass w was varied over the range 0.002 ⩽ w ⩽ 27 mg, leading to pronounced changes in separation. Similar studies were carried out with mixtures of methyl, ethyl, propyl and butyl parabens, varying total sample mass over the range 0.004 ⩽ w ⩽ 20 mg. Resulting separations were generally in good agreement with predictions (“model simulations”).

Simplified description of high-performance liquid chromatographic separation under overload conditions, based on the Craig distribution model : V. Gradient elution separation

Abstract The present model of mass-overloaded high-performance liquid chromatographic separation has been extended to the case of gradient elution. It is shown that the separation of two compounds by gradient elution varies with mass overload in the same way as in isocratic elution. If gradient conditions are selected to be equivalent to those in isocratic elution [for small samples, value of k (gradient) equal k′ (isocratic)], resolution is the same in either gradient or isocratic runs when the sample size is the same. Therefore gradient separations in a mass-overload mode can be predicted by means of model simulations for corresponding isocratic systems (where ko  k ). Additional relationships are derived for use in optimizing gradient separations in a mass-overload mode.

Simplified description of high-performance liquid chromatographic separation under overload conditions, based on the craig distribution model

Abstract We have previously described a model for predicting how mass overload affects the elution of a single solute-band in high-performance liquid chromatography (HPLC). This model was developed from initial computer simulations (Craig distribution plus Langmuir isotherms), and was subsequently modified on the basis of experimental data for several solute—column systems (reversed-phase HPLC). Reasonable agreement was found between experimental elution bands and data predicted by the final computer model (“model simulation”). In the present study we have extended model simulation to the case of two co-eluting solutes. Isotherms were derived for Langmuir-type sorption of two competing solutes (“mixed isotherms”); these data were then used in computer simulations based on Craig distribution. Resulting data for the mass-overloaded two-band case could be generalized in terms of our previous model for a single elution band. It appears that each of the two solutes effectively “blocks” the front of the column, thus reducing the column length available for sorption of the other solute. The extent of column-blockage can be related to the mass of each solute in the sample, and the ratio of k′ values for the individual solutes (blockage by the more strongly retained solute is greater; blockage is also greater when the two solutes have more similar k′ values). On the basis of this column-blockage model, a computer program has been developed that predicts the elution of each band as a function of mass overload for two f-solute samples. The latter model simulations agree reasonably well with Craig simulations for the same system.

The use of vanadium(II), manganese(II), and (cobalt(II) chlorides as packings to separate alkanes, alkenes, and alkynes by gas chromatography

Abstract The anhydrous chlorides of vanadium(II), manganese(II) and cobalt(II) were studied for possible use as packings in gas-solid chromatography. These compounds differ primarily in their number of available “3d” electrons, viz. V(II)(3d3), Mn(II)(3d5) and Co(II)(3d7). Various saturated and unsaturated organic compounds were investigated as adsorbates to observe the influence of varying π-electron densities. The heats of sorption were calculated and found to vary directly with the π-electron density of the adsorbate and vary inversely with the number of 3d electrons of the adsorbent. Separations achieved were a result of the π-electron density of the samples. Conjugated systems adsorbed on the salts were studied, but the column appeared to be best utilized for compounds having isolated π-bonds. A highly electronegative group, not near to a π-bond, appeared to have little effect on the degree of sorption. Although it was not our purpose to determine absolutely the mechanism of sorption, our data indicate that chemisorption plays a major role in the interaction between adsorbent and adsorbate.

Headspace determination of solubility limits of the base neutral and volatile components from the environmental protection agency's list of priority pollutants

Abstract Static headspace or vapor equilibrium analysis where the aqueous solution is allowed to equilibrate with the gaseous phase above it, was recently reported as a simple method for determining the solubility limits of some volatile aromatic priority pollutants and a few purgeable halocarbons 1 . The technique used for those analyses can be used to determine the solubility limit of other compounds on the Environmental Protection Agencys list of priority pollutants. We report solubility limits of halogenated alkanes and alkenes of the volatile fraction of the priority pollutant list and additional compounds of the base neutral fraction as determined by the gas chromatographic headspace analysis technique.

Determination of the solubility limits of organic priority pollutants by gas chromatographic headspace analysis

Abstract Up to now, most data tables and literature references listing solubility limits have used g per 100 ml or mg/ml as the units. Compounds which have solubility limits that fall within the parts per million (ppm) range have generally been listed as insoluble, with no numerical value given. Previously, such low solubility limits were not needed, nor was it possible to detect accurately these quantities. Headspace or vapor equilibration analysis, where the aqueous solution is allowed to equilibrate with the gaseous phase above it, can be used to determine the solubilities of compounds with even moderate volatility. Organic priority pollutants are a class of compounds where this information is not only useful but is needed. The use of the headspace or vapor equilibration method to determine these solubilities is discussed.

Indirect method for the determination of water using headspace gas chromatography

Abstract This research investigated an analytical method for the quantitative determination of water using headspace gas chromatography (HSGC) and the reaction of calcium carbide and water to produce acetylene. Samples containing water were transferred to a dry vial containing calcium carbide, after which the vial was sealed. The acetylene which formed inside of the vial was then measured by HSGC and used to calculate the original concentration of water in the sample. The investigation indicates that the method is a viable alternative for the determination of water in various organic solvents, for concentration ranging from 60 to 400 ppm (μl −1 .