John G. Dorsey

Micelles in Analytical Chemistry

Abstract Molecules which possess both hydrophilic and hydrophobic structures may associate in aqueous media to form dynamic aggregates commonly called micelles. Interest in these aggregates commonly called micelles. Interest in these aggregates has grown over the years from the original work of Hartley,1 which describes what is still accepted as a functional model of the geometry of micelles. Fortunately, our understanding of the geometry as well as other features of micelles has increased markedly over the intervening years as evidence by the abundance of reviews concerning micelles just since 1980 (over 350 reviews as searched via online CAS). While mose of these papers deal with micelle structure, catalysis, or dynamics, this report continues a tradition of reviews concerning the utility of micelles in analytical chemistry.

Clarification of the limit of detection in chromatography

SummaryCurrent problems with the limit of detection (LOD) concept in chromatography are reviewed. They include the confusion of the LOD with other separate, distinct concepts in trace analysis such as the minimum detectability (MD); the use of arbitrary, unjustified models for the calculation of the LOD; the use of concentration units instead of units of amount; and the failure to account for differences in chromatographic conditions when comparing LODS.Solutions to these LOD problems are discussed. Two models are proposed for calculating the chromatographic LOD. A new concept, the standardized chromatographic LOD, is introduced to account for differences in chromatographic bandwidths of experimentally measured LODs. The standardized chromatographic LOD is shown to be a more reliable parameter than the conventional (non-standardized) chromatographic LOD.

The measurement and meaning of void volumes in reversed-phase liquid chromatography

The seemingly simple process of measuring the mobile phase volume, V0, in reversed-phase liquid chromatography has eluded unambiguous agreement for over 25 years. Examples exist in the literature where the reported volume is physically impossible, either equal to or larger than the empty column volume, or being so small that it would represent a total porosity of half the theoretical limit for well-packed columns. Here we review the many proposals for methods of measurement, and compare and critique them. At this time, there is still no consensus for the best method of measurement, and workers are urged to critically examine values they measure, to insure they are at least physically possible.

Retention mechanisms in reversed-phase chromatography : Stationary phase bonding density and solute selectivity

Chromatographic selectivity for small, non-polar solutes has been determined as a function of monomeric octadecyl stationary phase bonding density over the range 1.74-4.07 mumol/m2. Phenyl or shape selectivity increases with increasing bonding density, whereas methylene selectivity remains approximately constant. These findings are in agreement with the mean field statistical thermodynamic theory of Dill, which predicts that increased stationary phase chain density should lead to increased anisotropic chain ordering and increased solute-shape selectivity. These studies provide further evidence that partitioning, not adsorption, is the dominant mode of retention for small, non-polar molecules in reversed-phase liquid chromatography.

Polyoxyethylene alkyl ether nonionic surfactants: physicochemical properties and use for cholesterol determination in food.

Polyoxyethylene alkyl ethers, C(n)E(m), are nonionic surfactants made of an alkyl chain with n methylene groups and a hydrophilic part with m oxyethylene units. C(n)E(m) nonionic surfactants are very useful in chemical analysis. The commercially available products are often a mixture of several C(n)E(m) molecules with different m values. Pure C(n)E(m) surfactants are now available. The physicochemical parameters: critical micelle concentration (c.m.c.), molar volume, density, cloud-point temperature and hydrophile-lipophile balance value for pure C(n)E(m) surfactants were collected from the literature. Regression analyses were carried out on the data. They showed that strong correlations existed between the structure of the molecule (n and m values) and its physicochemical properties. General equations linking the c.m.c., molar volume, density and cloud-point temperature of the C(n)E(m) surfactants and their structure (n and m values) are proposed and discussed. The use of these surfactants in chemical analysis is illustrated by the determination of cholesterol in egg yolk. Cholesterol was separated from the bulk yolk by cloud-point extraction using the C(12)E(10) surfactant. It was quantitated using micellar liquid chromatography. The C(12)E(23) surfactant was used to prepare the micellar mobile phase that allowed the separation of cholesterol and the use of an enzymatic detector.

Evaluation of methods for measuring amino acid hydrophobicities and interactions.

The concept of hydrophobicity has been addressed by researchers in all aspects of science, particularly in the fields of biology and chemistry. Over the past several decades, the study of the hydrophobicity of biomolecules, particularly amino acids has resulted in the development of a variety of hydrophobicity scales. In this review, we discuss the various methods of measuring amino acid hydrophobicity and provide explanations for the wide range of rankings that exist among these published scales. A discussion of the literature on amino acid interactions is also presented. Only a surprisingly small number of papers exist in this rather important area of research; measuring pairwise amino acid interactions will aid in understanding structural aspects of proteins.

Gradient elution micellar liquid chromatography

Abstract The usefulness of micellar mobile phases for gradient elution reversed-phase chromatography is demonstrated. Adsorption isotherms are shown to prove that no further stationary phase modification occurs above the critical micelle concentration. Micelle concentration gradients are then possible with no column re-equilibration necessary between injections. The strength of common micellar solutions is calculated and compared to methanol. Micellar concentration gradients are also shown to be compatible with electrochemical detection with minimal background current shifts.

Speciation of inorganic and organotin compounds in biological samples by liquid chromatography with inductively coupled plasma mass spectrometric detection

This paper describes the effect of inorganic tin chloride on the separation of trimethyl-, tributyl- and triphenyltin-chlorides by reversed-phase ion-pair high-performance liquid chromatography with detection by inductively coupled plasma mass spectrometry. The detection limits are 1.6 pg, 1.5 pg and 2.3 pg as tin for trimethyltin, tributyltin and triphenyltin, respectively. The relative standard deviation for ten injections of 20 ng of the tin compounds was less than 5%. Inorganic tin was held strongly on the columns used, to a greater extent on the silica column compared to the polymer column. Extraction and determination of tributyltin and triphenyltin as chlorides in fish tissue (certified reference material) and tuna fish (grocery store) were performed. The recovery study from fish tissue showed an efficiency of over 90% for both tributyltin and triphenyltin and over 60% recovery for spiked tuna.

Determination of cisplatin and some possible metabolites by ion-pairing chromatography with inductively coupled plasma mass spectrometric detection

A sensitive method is described for measuring cisplatin and some possible metabolites. The method combines reversed-phase ion-pairing liquid chromatography (LC) with inductively coupled plasma mass spectrometry (ICP-MS) for platinum-specific detection. Separation conditions for cisplatin hydrolysis products and the reaction products of cisplatin with methionine, cysteine, and glutathione have been investigated with sodium dodecylsulfate or heptanesulfonate as the ion-pairing agent. The detection limit for cisplatin was found to be 0.1 ng, corresponding to a concentration detection limit of 1 ng/ml when using an injection volume of 100 microliters. This study has demonstrated the usefulness of LC-ICP-MS for cisplatin metabolism studies.

Characterization of micellar mobile phases for reversed-phase chromatography

Abstract Cationic, anionic, and nonionic surfactants are characterized for their usefulness as micellar mobile phases in reversed-phase chromatography. Conditions found previously to provide optimum chromatographic efficiency for sodium dodecyl sulfate also provide high efficiency for the cationic and nonionic surfactants studied. The use of 3% n-propanol in the micellar mobile phase and column temperatures of 40°C appear to offer a broadly applicable solution to the low efficiency previously reported for micellar mobile phases. A chromatographic method for the determination of critical micelle concentrations is reported; it compares favorably with literature methods. Micellar mobile phases are shown to mimic ion-pairing mobile phases, allowing the separation of neutral solutes as well as solutes charged oppositely to the surfactant and offer a more rugged method of analysis than hydro-organic ion-pairing methods.