Roy D. Rocklin

Determination of carbohydrates by anion exchange chromatography with pulsed amperometric detection

Abstract Carbohydrates such as sugar alcohols, monosaccharides, disaccharides, and other oligosaccharides are separated as anions by ion exchange chromatography with a sodium hydroxide eluent. Retention time and selectivity are controlled by varying eluent strength and column temperature. The carbohydrates are detected by oxidation at a gold electrode. A repeating sequence of three potentials electrochemically cleans the electrode surface of oxidation products and other interfering species. Detection limits are as low as 30 ppb for sugar alcohols and monosaccharides, and about 100 ppb for oligosaccharides. Other species containing CHOH groups can also be detected, such as alcohols and glycols.

Gradient elution in ion chromatography

Abstract Both theoretical and practical considerations for the use of gradient elution in ion chromatography are discussed. The theory for the dependence of an analytes retention on eluent concentration for both isocratic and gradient elution is presented and shown to agree with experimental results. Practical solutions to problems not encountered in isocratic ion chromatography are presented. Background conductivity changes caused by increasing eluent concentration are greatly minimized by the use of eluents containing salts of weak acids with p K a values greater than 7, which are converted to low-conductivity free acids in the suppressor. Sodium hydroxide based eluents produce a minimal baseline shift. Small baseline shifts produced by other eluents can be compensated by chemical means. Using a gradient, as many as 36 ions can be separated and eluted in one run.

Capillary electrophoresis of inorganic anions and organic acids using suppressed conductivity detection strategies for selectivity control

The use of suppressed conductivity as a detection scheme for capillary electrophoresis (CE) is described. A comparison is made between several electrolytes for CE with suppressed conductivity detection (CESC) in terms of efficiency of separation and peak shape. The ability to modify electrophoretic mobility and selectivity as a function of temperature and electrolyte ionic strength is demonstrated. The separation of a variety of low-molecular-mass organic acids is optimized using the addition of metal ions to the separation electrolyte.

Separation and Detection of Carboxylic Acids by Ion Chromatography

Abstract Common carboxylic acids such as most Krebs cycle acids and other organic acids can be separated by either anion exchange or ion exclusion chromatography and detected by conductivity with chemical suppression. This is accomplished using a new generation of high dynamic capacity suppressors which allow separations on moderate and high capacity resins with concentrated eluants. For both anion exchange and ion exclusion chromatography, cation exchange membrane based suppressors are used to reduce the conductivity of the eluant while enhancing the conductivity of the organic acid analytes. The ability to choose between anion exchange and ion exclusion chromatography expands the versatility of ion chromatography by increasing the analysts ability to solve co-elution problems.

Detection in ion chromatography

Abstract Species determined by ion chromatography are nearly all ionic, so that conducitivity detection has become the workhorse detector. However, there are several other important types of detectors which offer major advantages for determining many species by ion chromatography. These include other forms of electrochemical detection, specifically d.c. and pulsed amperometry, as well as optical methods of absorbance and flourescence. Using these forms of detection, nearly all forms of ionic species can be detected, ranging from inorganic ions to carbohydrates and peptides. In this paper, the factors to be considered when selecting a detection method are reviewed, for example, the properties of different classes of ions which make them amenable to a specific form of detection.

Maximizing signal-to-noise ratio in direct current and pulsed amperometric detection

Abstract The magnitude of signal obtained during d.c. and pulsed amperometric detection using a thin-layer type cell is dependent on several factors, two of which are controlled by the cell design. These two factors are the surface area of the working electrode and the mobile phase velocity over the surface of the working electrode. Mobile phase velocity is controlled by the thickness and width of the thin-layer channel gasket. In this report, the effect of varying working electrode size and gasket dimensions are studied. Using 1 mm diameter working electrodes and a 25 μm × 1.3 mm gasket, the minimum detection limit for dihydroxybenzylamine is about 6 femtomoles and for glucose, about 200 femtomoles.

Linearity in Ion Chromatography

Abstract Using ion chromatography with chemical eluant suppression, peak heights and peak areas are determined for the anions chloride and sulfate and the cations sodium and potassium in standard solutions. Calibration curves are prepared for the ions and shown to be linear over the tested range, nearly five orders of magnitude.