fOII*
IWICNEIICPIITI©'
T h e Λ ι * «Γ.*:- "κ
ed by Johnson and his students at Iowa State University (20,21) is particularly attractive for organic compounds and ions that are difficult to detect by other means. This detector makes construc tive use of a long-standing enemy of electrochemists—the adsorption of or ganic compounds on a noble metal elec trode. The PAD detects organic mole cules and radicals using the faradaic signal resulting from their oxidative desorption. The separation of 13 carbo hydrates on an anion exchange column with electrochemical detection is a good example of the usefulness of the PAD detector (22). Programming
Richard P. Steiner, Editor Takes the mystery out of miracle cures. Explores the medical prac tices of nonwestern cultures to establish a scientific basis for the successes of folk remedies. Ex plains why western medical re searchers are increasingly turn ing their attention to folk medicine for new drugs. Brings together work from many coun tries and a variety of cultures. CONTENTS Aztec Sources of Some Mexican Folk Medicine · Zuni Indian Medicine: Folk lore or Pharmacology, Science or Sor cery? · Ayurveda: The Traditional Medicine of India · Fijian Medicinal Plants · Medicinal Plants of Papua New Guinea · Australian Medicinal Plants · Plants Used in African Traditional Medi cine · Antithrombotic Agent of Garlic: A Lesson from 5000 Years of Folk Medi cine · Scientific Basis of the Therapeu tic Effects of Ginseng · Anticancer Chinese Drugs: Structure-Activity Relationships · Some Recent Biological Characterizations of Chinese Herbal Preparations · Bioactive Compounds from Three Chinese Medicinal Plants · Zingiberaceious Plants · Alkaloid Com ponents of Zizyphus Plants 215 pages Clothbound (1985) LC 85-22904 ISBN 0-8412-0939-1 US & Canada $22.95 Export $27.95 Order from: American Chemical Society Distribution Dept. 99 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your credit card!
The use of temperature programming in gas chromatography and solvent (eluent) programming in high-perfor mance liquid chromatography (HPLC) is considered to be almost essential for resolving complex samples. The major ity of ion chromatographic separations up to now, however, have been per formed using isocratic elution. Main taining a steady baseline with a con ductivity detector while gradually in creasing the eluent concentration has proved to be difficult. Some success is now being achieved in suppressed anion chromatography with a basic eluent such as sodium hy droxide. The newer membrane sup pressors can handle a higher concen tration of sodium hydroxide (convert ing it to H2O) and still maintain a reasonable baseline. It is difficult, how ever, to prepare and maintain a solu tion of sodium hydroxide that is com pletely free of carbonate. As the pro grammed concentration of sodium hydroxide increases, the base line conductance increases because of higher concentrations of carbonic acid from the eluent impurities. Eluent programming is fairly easy in ion chromatography when the sample ions are detected by a direct method that is not affected by changes in the eluent composition. Detection of metal cations using a postcolumn reactor and spectrophotometry detector lends it self to eluent programming (9). Eluent programming has also been successful in the separation and potentiometric detection of halides and pseudohalides (23). The time required for separation of five anions was reduced from 8 min to just over 4 min by eluent program ming. Sample pretreatment Anions and cations can be separated and measured in extremely dilute sam ples, such as condensed high-purity steam and rainwater, provided a preconcentration step is included in the analytical procedure. Separation is usually accomplished by passing a rela tively large aqueous sample through a
342 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
small ion exchange precolumn. Then a switching valve is turned so that the eluent sweeps the accumulated ions from the precolumn onto the separa tion column. Ion concentrations down to the very low parts-per-billion range can be determined by this method. Concentration of dilute samples can also be achieved by injecting a larger than usual sample volume directly onto the separation column, but this may cause a broad dip or peak in the chromatogram as the sample plug flows through the separation column. Organic matter in samples to be ana lyzed by ion chromatography may foul the column by being irreversibly ad sorbed. This result can often be pre vented by first passing the sample through a resin column (such as Rohm and Haas XAD-2) that adsorbs organic material and permits inorganic ions to pass through unchanged. Applications Ion chromatography is no stranger to the "real world" of chemical analysis. It is being used extensively for the deter mination of ions in drinking water, nat ural water, wastewater, air, aerosols, detergents, polymers, food and plant matter, soils, sediments, and rocks. Ion chromatography is also used in clinical analysis and in industries such as phar maceuticals, pulp and paper, and metal plating, to name just a few. One reason for such popularity is that the technique is rapid and easy to perform. Analysis takes only a few min utes, even for complex samples, and several ions can usually be determined in each chromatogram. Sample prepa ration is usually simple—just "dilute and shoot." Before ion chromatogra phy, determination of ions sometimes took hours for each sample, and usually only one ion could be measured at a time. Another point worth mentioning is that ion chromatography has brought ion analysis to the "masses"—to people who previously had no other methods for measuring ions. One example is its use in studies on acid rain. There are two problems here: Ion concentrations in the samples are often so low that a very sensitive analytical method must be used, and analysis of a large number of samples is usually necessary. With ion chromatography, samples can be concentrated in the field using small ion exchange cartridges. Later, a car tridge can be placed in the injection valve of an ion chromatograph, and a chromatogram of the sample can be generated. In power plants it was impossible to measure chloride in water at low con centration levels before ion chromatog raphy came into use. Because the levels of chloride in the boiler were not known, the harmfulness of corrosive
