Comparison Study between Indirect Photometric and Direct Conductivity Detection for Anion Exchange Chromatography Using Naphthalenesuifonate Derivatives as Mobile Phases S.A. Maki / N.D. Danielson* Department of Chemistry, Miami University, Oxford, OH 45056, USA Key Words Column liquid chromatography Anion exchange Indirect photometric and direct conductivity detection Naphthalenesulfonate Inorganic anions Summary Indirect photometric and unsuppressed direct conductivity detection modes are examined using naphthalene mono-, di-, and tri-sulfonate as mobile phases for the separation of Several anions such as F-, CI-, NO2", Br-, NO3", SO4 =, I-, and SCN- using a commercial anion exchange column. With all three mobile phases, conductivity detection shows better Sensitivities and detection limits than indirect photometry. Conductivity detection is 5 to 16 times more sensitive than Indirect photometry for all analytes. Detection limits achieved using these mobile phases are, for example, 0.04 ng and 0.1 ng for chloride ion with conductivity and in- direct photometry, respectively. Both detection modes give Wide linear ranges extending from at least 100 ppm to the detection limit of each anion which is generally about 0.02 Pprn. Sulfur oxide anions such as dithionate and tetra- thionate are separated using flow programming with naph- thalenetrisulfonate as the mobile phase in less than 20 minutes. With both detection modes, desired chromato- graphic performance of these three eluents is achieved Without pH adjustment of the mobile phase. Introduction Ion exchange chromatography (IEC) has gained a wide ac- Ceptance in recent years especially after utilizing low CaPacity ion exchange columns for the determination of in- Organic and organic anions using conventional HPLC equipment [1]. Two detection techniques have been very POpular in anion exchange chromatography, namely, un- SUppressed direct conductivity and indirect photometric detection. Chromatographia Vol. 33, No. 1/2, January 1992 Beside the importance of the ion exchange column, the eluent in these detection modes plays crucial roles with respect to both the separation and detection of analytes. Eluents with high molar absorptivities will certainly en- hance indirect photometric detection sensitivity by allow- ing the use of relatively dilute mobile phase concentrations [2]. In conductivity detection, on the other hand, an eluent with a low background conductivity is necessary in order to detect the weak analyte signal [3]. In addition, the eluent effective charge is responsible for the elution time and the chromatographic resolution in both detection modes. Eluents with -1, -2, -3, and -4 charge have been characterized as mobile phases for ion exchange chromatography of inorganic anions such as chloride, bromide, nitrate, and sulfate. Most of the reported eluents are the salts of weak organic acids, and consequently, their effective charge and/or chromatographic reproducibility is pH dependent. Salicylate, benzoate, phthalate, sulfoben- zoate, trimesate, and pyromellitate are among those weak acid eluents commonly used for indirect photometric and direct conductivity detection modes [3-11]. Although these salts provide good chromatographic separation and sen- sitivity, most of them generate extraneous or system peaks not related to the analyte peaks [12, 13] when indirect photometric detection is chosen. These system peaks are believed to be the result of the elution of the neutral species or the undissociated form of these eluents [13, 14]. System peaks have also resulted when amino- or hydroxy-sub- stituted naphthalenesulfonate eluents are used for indirect photometric detection [15,16]. More recently, the use of un- substituted strong acid salts such as benzenesulfonate [17] and naphthalenesulfonate derivatives [18-20] has proven to eliminate both the need of controlling the mobile phase pH, as well as the presence of system peaks. Rapid separa- tions of typically highly retained analytes such as I- and SCN" were possible with naphthalenetrisulfonate as the eluent. The analyte sensitivity depends not only upon the mode of detection, but also on the type ofeluent used for that detec- tion. Sato reported that the indirect photometric sensitivity is much higher than that for conductivity detection when the salt form of dihydroxybenzenedisulfonic acid is used. He obtained, however, the opposite result when the acid form was used as the eluent [17]. A short comparison study of the separation of chloride, nitrate, and sulfate using a Originals 25 0009-5893/92/1 0025-07 $ 03.00/0 9 1992 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH