Gradient elution ion chromatographic determination of inorganic anions using a continuous gradient

Abstract

A method is described for the production of a gradient elution solvent delivery system for ion chromatography. The system uses elther a two pump or a three pump configuration wlth a pressure of less than 400 psi. A 100-mm separator column and a fiber suppressor column are used along with both etectrochemical and conductometrlc detectlon. The Initial eluent is a 0.003 M NaHCO, to which a strong eluent (elther 0.004 M Na,CO, or 0.002 M Na,CO, + 0.002 M NaOH) is added. With the component arrangement described, F-, Ci-, and NO2-elute under essentlaily isocratic conditions. I-, SCN-, Cr2072-, and S20: -ail elute within 22 min. Retention times of less than 22 min are given for 18 different anions. The preparatlon and chemical composltlon of the gradients are discussed and gradlent proflles are presented. Ion chromatography as developed by Small et al.(1) has significantly altered the determination of inorganic and organic ions (2). One of the limitations to ion chromatography that restricts its use is that the relative retention times for species of interest can vary widely reducing the number of species amenable to analysis in any one given sample injection. A prominent example of this variation is in the analysis of the halide ions. The conditions necessary for the analysis of Fand Cl-ions result in an unacceptably long retention time and peak shape for the I-ion. Conversely, the conditions necessary for the efficient analysis of I-results in little or no separation between the F-and C1-peaks (3).Three common approaches, when confronted with samples containing species with widely varying retention times, are to use multiple injections under different analytical conditions (4,5), stronger eluents (6), or traditional eluents with a shorter separator column (7). These methods result in either extended analysis time or significant changes in the early portion of the chromatogram including loss of peaks. Silica-coated polyamide crown resins have been employed (8) but the resolution of common ions suffers to some extent. A method developed by Wang et al. (3) used a sequential electrochemical and conductometric detection scheme to analyze for F-, C1-, Br-, and I-in one sample injection. The analysis time reported by Wang et al. was approximately 30 min, but the method did require that the operator switch a valve a t a critical time in the analysis to redirect the fluid flow and thus minimize the analysis time.The concept of gradient elution chromatography has been widely and successfully employed in high-performance liquid chromatography for the analysis of species with widely varying retention times (9). Gradient elution has been applied to the analysis of SOS2-, Sod2-, and Sz02-containing solutions by Sunden et al. (10) but no attention was given to the early eluting species such as F-or C1-.

Gradient anion chromatography with hydroxide and carbonate eluents using simultaneous conductivity and pH detection

“…In fact, even without the intentional introduction of any electroactive species, the background current of electrochemical detectors with aqueous eluents is acutely pH dependent in the absence of large amounts of a supporting electrolyte and increases with decreasing pH. Although it has not been clearly identified as pH detection, Tarter's observations (5,16) that an electrochemical detector placed after the suppressor responds to anions that are not electroactive are thus readily explicable.…”

Three-Dimensional Calibration for Routine Analyses of Bromide and Nitrate Ions as Indicators of Groundwater Quality in Coastal Territories

“…However, these methods often suffer interference by high nitrate concentrations (>100 mg·L −1 ) [38]. Electrochemical detectors may be used for determination of bromide in aqueous samples [42,43,44,45,46] but not for nitrate ions, which do not produce any signal. Electrostatic ion chromatography [47] and capillary zone electrophoresis [48] have been also used to separate bromide and nitrate ions in complex matrices but these apparatuses are not generally available for routine analyses.…”

Simultaneous determination of bromide and nitrate in contaminated waters by ion chromatography using amperometry and absorbance detectors

“…Several studies have reported the application of electrochemical detector (DC Amperometry) for determination of oxidizable species like Cl − , Br − , I − , SCN − , S 2 O 3 2− and S 2− in aqueous samples [23][24][25][26][27]. In DC amperometry, a constant voltage is applied to the working electrode and the resulting current due to ion oxidation or reduction is monitored.…”