Comparison of nonporous silica-based ion exchangers and monolithic ion exchangers in separations of inorganic anions.

Low capacity anion exchangers for IC have been prepared by modification of nonporous uniformed silica MICRA microbeads and by modification of the organic polymeric monolithic matrixes prepared in situ in quarz capillary. Due to the small particle size (1.5 microm) high-performance adsorbents were prepared allowing to obtain up to 190,000 tp/m. However, the column possesses a very high back-pressure and can be used in a short length up to 50 mm only to meet the requirements of conventional chromatographic equipment. An analysis of a test mixture of seven anions was completed within 3 min with a back column pressure of about 350 bar (HETP of about 5.5 microm, where HETP is the height equivalent to the theoretical plate). Monolithic capillary columns provide lower efficiency per column unit length than MICRA columns; however, they can be used at a longer length because of their low flow resistance. Monolithic column of ca. 40 cm length has workable pressure below 10 bar and allows separation of a five anions test mixture within less than 10 min. A better efficiency of monolithic column (HETP approximately 75 microm) can be achieved at reduced flow rates when the analysis time is not a critical parameter.