Buffered Chromate Electrolytes for Separation and Indirect Absorbance Detection of Inorganic Anions in Capillary Electrophoresis

Abstract

Chromate is the most commonly used carrier electrolyte for the determination of low molecular weight and inorganic anions by capillary electrophoresis. However, chromate electrolytes are usually prepared in the sodium form and consequently have poor buffering capacity. Addition of anionic buffers that introduce competing co-anions is undesirable because they can reduce the sensitivity of indirect detection and cause interfering system peaks. An approach is demonstrated to buffer a chromate electrolyte by using a suitable buffering base as a counter-ion of the chromate. This approach does not introduce any co-anions into the background electrolyte, which is prepared by titrating the acid form of chromate to the pK_a of the added base. Two buffered electrolytes were investigated containing tris(hydroxymethyl)amino- ethane (pK_a = 8.5) or diethanolamine, (pK_a = 9.2). The analytical performance characteristics of the buffered electrolytes were compared with the unbuffered chromate electrolyte. Both systems showed similar separation selectivity, efficiency and detection sensitivity, but the buffered electrolytes showed superior repeatability for migration times and peak areas, as well as a significantly greater tolerance to alkaline sample matrices.