An ion chromatography (IC) method with a double cell bulk acoustic wave (DCBAW) detector was used for the detection of magnesium concentration decrease during the culture of whole cells. The detector parameters were optimized according to its theoretical equation, including the cell constants and the background conductance of the adjusting cell. A low temperature coefficient and thus high sensitivity were obtained by this method. The detection limit is 0.01 ppm, and the linear range is 0.05-50 ppm. A new mathematical model was developed to describe the growth of the cell. The baseline for this model is the concept that a noninteractive growth process occurs among perfectly substitutable nutrients if the locus of points of the substrate concentrations producing equal growth rate is linear. The monitoring results agreed well with the calculated results.