A linear calibration for ppm levels of Mg2+ yielded a slope of 23.1 mV/ppm and a y-intercept of 11.2 mV, where the measured response was in units of mV. Linear regression was used with the results of 5 calibration samples to yield the slope and intercept. A blank sample was measured 20 times and yielded a mean response of 10.9 mV and a corresponding standard deviation of 1.47 mV. Calculate the limit of detection for this method in units of ppm Mg2+.