There is provided a differential signal comparator which maintains the duty ratio of complementary input signals. In the present invention, a feces sample is purified using a quaternary ammonium salt such as, for example, hexadecyltrimethylammonium bromide. Variances in the spectral data attributable to the analyte are isolated from spectral variances from other causes, such as compositional changes associated with different samples that are independent of the analyte. The system allows for removal of a filled leading harp rack, by designating a new leading harp rack for the control system, which then dynamically adjusts the bias and associated cutting scheduling.