Apparatuses, systems, and methods for sample testing

Method of identifying the condition of sample channel (13403A) based on the measurement of the refractive index change as one or more preservative chemicals present in the sample channel (13403A) are washed off from the sample channel (13403A) by a buffer solution. The measurement is based on using laser beams (13401A) that pass through the sample channel (13403A) of a waveguide and impinge upon an imaging sensor (13405A) to determine the change in refractive index. Based on the refractive index, a substance in the fluid can be identified. If two or more substances are expected in the observation area at different times, then a fluid transition completion can be identified. The present disclosure can optimize cycle time and fluid requirements and measure the variation in the wash process. A solution of buffer and dissolved preservative have a different refractive index than the buffer solution alone. As such, when the transition of the sample channel (13403A) from a condition where it is covered in preservative to a "purge complete" condition can be observed based on the refractive index change. Similarly, the refractive indexes of when a channel is dry and when a channel is wet are different. As such, the transitions of the dry-to-wet in a channel can also be observed based on the refractive index change.