Does a negative control have to have zero absorbance if does an experiment with

Question

Does a negative control have to have zero absorbance if does an experiment with a spectrophotometer or does it simply have to have zero reactivity?

Example, in the below graph, can't glucose be a negative control OR can it only be water? This question has a tiny intro but the question is short. Thank you!

Iodine will turn a deep reddish-brown color when added to starches, but remains lighter brown when added to simple sugars or monosaccharides. The degree of color change can be quantified by measuring the absorbance of light at 492nm using a spectrophotometer. An assay can be performed where the progress of a reaction that turns glucose into glycogen can be monitored by the increase of dark signal (measured at 492nm). You are presented with 3 unlabeled experimental samples and 5 labeled control samples. First, you incubate the unreacted control samples with iodine, measure their absorbance on the spectrophotometer, and get the following results ontrol samples iodine test response 0.3 0.25 0.2 0.15 water 5mg/ml 5mg/ml 1mg/m 1mg/ml glycogen glucose glycogen glycogen+ 1mg/ml glucose What sample(s) is/are negative controls for this experiment

Explanation / Answer

The sample contains water only is the negative control for this experiment because negative control is the sample which does not show any reactivity during the spectrophotometer based experiment to determine the concentration of experiment. theoretically, the absorbance of negative control will be zero but practically it absorbs some amount of light. The glucose cannot be used as a negative control because it acts as a reactant in this experiment.

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