One can use analytical instruments to make rough measurements of the chemicals in a sample of water but adjustments need to be made to these measurements to compensate for complicating factors. Fluorescence spectroscopy is capable of measuring quantities in a substance with concentrations down to the level of parts per billion. Samples cells are compared with standard cells of known concentration. When one is working near the limits of detection (LOD) the accuracy of the results may suffer. When a substance is present below the LOD false negatives are likely. With a bulk sample there is also the possibility of interactions between the molecules present which will alter results somewhat.
One encounters similar problems with human perception. For the eye the limit is the dark adaption threshold. At low light levels an image becomes pointilistic and scintillating. The eye is not equally sensitive to all colors of the same intensity and this is the subject of colorimetry.