The authors reply to Ellison.
It was not our intention to imply that data may be invented or manipulated; such a practice is intolerable. If we had given more explanation with the statement in our previous letter “one often tends to use the origin point (0,0) in the data. However, whether that is the best practice or not is totally arguable depending on various conditions of the measurements”, it may not have incurred criticism from Ellison. Thus we believe that he provided us with an opportunity for clarification.
When using an absorption spectrophotometer, it is necessary to use a blank to zero the instrument. The appropriate blank may be one of two choices, as appropriate: (i) water or other solvent or (ii) a background solution containing all solutes except the analyte. When using a single-beam instrument, a measurement must be taken at zero concentration so that there is a data point at zero concentration; this is necessary owing to possible fluctuation in the background signal. However, if a double-beam spectrophotometer is used, it is not necessary to take a measurement at zero concentration. For most of the double-beam spectrophotometers, background currents are instantly compensated. Measuring absorbance at zero concentration after zeroing with the blank is redundant. An exception to this is a case where water (or other solvent) is used as a blank instead of the background solution (described in ii) when zeroing an instrument: in this case, a measurement at zero analyte concentration must be obtained because of a possible absorption from other solutes present in the background solution.
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