The maximum at 560 nm is characteristic for glyoxylic acid. The other two maxima are common for all compounds, including the blank sample. Thus they can be attributed to the interaction with ferric chloride, see paragraph 2.2. Accordingly, the Hopkins-Cole reaction test by means of proposed method is selective only to glyoxylic acid without any interference of other compounds of the studied mixture including glyoxal containing aldehyde group. The baseline, however, has to be zero set using the blank sample in order to compensate the minor effect of ferric chloride interaction with tryptophan.