The text was revised. Each single measurement was based on internally averaged of 50 spectra (3 times). Each sample was measured from four positions (90°rotating the Petri dish each time). Consequently, 4 rotation x 3 time = 12 spectra were obtained. It is one of the commonly used method and several references are available.
Line 174: “…resulting in 12 spectra”. It is not clear how the 12 spectra were obtained here.