ðɪs ɪz kˈɔːld ɐ sˌiːˈoʊ dˈɪfɹəns spˈɛktɹəm ɐsˈeɪ: ðɪ ɐbsˈoːɹbəns spˈɛktɹəm ʌvðə səlˈuːʃən ɪz sʌbtɹˈæktᵻd fɹʌmðə spˈɛktɹəm ˈæftɚ sˌætʃɚɹˈeɪʃən wɪð kˈɑːɹbən mʌnˈɑːksaɪd.
This is called a CO difference spectrum assay: the absorbance spectrum of the solution is subtracted from the spectrum after saturation with carbon monoxide.