methods and apparatuses of determining the ph of a sample . a method can comprise determining an infrared spectrum of the sample , and determining the hemoglobin concentration of the sample . the hemoglobin concentration and the infrared spectrum can then be used to determine the ph of the sample . in some embodiments , the hemoglobin concentration can be used to select an model relating infrared spectra to ph that is applicable at the determined hemoglobin concentration . in other embodiments , a model relating hemoglobin concentration and infrared spectra to ph can be used . an apparatus according to the present invention can comprise an illumination system , adapted to supply radiation to a sample ; a collection system , adapted to collect radiation expressed from the sample responsive to the incident radiation ; and an analysis system , adapted to relate information about the incident radiation , the expressed radiation , and the hemoglobin concentration of the sample to ph .