Method of identifying an unidentified substance from a set of preselected substances during breath analysis of a human breath sample in a measuring cell using non-dispersive spectroscopy in a preselected wavelength range, the method comprising the steps of: - recording at least a first signal from a first infrared detector and a second signal from a second infrared detector, wherein the first signal represents the absorption in a first wavelength band in the preselected wavelength range and the second signal represents the absorption in a second wavelength band in the preselected wavelength range, wherein the first and second wavelength bands are at least to a major part separated by a preselected transition wavelength, λ t; - determining an absorption comparative value representing a comparison of at least the absorption in the first wavelength band and the absorption in the second wavelength band; and - determining a total absorption value representing a total absorption in the combined first wavelength band and the second wavelength band; - comparing the absorption comparative value and the total absorption value with tabulated data for the preselected set of substances arranged with corresponding values, and - identifying the unidentified substance as the substance from the set of preselected substances representing the best match in terms of the absorption comparative values and the total absorption values, and - wherein the first infrared detector is provided with a first interference filter with a transition wavelength, λ 1, and the preselected transition wavelength of the measuring cell, λ t, is given by transition wavelength of the first interference filter, λ 1, the first interference filter transmitting wavelengths above the preselected transition wavelength, λ t, to the first infrared detector and passing wavelengths below the preselected transition wavelength, λ t, to at least the second infrared detector, or - wherein the first infrared detector is provided with a first interference filter with a transition wavelength, λ 1, and the preselected transition wavelength of the measuring cell, λ t, is given by transition wavelength of the first interference filter, λ 1, the first interference filter transmitting wavelengths below the preselected transition wavelength, λ t, to the first infrared detector and passing wavelengths above the preselected transition wavelength, λ t, to at least the second infrared detector. - recording at least a first signal from a first infrared detector and a second signal from a second infrared detector, wherein the first signal represents the absorption in a first wavelength band in the preselected wavelength range and the second signal represents the absorption in a second wavelength band in the preselected wavelength range, wherein the first and second wavelength bands are at least to a major part separated by a preselected transition wavelength, λ t; - determining an absorption comparative value representing a comparison of at least the absorption in the first wavelength band and the absorption in the second wavelength band; and - determining a total absorption value representing a total absorption in the combined first wavelength band and the second wavelength band; - comparing the absorption comparative value and the total absorption value with tabulated data for the preselected set of substances arranged with corresponding values, and - identifying the unidentified substance as the substance from the set of preselected substances representing the best match in terms of the absorption comparative values and the total absorption values, and - wherein the first infrared detector is provided with a first interference filter with a transition wavelength, λ 1, and the preselected transition wavelength of the measuring cell, λ t, is given by transition wavelength of the first interference filter, λ 1, the first interference filter transmitting wavelengths above the preselected transition wavelength, λ t, to the first infrared detector and passing wavelengths below the preselected transition wavelength, λ t, to at least the second infrared detector, or - wherein the first infrared detector is provided with a first interference filter with a transition wavelength, λ 1, and the preselected transition wavelength of the measuring cell, λ t, is given by transition wavelength of the first interference filter, λ 1, the first interference filter transmitting wavelengths below the preselected transition wavelength, λ t, to the first infrared detector and passing wavelengths above the preselected transition wavelength, λ t, to at least the second infrared detector.