An apparatus for determining a concentration of a first component in a sample, the first component being responsive, in terms of absorption, to at least radiation of a first wavelength, the sample comprising the first component and a second component responsive, in terms of absorption, to at least radiation of a second wavelength, the apparatus comprising: a first radiation source (101, 201) configured to produce at least radiation at the first wavelength; a second radiation source (102, 202) configured to produce at least radiation at the second wavelength; a beam combiner (103, 203) configured to direct radiation from the first radiation source and radiation from the second radiation source along a common radiation path through the sample; a first radiation detector (106, 206) configured to detect at least radiation at the first wavelength, having propagated along the radiation path through a portion of the sample; a second radiation detector (107, 207) configured to detect at least radiation at the second wavelength, having propagated along the radiation path through a portion of the sample; and a processing unit operable to receive a detector signal from the first and second radiation detector indicative of the detected radiation at the first and second wavelength, and to - determine an estimated path length of the radiation path from a determined absorbance by the sample of radiation at the second wavelength; - determine an estimated concentration of the first component from a determined absorbance by the sample of radiation at the first wavelength and from the estimated path length; - determine a corrected concentration of the first component from the estimated concentration by multiplying the estimated concentration with a correction factor, wherein the correction factor is determined from a function that models the impact of the concentration of the first component on the absorbance at the second wavelength, thereby compensating for the inaccuracy of the initially estimated path length, wherein the first wavelength is different from the second wavelength, and any absorption peak in the absorbance spectrum of the second component is displaced from the first wavelength. a first radiation source (101, 201) configured to produce at least radiation at the first wavelength; a second radiation source (102, 202) configured to produce at least radiation at the second wavelength; a beam combiner (103, 203) configured to direct radiation from the first radiation source and radiation from the second radiation source along a common radiation path through the sample; a first radiation detector (106, 206) configured to detect at least radiation at the first wavelength, having propagated along the radiation path through a portion of the sample; a second radiation detector (107, 207) configured to detect at least radiation at the second wavelength, having propagated along the radiation path through a portion of the sample; and a processing unit operable to receive a detector signal from the first and second radiation detector indicative of the detected radiation at the first and second wavelength, and to - determine an estimated path length of the radiation path from a determined absorbance by the sample of radiation at the second wavelength; - determine an estimated concentration of the first component from a determined absorbance by the sample of radiation at the first wavelength and from the estimated path length; - determine a corrected concentration of the first component from the estimated concentration by multiplying the estimated concentration with a correction factor, wherein the correction factor is determined from a function that models the impact of the concentration of the first component on the absorbance at the second wavelength, thereby compensating for the inaccuracy of the initially estimated path length, - determine an estimated path length of the radiation path from a determined absorbance by the sample of radiation at the second wavelength; - determine an estimated concentration of the first component from a determined absorbance by the sample of radiation at the first wavelength and from the estimated path length; - determine a corrected concentration of the first component from the estimated concentration by multiplying the estimated concentration with a correction factor, wherein the correction factor is determined from a function that models the impact of the concentration of the first component on the absorbance at the second wavelength, thereby compensating for the inaccuracy of the initially estimated path length,