The present invention relates to a method of determining petroleum hydrocarbon fractions (Cn) in a sample, the method including: inputting the sample into a chamber; emitting infrared light from an optical light source into the chamber with the sample; detecting at a detector a detected infrared light from the chamber; transforming the detected infrared light to a Fourier Transform Infrared (FTIR) spectrum of the sample at a processor, wherein the FTIR spectrum has wavenumbers between 4000 and 400 cm−1; processing the FTIR spectrum to identify sub-bands each having at least one doublet of sub-band peaks at respective wavenumbers in a second derivative curve of the FTIR spectrum using a second derivation algorithm implemented by the processor; comparing the at least one doublet of sub-band peaks to data indicative of known doublets of sub-band peaks at known wavenumbers for petroleum hydrocarbon fractions in the FTIR spectrum to classify the petroleum hydrocarbon fractions in the sample; and determining a dominant petroleum hydrocarbon fraction of the petroleum hydrocarbon fractions in the sample based on a ratio of intensities of the sub-band peaks of the at least one doublet for each of the petroleum hydrocarbon fractions in the sample.

The present invention relates to a method of determining petroleum hydrocarbon fractions (Cn) in a sample, the method including: inputting the sample into a chamber; emitting infrared light from an optical light source into the chamber with the sample; detecting at a detector a detected infrared light from the chamber; transforming the detected infrared light to a Fourier Transform Infrared (FTIR) spectrum of the sample at a processor, wherein the FTIR spectrum has wavenumbers between 4000 and 400 cm1; processing the FTIR spectrum to identify sub-bands each having at least one doublet of sub-band peaks at respective wavenumbers in a second derivative curve of the FTIR spectrum using a second derivation algorithm implemented by the processor; comparing the at least one doublet of sub-band peaks to data indicative of known doublets of sub-band peaks at known wavenumbers for petroleum hydrocarbon fractions in the FTIR spectrum to classify the petroleum hydrocarbon fractions in the sample; and determining a dominant petroleum hydrocarbon fraction of the petroleum hydrocarbon fractions in the sample based on a ratio of intensities of the sub-band peaks of the at least one doublet for each of the petroleum hydrocarbon fractions in the sample.

Disclosed herein is a method for retrieving at least one alternating current (AC) signal S.sub.AC from at least one measurement signal S.sub.meas of at least one detector. The measurement signal S.sub.meas includes the AC signal S.sub.AC and at least one direct current (DC) signal S.sub.DC. The AC signal S.sub.AC has at least one predefined frequency f.sub.0. The method includes the following steps: a) monitoring the measurement signal S.sub.meas over time by using the detector; b) determining the DC signal S.sub.DC by using at least one evaluation unit; and c) determining the AC signal S.sub.AC by subtracting the DC signal S.sub.DC from the measurement signal S.sub.meas by using the evaluation unit.

Also disclosed herein are a method for determining at least one item of information on at least one measurement object, a photodetector and a spectrometer.