A method of detecting at least first and second species in a sample, comprising: generating probe light using a single laser source such that the probe light completes one or more frequency sweeps, each frequency sweep being across at least one infra red absorption spectrum feature of each of the at least first and second species; providing a plurality of absorption volumes, comprising a sample volume containing said sample, and two or more reference volumes (18-1, 18-2) containing known concentrations of said at least first and second species, wherein each reference volume contains a known concentration of at least one of the at least first and second species, which may be a zero concentration, which is different to the known concentration of that species in each of the other reference volumes; directing said probe light from the single laser source along a path to a single detector element to output a detector signal from the single detector element, wherein the complete path from the single laser source to the single detection element passes through only one of the absorption volumes at any one time; switching the path to the single detector element to sequentially select each of the absorption volumes (16, 18-1, 18-2) a plurality of times during each frequency sweep, so that the detector signal for a particular frequency sweep comprises a plurality of sample segments (S) when the path comprises the sample volume, and a plurality of reference segments (R1, R2) when the path comprises a reference volume; and detecting the at least first and second species from the sample segments including calibration using the reference segments from the two or more reference volumes, including detecting the first species from sample segments representing the at least one infra red absorption feature of the first species and detecting the second species from sample segments representing the at least one infra red absorption feature of the second species. generating probe light using a single laser source such that the probe light completes one or more frequency sweeps, each frequency sweep being across at least one infra red absorption spectrum feature of each of the at least first and second species; providing a plurality of absorption volumes, comprising a sample volume containing said sample, and two or more reference volumes (18-1, 18-2) containing known concentrations of said at least first and second species, wherein each reference volume contains a known concentration of at least one of the at least first and second species, which may be a zero concentration, which is different to the known concentration of that species in each of the other reference volumes; directing said probe light from the single laser source along a path to a single detector element to output a detector signal from the single detector element, wherein the complete path from the single laser source to the single detection element passes through only one of the absorption volumes at any one time; switching the path to the single detector element to sequentially select each of the absorption volumes (16, 18-1, 18-2) a plurality of times during each frequency sweep, so that the detector signal for a particular frequency sweep comprises a plurality of sample segments (S) when the path comprises the sample volume, and a plurality of reference segments (R1, R2) when the path comprises a reference volume; and detecting the at least first and second species from the sample segments including calibration using the reference segments from the two or more reference volumes, including detecting the first species from sample segments representing the at least one infra red absorption feature of the first species and detecting the second species from sample segments representing the at least one infra red absorption feature of the second species. <EOS>