The apparatus for detecting the presence of a particular gas within a mixture of gases comprises first and second emitter means for emitting measurement and reference electromagnetic radiation beams respectively, which beams are activated in alternation, a measurement cell, a filter cell, first and second detector means for detecting electromagnetic radiation beams, a beam splitter, and acquisition and processing means for synchronously acquiring and processing the four signals <math-cwu id="MATH-US-00001"> <number>1</number> <mathematica-file id="MATHEMATICA-00001" file="US20020050567A1-20020502-M00001.NB"/> <image id="EMI-M00001" wi="216.027" he="11.02815" file="US20020050567A1-20020502-M00001.TIF" imf="TIFF" ti="MF"/> </math-cwu> delivered by the first and second detector means in succession when the first and second emitter means are respectively activated so as to determine the absolute concentration of the gas to be detected on the basis of the ratio <math-cwu id="MATH-US-00002"> <number>2</number> <mathematica-file id="MATHEMATICA-00002" file="US20020050567A1-20020502-M00002.NB"/> <image id="EMI-M00002" wi="216.027" he="11.02815" file="US20020050567A1-20020502-M00002.TIF" imf="TIFF" ti="MF"/> </math-cwu> between the four signals, in which <math-cwu id="MATH-US-00003"> <number>3</number> <mathematica-file id="MATHEMATICA-00003" file="US20020050567A1-20020502-M00003.NB"/> <image id="EMI-M00003" wi="216.027" he="11.02815" file="US20020050567A1-20020502-M00003.TIF" imf="TIFF" ti="MF"/> </math-cwu> respectively represent the signals delivered by the first and second detector means when the first emitter means is activated, and <math-cwu id="MATH-US-00004"> <number>4</number> <mathematica-file id="MATHEMATICA-00004" file="US20020050567A1-20020502-M00004.NB"/> <image id="EMI-M00004" wi="216.027" he="11.02815" file="US20020050567A1-20020502-M00004.TIF" imf="TIFF" ti="MF"/> </math-cwu> respectively represent the signals delivered by the first and second detector means when the second emitter means is activated.