The invention relates to a gas chromatography instrument (2) for determining a concentration of a volatile marker. The instrument (2) comprises a first chromatographic column (8) and a first detector (12). It has been found that retention times and peak heights of all compounds in a chromatogram are not universal constants of nature, but depend strongly on many parameters, like thecolumn dimensions, materials and operating conditions. By utilizing an additional detector (22) or an additional chromatographic column (24), wherein the additional detector (22) differs from the first detector (12) by a detector property and/or the additional chromatographic column (24) differs from the first chromatographic column (8) by a column property, these redundancies can be exploited to determine a reliable consensus concentration value for the marker and allows for determining a confidence level thereof.