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 the column 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.