A method for determining the content of a substance to be detected in a sample, wherein a) a sampling receptacle is provided, b) a first predetermined quantity of a sample is introduced into the sampling receptacle, c) a quantitative analysis is carried out after an unspecified interval of time, wherein: c1) the entire quantity or a predetermined portion of the sample contained in the sampling receptacle is mixed with a second predetermined quantity of a first comparison substance which is chemically identical to the substance to be detected, wherein a first predetermined number of given atoms of the molecules of the first comparison substance constitutes an isotope of the corresponding atoms of the molecules of the substance to be detected, wherein the isotope does not occur or only rarely occurs in the substance to be detected, c2) at least one respective concentration-dependent parameter is determined both for the substance to be detected and also for the first comparison substance in the mixture which is formed thereby, and the quantity or concentration of the substance to be detected in the sample is determined as a function of these parameters, characterized in that in step a), a third predetermined quantity of a second comparison substance which is chemically identical to the substance to be detected is introduced into the sampling receptacle, wherein a second predetermined number of given atoms of the molecules of the second comparison substance constitutes an isotope of the corresponding atoms of the molecules of the substance to be detected, wherein the second predetermined number is not identical to the first number, in that the sample introduced in step b) is mixed with the second comparison substance, and in that in step c2), in the mixture formed in step c1), a concentration-dependent parameter is also determined for the second comparison substance and as a function of this at least one parameter and of the at least one parameter of the first comparison substance, a determination as to whether the content of the second comparison substance and therefore also that of the substance to be detected has reduced in the unspecified interval of time is made. a) a sampling receptacle is provided, b) a first predetermined quantity of a sample is introduced into the sampling receptacle, c) a quantitative analysis is carried out after an unspecified interval of time, wherein: c1) the entire quantity or a predetermined portion of the sample contained in the sampling receptacle is mixed with a second predetermined quantity of a first comparison substance which is chemically identical to the substance to be detected, wherein a first predetermined number of given atoms of the molecules of the first comparison substance constitutes an isotope of the corresponding atoms of the molecules of the substance to be detected, wherein the isotope does not occur or only rarely occurs in the substance to be detected, c2) at least one respective concentration-dependent parameter is determined both for the substance to be detected and also for the first comparison substance in the mixture which is formed thereby, and the quantity or concentration of the substance to be detected in the sample is determined as a function of these parameters, c1) the entire quantity or a predetermined portion of the sample contained in the sampling receptacle is mixed with a second predetermined quantity of a first comparison substance which is chemically identical to the substance to be detected, wherein a first predetermined number of given atoms of the molecules of the first comparison substance constitutes an isotope of the corresponding atoms of the molecules of the substance to be detected, wherein the isotope does not occur or only rarely occurs in the substance to be detected, c2) at least one respective concentration-dependent parameter is determined both for the substance to be detected and also for the first comparison substance in the mixture which is formed thereby, and the quantity or concentration of the substance to be detected in the sample is determined as a function of these parameters,