compositions of this invention are also useful as reagents in methods for determining the concentration of bosentan in solution or biological sample such as plasma examining the metabolism of bosentan and other analytical studies according to one embodiment the invention provides a method of determining the concentration in a solution or a biological sample of bosentan comprising the steps of a adding a known concentration of a compound of formula i to the solution of biological sample b subjecting the solution or biological sample to a measuring device that distinguishes bosentan from a compound of formula i c calibrating the measuring device to correlate the detected quantity of the compound of formula i with the known concentration of the compound of formula i added to the biological sample or solution and d measuring the quantity of bosentan in the biological sample with said calibrated measuring device and e determining the concentration of bosentan in the solution of sample using the correlation between detected quantity and concentration obtained for a compound of formula i measuring devices that can distinguish bosentan from the corresponding compound of formula i include any measuring device that can distinguish between two compounds that differ from one another only in isotopic abundance exemplary measuring devices include a mass spectrometer nmr spectrometer or ir spectrometer in another embodiment the invention provides a method of evaluating the metabolic stability of a compound of formula i comprising the steps of contacting the compound of formula i with a metabolizing enzyme source for a period of time and comparing the amount of the compound of formula i with the metabolic products of the compound of formula i after.