Abstract: A method for determining a concentration of an analyte in a sample stream containing the analyte and a solvent includes: preparing a measured stream by diluting the sample stream with a solvent stream; ascertaining a concentration of the analyte in the measured stream; and ascertaining the concentration of the analyte in the sample stream as a function of the measured stream concentration and a dilution ratio of sample stream and measured stream. An apparatus of the invention includes: a first Coriolis mass flow measuring device for measuring a solvent stream as a function of the sample stream for diluting the sample stream; a second Coriolis device for ascertaining a mass flow of the measured stream; a density measuring device for ascertaining a density of the measured stream; and a computing and operating unit for computing the concentration of the analyte in the sample stream.