A method for monitoring of amino acids in a biological material, comprising the steps of: a) preparation of a sample of a biological material which is an exhaled breath condensate, wherein 1 mL of an internal standard is added to the sample, the internal standard is norvaline and homoarginine, labelled deuterated leucine Leu-d3, b) centrifugation of a sample at 1000 rpm / 8 minutes, c) subsequent evaporation to dryness at 45-60°C during 1h, d) dissolving of a dry residue in 100 μL of a mixture of acetonitrile and H 2 O in a ratio of 8:2 (v/v) and subsequent centrifugation at 10000 rpm /8 minutes, and e) chromatographic analysis with a mass detector by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) performed under conditions wherein: - 21 amino acids are separated by hydrophilic interaction chromatography (HILIC), with the use of a column packed with polar sorbent, - a mobile phase consists of phase A): 10 mM ammonium formate (pH 3) in water and B): 10 mM ammonium formate in acetonitrile under gradient elution conditions, - the mobile phase flow is 0.8 - 1 mL/min, - the column temperature is 25°C, - a drying gas temperature:250-350°C, flow of the drying gas: 8-12 L/min, the drying gas temperature is: 350°C, flow of the drying gas: 11 L/min, collision energy depending on the monitored production (range from 5 to 70), - the duration of a single analysis of 21 amino acids with column equilibration is 25 - 30 min, and amino acid separation occurred under gradient elution conditions; a) preparation of a sample of a biological material which is an exhaled breath condensate, wherein 1 mL of an internal standard is added to the sample, the internal standard is norvaline and homoarginine, labelled deuterated leucine Leu-d3, b) centrifugation of a sample at 1000 rpm / 8 minutes, c) subsequent evaporation to dryness at 45-60°C during 1h, d) dissolving of a dry residue in 100 μL of a mixture of acetonitrile and H 2 O in a ratio of 8:2 (v/v) and subsequent centrifugation at 10000 rpm /8 minutes, and e) chromatographic analysis with a mass detector by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) performed under conditions wherein: - 21 amino acids are separated by hydrophilic interaction chromatography (HILIC), with the use of a column packed with polar sorbent, - a mobile phase consists of phase A): 10 mM ammonium formate (pH 3) in water and B): 10 mM ammonium formate in acetonitrile under gradient elution conditions, - the mobile phase flow is 0.8 - 1 mL/min, - the column temperature is 25°C, - a drying gas temperature:250-350°C, flow of the drying gas: 8-12 L/min, the drying gas temperature is: 350°C, flow of the drying gas: 11 L/min, collision energy depending on the monitored production (range from 5 to 70), - the duration of a single analysis of 21 amino acids with column equilibration is 25 - 30 min, and amino acid separation occurred under gradient elution conditions; - 21 amino acids are separated by hydrophilic interaction chromatography (HILIC), with the use of a column packed with polar sorbent, - a mobile phase consists of phase A): 10 mM ammonium formate (pH 3) in water and B): 10 mM ammonium formate in acetonitrile under gradient elution conditions, - the mobile phase flow is 0.8 - 1 mL/min, - the column temperature is 25°C, - a drying gas temperature:250-350°C, flow of the drying gas: 8-12 L/min, the drying gas temperature is: 350°C, flow of the drying gas: 11 L/min, collision energy depending on the monitored production (range from 5 to 70), - the duration of a single analysis of 21 amino acids with column equilibration is 25 - 30 min, and amino acid separation occurred under gradient elution conditions; <EOS>