Targeted metabolomics identifies perturbations in amino acid metabolism that sub-classify patients with COPD by Ubhi, Baljit K., Kian, Kai Cheng, Dong, Jiyang, Janowitz, Tobias, Jodrell, Duncan, Singer, Ruth Tal, MacNee, William, Lomas, David A., Riley, John H., Griffinai, Julian L., Connora, Susan C.