VTSEGAGLQLQK- 13 C 6 , 15 N 2 (TFA) is the 13 C- and 15 N-labeled VTSEGAGLQLQK. VTSEGAGLQLQK is an amino acid sequence in the C-terminal region of recombinant human alpha-acid glucosidase (rhGAA), which can be used to bind anti-drug antibodies in plasma and quantitatively analyze the therapeutic effect.