Measurement of 2H-labeling of body water and protein-bound alanine using gas chromatography-mass spectrometry. Panel A shows that 2H-labeling of body water (plasma) is determined by 2H exchange with carbon-bound hydrogens of acetone. Acetone has a mass-to-charge ratio (m/z) of 58, thus m/z 59 represents 2H-labeling. Panel B shows the methyl-8 derivative upon reacting the protein hydrolysate with N, N- dimethylformamide dimethyl acetal. Three ions that have been characterized are m/z 99 (the base peak that includes α and β-hydrogens of alanine), m/z 143 (includes only the α-hydrogen of alanine) and m/z 158 (represents the molecular ion). Since the precursor (water) labeling is relatively low (e.g., 0.5 to 2%) one typically only measures changes in M+1/M0 ratio, although in theory M+2, M+3, etc. mass isotopomers are generated, their abundance is typically not measurable (e.g., when the water labeling is 0.5%, the abundance of M+6 acetone is ~0.0056, likewise the abundance of M+4 alanine is ~0.0053.7).