  Isotope labeling is a fundamental staple for the study of cellular metabolism
and protein function. The conventional techniques that allow resolution and
identification of isotopically-labeled biomarkers, such as mass spectrometry
and infrared spectroscopy, are macroscopic in nature and have the disadvantage
of requiring relatively large quantities of material and lacking spatial
resolution. Here, we record the vibrational spectra of an {\alpha}-amino acid,
L-alanine, using spatially-resolved monochromated electron energy loss
spectroscopy (EELS) to directly resolve carbon-site-specific isotopic labels in
a scanning transmission electron microscope. The EELS is acquired in aloof
mode, meaning the probe is positioned away from the sample (~20 nm) sparing the
sensitive biomolecule from the high-energy excitations, while the vibrational
modes are investigated. An isotopic red-shift of 5.3 meV was obtained for the
C=O stretching mode in the carboxylic acid group for 13C-enriched L-alanine
when compared with naturally occurring 12C L-alanine, which is confirmed by
macroscopic infrared spectroscopy measurements and theoretical calculations.
The EELS experiments presented here are the first demonstration of
non-destructive resolution and identification of isotopically-labeled amino
acids in the electron microscope, opening a new door for the study of
biological matter at the nanoscale.
