Table 1.

Summary of isotopic labeling approaches [6].

Approaches	Description	Example
Isotopic dilution (or enrichment)	Grow cells with multiple carbon sources (some of them are labeled); then measure labeling of the metabolic products. This method is used for studying cell nutrient utilizations.	In a culture with 13C-glucose and yeast extracts, analysis of 13C-enrichment in proteinogenic amino acids reveals the contributons of yeast extract to biomass synthesis.
Isotopic tracing	Expose cell culture to a labeled compound (pulse); then measure change of labeling in downstream metabolites over time (chase). Pulse-chase tracing allows isotopic non-stationary MFA to quantify cell fluxomes [16].	The kinetics of isotopic incorporation from a nutrient into a downstream metabolite can detect and quantify functional pathways (e.g., kinetic flux profiling) [17]
13C-fingerprinting	Use specified labeled 13C-substrates to create steady state and position specific labeling patterns in metabolites, which delineate functional pathways. 13C-fingerprint allows 13C-MFA to quantify cell fluxomes.	If cell grows with 1st position labeled glucose, labeling patterns in serine and alanine can examine the Entner-Doudoroff pathway function.