Bw. Patterson et al., IMPROVED ACCURACY AND PRECISION OF GAS-CHROMATOGRAPHY MASS-SPECTROMETRY MEASUREMENTS FOR METABOLIC TRACERS, Metabolism, clinical and experimental, 47(6), 1998, pp. 706-712
The use of stable-isotope tracer methodology to study substrate metabo
lic kinetics requires accurate measurement of the tracer to tracee rat
io (TTR), often by gas chromatography/mass spectrometry (GC/MS). Many
approaches for measurement of the TTR by GC/MS do not use standards of
known isotopic enrichment to control for variability in instrument re
sponse. In addition, most GC/MS applications exhibit some degree of co
ncentration dependency whereby the measured ion abundance ratio varies
with the quantity of sample analyzed, thereby placing a limitation on
the accuracy of isotopic enrichment standard curves unless the quanti
ties of standards and samples analyzed are closely matched. We documen
t the degree to which day-to-day variability can affect the instrument
response for several GC/MS analyses of metabolic tracers when isotopi
c enrichment standards are not used to control for variable instrument
response. Furthermore, we report a new approach that incorporates con
centration dependencies within a standard curve to improve the accurac
y and precision of TTR measurements over a range of sample quantities
analyzed. The new approach was applied to plasma samples obtained from
experimental protocols performed in human subjects with three commonl
y used tracers: H-2(2)-palmitate, N-15(2)-urea, and C-13-leucine. Vari
ability in the day-to-day instrument response was 84% and 26% for H-2(
2)-palmitate and N-15(2)-urea, respectively; in addition, up to 10% va
riability due to concentration dependency was noted for these applicat
ions, The new approach virtually eliminated these sources of variabili
ty. After controlling for concentration dependency, a threefold reduct
ion in the standard error was noted when the enrichment of C-13-leucin
e measured by electron-impact (El) ionization GC/MS was correlated aga
inst negative chemical ionization (NCI) GC/MS. These data demonstrate
that our new approach decreases the errors in TTR determination caused
by variations in instrument response and concentration dependency. Th
is approach is generically applicable, and can improve the accuracy an
d precision of TTR determinations for most GC/MS analyses. Copyright (
C) 1998 by W.B. Saunders Company.