Preparation of CO2 from blood and protein-bound amino acid carboxyl groups for quantification and 13C-isotope measurements.

Measurements of protein and amino acid metabolism in man using stable isotopes and selected ion monitoring gas chromatographic/mass spectrometric techniques are limited by the requirement of relatively high levels of labelling for adequate precision (greater than 0.05 at % excess). We describe here a means of extending the scope of such studies by measurement of lower levels of enrichment achieved in gaseous CO2 derived from whole blood or protein-bound amino acids following the administration of tracer amounts of appropriately labelled substrates. Construction and operation of a novel glass vacuum line required for this work are described in detail and specific applications relevant to clinical investigations are outlined. Measurements of both the total amount of CO2 and its 13C enrichment are performed in an isotope-ratio mass spectrometer which provides acceptable levels of accuracy and reproducibility for both measurements (+/- 0.1% and +/- 0.0001 at % excess respectively).