title: Practical recommendations for the reduction of memory effects in compound-specific 15N/14N-ratio analysis of enriched amino acids by gas chromatography/combustion/isotope ratio mass spectrometry.
abstract: Gas chromatography/combustion/isotope ratio mass spectrometry (GC-C-IRMS) is a highly sensitive approach which allows the analysis of the (13)C/(12)C and (15)N/(14)N isotope composition of amino acids in the range of natural abundance or in slightly (13)C- and (15)N-enriched samples. However, the accuracy of measurements remains a permanent challenge. Here we show the effect of the presence of slightly (15)N-enriched compounds in physiological samples on the accuracy and reproducibility of (15)N-abundances of amino acids within or between analytical runs. We spiked several individual amino acids with the respective (15)N-labelled isotopomer and measured the (15)N/(14)N ratios of other amino acids in the same sample or in the following analytical runs. Intra- and inter-run memory effects can be observed in (15)N/(14)N ratios of amino acids. Sample throughput is reduced when cleaning runs using standard mixtures are required to restore initial conditions after runs of samples with (15)N-enriched analytes. Possible reasons for the observed phenomenon and its implications for work in the lower (15)N-enrichment range (<0.5 APE) are discussed and include different aspects of gas chromatography, derivatisation, and hot catalytic metal surface effects. Results need to be interpreted with caution if complex physiological samples contain (15)N-enriched amino acids beyond 500‰ δ(15)N (~0.18 APE).
authors.affiliation: German Institute of Human Nutrition in Potsdam-Rehbruecke (DIfE), Arthur-Scheunert-Allee 114-116, 14558 Nuthetal, Germany. petzke@dife.de
authors.forename: Klaus J
authors.initials: KJ
authors.lastname: Petzke
authors.forename: Cornelia C
authors.initials: CC
authors.lastname: Metges
mesh_terms: D000596:Amino Acids / Q000032:analysis / Q000097:blood / Q000737:chemistry*; D001426:Bacterial Proteins / Q000737:chemistry; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods* / Q000592:standards; D006801:Humans; D007422:Intestines / Q000382:microbiology; D009587:Nitrogen Isotopes / Q000032:analysis* / Q000097:blood; D012015:Reference Standards; D015203:Reproducibility of Results
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D009587:Nitrogen Isotopes