15N/14N Analysis of Amino Acids with GC-C-IRMS - Methodical Investigations and Ecotoxicological Applications

Abstract

In order to perform the ¹⁵N/¹⁴N analysis of amino acids using a gas chromatograph and isotope ratio mass spectrometer linked up via a combustion interface (GC-C-IRMS), the amino acids must be derivatized. Tert-butyldimethylsilylation is examined using various techniques (direct conversion to nitrogen gas, ConFlo-IRMS [1], GC-C-IRMS [2]) and subsequently applied to isotopic characterization of amino acids from wheat protein hydrolyzate obtained from plants exposed to ozone. The method provides a reliable tool for studying ecotoxicological effects on plants at a molecular level in addition to the investigation of the natural variations of different N fractions.     

Brassicaceae seed oil identified as illuminant in Nilotic shells from a first millennium AD Coptic church in Bawit,Egypt

Burned greasy deposits were found inside shells of the large Nile bivalve Chambardia rubens, excavated in an eight- to tenth- century AD church of the Coptic monastery of Bawit, Egypt, and supposedly used as oil lamps. The residues were subjected to a combination of chromatographic residue analysis techniques. The rather high concentrations of unsaturated fatty acids, as analysed by gas chromatography (GC) in the methylated extract, suggest the presence of a vegetal oil. Analysis of the stable carbon isotopes (δ ¹³C values) of the methyl esters also favoured plants over animals as the lipid source. In the search for biomarkers by GC coupled to mass spectrometry on a silylated extract, a range of diacids together with high concentrations of 13,14-dihydroxydocosanoate and 11,12-dihydroxyeicosanoate were found. These compounds are oxidation products of erucic acid and gondoic acid, which are abundantly present in seeds of Brassicaceae plants. Liquid chromatography coupled to mass spectrometry analysis showed low concentrations of unaltered triglycerides, but revealed sizeable amounts of triglycerides with at least one dihydroxylated acyl chain. The unusual preservation of dihydroxylated triglycerides and α,ω-dicarboxylic acids can be related to the dry preservation conditions. Analysis of the stereoisomers of the dihydroxylated fatty acids allows one to determine whether oxidation took place during burning of the fuel or afterwards. The results prove that the oil of rapeseed (Brassica napus L.) or radish (Raphanus sativus L.) was used as illuminant in early Islamic Egypt, and that not only ceramic lamps but also mollusk shells were used as fuel containers.     

The Measurement of Muscle Protein Synthesis in Broilers with a Flooding Dose Technique: Use of 15N-Labelled Phenylalanine,GC-MS and GC-C-IRMS

Abstract

An experiment was carried out to measure fractional muscle protein synthesis rates (k _s ) in broilers with injection of a flooding dose of phenylalanine (1 ml/100 g body weight of 150 mM phenylalanine; 38 atom percent excess (APE) [¹⁵N]phenylalanine). K _s was calculated from the [¹⁵N] enrichment in phenylalanine of tissue-free and protein-bound phenylalanine using both gas chromatography mass spectrometry (GC-MS) and gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) for measurements after a 10 min isotope incorporation period.

The tertiary-butyldimethylsilyl (t-BDMS) derivatives of phenylalanine were used for gas chromatographic separation in both systems. GC-MS and GC-C-IRMS were calibrated for a range of 7 to 37 [¹⁵N]APE and 0 to 0.62 [¹⁵N]APE, respectively, and for sample sizes of 0.45 to 4.5 nmol phenylalanine and 7 to 40 nmol phenylalanine, respectively. Reproducibility of standards as a measure of precision varied from 0.06 to 0.29 [¹⁵N]APE and from 0.0004 to 0.0018 [¹⁵N]APE in GC-MS and GC-C-IRMS, respectively.

K _s was measured in the m. pectoralis major of broilers fed rye based diets (56%) which were provided either unsupplemented (-) or supplemented (+) with an enzyme preparation containing xylanase. K _s in breast muscles was significantly increased from 21.8%/d to 23.9%/d due to enzyme supplementation.

It can be concluded from the study that the measurement of protein synthesis in broilers with the flooding dose technique can be carried out by using [¹⁵N]phenylalanine, GC-MS and GC-C-IRMS.     

Determination of 13C Enrichment by Conventional GC-MS and GC-(MS)-C-IRMS

Abstract

Isotopic enrichment of branched-chain L-amino acids (BCAA) and branched-chain 2-oxo acids (BCOA) in standard preparations and in human plasma samples withdrawn after oral loads with 1-¹³C labelled BCAA were measured on a conventional GC-MS system and an on-line GC-C-IRMS employing O-TMS quinoxalinol derivatives. It was concluded that the recently introduced GC-C-IRMS, owing to its high sensitivity, is the adequate analytical tool when tracer doses of stable isotope labelled compounds of low enrichment are to be used in biomedical in vivo studies.     

Incorporation of 15NO2 Nitrogen into Individual Amino Acids by Sunflowers Using Gc-C-Irms

Abstract

The nitrogen isotopic composition of individual amino acids in sunflower leaves after exposures to ¹⁵NO₂ in the range of ambient NO₂ concentrations (5–37 ppb) was analysed by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS). Amino acids as well as the amides glutamine and asparagine were converted with MTBSTFA (N-methyl-N-(tert.-butyldimethylsilyl)-tri-fluoroacetamid) in pyridine to their corresponding TBDMS derivatives (N, O-tert.-butyldimethylsilyl) in a simple one-step silylation reaction. The derivatized amino acids were separated by gas chromatography, combusted on-line, and the products were sent continuously to an isotope ratio mass spectrometer. Accurate measurements were obtained, when more than 7 nmol N₂ were introduced into the ion source of the mass spectrometer per gas chromatographically separated and combusted compound. No interferences of the silicate and fluor containing derivatization agents on the performance of the system were observed.

In the range of ambient NO₂ concentrations sunflower leaves predominately incorporate the nitrogen derived from atmospheric NO₂ into soluble amino acids. The highest δ¹⁵N values were measured for alanine. The ¹⁵N enrichments of the detectable amino acids increased with increasing ¹⁵NO₂ concentration.