Selective detection of carbon-13, nitrogen-15, and deuterium labeled metabolites by capillary gas chromatography-chemical reaction interface/mass spectrometry

Abstract

We have applied a new chemical reaction interface/mass spectrometer technique (CRIMS) to the selective detection of 13C-, 15N-, and 2H-labeled phenytoin and its metabolites in urine following separation by capillary gas chromatography. The microwave-powered chemical reaction interface converts materials from their original forms into small molecules whose mass spectra serve to identify and quantify the nuclides that make up each analyte. The presence of each element is followed by monitoring the isotopic variants of CO2, NO, or H2 that are produced by the chemical reaction interface. Chromatograms showing only enriched 13C and 15N were produced by subtracting the abundance of naturally occurring isotopes from the observed M + 1 signal. A selective chromatogram of 2H (D) was obtained by measuring HD at m/z 3.0219 with a resolution of 2000. Metabolites representing less than 1.5% of the total labeled compounds could be identified in the chromatogram. Detection limits from urine of 380 pg/mL of a 15N-labeled metabolite, 7 ng/mL of a 13C-labeled metabolite, and 16 ng/mL of a deuterium labeled metabolite were determined at a signal to noise ratio of 2. Depending on the isotope examined, a linear dynamic range of 250-1000 was observed using CRIMS. To identify many of these labeled peaks (metabolites), the chromatographic analysis was repeated with the chemical reaction interface turned off and mass spectra obtained at the retention times found in the CRIMS experiment. CRIMS is a new analytical method that appears to be particularly useful for metabolism studies.

Nitrogen isotopic analyses by isotope-ratio-monitoring gas chromatography / mass spectrometry

“…Removal of CO 2 from the effluent stream. Since CO + fragments are generated from COz in the ion source and produce ion currents at mrz 28-:n which interfere with measurement of ion currents for N 2 (m/z [28][29][30], CO 2 produced during the combustion of organic material must be removed from the effluent stream. In early experiments, CO 2 was removed from the effluent by sorption in a microbore chromatographic column containing molecular sieve (Molsieve 5A PLOT column) or in packed tubes containing molecular sieve or ascarite.…”

“…Although diverse spectroscopic techniques have been used to measure nitrogen isotopic abundances, highly precise and accurate measurements (±O.5%o) in small quantities of naturally occurring materials depend mainly on isotope-ratio mass spectrometry. Typi-cally, potentially interfering elements are removed before analyses of nitrogen isotopic abundances in natural materials by converting organic nitrogen to N 2 after Kjeldahl digestion [22] or combustion [20,[23][24][25][26], although different techniques, such as inductively-coupled or microwave-induced plasmas [27,28], have been used to transform organic materials to suitable forms for mass spectrometric measurements.…”

Selective detection and characterization of chlorine- and bromine-containing compounds in complex mixtures using microwave-induced plasma/chemical reaction interface mass spectrometry

“…The GC/MIP/CRI mass spectrometer used for this study is a modified version of the previously reported GC/MIP/CRI mass spectrometer. 16 Briefly, it consists of a Varian 3400 gas chromatograph that is modified by a $-inch hole in the top of the gas chromatograph oven to accommodate the installation of a microwave cavity inside the GC oven. The microwave cavity consists of a half-wave cavity (1.75 inches diameter) connected to a Side View section of a high-temperature, air dielectric coaxial cable, fabricated from a 6-inch long, +-inch o.d., and $-inch i.d.…”

Non-targeted Tracer Fate Detection

“…Alternatively, isotopic enrichment can be determined using isotope ratio mass spectrometry (IRMS) [19] with high precision in a non-targeted manner. Unlike conventional mass spectrometric methods, IRMS cannot be used to determine MIDs.…”