Comprehensive Two-Dimensional Gas Chromatography Combustion Isotope Ratio Mass Spectrometry

Abstract

We report the first coupling of comprehensive two-dimensional gas chromatography (GC x GC) to online combustion isotope ratio mass spectrometry (C-IRMS). A GC x GC system, equipped with a longitudinally modulated cryogenic system (LMCS), was interfaced to an optimized low dead volume combustion interface to preserve <300 ms full width at half-maximum (fwhm) fast GC peaks generated on the second GC column (GC2). The IRMS detector amplifiers were modified by configuration of resistors and capacitors to enable fast response, and a home-built system acquired data at 25 Hz. Software was home-written to handle isotopic time shifts of less than one bin (40 ms) and to integrate peak slices to recover isotope ratios from cryogenically sliced peaks. The performance of the GC x GCC-IRMS system was evaluated by isotopic analysis of urinary steroid standards. Steroids were separated by a nonpolar GC1 column (30 m x 0.25 mm, 5% phenyl), modulated into multiple 4- or 8-s cryogenic slices by the LMCS, and then separated on a polar GC2 column (1 or 2 m x 0.1 mm, 50% phenyl). GC2 peak widths from a 1-m column averaged 276 ms fwhm. Steroid standard sliced peaks were successfully reconstructed to yield delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.30 per thousand and average accuracies within 0.34 per thousand, at 8 ng on column. Two steroids, coeluting in GC1, were baseline separated in GC2 and resulted in delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.86 per thousand and average accuracies within 0.26 per thousand, at 3 ng on column. Results from this prototype system demonstrate that the enhanced peak capacity and signal available in GC x GC is compatible with high-precision carbon isotope analysis.

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…An algorithm has therefore been developed to reconstruct isotope ratios of target compounds from the slices of several fractions [144].…”

Theory and Practice of UHPLC and UHPLC–MS

“…The validated method showed good results, except for the steroid oxymetholone, which because of the particular chemical structure, interacted more strongly with PSA (through hydrogen bonding), causing low recovery. Tobias et al (2008) coupled a GC Â GC system with an isotope ratio MS (IRMS) system for the measurement of steroid 13 C/ 12 C ratios. In carbon-13 IRMS analyses, all the organic compounds are transformed into CO 2 , isotopic abundance measurements are achieved, and thus fragmentation profiles are not generated.…”

Microfabrication of high temperature micro-reactors for continuous flow isotope ratio mass spectrometry

“…A MAT 252 IRMS (Thermo Finnigan, Bremen, Germany) was used to measure carbon isotope ratios, 13 C/ 12 C. The MAT 252 IRMS was run with the variable ion source conductance window open (by three turns). The RC time constants of all three faraday cup detectors were modified to be 30 ms (Tobias et al 2008), in order to facilitate accurate data acquisition for the narrow peak shapes observed in this study. The MFMR was placed partially inside an open tubular glass fiber furnace (Thermcraft Inc., Winston-Salem NC).…”

“…In order to improve peak shapes for chromatography by reducing dead volumes associated with connections, a continuous fused silica capillary design was reported and refined Ellis and Fincannon 1997;Goodman 1995;Goodman 1998;Sacks et al 2007;Tobias et al 2008) and is in use in several laboratories. Apart from the input and output ends, the reactor is held in a high-temperature furnace.…”

“…GC fused silica capillary reactor, to enable more rapid separation by fast-GCC-IRMS (Sacks et al 2007) and improved separations of complex sample mixtures by comprehensive 2D GC (GCxGCC-IRMS) (Tobias et al 2008). These techniques require the preservation of very narrow peak shapes produced in the gas-phase separations; therefore, post-column band broadening can be further minimized by reducing the i.d.…”