Nickel Wall-coated Microreactors for Enhanced Sensitivity and Peak Resolution in Compound-specific Carbon and Nitrogen Isotope Analysis

Online combustion of analytes through gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) has facilitated compound-specific isotope analysis (CSIA) for various applications like tracing environmental contamination and doping in sports. However, GC-C-IRMS critically depends on complete peak separation, where current combustion tubes with an inner diameter (i.d.) of 0.5 mm and threaded nickel wires are a notorious source of peak broadening. To reduce the reactor volume by 3–6 fold, we instead produced Ni wall-coatings in catalytic microreactors of alumina and quartz (0.2 mm–0.3 mm i.d.). Electroless plating coated the tube wall over a length of 18–20 cm covering the hot zone of the furnace. Coating thickness, characterized by scanning electron microscopy, was (1.23 ± 0.12 μm, n = 7) and the total mass of deposited Ni was 0.10 mg/cm. A seed oxidation of 2 min before each chromatographic run facilitated accurate carbon and nitrogen isotope analysis of caffeine: Δδ13C = −0.16‰ ± 0.12‰ (quartz capillary, i.d. 0.30 mm, splitless injection, n = 232); Δδ13C of 0.05‰ ± 0.09‰ and −0.16‰ ± 0.13‰ (two replicate alumina reactors, i.d. 0.20 mm, split injection, n = 336 and 347 respectively) and Δδ15N = 0.23‰ ± 0.22‰ (alumina reactor, i.d. 0.20 mm, split injection, n = 29). Increasing the residence time in the reactor enabled even accurate isotope analysis of atrazine (Δδ13C = 0.03‰ ± 0.21‰, n = 30; Δδ15N = −0.05‰ ± 0.15‰, n = 15) as a structure notoriously resistant to complete oxidation. A proof-of-principle test with n-alkanes demonstrated a 10-fold higher sensitivity and 2-times better peak resolution compared to the current commercial configuration. Hence, our setup paves the path to developments for which sensitivity and peak width are critical like environmental trace analysis and comprehensive two-dimensional gas chromatography (GCxGC-C-IRMS).