Development of Alditol Acetate Derivatives for the Determination of ¹⁵ N-Enriched Amino Sugars by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry

Amino sugars can be used as indices to evaluate the role of soil microorganisms in active nitrogen (N) cycling in soil. This paper details the assessment of the suitability of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) for the analysis of ¹⁵ N-enriched amino sugars as alditol acetate derivatives prior to application of a novel ¹⁵ N stable isotope probing (SIP) approach to amino sugars. The efficient derivatization and cleanup of alditol acetate derivatives for GC was achieved using commercially available amino sugars, including glucosamine, mannosamine, galactosamine, and muramic acid, as laboratory standards. A VF-23ms stationary phase was found to produce optimal separations of all four compounds. The structure of the alditol acetate derivatives was confirmed using gas chromatography/mass spectrometry (GC/MS). For GC-C-IRMS determinations, implementation of a two-point normalization confirmed the optimal carrier gas flow rate to be 1.7 mL min ^-1 . Linearity of δ ¹⁵ N value determinations up to δ ¹⁵ N _t of 469 ± 3.1‰ (where δ ¹⁵ N _t is the independently measured δ ¹⁵ N value) was confirmed when 30 nmol N was injected on-column, with the direction of deviation from δ ¹⁵ N _t at low sample amount dependent on the ¹⁵ N abundance of the analyte. Observed between- and within-run memory effects were significant (P < 0.007) when a highly enriched standard (469 ± 3.1‰) was run; therefore, analytical run order and variation in ¹⁵ N enrichment of analytes within the same sample must be considered. The investigated parameters have confirmed the isotopic robustness of alditol acetate derivatives of amino sugars for the GC-C-IRMS analysis of ¹⁵ N-enriched amino sugars in terms of linearity over an enrichment range (natural abundance to 469 ± 3.1‰) with on-column analyte amount over 30 nmol N.