Dopant-Assisted APCI for Enhanced Sugar Analysis: Overcoming Ionization Bottlenecks

The investigation of lignocellulosic biomass by atmospheric-pressure chemical ionization (APCI) mass spectrometry (MS) has revealed significant issues for carbohydrate ionization, particularly during biomass pyrolysis analyses. Unequal ionization of carbohydrates and significant fragmentation have been reported. The introduction of suitable dopants into the ion source shows promise for limiting fragmentation.

To investigate these ionization disturbances, both direct infusion (DI) and modified direct insertion probe (DIP) APCI coupled with a Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were employed. Solvents with increasing proton affinity (PA) were used to assess solvent PA influence in DI-APCI ionization processes. The DIP system was modified to enable the introduction of an inert gas stream passing through a dopant-rich atmosphere prior to entering the ion source.

A significant PA difference between the solvent and the analyte promotes carbohydrate dissociation after their ionization. Experiments without solvent and with deuterated compounds showed that carbohydrate ionization and dissociation may be self-induced by water formed during carbohydrate dehydration. The introduction of ammonia in the ion source promoted the [M + NH₄]⁺ adduct formation, reducing ionization-induced dissociation and increasing the intensity of the carbohydrate signal by at least one order of magnitude in DI-APCI. The method was successfully applied to both standards and cellulose pyrolysis samples.

The dopant-assisted APCI (dAPCI) approach appears highly promising for the rapid assessment of biomass fast pyrolysis, particularly when highly oxygenated compounds are key markers. Careful selection of dopants and solvents is essential in carbohydrate analysis to preserve analyte structural integrity during ionization.