Rapid online analysis of n-alkanes in gaseous streams via APCI mass spectrometry

Online monitoring of dynamic chemical processes involving a wide volatility range of hydrocarbon species is challenging due to long chromatographic measurement times. Mass spectrometry (MS) overcomes chromatographic delays. However, the analysis of n-alkane mixtures by MS is difficult because many fragment ions are formed, which leads to overlapping signals of the homologous series. Atmospheric pressure chemical ionization (APCI) is suitable for the analysis of saturated hydrocarbons and is the subject of current research. Still, although APCI is a "soft ionization" technique, fragmentation is typically inevitable. Moreover, it is usually applied for liquid samples, while an application for online gas-phase monitoring is widely unexplored. Here, we present an automated APCI-MS method for an online gas-phase analysis of volatile and semi-volatile n-alkanes. Mass spectra for n-heptane and n-decane reveal [M-H]⁺, [M-3H]⁺ and [M-3H+H₂O]⁺ as abundant ions. While [M-H]⁺ and [M-3H]⁺ show an excessive fragmentation pattern to smaller C_nH_2n+1⁺ and C_nH_2n-1⁺ cations, [M-3H+H₂O]⁺ is the only relevant signal within the C_nH_2n+1O⁺ ion group, i.e., no chain cleavage is observed. This makes [M-3H+H₂O]⁺ an analyte-specific ion that is suitable for the quantification of n-alkane mixtures. A calibration confirms the linearity of C₇ and C₁₀ signals up to concentrations of ~1000-1500 ppm. Moreover, validated concentration profiles are measured for a binary C₇/C₁₀ mixture and a five-alkane C₇/C₁₀/C₁₂/C₁₄/C₂₀ mixture. Compared to the 40-min sampling interval of the reference gas chromatograph, MS sampling is performed within 5 min and allows dynamic changes to be monitored.