Estimating the intermediate precision in petroleum analysis by (±)electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

Rationale: Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) is an important analytical technique used for the elucidation of crude oil polar compounds at the molecular level, providing thousands of heteroatom compounds in a single analysis. Due to the high resolution, the complexity of data produced, and steps involved in spectra acquisition and processing, it is necessary to estimate its intermediate precision.

Methods: Intermediate precision was estimated for positive- and negative-ion ionization modes (ESI(±)) using Composer® software for two Brazilian crude oil samples. The analytical parameters evaluated were the class distribution histogram, the double bond equivalent (DBE) distribution, and the DBE versus carbon number. The statistical parameters used to study the intermediate precision were calculated from the average, standard deviation, confidence interval (significance level at 5%), coefficient of variation (CV), intermediate precision limit (ISO 5725), and principal component analysis (PCA).

Results: Two crude oil samples (A and B) were analyzed, in triplicate, for seven consecutive days by ESI(±) FT-ICR MS. The assigned class limit by ESI(+) for crude oil A was 0.42% (O S[H] class) and for crude oil B was 0.04% (N O S[H] class). The assigned DBE intensity limits for the two crude oils were 0.04% for ESI(+) and 0.013% for ESI(-). The PCA for ESI(-) and ESI(+) modes presented better precision for crude oils B and A, respectively.

Conclusions: The most abundant classes and DBE of the majority class (i.e., with the highest intensity) are the parameters produced from the Composer® software that had the highest precision and can be used to estimate crude oil properties. The DBE values presented lower intermediate precision limit values (0.04%) than the assigned class values (0.4%). According to CV and PCA, ESI(+) was more precise for crude oil A (83% precision) and ESI(-) for crude oil B (84% precision).