Optimisation of LDI(+)-FT-ICR MS analysis of asphaltenes to prevent the formation of fullerenes

Abstract

The laser desorption and ionisation (LDI) technique coupled with mass spectrometry (MS) equipment is a common tool for determining the chemical composition of asphaltene molecules chemical composition of asphaltene molecules. However, the laser radiation that impacts the sample can lead to the formation of fullerene artifacts, which has generated numerous controversies. In this work, we employed a 2(6-2) fractional factorial design and central composite design to optimise the assay of the positive-ion mode LDI technique coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI(+) FT-ICR MS) for analysing asphaltenes, aiming to avoid the formation of fullerenes during ionisation. We evaluated the experimental conditions such as: analyte concentration, number of shots, power, frequency and focus. Subsequently, we analysed the same asphaltene sample using the positive mode-ion atmospheric pressure photoionisation ionisation (APPI(+)) technique to compare and complement the data obtained using the LDI(+) technique. We found that higher values of power and focus, as well as their interaction at a high level, favoured effective ionisation of asphaltenes, without the formation of artifacts such as fullerenes. Additionally, minimal focus favoured the formation of fullerenes, which were identified starting at m/z 528 Da (C44) to m/z 1488 (C124). After optimisation, LDI(+) identified 1,486 polar compounds and five classes (HC; N; N2; N4[H]; and N4O4[H] classes). In comparison, the APPI(+) technique identified only 307 polar compounds and the same number of classes (HC[H]; HC; N[H]; O[H]; and S classes). These data demonstrate that LDI(+) technique has a greater ability to ionise nitrogen compounds.