The detection of high-mass aliphatics in petroleum by matrix-assisted laser desorption/ionisation mass spectrometry

RATIONALE The development of simplified procedures for isolating high-mass alkanes present in crude oils is described. The new procedures, which bypass the sample recovery step with hot toluene in the conventional alkane-isolation procedure, also provide an effective sample preparation route, prior to analysis by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). METHODS Urea-alkane adducts are formed by mixing sample and urea solutions on chromatographic paper or silica-coated plates. Unreacted hydrocarbons are removed by developing the plates with chloroform. In a second development with water, adducts are broken up in situ and the liberated urea removed, leaving bands of isolated alkanes behind. For MALDI-MS, strips of paper/plates, carrying the isolated alkanes, are fixed on metal target plates. The samples are treated with matrix (AgNO₃) and analysed by MALDI-MS. RESULTS The observed signal represents silver ion adducts of the isolated alkanes. Silver appears to work without much fragmentation and to generate whole silver adduct ions. Much improved MALDI-MS detection sensitivity and a wider range of masses was observed when samples were ablated from paper/plate surfaces, than by ablation from bulk samples spread over a smooth surface - the conventional method. Chromatographic paper gave better resolution and a broader range of masses than silica-coated plates. CONCLUSIONS The analytical sequences have been confirmed using standard alkanes (C₂₀-C₆₀) and Polywax. The proposed procedures enhanced the sensitivity and detection range of the MS analysis. The method was useful in detecting n-alkanes to m/z 1500 (C₁₀₀) and required relatively small quantities of sample and reagents. It provides a promising qualitative analysis route for the rapid isolation and reliable determination of alkanes in crude oils.