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. Copyright © 2012 John Wiley & Sons, Ltd.