The detection of high-mass aliphatics in petroleum by matrix-assisted laser desorption/ionisation mass spectrometry.
BACKGROUND
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 (AgNO(3)) 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 (C(20)-C(60)) 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 (C(100)) 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.
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK. elorente@imperial.ac.uk
Esther
E
Lorente
Cesar
C
Berrueco
Alan A
AA
Herod
Marcos
M
Millan
Rafael
R
Kandiyoti
D000473:Alkanes / Q000737:chemistry / Q000302:isolation & purification; D005591:Chemical Fractionation; D002855:Chromatography, Thin Layer; D008970:Molecular Weight; D010578:Petroleum / Q000032:analysis*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods
D000473:Alkanes; D010578:Petroleum