Changes in the molecular composition of crude oils during their preparation for GC and GC-MS analyses

Rotary evaporation and nitrogen blowing are the two frequently used procedures in organic geochemistry laboratories to prepare crude oils and extractable organic matter for gas chromatography (GC) and GC-mass spectrometry (GC-MS) analyses. In this work, the effects of these preparatory procedures on the molecular composition have been comprehensively assessed for the first time, by evaporating 34 aliquots of North Sea Oil-1 dissolved in dichloromethane under a variety of conditions: (a) rotary evaporation with a reduced pressure of 80 to 60 kpa, and water bath temperatures of 30-60 degreesC, (b) nitrogen blowing, with flow rates of 130 to > 850 ml/min and heater block temperatures of 30-60 C, and (c) open vial evaporation in a refrigerator at 3 C and in a fume cupboard at 22 C. Analyses of the unaltered original oil solution and the evaporated oil aliquots for 215 target compounds, from benzene to n-C-32, indicate that (1)<n-C-15 compounds are very susceptible to all the evaporation procedures applied, (2) the loss of variable amounts of <n-C-12 compounds is almost inevitable, and (3) rotary evaporation is less destructive to compound distributions than nitrogen blowing. During rotary evaporation, the extent of compound loss depends on water bath temperature, the amount of solvent reduction, the level of reduced pressure and the length of drying. During nitrogen blowing the extent of compound loss depends on the heater block temperature, the amount of solvent reduction and the length of drying. If a sample is blown to dryness, higher nitrogen flow rates also cause more loss. The extent of compound loss generally increases with decreasing boiling points and molecular weights. However, some compounds such as C-0-C-2 alkylbenzenes do not follow this trend and are particularly resistant to laboratory evaporation processes. Nitrogen blowing evaporation to dryness can result in substantial losses of <C-21 n-alkanes, alkylnaphthalenes, methylbiphenyls, methyldibenzothiophenes and methylphenanthrenes, and their non-alkylated parents. The Pr/Ph, Pr/n-C-17, methylnaphthalene and dimethylnaphthalene ratios are reduced if evaporation is particularly severe, whilst the methylbiphenyl ratio is increased by evaporation. Despite overall loss of trimethylnaphthalenes, tetramethylnaphthalenes, methylphenanthrenes and methyldibenzothiophenes during evaporation, ratios based on these compounds appear to be unaffected by the extent of evaporation. Light (<n-C-9) hydrocarbon distributions and their ratios are severely altered by any evaporation, suggesting the requirement of special care in sample handling and data interpretations. The evaporated oil aliquots, like the residual oils formed by natural evaporative fractionation, exhibit an increase in the aromatic hydrocarbons relative to normal alkanes, an increase in normal alkanes relative to branched alkanes, and a decrease in linear alkanes relative to cyclic alkanes. Crown Copyright (C) 2003 Published by Elsevier Ltd. All rights reserved.