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 °C, (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-C32, indicate that (1)