39 
Fig. 32: Overlay of the hopanes of Oseberg C - and Oseberg S crude oil (below), subtracted 
mass-chromatograms (above). 25-norhopane indicated. 
If the high value of this compound for discriminating between oil samples is known, the 
responsible analyst must come to the conclusion “non-match” already by the visual inspection 
of the mass-chromatograms. 
A in order to demonstrate and pronounce differences properly it might be much 
more convenient in Oil Spill Identification to calculate the (absolute) difference of 
compound relations between the samples and express this difference in form of 
percentages of the corresponding compound relations of the sample, which has to 
be identified, i.e. the spill sample. In the example given in Figure 32 this would lead to 
differences of about 28% (28-bisnorhopane) and about 85% (25-norhopane), if the higher 
values of these components were found in the spill sample. Especially the latter value is by far 
out of the range of analytical errors. Of course, these percentages may become very high or 
even infinite, in case a compound is not found in the spill sample but present in a comparison 
sample. But these cases have to be explained in any way: the responsible analyst must have 
come to the conclusion “non-match” already by the visual inspection of the mass- 
chromatograms in these cases.