44 
Oil Spill identification - Round Robin 20045 
Fig 19. 
RIZA additional weathering check 
This must be regarded as a highly uneconomical practice. 
What cannot be said about the C17/pristane, C18/phytane and 
pristane/phytane ratios is much more likely for compounds boiling from C13: 
in spill samples, these might nearly always be affected by evaporation. 
Generally, it is given in the literature that sesquiterpanes may be useful in 
distinct cases for source correlation (i.e. for finding “general sources”) 
because of their resistance to biodegradation. A case such as the one, which is 
given in this Round Robin here, is, of course, neither mentioned nor meant, 
when the usefulness of sesquiterpanes is discussed in literature. 
It is indeed true that the sesquiterpanes should not have been used in 
this case . The CEN working group has recently decided to add some 
ratios of the sesquiterpanes, because this group of biomarkers is well 
known and often mentioned in source specifications. See the 
contribution of Wang in RR2004, the contribution of Petrobas in 
RR2005 and CEN Guideline Part II 6.3.5.4. In the guideline the 
sesquiterpanes are mentioned optionally with the restriction: 
As any other low-boiling compounds, the sesquiterpanes are subject to 
evaporative weathering, a fact which has to be taken into account before 
using them for diagnostic purposes. 
It is nice to see that all labs working on the guideline have individually 
decided to analyze and integrate the sesquiterpanes although it is 
mentioned in their reports, that they are weathered. 
Remarkable is the difference of 22% in one of the ratios found by 
Sintef (DM3/DM5). But RIZA and NERI, having also given the 
DM3/DM5 ratio, find only differences of this ratio in the two source 
samples of well below the repeatability limit of 14%.