42 
Oil Spill identification - Round Robin 20045 
Here also the absence of the triaromatic steranes in lubrication oil is 
mentioned. 
3.5 Number of ratios 
The labs using ratios apply a varying number of ratios. E.g. LVA has 
used 5 ratios from m/z 216 to show the difference between source II 
and the extract samples, NFI has used 21 ratios and Petrobas 22 ratios 
to test weathering and identity and NERI has calculated the highest 
number of 57 ratios. 
Ratios can be used to show differences, but also to "proof " identity. 
Gerhard Dahlmann has build a database containing the chromatograms 
of more than 1000 cases (see Oeldatenbank-COSI.ppt on the CD for a 
presentation). The database is able to sort on identity and makes it 
possible to test how many ratios are needed to specify a sample. 
Gerhard has learned that for most samples 5 to 6 ratios are sufficient to 
eliminate non-matching samples. 
Without a database it becomes more difficult to "proof" identity on a 
limited number of ratios. Therefore 29 ratios are mentioned in the CEN 
guideline to be used for comparison. The ratios should be used in a 
flexible way, based on presence and stability of the compounds 
involved. 
Finally it must be stated that a comparison without using ratios to proof 
identity is highly susceptible for (mis)interpretation, while a comparison 
only based on ratio comparison can easily lead to false matches, 
because the differences could be present elsewhere. Therefore ratios 
must be seen as a tool, 
to force analysts to compare a large range of compounds, 
to be able to set a quality criterion for the comparison and 
to compare samples unaffected by personal interpretation. 
3.6 Sesquiterpanes. 
Comments of Gerhard Dahlmann on the use of the sesquiterpanes by 
some of the participants: 
RIZA indicates: 
Changes compared to the round robin of last year: 
The sesquiterpanes are added as compound group to be analysed with GCMS 
(m/z 123). 
Thus, although it is found in the weathering check that alkanes are evaporated 
up to C20 in the spill samples, sesquiterpenes, which are