35 
Oil Spill identification - Round Robin 20045 
3.4 Comparison of compound ratios 
Besides the visual comparison of chromatograms and plots of peak 
heights or area's of ranges of compounds (e.g. see Fig 6 and 7), all labs 
used, to a certain extend, ratio's to improve the foundation of the final 
conclusions. A summary of the methods reported is given in Table 2. 
Labs 
Methods 
GC-FID 
GC-MS 
visual 
height or 
PW 
ratio 
visual 
height or 
PW 
ratio 
numerical 
FID 
MS 
inspect. 
area plots 
plots 
calc. 
inspect. 
area plots 
plots 
calc. 
limitations 
BMM 
X 
X 
X 
X 
X 
X 
X 
Cedre 
X 
X 
X 
X 
X 
LAS EM 
X 
X 
X 
X 
X 
LVA 
X 
X 
X 
X 
X 
X 
X 
NBI 
X 
X 
X 
X 
X 
X 
NERI 
X 
X 
X 
X 
X 
X 
X 
X 
NFI 
X 
X 
X 
X 
X 
Petrobas 
X 
X 
X 
X 
X 
X 
X 
RIZA 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
Sintef 
X 
X 
X 
X 
X 
X 
X 
SKL 
X 
X 
X 
X 
X 
X 
WRD 
X 
X 
X 
X 
Table 2 
Methods to compare samples a type of ratio calculation is the percentage-weathering (PW) plot. 
After normalization on compounds not weathered, the percentage of a 
compound in the spill sample compared to the source sample is 
calculated for a range of compounds and shown in a sequence based 
on retention time. These plots were produced by RIZA and SKL: 
Weathering is investigated with help of the PW plot, where the area of each 
of the n-alkanes is compared with the average of the area of five unweathered 
n-alkanes. For slightly weathered oil samples we use the average area of C17 
- C21 and for more highly weathered samples we use C20 - C24. 
These plots are very helpful to estimate evaporation and to find 
differences in patterns. For examples see Fig. 5 and 19 in this report 
and Fig. 19 and 20 in the RIZA report. 
Very common is also the calculation of ratios between compounds. 
Assuming that a spill and source sample are coming from the same 
source, it can be assumed that ratio's between compounds are also 
identical, although it must be keep in mind that weathering, 
contamination and homogeneity can cause differences. 
An example is the calculation of the C17/pristane, C18/phytane and 
pristane/phytane ratios from the GC-FID and/or GCMS results. 
In this case however the value of these ratio's was limited. First of all 
the differences were very small and additionally the ratio's were 
influenced by weathering. SKL has applied a calculation to compensate 
for weathering, but no conclusions based on the results are mentioned.