36 
Oil Spill identification - Round Robin 20045 
Table 3. 
Normalizing the peak heights on peak 
4-M-pyrane (e) of mass 216 (LVA) 
Fig 12 
Ratio plot of Cedre including error bars 
showing a 95% interval calculated 
from 
Another example is the ratio calculation of LVA shown in Table 3. The 
table highlights the difference in ratio between B(a)F/ 4-methylpyrene 
for Source II. 
Ratio 
EXTR I 
EXTR II 
Source I 
Source II 
a/e 
0.23 
0.23 
0.25 
0.16 
b/e 
1.0 
0.94 
0.93 
0.36 
c/e 
0.19 
0.16 
0.16 
0.1 
d/e 
0.79 
0.76 
0.8 
0.72 
f/e 
0.73 
0.70 
0.75 
0.60 
Such tables show differences, but what is acceptable is still a personal 
decision. 
In the CEN guideline it is defined that differences caused by the 
analytical method may not lead to the conclusion that samples are 
different: 
positive match 
differences in chromatographic patterns and diagnostic ratios of the samples 
submitted for comparison are lower than the analytical variance of the method 
or can clearly be explained by weathering. The samples are identical beyond 
reasonable doubt 
Several labs have used numerical limitations to estimate the acceptable 
difference. 
70 
60 
50 
40 
30 
20 
10 
0 
Figure 5 Aromatics distribution of 4 samples (in/z = 216 and 234) (error bars represent 95% 
confidence level intervals, N=3) 
m Source I ■ Source II □ Extract I □ Extract II 
B(a)F/4-Mpy B(b+c)F/4-Mpy 2-Mpy/4-Mpy 1-Mpy/4-Mpy Retene/C4-Phe 
Fig 12 shows a bar graph of Cedre with error bars: 
The analyses were run in triplicates for each sample, and error bars 
considering 95% confidence level intervals were plotted on the histogram. 
In the report of Cedre also graphs with a 95% and 98% confidence are 
shown and used to estimate, whether differences are significant or not.