13 
This cluster represents mainly six compounds from different compound classes of aromatic 
hydrocarbons (Fig. 6). It is, therefore, much more suitable for discriminating between oils 
than clusters representing only the isomers of a compound within a single compound class. 
a: 2-M-fluoranthene 
b: benzo(a)-fluorene 
c: benzo(b)-fluorene 
d: 2-M-pyrene 
e: 4-M-pyrene 
f: 1 -M-pyrene 
31 33 
35 37 nw 39 
Fig. 6: Mass-chromatograms of mass 216 of two different samples and peak identification. 
By normalizing the peak heights on one of these peaks, e.g. on peak “e” (4-M-pyrene), which 
is often the highest peak of this cluster, a set of five parameters (compound ratios) can be 
calculated. 
A in general, compound ratios for verification of the visual findings should be 
produced by dividing (normalizing) peaks on a peak that is present within the 
same mass chromatogram and within a narrow boiling range. Variations due to 
instabilities on the mass spectrometric side as well as variations due to possible compound 
discriminations on the gas chromatographic side are thus minimized. A high reproducibility is 
achieved and the resulting compound ratios are even comparable if measurements are made 
on different instruments (e.g. in interlaboratory comparisons). 
In the same boiling region, further masses exist which represent different compound classes 
within the same mass chromatogram. In the mass-chromatograms of mass 234 and mass 240 
even a comparison of aromatics with aromatic sulfur compounds is possible. Here, higher- 
methylated phenanthrenes and M-benzo-naphtho-thiophenes (mass 234) and higher- 
methylated naphthalenes and higher methylated dibenzo-thiophenes (mass 240) are indicated 
(Fig. 7).