36 
2.5.1.1 GC screening 
Crude oil washing residues from tankers often can be identified by an additional hump of the 
n-alkanes in the higher boiling region of the GC (“bimodal distribution”, Figure 29 upper 
samples). Here, high boiling residues sticking to the tank walls after unloading have been 
washed out by means of or together with residual loading. 
Fig. 29: GCs of two samples from massive oil pollution taken from German and Dutch 
beaches (above) and possible sources (below). 
Crude oils are generally roughly categorised into n-paraffin (wax) based and naphthene based 
crudes and mixtures of these (Figure 30).