20 
Organization in the late 80s, such residues account for about 3% of the originally bunkered 
fuel oil. Generally, refinery residues appear where they are least usable, i.e. on board ships. 
Obviously, these residues are often illegally discharged into the sea. 
2.3.1 Analysis 
As HFOs are blended products, the contents of different compound classes in HFOs may 
differ widely. 
A HFOs contain high-boiling compounds which are not analyzable by the GC 
technique. Therefore, in order to prevent contamination and pollution of the 
injection system, a sample clean-up is advisable whenever HFO is involved. As a 
general rule, the extracts of all black samples (“black oil”) should be treated in this way 
before injecting them into the instruments. 
As variable parts of HFO samples thus are not covered by the analysis, it may be hardly 
possible to differentiate between higher grades of HFOs by application of GC techniques. 
2.3.1.1 GC screening 
Breaking of the bonds of high-molecular crude oil compounds produces primarily olefins and 
aromatic hydrocarbons. While olefins react rapidly and disappear, aromatics are stable and 
remain present. 
Therefore, a high content of aromatic hydrocarbons in an oil sample is a strong indicator of a 
heavy fuel oil (Fig. 13 and 14). However, depending on the different products used for 
blending and on their amounts, the composition of HFOs may vary widely (Fig. 15). GC-MS 
analysis thus may have to be performed in order to properly identify the product.