General Information

Hydrocarbons are usually fairly easy to be characterized by mass spectrometry.  The sample's volatility will largely determine the best ionization method.  Volatility is to a large degree a function of molecular weight.

Note that other species with a large hydrocarbon - like component (example: porphyrins) also tend to follow these guidelines.
 

Information on low mass (< 500) hydrocarbons.

Information on high mass hydrocarbons.
 
 
 
 

Low molecular weight hydrocarbons

Most hydrocarbons with masses under 500 (approximately) are reasonably volatile under high vacuum conditions.  The most effective way to ionize these materials is using ELECTRON IMPACT ionization.  Linear, saturated hydrocarbons tend to give small molecular peaks and intense fragment ions.  Unsaturated species tend to give relatively larger molecular ions.

CHEMICAL IONIZATION is generally less effective. However, for highly unsaturated species, a reasonable molecular peak may sometimes be found.
 
 

High molecular weight hydrocarbons

Once the molecular weight exceeds 500, hydrocarbons are far less volatile, even under high vacuum conditions.  For materials of this type, MATRIX-ASSISTED LASER DESORPTION IONIZATION (MALDI) is the best bet for ionization. In particular, species with a substantial aromatic portion (e.g. fullerenes) tend to work well, because they generally are excellent absorbers of the UV laser radiation used in ionization.