F4 gravimetric (F4G) analysis is recommended (and often required under regulation) every time the chromatogram of a sample does not reach baseline at C50, following routine F2-F4 petroleum hydrocarbon analysis. Where F2, F3, F4 denotes the chain length of that carbon fraction.
This means that there are hydrocarbons present with molecular weight larger than C50. An F4G analysis is meant to quantify these compounds that the routine F2-F4 PHC method cannot capture.
This analysis does not utilize instrumentation, rather the extract is oven dried and the remaining material is weighed. An F4G result represents a combination of (1) all hydrocarbons >C50 + (2) the majority of the F4 (C34-C50) fraction and (3) some components of the F3 (C16-C34) fraction. Therefore, an F4G result will always be higher than the F4 result.
In order to consider these heavier hydrocarbons when comparing to a standard, the F4G result should be used, rather than the F4 (when applicable).