Problem is - most analytical methods give you information about individual compounds. Gasoline is a mixture of different hydrocarbons and it is not clear to me how to translate individual compounds into "gasoline amount". Additionally, these different hydrocarbons have different volatility, so they will evaporate from the soil with different speeds - so their composition in soil will change with time, making problem even more complex.
I suppose that there are some norms and methods defined by gov agencies that allow for such measurements, but the result is - at least partially - a matter of convention (just like soil pH
is). As long as you follow the convention you may compare results of measurements done in the different places, but measurement doesn give exact answer to the "gasoline amount" question, as the question is not exact. Perhaps if you will ask for "total hydrocarbons" it will be much easier - it will be enough to add all HC concentrations to get some number of more or less well defined meaning.
I suppose that would work if there is a small sample.
You always work this way - you get small sample to analyze.
Is there an instrument that would detect and calculate the amount of the fumes from the gas?
What do you mean by "amount of the fumes from the gas"? But perhaps I have already answered your question above?