You need the response factor (detector response versus concentration) for each relevant FAME you want to use.  Thus, from that curve;

100 x [mg/L x L (volume)]/mg (sample weight) = % FAME

In order to determine the 'dry weight (DW)' use a Karl Fischer reaction (KFR) to determine the amount of water.

Dear Bruce, many thanks for your answer. Would it be possible for you to provide me with some clarification.

I have constructed a calibration curve ranging from 1000 mg/L to 50 mg/L and excel gave me this equation; y = 762013x + 2E+06.

Based on this equation, the unknown area is 5343039376, giving the concentration of 6958.754041 mg/L.

Would it be possible to change the concentration (mg/L) to mg/g?

It depends on several other factors.  

What is your sample volume?  It's usually 100 mL or 0.1 L.  

What is your initial sample weight in grams?

What is the 'dry weight' of the biomass in %?

Are you sure you're in the linear range of the method?

I have concentrated all my sample by evaporating the solvents and reconstitute with 1 mL of hexane.

the initial weight of the biomass is 0.6949g.

Good.  The answer is;

[6958.754041 mg/L x 0.001 L (which is 1 mL)]/0.6949 g = 10.01404 mg/g

Biomass, in this case is 'as is' and not dried.

Many thanks for your help Bruce, would it be possible for you to provide me with some reference regarding this calculation using external standard calibration curve?

thank you.

Try the USP (monographs on corn, safflower, and soybean oils) as well as AOCS (Association of Oil Chemists Society) on FAMES.

I am very sad that I must repeat "MS is not a quantitative detector at all".

Fatty acid ester should be hydrolyzed, and fatty-acid-ADAM derivative is measured quantitatively using RP-HPLC-photometric or -fluorimetric detection (please see file; Thioctic acid Determination).