one thing i want to mention here is samplr preparation is very important for PHB GC analysis, if sample not prepared well all PHB Will be evaporated during heating stage. So keep in mind that use every time new tubes with its cork/lead.

is for hydrolysis of PHA, to obtain short molecules 

Hello, I perform PHA sample preparation for GC as follwo: 15 % H2SO4 (w/v) in methanol 0.8 ml + 1.0 ml of sample in the CHCl3, 100°C, 2.5 h.

Hi, as per Stanislav's response a 15% H2SO4 85% Methanol (v/v) is the standard you will see in most publications, we tested this in our lab a few years back and between 3 and 15 % made no difference to the monomer content we detected. 

A solution of 2 ml of chloroform and 2 mL of acidified methanol (3% sulfuric acid) were added to the cell pellet in a vial with Teflon screw cap and heated at 100 ◦C

for 3.5 h. Please find the following publication:

Daneshi A, Younesi H, Ghasempouri SM, Sharifzadeh M. Production of poly-3-hydroxybutyrate by Cupriavidus necator from corn syrup: statistical modeling and optimization of biomass yield and volumetric productivity. J. Chem. Technol. Biotechnol. 2010; 85(11): 1528-1539.

In our lab, we prepared methyl benzoate with the following procedure:

Benzoic acid was methylated in acidified methanol (3% sulfuric acid) as acidifying agent and heated in water  bath at 100°C for 3 h. Then 15 microliter (µl) of methylbenzoate was added into standard solution of PHAs purchased from Sigma or unknown sample of PHAs from bacteria samples. Calibration curve of standard PHAs solutions should be prepared before methylation of the PHA composition of the cells.