The methylene blue method may not be very ideal in this case. You may try the gravimetric method of conversion to lead sulfide using lead nitrate as the starting material.

Hello Vaios - we work with emissions of H2S from livestock manure (liquid slurry).  H2S losses to headspace can affect results depending how fecal samples are handled, pH, surface area and solid-liquid-headspace mass transfer, to name a few.  Elemental S analyzers are available and that can help with mass balance and accounting for losses.  There is some interesting partitioning of S into H2S, sulfur VOCs, and SOx occurring.