Dear Nouha,

In marine sediments, AVS is usually measured by distillation using 6M HCl as extractant (first step of Canfield et al (1986) technique). The H2S released during distillation can be trapped with ZnAc(20%) and subsequent measured by spectrophotometry (Cline 1969), or be precipitated as Ag2S using a solution of 1M AgNO3. For latter, you need to weight the Ag2S and then to convert the weight to FeS. A 500 mg of dry sediment is enough for the extraction. 

Best regards,

Juan