This part of ISO 6978 specifies a method for the determination of total mercury content in natural gas using a sampling method at pressures up to 40 MPa by chemisorption on iodine-impregnated silica gel. This sampling method is suitable for the determination of mercury contents within the range of 0,1 μg/m3 to 5 000 μg/m3 in natural gas. This method is applicable to sampled gas volumes containing less than 20 mg hydrogen sulfide (absolute content) and less than a total liquid hydrocarbon condensate of 10 g/m3 under the sampling conditions. The collected mercury is determined by measuring the absorbance or fluorescence of mercury vapour at 253,7 nm.