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 5000 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. NOTE ISO 6978-2 gives a sampling method suitable for the determination of mercury on gold/platinum alloy thread at atmospheric pressure for the range of mercury from 0,01 µg/m3 to 100 µg/m3 and for sampling at high pressure (up to 8 MPa) from 0,001 µg/m3 to 1 µg/m3.