ISO 6978-1:2003 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 microgram/m3 to 5 000 micrograms/m3 in natural gas. This method is applicable to sampled gas volumes containing less than 20 mg hydrogen sulfide 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.