Abstract: A sampling apparatus for mercury in natural gas lines includes a source line fluidly connected to a natural gas line and configured for delivering a sample gas flow, a directional valve fluidly connected with the source line and configured for selecting a flow direction of the sample gas flow, a bypass line connected to the directional valve and configured for drawing off the sample gas, and a sample line assembly fluidly connected to the directional valve. The sample line assembly includes a plurality of sample trains, and each of the sample trains includes (i) a shut-off valve at an upper end thereof configured to control the sample flow therethrough, (ii) a measuring device at a lower end thereof configured for measuring flow therethrough, and (iii) a sample trap assembly intermediate the shut-off valve and the measuring device configured for collecting constituents in the sample gas flow. A method of using the sampling apparatus for mercury in natural gas lines is also disclosed.