Type HGR(R) activated carbon is frequently employed to protect aluminum heat exchangers in liquefying natural gas. Residual mercury levels in the treated gas are frequently at or near the limits of detection, challenging the analytical methods employed. Additionally, process variables and variations in the testing environment can affect the analytical procedures used to measure low levels of mercury. This paper will describe the analytical procedures used to routinely measure mercury levels to below 0.01 mu g/Nm(3) under challenging process and field conditions. The impact of process variables on adsorbent performance will also be described.