Characterising and reducing the blank response from mercury vapour sorbent tubes

An investigation into the factors contributing to the blank response of sorbent tubes used for sampling and measuring mercury vapour is presented. These contributing factors are quantified and strategies to mitigate or remove their effects have been proposed - the most effective of which on a routine operational basis is the cleaning of the sorbent tubes in air to remove surface adsorbed mercury and any organic contaminants that can be oxidised. Contributions of up to 175 pg of mercury, or mercury equivalent mass, were identified and removed. The largest contributors were deeply absorbed mercury and hydrocarbons and other organic compounds oxidised and removed by heating in air. Decreasing the blank response resulted in an improvement in detection limit of a factor of two. This estimate was corroborated by a novel technique for assessing the detection limit of analytical methods employing multiple desorptions that relies on determining when the ratio of the third desorption response was equivalent to the first desorption response.