Online detection and measurement of elemental mercury vapor by ion mobility spectrometry with chloroform dopant.

A rapid and simple method is proposed for detection of elemental mercury (Hg) vapor by ion mobility spectrometry (IMS). Negative corona discharge (CD) as the ionization source and chloroform as the dopant gas were used to produce Cl- reactant ion. A mass spectrum of the product ions confirmed that the mechanism of ionization is based on Cl- anion attachment to Hg and formation of HgCl- ion. It was found that the optimum drift gas temperature for Hg detection was about 160 °C and the drift gas flow rate should be minimized and just sufficient to clear contaminants and carry-over from the drift cell. The drift time of the HgCl- peak relative to that of the Cl- peak at 160 °C is 1.52 ms corresponding to the reduced mobility of 1.90 cm2/Vs. Because many volatile organic compounds (VOCs) such as alcohols, amines, aldehydes, ketones, and alkanes are not ionized in the negative mode of CD-IMS, these compounds do not interfere with the detection of Hg. Mercaptans peaks also did not show any interference with the Hg signal. Hence, the method is highly selective for detection of Hg in natural gas containing sulfur compounds. The detection limit of Hg obtained by the proposed method was 0.07 mg/m3. The method was successfully verified in determination of the mercury vapor content of a fluorescent lamp, as a real sample.