He MIP-MS detection of gas phase halogens

Satzger and co-workers [97] used an atmospheric-pressure He MIP at moderate powers (360 W) with a laboratory constructed plasma MS instrument to detect Br, Cl and I as positive ions. In order to reduce air entrainment in the plasma plume, a torch was designed which provided a tangential sheath flow of nitrogen. The major background ions in the He mass spectrum were found to be N 0 and. To 

Increased sensitivity for Br, Cl and I as positive ions was reported when using the bonnet and sheath gas. The apparent ionization potential of the plasma seemed to have been increased above 9.25 eV (the ionization potential for NO) when using the combination [97]. The ionization potential for (the principal background ion in the shielded plasma) is 15.6 eV while the first ionization potentials for Br, Cl and I are 11.84, 13.01 and 10.45 ey respectively. 

Gaseous mixtures of CHaBr, CH3CI and CH3I in helium were used for this preliminary study. The detection limits reported were 1.2, 21, and 1.8 pgs, respectively. These results were encouraging for they showed an improvement over Ar ICP-MS for Br and Cl and compare well with results obtained using He plasmas with optical emission spectrometry. 

The detection of gas phase halogens by He MIP-MS has also been investigated by Brown and co-workers [98]. They utilized a commercially available ICP-MS instrument. The ICP torch box was removed and replaced with an internally tuned TMqio microwave cavity. The He plasma was formed using a demountable tangential flow torch. This was used in an effort to create a central region in the plasma, rich in analyte ions which allows for maximum sensitivity when sampling the plasma with the mass spectrometer. 

The optimum power was approximately 200 W depending on the element of interest. Sampling position was generally found to be optimal in the center of the plasma and at small sampling depths (tip of sampler even with the face plate of the cavity). Tangential He flows were kept between 6 and 8 1 min.  

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