Low Pressure Inductively Coupled Plasma Research Articles

Low Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry: The Effect of Reagent Gases

A low pressure inductively coupled plasma (LP-ICP) source, sustained at only 6 W and utilising 6 ml min–1 helium, has been investigated as an ionisation source for molecular and atomic mass spectrometry. Iodobenzene and dibromobenzene were introduced to the LP-ICP via gas chromatography and yielded purely atomic ion signals for the iodine and bromine present, with detection limits of 4 and 76 pg for iodobenzene and dibromobenzene, respectively. The addition of nitrogen to a LP helium ICP increased the molecular ion signal for chlorobenzene, with a detection limit of 2 pg. However, the addition of nitrogen did not aid the production of molecular ions of iodobenzene and dibromobenzene. A study of the effect of skimmer spacing and forward power revealed considerable spatial separation of ionisation processes within the expansion chamber of the molecular beam interface. On the addition of 0.07 ml min–1 isobutane the LP-ICP yielded mass spectra similar to those obtained by an electron impact source. However on the addition of more isobutane only the molecular ions (M+) for chlorobenzene, iodobenzene and dibromobenzene were observed. The detection limits for the instrument operating in the molecular mode were 100, 140 and 229 pg for chlorobenzene, iodobenzene and dibromobenzene, respectively.

Reduced-Pressure Inductively Coupled Plasma Mass Spectrometry for Nonmetallic Elements

A reduced-pressure argon inductively coupled plasma (ICP) is interfaced to a mass spectrometer to evaluate its possibility of increasing the sensitivity of nonmetallic elements. An electrostatically shielded water-cooled torch is used for the investigation of the secondary discharge at the sampling orifice. Iodine vapor is continuously introduced into the torch as an analyte by using a peristaltic pump. The effects of plasma operating parameters such as gas flow rate, pressure, and power on the intensities of background and iodine ions are studied. It is shown that when the pressure is less than about 30 Torr, an intensive secondary discharge occurs at the sampling orifice if the torch shield is not grounded. The background ion intensity and secondary discharge effect decrease with increasing pressure. The pressure in the torch has an important effect on both polyatomic and analyte intensities. At about 130 Torr of torch pressure, the iodine signal is more than one order of magnitude higher than that obtained at atmospheric pressure, which suggests that low-pressure ICP provides a sensitive ion source for the elements with high ionization potential.