Explicit chemical speciation by microsecond pulsed glow discharge time-of-flight mass spectrometry: concurrent acquisition of structural, molecular and elemental information

A microsecond pulsed glow discharge (mu s-pulsed GD) was used for elemental, structural and molecular analysis. This plasma was generated repeatedly at 100 Hz with a 1 kV electrical pulse lasting 20 mu s. Samples were continuously introduced into the mu s-pulsed discharge and allowed to interact with the plasma. Depending on the extent of this interaction, which is controllable at the discretion of the user, the samples may undergo soft chemical ionization yielding molecular ions or may be completely atomized and ionized yielding elemental information. The operator could choose the above interaction zones or anything in between, which provides structural information through an electron ionization type fragmentation pattern. Detection of plasma produced ions was done by time-of-flight mass spectrometry (TOF-MS), which is most suitable for complete mass spectrometric analysis of time-gated transient events. Using ion energetic information and the so-called 'thermometer molecules', the magnitude of ionization energy available in the discharge was calculated. At 60 mu s after the plasma ignition the plasma had an equivalent ionization energy of about 17 eV. The plasma energy decreased as a function of time. At about 300 mu s the plasma reached a relatively constant ionization energy of 8-9 eV, which lasted for nearly 200 mu s. Toluene, p-xylene, ethylbenzene and tungsten hexacarbonyl are among the molecules analyzed using time-gated mu s-pulsed GD-TOF-MS.