| A time-of-flight mass spectrometer (TOFMS) conceived particularly for plasma-source atomic mass spectrometry is theoretically described and experimentally characterized. When coupled with the inductively-coupled plasma (ICP) ionization source, the system is capable of the complete and simultaneous acquisition of the entire atomic mass range at a rate of 30,000 mass spectra per second, achieving atomic limits of detection of 10-12 g/g and a linear dynamic range exceeding 6 orders of magnitude. The simultaneous extraction of all mass-to-charge ratios (m/z) of interest is shown to facilitate improvements in measurement precision and to overcome artifacts attributable to sequentially-scanning types of mass analyzers. The unique geometry of this on-axis TOFMS improves upon previous designs by providing uniform response across the atomic m/z range and in attaining greater sensitivity. This on-axis TOFMS design places the flight tube axis and the continuous ion source along a single axis, in contrast to the traditional orthogonal-extraction TOFMS geometry in which the two are arranged perpendicularly. Salient design principles pertinent to the on-axis design are described and contrasted with those of the traditional orthogonal-extraction TOFMS geometry. Advantages attributable to the on-axis TOFMS geometry are demonstrated experimentally, including an increase in sensitivity of approximately 10-times over previous ICP-TOFMS designs. Limitations of the design are also examined, particularly a residual chemical background noise source. The contributing cause is studied and a mechanism suggested based upon an ancillary charge-exchange reaction occurring within the mass analyzer. A means of overcoming this noise source consisting of a novel off-axis detector assembly for TOFMS is theoretically described and experimentally characterized. Finally, a unique ionization source based on a tandem arrangement of an atmospheric-sampling glow discharge (ASGD) and a microwave plasma torch (MPT) is described. The tandem pair is shown to be capable of generating atomic mass spectra, and molecular mass spectra that are similar to electron impact ionization, in an alternating fashion. In this way, both the atomic composition and molecular form of a sample can be determined on a chromatographic time scale. |
