Conducting multi-elemental analyses with an inductively coupled plasma mass spectrometer using electrothermal vaporization sample
This research focuses on the use of electrothermal vaporization (ETV) as a sample introduction source to an inductively coupled plasma mass spectrometer (ICPMS) and the ability to perform multi-metal analyses. The ETV produces skewed Gaussian shaped signals that are typically 1-3 s in duration. The short signal duration can adversely affect precision and accuracy when attempting quantitative analysis on many elements or, in the extreme, when conducting full mass scans (2-260 amu) with a quadrupole scanning mass analyzer. Therefore, a novel peak broaden device was designed to convert the typical ETV signal to a variable length square wave signal, which achieved analyte precisions of < 10% while monitoring 248 masses on a conventional quadrupole mass analyzer. Although quadrupoles have generally been the mass analyzer of choice in ICPMS, the coupling of an ETV to a recently purchased GBC Optimass 8000 inductively coupled plasma time-of-flight mass spectrometer ICP(TOF)MS was investigated. The TOFMS, unlike the quadrupole, offers the ability to monitor one or many masses without altering the duty cycle. While a complete spectrum is gathered in ca. 30 µs, several spectra are accumulated to improve the S/N ratio. The rapid data collection technique abrogates the need for peak broadening devices when using the ETV. The GBC Optimass 8000 exhibited low picogram detection limits, high isotopic precision and a linear dynamic range of 3-4 orders of magnitude with combination of ion and analog counting modes. In the course of interfacing the ETV to the ICP(TOF)MS, the use of a dry plasma produced an electrical discharge between the 1st and 2nd skimmer cones in the GBC Optimass 8000. The nature of this discharge was explored. The discharge was found to cause a loss in analyte sensitivity, 2nd skimmer cone sputtering, and eventual loss of 2nd skimmer voltage adjustment. The discharge was alleviated by reducing the r.f. power to 700 W when using 1.2 L/min central channel gas flow rate with ETV sample introduction. Although the ICPMS is a sensitive technique in determining trace metals, it can be highly susceptible to “matrix effects” when large concentrations of concomitant elements are present in the sample matrix. The utility of a mixed Ar-N2 plasma was investigated with ETV-ICP(TOF)MS. The mixed gas plasma was shown to minimize analyte suppression at the expense of 83% loss in analyte sensitivity. The immunity to concomitant elements was attributed to reduced space charge effects in the mass analyzer when the ion introduction rate was lowered as a result of the use of either the mixed gas plasma or altering the sample cone-to-plasma distance for the Ar plasma.