The isotope ratio performance of an axial time-of-flight ICP mass spectrometer (Renaissance TOF-ICPMS, LECO Corp.) was evaluated. The isotope ratio precision, expressed as the relative standard deviation (RSD) for 10 successive measurements, was evaluated using multielement standard solutions with analyte concentrations of 50-500 μg/L. The influence of the acquisition time per replicate measurement was studied by varying it between 0.5 and 300 s. For an acquisition time of 30 s per replicate and an elemental concentration of 500 μg/L, typical isotope ratio precisions of ≤0.05% RSD were obtained. The fact that this isotope ratio precision can be obtained for many ratios simultaneously is an especially attractive feature of TOF-ICPMS. In contrast to what was expected, increasing the acquisition time per replicate to values of >30 s resulted in a slightly deteriorated isotope ratio precision. At short acquisition times (<10 s), isotope ratio precisions similar to, or better than, the best values ever reported for quadrupole-based instruments were obtained. The latter observation remained valid when working with transient signals of corresponding duration. Mass discrimination was observed to be analogous to that observed with other types of ICPMS instrumentation (∼1% per mass unit at midmass). The accuracy attainable was evaluated by comparing Pb isotopic results for a "natural" Pb standard solution obtained by TOF-ICPMS with those obtained by thermal ionization mass spectrometry.

No abstract

Pulses of analyte and matrix ions from individual drops are measured simultaneously using a twin quadrupole inductively coupled plasma mass spectrometer (ICP-MS). The sample solution is introduced by monodisperse dried microparticulate injection (MDMI). At modest Pb concentrations (500 ppm), a shoulder on the leading edge of the Li(+) signal appears. At higher matrix concentrations (1000 to at least 1500 ppm), a dip in the leading edge of the Li(+) signal develops. These changes in the shapes of the Li(+) pulses are attributed to space charge effects in the extraction system and ion optics of the mass spectrometer. A qualitative depiction for this behavior is proposed, in which the Li(+) ions are deflected out of the preferred ion path and then refocused by the ion optics. Part of the Li(+) ion cloud is driven ahead of the Pb(+) cloud, and part is trapped behind the Pb(+) cloud. The result is a shoulder on the leading edge of the Li(+) signal. With the Pb matrix present, the shapes of the analyte ion pulses are sensitive to the voltages applied to the first two ion lenses, especially the extractor lens. This observation shows that the part of the matrix effect that occurs in the ion optics takes place mainly in the first two lenses.