Isotope ratio precision with transient sample introduction using ICP orthogonal acceleration time-of-flight mass spectrometry

Abstract

An assessment of inductively coupled plasma orthogonal acceleration time-of-flight mass spectrometry (ICP-oa-TOF-MS) for measurement of isotope ratios arising from steady-state and transient signal detection is presented. An HPLC pump and autosampler were used to introduce microliter volumes of analyte solution into a carrier flow to generate precise and repeatable transient signals having a FWHM of approximately 1 s. Analog and pseudo pulse data processing modes were evaluated and found to obey Poisson statistics for prediction of isotope ratio performance. Use of pseudo pulse counting is restricted by the limited linear range of response and cannot exceed approximately 0.2 counts per pushout. This limitation does not impede analog detection, which thus permits precise isotope ratio measurements to be achieved at higher analyte concentrations using shorter integration times. For transient signals, area measurements are superior to averaging point-to-point measurements across the peak for optimum ratio precision. Although the TOF system provides for simultaneous sampling of the extracted pulse, thereby permitting elimination of correlated noise through ratioing techniques, detection remains sequential and noise components present in the detection system limit isotope ratio precision in this study to 0.04% RSD.