ISOTOPE RATIOS AND ABUNDANCE SENSITIVITY OBTAINED WITH AN INDUCTIVELY-COUPLED PLASMA-TIME-OF-FLIGHT MASS-SPECTROMETER

Isotope ratios and abundance sensitivities have been determined with a n inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOF MS). Abundance sensitivities are at least in the 10(6) range for low a bundance ions that precede high abundance ions. Three methods of detec tion for isotope-ratio measurement have been compared. The three syste ms involve gated detection followed by analog integration, analog aver aging, or ion counting. Gated ion counting offers excellent precision- between 0.64 and 1.00% relative standard deviation (RSD). These values approach those predicted from counting statistics and are comparable to those reported for other inductively coupled plasma-mass spectromet ry (ICP-MS) instruments. In addition, a greater number of accumulated counts or longer analysis times would afford precisions of 0.1% with s table gating electronics. The accuracy of the counting method is in th e 1-10% range if no correction for mass bias is performed. However, th is ion counting method suffers from a limited dynamic range due to pul se pileup. Constant-fraction discrimination gated integration and comm ercial boxcar averager techniques offer a broader dynamic range becaus e of their analog nature, but the attainable RSD values are limited by drift in the detection systems and by the methods employed to calcula te an accurate ratio. Overall, mass bias in the ICP-TOFMS is more seve re than previous work in ICP-MS due primarily to detection system bias .