AN INDUCTIVELY-COUPLED PLASMA TIME-OF-FLIGHT MASS-SPECTROMETER FOR ELEMENTAL ANALYSIS .2. DIRECT-CURRENT QUADRUPOLE LENS SYSTEM FOR IMPROVED PERFORMANCE

An electrostatic quadrupole lens has been substituted for a cylindrica l lens system used in the original inductively coupled plasma-time-of- flight mass spectrometer (ICP-TOFMS). With an improved vacuum system a lso installed, the cylindrical and quadrupole lenses are compared to e ach other and to the performance of the prototype ICP-TOFMS. The quadr upole lens requires no tradeoff between ion throughput and resolving p ower as was encountered with cylindrical lenses. The background noise in both ion-optical systems is within the same order of magnitude. Ima ges of the ion beam formed by each ion-optical system have been obtain ed on a microchannel plate-phosphor screen. The quadrupole lens shows a higher ion-beam flux and produces a slitlike focus required in the o rthogonal ICP-TOFMS instrument. Signal-to-noise ratios in the ICP-TOFM S can be improved by using a technique called pulsed-ion injection tha t is particularly convenient with the quadrupole lens. In this techniq ue, one quadrupole electrode is pulsed to preventions from entering th e extraction zone except when an ion packet is to be extracted for mas s analysis. This technique significantly reduces the noise over contin uous ion injection. In the orthogonal ICP-TOFMS with pulsed-ion inject ion, 0.5 fmol of analyte could be detected in 1.4 ms with a proper dat a acquisition system. Overall, the combination of a quadrupole lens an d pulsed-ion injection may provide detection limits for the ICP-TOFMS that are competitive with those of quadrupole inductively coupled plas ma-mass spectrometry instruments.