The PNL high-transmission three-stage mass spectrometer

Abstract

We have constructed a three-stage isotope-ratio mass spectrometer of unique ion-optical design that achieves high ion transmission efficiency and high abundance sensitivity. The spectrometer has tandem 90[degrees] -deflection magnets with boundaries 18[degrees] off normal. The magnet drift lengths are 1.48 times the 27-cm radius of deflection. This extended geometry gives mass dispersion equivalent to a 40-cm-radius magnet with normal boundaries. The first magnet renders the ion beam parallel in the vertical plane and provides a focus in the horizontal plane of mass dispersion. The second magnet brings the beam to a stigmatic focus. This novel ion-optical design gives 100% transmission without the need for intermediate focusing lenses. It also provides a 16% increase in mass resolution over the traditional tandem geometry with normal magnet boundaries. Complete transmission of ions is maintained through a third-stage cylindrical electric sector of 38-cm radius, which provides increased isotope-abundance sensitivity. The isotope-abundance sensitivity of the new mass spectrometer is an order of magnitude better than similar instruments with normal magnet boundaries. This is because the vertical focusing of the ion beam prevents ion scattering from the top and bottom of the flight tube. The measured values of the isotope-abundance sensitivity one-half mass unit away frommore » the rhenium ion peaks at masses 185 and 187 are M [minus] 1/2 = (6.5 [plus minus] 0.5)[times] 0[sup [minus]10] M + 1/2 = (3.1 [plus minus] 0.8) [times] 10[sup [minus]10]. By extrapolation, the uranium isotope-abundance sensitivity is m [minus] 1 = 1 [times] 10[sup [minus]10]. Construction of the instrument was facilitated by using standard commercial mass spectrometer components.« less