Application of glow discharge Fourier-transform ion cyclotron resonance mass spectrometry to isotope ratio measurements

The combination of a glow discharge ionization source with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer provides several advantages for elemental analysis; among these is an isotope ratio measured at the highest possible mass resolving power. In this report, lead ratios (Pb-206/Pb-208, Pb-207/Pb-208, and Pb-207/Pb-206) in a sample composed of 10% lead oxide, 10% mercuric oxide, and 80% silver powder (w/w) were observed to be precise to better than +/-3% relative standard deviation (RSD at 1 sigma) in a commercial FTICR instrument that used a conventional elongated cell. Upon incorporation of a cell whose excitation voltages approximated those in an ideal cell of infinite length, the precision improved to better than +0.4% RSD. The isotopic bias between measured and known values was also evaluated. Using a cathode composed of 5% National Institute of Standards and Technology (NIST) SRM 987 SrCO3 in silver powder, biases that averaged less than 2.5% were detected using the cell of improved design. These results compared favorably with those obtained using a commercial magnetic sector glow discharge mass spectrometer, although it is still unclear how glow discharge mass spectrometry biases vary in general. (Int J Mass Spectrom 178 (1998) 73-79) (C) 1998 Elsevier Science B.V.