Carbon isotope ratios ( 12 C/ 13 C) are measured for aqueous solutions of tryptophan, myoglobin, and ␤-cyclodextrin using C ϩ ions from an inductively coupled plasma (ICP) and a prototype twin quadrupole mass spectrometer (MS). 13 C/ 12 C ratios can be determined with a relative standard deviation (RSD) of ϳ1%. This precision is close to the limiting value predicted by counting statistics (1.16%). Spectral interference on 13 C ϩ , presumably from 12 C 1 H ϩ , comes from the incomplete dissociation of myoglobin and/or ␤-cyclodextrin, but not tryptophan. Decreasing the aerosol gas flow rate slightly from that which yields maximum signal eliminates this 12 C 1 H ϩ interference. The count rate of the minor isotope ( 13 C ϩ ) can be artificially enhanced by increasing the voltage of the 13 C ϩ detector, and/or by shifting the ion beam splitter offset from the central axis. Instrumental modifications to the MS that improve the sensitivity are also described. I nductively coupled plasma mass spectrometry (ICP-MS) is a versatile technique for elemental and isotopic analysis. Applications of ICP-MS range from elemental determinations to isotopic analysis in nutrition, clinical studies, geochemistry, environmental studies, and the semiconductor and nuclear industries. ICP-MS has long linear dynamic range (eight orders of magnitude) and detection limits at part per trillion levels for many elements. Most ICP-MS instruments contain a quadrupole or magnetic sector mass spectrometer with a single detector. These are sequential devices that do not measure the various m/z values of interest at precisely the same time. They provide moderate to good precision in isotope ratio measurements, typically 0.05% relative standard deviation (RSD) on ratios close to unity, with substantially poorer precision on large or small ratios [1][2][3][4].Much of the instability in the ion signal is caused by flicker noise from the ICP. This source of noise can be eliminated by measuring the various m/z values at the same time using a multicollector magnetic sector device [5,6]. We have described a unique instrument with two quadrupole mass filters and an ion beam splitter that also eliminates the flicker noise contribution to isotope ratios [7]. Allen and co-workers have analyzed solid copper and steel samples using laser ablation with this twin quadrupole device. The isotope ratio precision reported by Allen et al. was 0.06 to 0.1% for 52 Cr/ 53 Cr, depending on the dwell time and averaging method used [8].This paper evaluates the performance obtained for measurement of carbon isotope ratios from organic compounds at natural isotopic abundance. The ultimate goal is to provide a method for carbon isotopic analyses in bioorganic molecules in aqueous solution. This is an ambitious goal, since carbon is only 1 to 5% ionized in the ICP [9]. Thus, the sensitivity (i.e., count rate per unit concentration) is much lower than that for the metals and metalloids usually determined by ICP-MS, and 13 C is only ϳ1% of 12 C. No more than 0.1% of the analyte ion...

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