Determination of carbon isotope ratios in amino acids, proteins, and oligosaccharides by inductively coupled plasma-mass spectrometry

Carbon isotope ratios ( 12C/ 13C) are measured for aqueous solutions of tryptophan, myoglobin, and β-cyclodextrin using C + ions from an inductively coupled plasma (ICP) and a prototype twin quadrupole mass spectrometer (MS). 13C/ 12C ratios can be determined with a relative standard deviation (RSD) of ∼1%. This precision is close to the limiting value predicted by counting statistics (1.16%). Spectral interference on 13C +, presumably from 12C 1H +, comes from the incomplete dissociation of myoglobin and/or β-cyclodextrin, but not tryptophan. Decreasing the aerosol gas flow rate slightly from that which yields maximum signal eliminates this 12C 1H + interference. The count rate of the minor isotope ( 13C +) can be artificially enhanced by increasing the voltage of the 13C + 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.