QUANTITATION OF METAL ISOTOPE RATIOS BY LASER-DESORPTION TIME-OF-FLIGHT MASS-SPECTROMETRY

Laser desorption time-of-flight mass spectrometry (LD/TOF-MS) is evalu ated for the determination of stable metal isotope ratios, The isotope ratios of five metal ions (Cu, Ca, Mg, Fe, Zn) in atomic absorption s tandard solutions and two metal ions (Ca, Mg) in human serum samples a re determined. With an existing LD/TOF-MS instrument we show that the technique can overcome the difficulties of the most commonly used meth ods for measuring metal isotope ratios: (1) all metals are ionizable w ithout surface treatment, thus overcoming the major drawback of therma l ionization mass spectrometry (TIMS); (2) there is no matrix involved to interfere with the metal ion detection, thus overcoming the major disadvantage of inductively coupled plasma mass spectrometry (ICPMS); (3) there is no interference from hydride ions, a major disadvantage o f fast atom bombardment secondary ionization mass spectrometry; (4) a mixture of metals can be detected simultaneously using a single laser wavelength, overcoming the major disadvantage of resonance ionization mass spectrometry; (5) accuracy and precision comparable to ICPMS can be achieved with the current instrumentation; (6) precision comparable to TIMS is feasible; and most importantly (7) high precision can be a chieved on very small quantities of material because the LD/TOF-MS ins trument permits all masses to be monitored simultaneously and very sma ll differences in isotope ratio can be detected.