Quantitation of metal isotope ratios by laser desorption time-of-flight mass spectrometry

Laser desorption time-of-flight mass spectrometry (LD/TOF-MS) is evaluated for the determination of stable metal isotope ratios. The isotope ratios of five metal ions (Cu, Ca, Mg, Fe, Zn) in atomic absorption standard solutions and two metal ions (Ca, Mg) in human serum samples are determined. With an existing LD/TOF-MS instrument we show that the technique can overcome the difficulties of the most commonly used methods for measuring metal isotope ratios: (1) all metals are ionizable without surface treatment, thus overcoming the major drawback of thermal 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 of 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 achieved on very small quantities of material because the LD/TOF-MS instrument permits all masses to be monitored simultaneously and very small differences in isotope ratio can be detected.