A method of multiple spiking isotope dilution mass spectrometry comprising: obtaining a mass spectrum of a chemical system having two or more inter-converting analytes of interest, the chemical system having been spiked with known amounts of isotopes of the analytes; determining systematic instrument biases corrected values of a mass spectrometric parameter of the analytes from the mass spectrum of the spiked chemical system; determining pure component contribution coefficients for each analyte in the mass spectrum by mathematically deconvoluting the corrected values of the mass spectrometric parameter using pure component mass spectra of the analytes; determining a property of one or more of the analytes in the chemical system from the pure component contribution coefficients determined for each analyte; and, estimating uncertainty in the property including estimating an increase in the uncertainty of amount of analytes due to inter-conversion of the analytes, characterized in that the increase in uncertainty of the amount of analyte is determined by: f ⇆ M k ≈ 1 + ∑ i ≠ k m F i → k n M i n M k δ i → k wherein f ⇆ is increase in uncertainty of amount of analyte M k due to inter-conversion of species M 1 -M m, n M i is initial amount of analyte M i, n M k is initial amount of analyte M k, F i → k is inter-conversion amount correction factor for interconversion of M i to M k, and δ i → k is: δ i → k = e F i → k F k → i 1 + 1 2 − F i → k − F k → i wherein F i → k is inter-conversion amount correction factor for interconversion of M ¡ to M k and F k → i is inter-conversion amount correction factor for interconversion of M k to M i. obtaining a mass spectrum of a chemical system having two or more inter-converting analytes of interest, the chemical system having been spiked with known amounts of isotopes of the analytes; determining systematic instrument biases corrected values of a mass spectrometric parameter of the analytes from the mass spectrum of the spiked chemical system; determining pure component contribution coefficients for each analyte in the mass spectrum by mathematically deconvoluting the corrected values of the mass spectrometric parameter using pure component mass spectra of the analytes; determining a property of one or more of the analytes in the chemical system from the pure component contribution coefficients determined for each analyte; and, estimating uncertainty in the property including estimating an increase in the uncertainty of amount of analytes due to inter-conversion of the analytes, characterized in that the increase in uncertainty of the amount of analyte is determined by: f ⇆ M k ≈ 1 + ∑ i ≠ k m F i → k n M i n M k δ i → k wherein f ⇆ is increase in uncertainty of amount of analyte M k due to inter-conversion of species M 1 -M m, n M i is initial amount of analyte M i, n M k is initial amount of analyte M k, F i → k is inter-conversion amount correction factor for interconversion of M i to M k, and δ i → k is: δ i → k = e F i → k F k → i 1 + 1 2 − F i → k − F k → i wherein F i → k is inter-conversion amount correction factor for interconversion of M ¡ to M k and F k → i is inter-conversion amount correction factor for interconversion of M k to M i.