Quantitative Determination of Isotope Ratios from Experimental Isotopic Distributions

Abstract

Isotope variability due to natural processes provides important information for studying a variety of complex natural phenomena from the origins of a particular sample to the traces of biochemical reaction mechanisms. These measurements require high-precision determination of isotope ratios of a particular element involved. Isotope Ratio Mass Spectrometers (IRMS) are widely employed tools for such a high-precision analysis, which have some limitations. This work aims at overcoming the limitations inherent to IRMS by estimating the elemental isotopic abundance from the experimental isotopic distribution. In particular, a computational method has been derived which allows the calculation of 13 C/ 12 C ratios from the whole isotopic distributions, given certain caveats, and these calculations are applied to several cases to demonstrate their utility. The limitations of the method in terms of the required number of ions and S/N ratio are discussed. For high-precision estimates of the isotope ratios, this method requires very precise measurement of the experimental isotopic distribution abundances, free from any artifacts introduced by noise, sample heterogeneity, or other experimental sources.

An Improved Measurement of Isotopic Ratios by High Resolution Mass Spectrometry

“…High resolution mass spectra were collected using a LTQ FT Ultra MC (Thermo Electron, Bremen, Germany), peptides were infused directly or using UltiMate 3000 nano-HPLC system (Dionex, Germering, Germany) [1]. The MID measurements were performed in the SIM mode with mass isolation window of 10 Da in the ICR cell at a target value of 5×10 4 , which allows statistical accuracy of the ion intensity [9], (Supplementary Table 1S). …”

Monitoring Membrane Lipidome Turnover by Metabolic 15N Labeling and Shotgun Ultra-High-Resolution Orbitrap Fourier Transform Mass Spectrometry

“…For resolving 15 N/ 13 C peaks, it was critical to optimize the number of ions filling the Orbitrap analyzer: for damping ion coalescence, it should be maintained at sufficiently low level, 29 although larger numbers of trapped ions provide better ion statistics required for the accurate measurement of abundance of isotopic peaks. 30 In shotgun FT MS spectra of total lipid extracts, the abundance of peaks of bona fide lipids differs by more than 1000-fold. 31,32 Therefore, direct acquisition of broad-band FT MS spectra at the unusually low AGC values required for separating 15 N/ 13 C peaks could impact both the sensitivity and dynamic range by failing to detect low-abundance species.…”

Atomic spectrometry update. Atomic mass spectrometry

“…A computational tool has been derived to calculate elemental isotopic abundance from experimental whole isotope distributions in order to overcome some of the limitations of SIRMS instruments. 233 The limitations of the method were discussed in terms of the required number of ions and S/N. For high-precision estimation of isotope ratios, the method required very precise measurement of the experimental isotopic distributions free from any artifacts introduced by, for example, noise and sample heterogeneity.…”

“…95 Concentrations in natural samples of 1-4 pg g À1 could be measured on a compact (300 kV) system with a reproducibility of better than 2% and an LOD in the low fg range. All samples were spiked with the short-lived (t1⁄ 2 ¼ 27 days) 233 Pa isotope prior to chemical purification in order to determine the yield. Purified Pa samples were baked with Fe(NO 3 ) 3 and the oxidized sample mixed with aluminium powder to form targets.…”

“…The Pa isotopes were extracted from the sputter source as PaO À ions which were stripped with Ar gas to the Pa 3+ ions. An interesting consequence of using 233 Pa as spike was that 233 U was produced rapidly in the samples and a 233 UO À ion beam was extracted together with the 233 PaO À beam. The enhancement factor of UO À over PaO À had to be determined for each individual run by measuring additional 233 U-spiked RM.…”