Accurate Isotopic Abundances

Isotopic abundance ratios are measured using mass spectrometry since the very beginning of MS in the 1910s (Aston, Thomson) and 1940s (Nier). Well known are accelerator mass spectrometers which utilize isotope ratio mass spectrometers that are used for radiocarbon dating.

For organic molecules we recently published a mathematical confirmation of the principle that isotopic abundances need to be used. The outcome of that study was that high mass accuracy or high resolution alone are not enough for unique elemental composition assignment. Especially fast scanning TOF mass spectrometers (JEOL AccuTOF, LECO Unique, Waters LCT, Bruker MicroTOF, Agilent TOF) coupled to LC would greatly profit from using isotopic abundances as orthogonal filter. Why? Because they are less expensive than FT-ICR mass spectrometers or Orbitraps. Compared to ion traps or quadrupole mass spectrometers they could provide accurate masses and accurate isotopic abundances together.

The same principle can be used for gas chromatography (GC) solutions. Any GC-MS (usually a fast scanning TOF) with accurate masses and accurate isotopic abundances and sufficient resolving power (> 5000, FWHM) and orthogonal gas chromatography (GCxGC) together with soft ionization techniques like chemical ionization (CI) and field ionization (FI) and field desorption (FD) option would outperform any existing GC-MS in terms of structure elucidation (Lets say this would be the ultimate “killer” GC-MS).

What is this principle about?

Assume you work with an inaccurate mass spectrometer, people call it a mass spectrometer with unit mass resolution. If you calibrate this instrument you can still achieve a sufficient mass accuracy (let’s say less than 100 ppm). The graphic below shows a calculation with more than 7692 possible molecular formulas which are obtained at 100 ppm mass accuracy at a mass of 867.5 with elements C,H,N,S,O,P and seven golden rules applied. If only the accurate mass would be used as input – these 7000 elemental compositions were the only result. However if the isotopic pattern filter is applied with a 2%-5% error only the few solutions which found in the red box are now under investigation. This is a tremendous reduction.

Why not search accurate masses in large database (PubChem) directly?

Well, the picture below clearly shows why this is possible but not very clever. The first problem is that some databases do not report isotopic masses, but integer masses or average masses. The chances that the mass can not be found is high. The second more severe problem is, that an important orthogonal filter is lost. Hence if no isotopic pattern filter is applied all 7692 solutions must be regarded as highly probable. That would lead to more than 54 solutions which could be found in the PubChem database.

Example: Isotopic patterns from Solanine, simulated at mass 867.5 with 100 ppm mass accuracy.

Example: Solanine C45H73NO15 (WIKI, PubChem) measured with high resolution ESI-FT-ICR-MS (LTQ-FT) (resolving power = 48,250 at m/z 868.5, peak width ~ 0.018; FWHM) maximum error of isotopic abundances +/- 4% for this example.

Additional Literature: