The Paclitaxel (Taxol) example is an example of the good performance of newly developed time-of-flight mass analyzers (TOF-MS). These mass spectrometers show an excellent accuracy for isotopic abundances together with accurate masses and the ability to perform multiple scans per second. The results presented here were obtained with a Bruker MicroTOF [PDF]. The advertised performance parameters are 3 ppm mass accuracy, 2%-5% isotopic abundance accuracy, 15000 mass resolving power, 20 scans/second over the full mass range.

Seven Golden Rules
Using data from the Bruker MicroTOF for Paclitaxel (C47H51NO14; MW = 853.33094) an accurate measured mass [M+H]+ of 854.3376 Da (-0.7 ppm error) was obtained. The isotopic pattern were [A+1] = 56.4%, [A+2] = 16.5%, [A+3] = 2.9% with a maximum absolute error of 3.9%.

The Seven Golden Rules (see supplement) calculated 229 formulae within ±5 ppm mass accuracy (elements CHNSOP) and kept 39 formulas within ±5% isotopic abundance accuracy. Only 2 results (marked blue) could be found in the existing formula database (750000 formulas covering more than 10 million compounds). Only one result was found in the Natural Compound formula database.

It is interesting to note that none of 14 other formulas which were ranked higher could be found in the CAS Database (which is not comprehensive, but pretty large and curated). The higher ranked formula refers to C48H47N5O10 a synthetic uridine. C47H51NO14 is the first formula where 46 entries can be found in the CAS DB. The CSLS lookup service lists 111 entries for C47H51NO14. But these 111 entries can be uniquified using the FICuS, uuuuu or INCHI and the result is 28 unique compounds. Download the example in the software section.

As mentioned very often such an annotation should not be confused with true identification of the substance unless the substance is confirmed by an external standard and GC-MS,LC-MS,MS/MS or better NMR. Pseudo-identifications of isomer structures just with mass spectrometry or FT-MS alone are highly speculative and should be avoided.


Taxol; PubChem CID: 36314; An anti-cancer drug isolated from the bark of the Pacific yew tree, TAXUS BREVIFOLIA. It stabilizes MICROTUBULES in their polymerized form leading to cell death.

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