I had one ESI mass spectrum of a porphyrin. The molecular ion appeared at 734.30. However additionally, i had also observed some peaks of low intensity around 850 and even around 1400 Dalton. My question is why these peaks corresponding to a much higher molecular weight appear other than the molecular ion peak? Even in literature many such mass spectra are reported, and researchers just report the molecular ion peak but hardly address such types of peaks.
Hi,
In ESI, impurities and adducts can be observed, as well as (non-covalently associated) dimers of your molecule of interest.
This could (partially) explain why you observe more than just the molecular ion you expect.
The reference, below, is the only available in the literature comprehensive analysis of MS spectra of your analyte:
1. G. van Berkel, G. Glish, S. McLuckey, Anal. Cham. 1990, 62, 786-793
The paper G. van Berkel, G. Glish, S. McLuckey, Anal. Chem. 1990, 62, 786-793 is not relevant. It is related to high pressure NH3 CI (chemical ionization) leading to new reduced species, and CID (collision induced dissociation) for structure sequencing. ESI should produce rather different type of ions.
It is not straightforward to find relevant ESI MS of porphyrins, as they are mostly analyzed as metal adducts (many references are related to crude oil analysis). See for example:
- Amy M. McKenna, et al. Advances and Challenges in the Molecular Characterization of Petroporphyrins, Energy Fuels 2021, 35, 18056−18077.
- L Nyadong, J Mendez-Arroyo, Sodium Cationization Electrospray Ionization Orbitrap Mass Spectrometry for Selective Determination of Crude Oil Porphyrins, Energy Fuels 2021, 35, 22, 18116–18124.
- Emily R. Sekera and Troy D. Wood, Examination of the Fragmentation Behavior of Hemin and Bilin Tetrapyrroles by Electrospray Ionization and Collision-induced Dissociation, Mass Spectrometry Letters, 2018, 9, 91-94.
I am thankful to all of you for your guidance and expert opinion that has really helped me a lot.
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