I was evaluating the collision cross section (CCS) values for various compounds using traveling wave ion mobility spectrometry (TWIMS). Quite beyond my expectation, some compounds seem to possess more than one CCS.
For example, for a molecular with a mass of 452.2046, two CCS values corresponding to its sodium adducts (m/z 475.1941) with the retention time of 6.20 min were observed. The CCS values were 321 and 224.
Is it possible that there are two kinds of molecules which have very similar retention time and mass-to-charge ratio but very different CCS values? Is it possible that a single kind of molecules showed different gas phase conformations and thus different CCS values? Are there any special consideration for CCS detection when using TWIMS?
The sample I used was a purified natural product and was believed to be a single compound based on its NMR data.
More information would help. You said you have NMR data - do you have a structure as well? What were the conditions for obtaining the CCS values?
Yes, two molecules with similar retention time and m/z can have different CCS values.
Yes, a single molecule can have different gas phase conformations, thus, different CCS. The molecule you are analyzing may be unfolding, one of the possible reasons you are acquiring different CCS for the same molecule.
You should compare CCS values with the same conditions and same buffer gas (nitrogen and helium can give different CCS).
Different conformers have different ion mobility CCS. You can actually differentiate that spectroscopically, as well. You can check out e.g. the research of my former colleague Patrick Weis, https://www.researchgate.net/profile/Patrick-Weis-4 :
Article Combining Ion Mobility Spectrometry, Mass Spectrometry, and ...
and of course many of his newer papers.
Edit: when we're on the fundamentals of the method: please be aware that in TWIMS you don't discuss "retention times", here we have "transit times", see:
Article Fundamentals of Traveling Wave Ion Mobility Spectrometry
Yes, we have a structure. It is an analogue of aphidicolin. The CCS values were obtained using a TWIMS-QTOFMS instrument (Waters Vion IMS QTOF). Nitrogen (>99.5%) was used as the trap and IMS buffer gas. Charles H Hocart
Thank you for your answer. Yes, we are comparing the CCS values that were obtained under the same conditions and using the same buffer gas (nitrogen). Thaís Pedrete
Thank you for your answer. I am going to check the research work you suggested. Jürgen Weippert
I should think it is quite possible a tetracyclic diterpene analogue of aphidicolin is sufficiently complex to have more than one stable conformer as you have observed. Good luck with your research.
I think you are talking about "carbon capture and storage". Interesting. Haseeb Javed
You give one value for retention time which suggests to me you are doing GC-IMS or LC-IMS and delivering a pre-separated sample to the IMS. Assuming that the sample delivered to the IMS is pure, several things can happen to cause it to have two or more ion mobility peaks or collision cross sections. One is that the sample may contain a mixture of isomers or stable conformations as mentioned above. Another is that the sample may dissociate upon ionization in the IMS reactor. Sometimes mass spectrometry is needed to sort out the details.
Glenn Spangler : there's no chromatography involved here, TWIMS is a proprietary method used by Waters e.g. in the Synapt tools. In these you have an (optional) pre-IMS quadrupole mass filter, then follows the IMS transit cell and in the end there's a final mass separation by a ToF. When the data is analyzed, you extract the ion mobility transit times for individual mass signals, so there is always mass separation in that method.
There is no retention or drift time here, that was a confusion made within the discussion, the correct definition of the TWIMS transit time is given in the article I referenced above.
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