Which method is better to analyze for example Vit D and its metabolites?
With DESI You work with an electrospray source that is directed atthe target. You produce a charged aerosol that then hits the sample and transfers protons on proton accepting molecules.- this is a very chemical, soft process. - however the spray cannot be focussed super well so You get pixels of 50 µm diameter.
With SIMS You produce energy-rich ions that are then accelerated and focused on a target. The transfer of energy is significantly higher, leading to immediate fragmentation. - on the other hand - the beam can be focussed on a nm scale (thus it can be considered to be 100 times higher focussed in comparison to DESI).
DESI is considered to be the softest and SIMS the hardest ionisation, MALDI being somewhat in between both in energy and resolution.
All three can in principle be operated on all kind of mass analysers- TOF, Orbitraps, Quadropoles, secrtorfields or FT instruments.
Dear Zainab Hashem,
I agree with the statements of Kai Bester. I also think that in the area of metabolites it would make sense to use an HPLC separation. Depending on the matrix, ion suppression can quickly occur with direct ion sources, so that relevant components can no longer be seen. With HPLC separation before the MS, this can be easily prevented. In this case, an ESI source would be the standard, but APCI is also conceivable. If HPLC separation is not possible, my choice would be the DESI variant, since fragmentation generates more masses in the spectrum, which then have to be assigned. In my experience, this would be very challenging.
Kind regards
Joachim Horst
Kai Bester & Joachim Horst
Thank you for your reply, I appreciate it.
I have now used the DESI-MS ion source because it was the one available in my university. But now to the question, I have received a spectrum with many masses on different lipids. I have compared diseased and healthy tissue in psoriasis patients and I want to analyze vitamin D and its metabolites. How can I measure the vitamin D and its metabolites, how do I know which they are from all these masses? Is it just to freely search for the mass of D vitamin and its metabolites and start from that or how?
Regards
Zainab Hashem
The first way to find vitamin D in your speltrum would be to simulate the isotope cluster. Either your MS software has the possibility to calculate the cluster or you can do this online on the internet. Since I don't know what MS you are working with and how high the resolution of the MS is, you can look for the accurate mass of vitamin D by generating an Exctrated Ion Chromatogram with this mass. But you should also measure a reference spectrum of vitamin D. To do this, make a vitamin D analysis standard from pure vitamin D and measure this standard with your MS. This will give you a reference spectrum.
To identify metabolites, you should know the degradation pathways of the vitamin and thus the structure and molecular formula of possible metabolites. From the molecular formula you can calculate the mass and then you may generate an extracted ion chromatogram.
With this procedure, metabolites can be identified with a little work and thought. If you also want to perform a quantification, you have to perform a calibration in the expected concentration range with the corresponding single substances before.
Many greetings
Joachim Horst
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