By Kambikudi Murugan on Friday, July 16, 2004 - 08:22 pm:

Dear All

Regarding the ionisation of compounds containing more than one similar functional groups in LCMS

1. In Positive ionisation mode, how many charged ions will form (M+1/1, M+2/2,...) for compounds having more than one functional froups (Easily protonating groups like amine)

2. In negative ionisation mode, how many charged ions will form (M-1/1, M-2/2,...) for compounds having more than one similar functional groups (Proton donating groups, like acid groups in the case of Tartaric acid)

3. Is there any special criteria for the ionisation in the case of proteins where there is a possibility of formation of multicharged ions
In the same condition followed for Tartaric acid, Is the result is M-2/2?

Thanks

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By MG on Tuesday, July 20, 2004 - 09:17 am:

1 & 2: This depends on the size of the molecule. I have rarely if ever seen doubly charged for molecules smaller than about 600, even with multiple sites available for charging. An exception might be if you had multiple quaternary amine groups, since these are pre-formed ions, although I've not analyzed such a compound. Another exception could be multiple sulfonic acid groups, since the pKa is so low, although the only examples of these I have analyzed were > 1000 mw (and I definitely saw multiple charging).

3: I may not fully understand the question. Generally, your optimum ion source conditions for proteins will be different from those of a small molecule. But proteins will form multiply charged ions without any special conditions. Of course your conditions for tartaric acid will have to be different: negative mode, versus positive mode for proteins. I would not expect tartaric acid to form a doubly charged ion in ESI; it is too small.

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By Kambikudi Murugan on Tuesday, July 20, 2004 - 10:24 pm:

Dear MG

Thanks for your valuable information. In my earlier query No:3, we will consider the charge sites only, irrespective of ionisation mode
(Sorry I have not mentioned earlier ).

I understand from your answer 1 & 2, the charge creation is only due to size of the molecule and not the availability of charge site. In malonic acid, a diacid leaves one proton from active methylene group at particular pH and we increase the pH we can remove both the protons from that group. Can we apply this rule to MS either by changing the pH of the mobile phase ( to create a pre-formed ions mentioned by you earlier ) or increasing the ionisation voltage( or other ms parameters).

Let me know about your opinion on this.

Thanks