By Anonymous on Wednesday, August 20, 2003 - 07:25 am:
Can anyone to help me clarify the following things?
In General, if your compound is an ionic compound (in its salt form and has one positive charge in solution), will you get M+ or (M+H)+ in ESI positive mode? Or will you get (M-H)- in ESI negative mode? If your compound is two positive charged, what mass signal should we expected? (M/2)+ or ((M/2)+H)+ ?
For ionic compounds, do you really need an ionization step in LC-MS measurement?
Your reply are greatly appreciated.
By M_Gardner on Wednesday, August 20, 2003 - 08:17 am:
If your compound is ionic, and you really only care about the cation, then you should use positive ESI. For example, suppose you've got a quaternary amine salt like tetrabutylammonium sulfide. In positive mode, you'll see TBA+ at m/z = 242, and it should be a very strong signal. Tetrabutylammonium already exists as a cation, and there's no additional protonation site, so you won't see a proton adduct.
If an analyte forms a doubly charged ion for whatever reason, you'll see m/2, where "m" is the mass of your analyte plus whatever adducts it formed. For example, if something is doubly protonated, you'll see (M+2H)/2. If your mass analyzer is a quadrupole operating in unit resolution mode, it might not resolve the 13C isotope peak from the main peak in a doubly charged ion (since they now differ by 0.5 m/z), and your mass peak may appear overly broad.
>>For ionic compounds, do you really need an ionization step in LC-MS measurement?<<
Yes and no. :-) The answer sort of depends on what you mean by "ionization step". In LC/MS, your ion source also functions as an inlet.
By Anonymous on Tuesday, November 25, 2003 - 03:45 am:
Sir
I am having one doubt.please explain in detail.
I never come across the multiple charged ions in LCMS. It is my assumption,for doubly charged ion of whose mw is odd number(Assume 187 dalton).what will be the ms signal. Is it {(187+2H)/2} 94.5
Thanks in advance
By Anonymous on Monday, December 1, 2003 - 09:04 am:
Yes that signal will be 94.5 Da/z
Multiple charge ions generally do not happen for small molecules and thus probably why you haven't witnessed one yet. They are more common with peptides and proteins.
I've come across a double charged small molecule before. It was detected by a M+H+Na peak 11Da/z higher than the M+2H and 11 lower than the M+2Na.