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I was working through a previous exam and came across the following question (with reference to the picture shown below):

"What MS ionization method was employed (MALDI or ESI)?"

I cannot seem to find the answer, so I'm guessing it is not possible to tell, but if there is a way to see which one was used, please enlighten me.

enter image description here

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It is probably ESI because, as already mentioned, ESI produces multiply charged precursors while Maldi would generally produce fewer charge states and +1 most frequently. But, you can not say that MALDI wont give multiply charged ions. So from just this picture you can not say for certain but can only make a guess. – ktyagi Feb 18 at 10:53
up vote 2 down vote accepted

I am not an expert in this topic but from what I have heard ESI generates multiply charged ions but MALDI usually does't. It is possible to find multiply charged species in the spectrum.

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They all have a charge of four it seems to me? (Assume one Da mass difference, and then divide by .25, the m/z difference.) Is that how you are able to tell it comes from ESI, namely that they have a charge of four, not one? If you confirm I will upvote and accept your answer. – The Unfun Cat May 1 '14 at 7:01
I was not actually looking at the spectrum that you attached.. I answered that in general ESi produces multiply charged ions. But when you commented, I noticed the spectrum and it seems that these are +4 charged ions of different molecules/peptides. Nothing else fits that neatly – WYSIWYG May 1 '14 at 8:45

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