The ~55 u loss is not correct. If it is a metastable fragment it will
appear at a larger mass than it actually is when you are at the original
reflectron voltage setting. You need to use the PSD technique to calibrate
the metastable ion. As a first guess it probably is loss of 60u.
Regards,
Damon
At 09:04 AM 6/19/98 +0200, you wrote:
>
>Hi,
>
>I've got a question for all you mass spec people out there.
>
>We use PSD-spectra to identify proteins. Yesterday I was doing a
>reflectron MALDI-MS analysis on a HPLC-fraction of a tryptic digest. At
>first sight, the fraction contained two peptides with masses of 1399.14
>and 1343.66 respectively. When I tried to select the peptide with a mass
>of 1343.66, I didn't succeed. However, it appeared again when putting a
>very small selection window on the 1399.14 peak. So it appears that the
>smaller peptide is actually a fragment of the 1399.14 one, however it's
>signal is around 50% higher! The rest of the PSD-spectrum is very bad
>since only minor fragmentations occur.
>
>Now, I was wondering if anybody has ever noticed a loss of around 55 u
>when working with tryptic peptides is PSD-mode. If I remember correctly
>there was a paper describing the loss of 60 u in reflectron mode when
>arginine was the C-terminal amino acid. However, to my knowledge I did not
>see anything on this 55 u loss. Also when looking at a list of possible
>modifications I don't see anything with an increase of 55 u.
>
>Has anybody out there seen this and can come up with an explanation?
>
>
>
>Thanks in advance and greetings from a cold Belgium!
>
>
>
>Kris Gevaert
>Flanders Interuniversity Institute for Biotechnology (V.I.B.)
>Department of Medical Protein Chemistry
>Universiteit Gent
>K.L. Ledeganckstraat 35
>B-9000 Gent
>Belgium
>
>Telephone: -- 32 / 9 2 64 52 91
>Fax: -- 32 / 9 2 64 52 93 or 53 37
>e-mail: krgev@gengenp.rug.ac.be
>
>
>
Damon C. Barbacci
The Laboratory for Biological Mass Spectrometry
Department of Chemistry - MS 3255
Texas A&M University
College Station, TX 77843