Sodium and potassium (and many other adducts) are often seen in either
MALDI or ESI mass spectra. If you have a lot of salts in your sample you
are going to see them, to a lesser or greater extend, depending somewhat on
the structure of your compound. In MALDI, a source of salt, external to
your sample, may be the matrix, which can be cleaned up by
recrystallization. Obviously, solvents and glassware are another source.
However, there are several problems with your data that are not related to
salts:
A. The tripeptide GM(O)G (Gly-Met(Ox)-Gly, I assume) has a calculated MW of
279.1 for the C-12 isotope. Possibly, for reasons unclear, your ESI-MS
analysis was carried out in the nagative ion mode, in which case the major
ion observed would have been the (M-H)- ion at m/z 278.1 [M is the
molecular mass of the neutral species]; this would be consistent with the
first set of m/z values you report. However, if the analysis were carried
out in the positive ion mode, which is most often used for peptides
analyzed by MALDI- or ESI-MS, then there is an unacceptable error in
measurement (2 u for the tripeptide with a free acid C-terminus, 1 u with
C-terminal amide) and, in this case, no reasonable interpretation of the
data can be made.
B. Your peptide has 9 carbon atoms, so the C-13 isotope peak (second peak
in the cluster) should be roughly 10% of the C-12 peak, and the third peak
in the cluster should be even smaller (I don't have a mass spec book handy
so I can't give you exact numbers). If there is significant discrepancy
between this pattern and your data then there is something else going on.
C. The sodium adduct should appear 22 u higher than the (M+H)+ or (M-H)-
cluster; the ratios of the isotopes are the same, as sodium has one stable
isotope. These adducts have the nominal structure (M+Na) or (M-2H+Na) for
positive and megative ion mass spectra data, respectively. Your second set
of peaks doesn't make much sense: The first peak (298.75) is about 20 u
higher than the presumed C-12 isotope peak, which is tricky to explain, as
+20 is one of those numbers that are difficult to write a reasonable
structure for. The peak at m/z 300.82 could, indeed, be the sodium adduct.
D. The fractional parts of the m/z values you report for the first peak of
the first cluster and the first two peaks of the second cluster are quite
high (0.3-0.7 u higher than theoretical), which may indicate that these
ions were saturated.
E. The third cluster may, indeed, be the potassium adduct.
I hope this helps.
Ioannis Papayannopoulos
At 12:37 PM 10/22/99 -0200, you wrote:
>Dear ABRFers,
>
>I'm used to use Maldi-tof as a MW quality control of the peptides I
>synthesize. A couple of weeks ago, I synthesized an GM(O)G wich
>MW=278. As I could not detect low MW in the Maldi I use, I asked a
>friend to do it in a Electron Spray, and it showed the following
>results:
>1) 278.45; 279.09; 280.12
>2) 298.75; 300.82; 301.04
>3) 316.28; 317.03; 318.06
>
> It looks for me that group 1 correspond to Na+ and group 2 =
>K+. I'm used to see Na+ and K+ when I analyse Maldi spectra. Is it
>common also to see them in ES?
> Can I realy attribute the difference, I would say 298.75-
>278.45=20.3 as the Sodium or I have to use the higher value of the
>isotopes of group 2 minus the lower value of group 1 (301.04-
>278.45=23.3)? Wich peak do I have to use to calculate the "real"
>MW? (same question for the possible K+ peaks).
>Thanks in advance,
>
>Leo
>
>----------------------------------------------------------
>Leo Kei Iwai - Chemist
>Dept of Biophysics
>Escola Paulista de Medicina - EPM - UNIFESP
>Lab of Transplantation Immunology
>Instituto do Coracao - Incor - FMUSP
>----------------------------------------------------------
>
>