Re: MassSpec/136Da contam

Beverly DaGue (bdague@bmb.med.uth.tmc.edu)
Tue, 21 Jul 1998 09:37:18 -0500

We have also observed a series of ions separated by 136 amu by
Electrospray MS. This series of ions appeared to be Na trifluoroacetate
clusters with the first ion of the series at 159 amu [ie., (Na
trifluoroacetate)+Na+] and subsequent ions at 295, 431, 567, 703, and
so on. The sample in which these clusters appeared was a peptide
isolated by typical reverse phase HPLC with TFA/Acetonitrile containing
solvents. The customer had decided to neutralize the peptide fraction
with NaOH solution prior to drying the fraction. We verified the source
of the cluster ions simply by mixing TFA and NaOH solutions, diluting
the mixture with our usual solvent for ESMS analysis of samples
introduced by infusion, and checking the result by ESMS.Further
comments:
Sources of sodium in samples for MS need not be as obvious as the
one above. A sample may contain a large concentration of a sodium
salt. On HPLC, the tailing of the sodium salt may be collected along
with the peptide of interest and then be concentrated along with the
peptide.

Sodium ions may arise from poor quality water used for preparing
HPLC solvents, contaminated glassware, alkaline reagents stored in glass
bottles.

An example: In our laboratory a simple tripeptide with only the
c-terminal COOH available for ionization is solubilized in ammonium
bicarbonate solution, then diluted with 50% methanol containing 0.5%
acetic acid for positive ion ESMS analysis. If the ammonium bicarbonate
solution has been prepared and stored in plastic containers, the peptide
spectrum shows the M+H+ and M+NH4+ ions. However, if the ammonium
bicarbonate solution has been prepared and stored in glass containers,
the peptide spectrum has a high proportion of sodium and potassium
adducts.

Beverly DaGue
Research Associate
Analytical Chemistry Center
University of Texas Medical School
Houston, Texas 77225
(713-500-6285)
bdague@bmb.med.uth.tmc.edu
Len Packman wrote:

> I have some sample contamination on mass spec analysis which appears
> as a
> polymer with a repeat unit of 136Da. Any ideas as to the nature of
> this? Is
> this another 'releasing agent' problem from the tube manufacture? All
> samples are ingel digests which have been prepared using the Keck
> Facility
> protocol and then cleaned up on a micro C8 cartridge.
>
> Thanks for any input
>
> Len
>
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