Asparagine could be detected with both ESI+ and ESI−.
The response was highest in positive ion mode but signal-tonoise
ratio was improved when ionisation was done in negative
ion mode. This correlates well with the fact that asparagine is
eluted in the beginning of a chromatogram where many matrix
components coelute and that much less noise is observed with
ESI− compared to ESI+. Interferences from matrix components
were also smaller in negative ion mode. A product ion scan of
asparagine in negative ion mode (Fig. 2) shows products ions
at, e.g. m/z 114, 113 and 70. m/z 114 corresponds to the loss
of ammonia, m/z 113 corresponds to the loss of water while at
m/z 70 the most likely fragmentation pattern would be loss of
ethylamide, as the corresponding peak at m/z 71 for the 15N2-
asparagine fragmentation shows that one nitrogen is lost. The
asparagine signal was improved when raising the desolvation
temperature to 400 ◦C but lost at 420 ◦C suggesting that the
asparagine molecule is fragmented in the atmospheric pressure
interface at elevated desolvation temperatures.