Micromass Quattro Ultima, User Manual

Page: 124 / 227

Electrospray Calibration with PEG
Caution should be used when calibrating with PEG in electrospray mode due to the
number of peaks which are produced. Although ammonium acetate is added to the
PEG reference solution to produce [M+NH
4
]
+
ions, under some conditions it is quite
usual to see [M+H]
+
, [M+Na]
+
and doubly charged ions.
The spectrum shown below demonstrates how the PEG spectrum can be dominated by
doubly charged ions (in this case [M+2NH
4
]
2+
) if the wrong conditions are chosen. In
this case the concentration of ammonium acetate in the reference solution is too high
(5mmol ammonium acetate is the maximum that should be used) and
Cone is too
low.
A low Cone voltage encourages the production of doubly charged ions. The voltage
should be at least 120V.
Doubly charged peaks can be identified because the
13
C isotope peak is separated from
the
12
C isotope by only 0.5 Da/e. If the instrument is set to unit mass and data are
acquired in continuum mode the doubly charged peaks appear broader as the isotopes
are not resolved.
Mass Calibration
Page 124
Quattro Ultima
User's Guide