Environmental Engineering Reference
In-Depth Information
Fig. 8.2 The main
physicochemical processes
of electrospray ionization in
positive ion mode
ESI capillary
Taylor cone
+
+
+
+
-
-
-
+
+
-
-
-
-
+
-
-
+
+
+ +
+
-
-
-
+
+ + +
++
-
-
-
-
+
+ +
+
+ + +
+ ++
++
+
++
+
+ + +
+ +
+
+
+ + +
+
++
++
+
++
oxidation
+
+
_
+
HV power supply
electrons
ESI mass spectrum contains multiple peaks corresponding to the different charge
states [
10
-
12
].
This feature brings complexity to the interpretation of the ESI mass spectra but
concomitantly, as a first advantage, it adds to the information and can be used to
improve the accuracy of the molecular-weight determination. The method of
deducing this way the molecular weight was presented in
the multiple charge
theory
described by Fenn. The theory showed that different charge states could
be interpreted as independent measurements of molecular weight and that an
averaging method based on the solution of simultaneous equations could provide
accurate molecular weight estimations for large molecules. The complex charge
pattern can simply be deconvoluted and the mass of the uncharged protein is
determined to dramatically higher accuracy than if the interpretation of data was
based on a single ion. The second advantage of multiple charging is the formation
of ions with reduced
m/z
ratio measurable with good resolution by almost any type
of analyzer with which ESI has been interfaced: magnetic sector, single or triple
quadrupole, time-of-flight (TOF), quadrupole ion trap (QIT), Fourier-transform ion
cyclotron resonance (FTICR) or hybrid quadrupole time-of-flight analyzer (QTOF).
All these make ESI the method of choice for large biopolymers and molecular
aggregates or complexes that only have weak non-covalent interactions, such as
protein-protein, enzyme-substrate or protein-ligand complexes [
13
-
16
].
The ESI process involves three main steps prior to mass analysis: the generation
and charging of the ESI droplets, the droplet division by solvent evaporation and
repetitive disintegrations leading ultimately to very small highly charged droplets
and the production of gas-phase ions from these droplets. They are subsequently
followed by the processes that modify the gas-phase ions in the atmosphere and the
sub-atmospheric pressure-sampling regions of the mass spectrometer.
