Introduction
Ionization Techniques
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_______________________ Finnigan LTQ Hardware Manual _______________________
You can use the ESI probe in either positive or negative ion polarity mode.
The ion polarity mode of choice is determined by the polarity of the
preformed ions in solution: Acidic molecules form negative ions in solution,
and basic molecules form positive ions. The ejection of sample ions from
droplets is facilitated if the ionic charge and surface charge of the droplet are
of the same polarity. Thus, a positively charged needle is used to analyze
positive ions and a negatively charged needle is used to analyze negative ions.
Sample ions can carry a single charge or multiple charges. The number of
charges carried by the sample ions depends on the structure of the analyte of
interest and the carrier solvent. (In ESI, the buffer and the buffer strength both
have a noticeable effect on sensitivity. Therefore, it is important to choose
these variables correctly.) In the case of higher molecular weight proteins or
peptides, the resulting mass spectrum consists typically of a series of peaks
corresponding to a distribution of multiply charged analyte ions.
Figure 1-3. ESI process in the positive ion polarity mode
Ion
Transfer
Capillary
Heater To Ion
Skimmer
Pressure
= 1 Torr
+10 V
Ion
Sweep
Cone
ESI Nozzle
ESI Nozzle
ESI Needle
(+5 kV)
–
+ +
+ + +
+
+
+
+
+ +
+
+
+
–
–
–
–
–
–
+ + +
+
+
+ +
–
– – –
––
–
Charged
Droplet
Solvent
Evaporates
From
Droplet
Ejected
Positive
Ions
+
–
