1
Introduction
Ionization Techniques
Thermo Scientific
Surveyor MSQ Plus Hardware Manual
5
mode to give a peak at an
m/z
value of M – 1. Because electrospray is a very soft ionization
technique, there is usually little or no fragmentation and the spectrum contains only the
protonated or deprotonated molecule.
Preformed ions can also include adducts. Adduct ions are produced by the interaction
reaction between a molecule and an ionic species to form an ion that contains all the
constituent atoms of the original molecule as well as one or more additional atoms.
Common adducts are ammonium ions (NH
4
+
), yielding an
m/z
value of [M + 18]
+
, sodium
ions (Na
+
), yielding an
m/z
value of [M + 23]
+
, and potassium ions (K
+
), yielding an
m/z
value of [M + 39]
+
.
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.
Because of multiple charging, you can use the ESI mode to analyze ions with molecular
weights greater than 100000 u. This makes ESI especially useful for the mass analysis of polar
compounds including biological polymers and industrial polymers. The mass spectra for these
compounds consist typically of a series of peaks corresponding to a distribution of multiply
charged analyte ions.
You can run ESI in three ion polarity modes: positive, negative, or positive-negative
switching. Because like charges repel each other, select the ion polarity mode that matches the
polarity of your analytes:
• For acidic compounds, which form negative ions in solution, select the negative ion
polarity mode.
• For basic compounds, which form positive ions in solution, select the positive ion polarity
mode.
• For unknown mixtures, select the positive-negative switching mode.
Droplet size, surface charge, liquid surface tension, solvent volatility, and ion solvation
strength affect the ESI process. Large droplets with high surface tension, low volatility, strong
ion solvation, low surface charge, and high conductivity prevent good electrospray. The buffer
type and buffer strength have a noticeable effect on sensitivity, making it important to choose
these variables correctly.
Organic solvents such as methanol, acetonitrile, and isopropyl alcohol are superior to water
for ESI. Volatile acids and bases can be used, but salt concentrations above 10 mM and strong
acids and bases are extremely detrimental to the mass spectrometer.
The rules for a good electrospray are as follows:
• Keep salts out of the solvent system.
• Use organic/aqueous solvent systems and volatile acids and bases.
• Optimize the pH of the solvent system.
