Introduction
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____________________________________________________________ Ionization Techniques
_______________________ Finnigan LTQ Hardware Manual ______________________
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The ESI process is affected by droplet size, surface charge, liquid surface
tension, solvent volatility, and ion solvation strength. Large droplets with high
surface tension, low volatility, strong ion solvation, low surface charge, and
high conductivity prevent good electrospray.
Organic solvents such as methanol, acetonitrile, and isopropyl alcohol are
superior to water for ESI. Volatile acids such as acetic acid (1% vol/vol) and
formic acid (0.1% vol/vol) and volatile bases such as ammonium hydroxide
and triethanolamine are good. Use volatile salts such as ammonium acetate or
ammonium formate at concentrations below 10 mM. Strong acids and bases,
mineral acids, and nonvolatile salts (such as those containing potassium or
sodium) are detrimental.
The rules for a good electrospray are:
•
Keep salts out of the solvent system.
•
Use the lowest possible HPLC flow rates.
•
Use organic/aqueous solvent systems and volatile acids and bases.
•
Optimize the pH of the solvent system.
Atmospheric Pressure Chemical Ionization
Atmospheric pressure chemical ionization (APCI) is a soft ionization
technique. APCI is used to analyze nonpolar compounds and compounds of
medium polarity that are relatively low in molecular weight and have some
volatility.
In APCI, ions are produced and analyzed as follows:
1. The APCI nozzle sprays the sample solution into a fine mist of droplets.
2. The droplets are vaporized in a high temperature tube (the vaporizer).
3. A high voltage is applied to a needle located near the exit end of the tube.
The high voltage creates a corona discharge that forms reagent ions
through a series of chemical reactions with solvent molecules and
nitrogen sheath gas.
4. The reagent ions react with sample molecules to form sample ions.
5. The sample ions pass through an ion transfer capillary, enter the MS
detector, and are analyzed.
In the LTQ, the sample tube in the APCI nozzle is orthogonal to the axis of the
ion transfer capillary that carries ions to the MS detector. This geometry keeps
the ion transfer capillary clean. The ion sweep cone serves as a mechanical
barrier that keeps large droplets and particulates from entering the ion transfer
capillary.
APCI is a gas phase ionization technique. Therefore, the gas phase acidities
and basicities of the analyte and solvent vapor play an important role in the
APCI process.
