1 Introduction
Is ESI or APCI Better for Analyzing My Samples?
4MSQ Plus Mass Detector Getting Started Guide Thermo Scientific
Electrospray (ESI)
ESI mode generally produces mass spectra consisting of singly charged ions, but it depends on
the structure of the analyte and the solvent. When multiply charged ions are produced, the
Xcalibur data system can mathematically transform the resulting mass spectrum to express the
molecular weight of the sample.
ESI mode transfers ions in solution into the gas phase. Because the ESI process does not use
heat to nebulize the analyte ions, you can use it to analyze heat-labile compounds or
high-molecular-weight compounds.
You can use ESI mode to analyze any polar compound that makes a preformed ion in
solution. Preformed ions can include adduct ions. For example, polyethylene glycols can be
analyzed from a solution containing ammonium acetate because of adduct formation between
the NH4+ ions in the solution and oxygen atoms in the polymer. With ESI, the MSQ Plus
Mass Detector can analyze compounds with molecular weights that are greater than
100000 Da because of multiple charging. ESI is especially useful for the mass analysis of polar
compounds, which include biological polymers (for example, proteins, peptides,
glycoproteins, and nucleotides), pharmaceuticals and their metabolites, and industrial
polymers.
You can use ESI mode in either the positive- or negative-ion polarity mode. Matching the
charge on the ESI probe to the charge on the preformed ions in solution increases the
sensitivity of the analysis. Therefore, select the ion polarity mode according to the polarity of
the preformed ions in solution. Acidic molecules form negative ions in high pH solution, and
basic molecules form positive ions in low pH solution.
You can vary the flow rate in ESI mode from the LC into the mass detector from 10 L/min
to 2000 L/min. (In ESI, the buffer type 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 typically consists
of a series of peaks corresponding to a distribution of multiply charged analyte ions.
The droplet size, surface charge, liquid surface tension, solvent volatility, and ion solvation
strength all affect the ESI process. Large droplets with high surface tension, low volatility,
strong ion solvation, low surface charge, and high conductivity prevent good electrospray.
Mixed organic-aqueous solvent systems that include organic solvents, such as methanol,
acetonitrile, and isopropyl alcohol, are superior to water alone for ESI. Volatile acids and bases
are good, but Thermo Fisher Scientific does not recommend salts above 10 mM. Strong
mineral acids and bases are extremely detrimental to the instrument.
Observe the following recommendations for generating stable electrospray:
• Refrain from using non-volatile salts and buffers in the solvent system. For example, avoid
the use of salts containing sodium or potassium and avoid the use of phosphates.
If necessary, use ammonium salts instead.
• Use organic-aqueous solvent systems.