1 Introduction
Is ESI, H-ESI, or APCI Better for Analyzing My Samples?
2TSQ Series Getting Started Guide Thermo Scientific
• Atmospheric pressure chemical ionization (APCI)
Typically, ESI or H-ESI provides the best analysis of polar compounds such as amines,
peptides, and proteins; and APCI provides the best analysis of non-polar compounds such as
steroids.
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, the mobile phase, and the
ionization mode.
Using ESI or H-ESI
The ESI and H-ESI modes generally produce mass spectra consisting of singly-charged ions,
but the charge depends on the structure of the analyte and the solvent. When
multiply-charged ions are produced, the resulting mass spectrum can be mathematically
transformed to express the molecular weight of the sample.
The ESI and H-ESI modes transfer ions in solution into the gas phase. ESI and H-ESI can
analyze any samples that previously were not suitable for mass analysis (for example,
heat-labile compounds or high molecular weight compounds). You can use ESI and H-ESI to
analyze any polar compound that makes a preformed ion in solution. The term preformed ion
can include adduct ions. For example, you can analyze polyethylene glycols 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 and H-ESI, the TSQ can analyze
compounds with molecular weights that are greater than 100000 u because of multiple
charging. ESI and H-ESI are 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 the ESI and H-ESI modes in either positive or negative ion polarity mode. The
polarity of the preformed ions in solution determines the ion polarity mode: Acidic molecules
form negative ions in high pH solution, and basic molecules form positive ions in low pH
solution. You use a positively-charged electrospray needle to generate positive ions and a
negatively-charged needle to generate negative ions.
You can vary the flow rate from the LC into the mass spectrometer from less than 1 μL/min
(using a Nanospray ion source) to 1000 μL/min (using the standard ESI and H-ESI sources).
See Ta b le 4. 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 ESI and H-ESI processes are 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. (In ESI and H-ESI, because the buffer and the buffer strength both have a
noticeable effect on sensitivity, take care to choose these variables correctly.)