What Do S+, S−, and As+ Mean in Protein Mass Spectrometry
In protein mass spectrometry, the terms s+, s-, and as+ typically refer to the following ion types:
1. s+
This usually denotes a singly charged, protonated ion carrying a single positive charge. In mass spectrometry, biomolecules must first be ionized. Ionization techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) enable molecules like proteins and peptides to acquire one or more charges. The s+ designation indicates that a protein or peptide fragment carries one positive charge.
2. s-
This typically represents a singly charged, negatively charged (anionic) ion. Similar to s+, s- denotes a protein or peptide fragment that carries a single negative charge.
3. as+
This term refers to a doubly charged, diprotonated ion bearing two positive charges. The as+ notation indicates that a protein or peptide fragment has acquired two protons.
These designations describe different ionization states encountered during mass spectrometric analysis. Mass spectrometers determine the composition of a sample by measuring the mass-to-charge ratio (m/z) of ionized particles. Variations in ionization methods and sample preparation can result in different degrees of protonation, making it essential to understand the meaning of these terms in protein mass spectrometry.
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