Determining Relative Molecular Mass via Mass Spectrometry: The Principle of Maximum Mass-to-Charge Ratio
When employing mass spectrometry (MS) to ascertain the relative molecular mass, the mass-to-charge ratios (m/z) of molecular ions and various fragment ions are typically observed. A molecular ion is an ion formed when a molecule loses or gains an electron without the loss or separation of other atoms or groups.In a mass spectrum, the molecular ion's m/z value in positive ion mode generally corresponds to the analyte's relative molecular mass. This is due to several factors:
1. Formation of Molecular Ions
Sample molecules are ionized into molecular ions within the ionization source. In certain ionization methods, molecular ions may form when the sample molecule loses or gains a charge without significant fragmentation.
2. Definition of Mass-to-Charge Ratio
The m/z value denotes the mass-to-charge ratio, where m represents the ion's mass and z denotes its charge. For molecular ions, which are typically singly charged, the m/z value corresponds to the actual molecular mass.
3. Mass of Fragment Ions
Fragment ions result from the further fragmentation of molecular ions during analysis. The masses of these fragment ions are necessarily less than that of the original molecule.
In summary, the molecular ion's m/z value in the mass spectrum is typically the highest among all observed ions, representing the analyte's relative molecular mass in positive ion mode. Experimental conditions (e.g., ionization methods and fragmentation energy) and the molecule's chemical properties can influence the formation and stability of molecular and fragment ions.
MtoZ Biolabs, an integrated chromatography and mass spectrometry (MS) services provider.
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