Mass-to-Charge Ratio (m/z)
Definition
What a mass spectrometer actually measures: mass divided by charge, not mass. A 4,000 Da peptide carrying three charges appears near m/z 1,334, which is why a raw spectrum rarely shows the molecular weight directly.
In more detail
A mass spectrometer separates ions by mass divided by charge, not by mass. For small molecules carrying one charge the difference rarely matters. For peptides it matters a great deal, because electrospray routinely puts several charges on one molecule.
A 4,000 Da peptide carrying three charges appears near m/z 1,334, and the same molecule carrying four appears near 1,001. A raw spectrum therefore shows a family of peaks for a single compound, and the molecular mass is not directly visible on it.
Terms used in this definition
Other entries this definition refers to.
Where this term comes up
Entries whose definition mentions Mass-to-Charge Ratio (m/z).
What this means for research sourcing
Purity and identity verification are central to research reproducibility. Every Volta compound ships with >99% HPLC-verified purity and a certificate of analysis.
Related Terms in Quality
Charge State
How many charges an ion carries in the mass spectrometer. Peptides commonly appear in several at once, and the series of peaks they produce is what allows the true molecular mass to be calculated.
Deconvolution
The arithmetic that turns a series of multiply-charged peaks back into one molecular mass. It is the step between what the instrument recorded and the single number printed on a certificate.
Adduct
An ion formed by the analyte plus something else it picked up, commonly sodium or potassium. An unrecognised adduct reads as a mass that does not match the expected one, and as an impurity peak that is not an impurity.
Parts Per Million (ppm)
One milligram per kilogram, or one microgram per gram. Residual solvents and elemental impurities are reported this way, and mass spectrometers also quote mass accuracy in ppm, which is a different use of the same unit.
As-Received Basis
A result calculated on the material exactly as it arrived, water and counterion included. It is the honest basis for what is in the vial and reads lower than the same measurement on a dry basis.
Anhydrous Basis
A result recalculated as though the water content had been removed, using a separately measured water figure. Two certificates can disagree by several percent purely because one reports anhydrous and the other as received.
Not Detected (ND)
The analyte produced no signal distinguishable from background at the method's limit of detection. It states the limit of what was looked for, not that the substance is absent.
Not Less Than (NLT)
A one-sided specification setting a floor. Purity written as NLT 99.0% is met at 99.0 and failed at 98.9, and nothing above the floor is more compliant than anything else above it.